| Akira Imai | Last modified date：2024.04.18 |
Professor /
Earth System Engineering /
Department of Earth Resources Engineering /
Faculty of Engineering
Papers
| 1. | Suhendra, R., Takahashi, R., Imai, A., Sato, H., Setiawan, N. I. & Agangi, A., Primary source of placer gold in the Luk Ulo Metamorphic Complex, Central Java, Indonesia, Resource Geology, 72, 2, e12300, 2022.10. |
| 2. | Sone, S. P., Yonezu, K., Imai, A., Watanabe, K., Tindell, T. & Sanematsu, K., Geological, mineralogical and ore fluid characteristics of the Tagun-Khin-Dan gold mineralization in Mogok-Mandalay-Mergui Belt, Central Myanmar, Resource Geology, 72, 1, e12298, 2022.09. |
| 3. | Pearlyn Manalo, Ryohei Takahashi, Akira Imai, Rhyza Ruth Parcon-Calamohoy, Mervin de los Santos, Leo Subang, Glenn Christian Alburo, Heterogeneity of mineral chemistry and sulfur isotopic composition of alunite in the Mankayan lithocap, northern Luzon, Philippines, ORE GEOLOGY REVIEWS, 10.1016/j.oregeorev.2022.104959, 146, 2022.07, The lithocap at Mankayan is a contiguous zone of alunite + quartz + pyrite that occur as pervasive alteration of the metavolcanic basement and dacitic pyroclastic rocks. Alunite + quartz + pyrite assemblage also occurs within hydrothermal gold-bearing veins and breccias. Several high-sulfidation epithermal gold orebodies have been previously delineated in the Lepanto Main Enargite orebody, and in the Northwest, Carmen, Florence West and Florence East quartz-pyrite-gold (QPG) veins. This study presents new data of mineral composition and sulfur isotopic ratios of alunites from different zones of the lithocap at Mankayan. Elemental composition maps and point analysis of alunites indicate compositional heterogeneity within a single alunite crystal. Common substituents to the K+ site are Na+ and H3O+, with few Ca2+ substitution in some sites. Limited occurrences of PO43- substitution to SO42- were also documented. Sulfur isotopic ratios (delta 34SCDT) of alunite range from + 13 %o to + 24 %o, which are typical of hypogene alunite. Sulfur isotopic ratios of coexisting pyrite are mostly negative, ranging from -5.4 to -1.0 %o. Few samples of alunite from the Northwest and Florence West quartz-pyrite-gold veins have sulfur isotopic ratios similar to the values of its coexisting pyrite. The estimated temperature of formation using sulfur isotope geothermometry of alunite-pyrite pairs ranges from 197 degrees C to 364 degrees C, with most of the samples varying within 220 to 270 degrees C. The calculated bulk delta 34S of the hydrothermal fluid was found to be + 5 to + 6 %o for the different mineralization events in Mankayan. The alunite crystals are heterogeneous even in the microscopic scale. Compositional maps show that K and Na concentration vary within single crystals, as well as among different crystals of a sample. The variations of K and Na content mostly follow the crystal growth structure, however, some alunite crystals with irregular variations are also present. Some samples contain aluminum-phosphate-sulfate (APS) minerals intergrown with alunite. Electron probe microanalysis of the alunite crystals showed a generally wide range of composition between the alunite - natroalunite solid solution. Using thermodynamic functions, a relationship between temperature and K+/Na+ of the hydrothermal fluids was determined across the range of alunite-natroalunite solid solution system. The model curves suggest that at temperatures less than ~ 250 degrees C a slight change in fluid composition and/or temperature can vary the number of Na atoms per formula unit by 0.3 units. At higher temperatures, a more significant change in the physicochemical conditions is required for a substitution to occur. This explains the wide range of Na content in the alunites from the different parts of the lithocap. The characteristics compiled in this study reflect the fluctuations in temperature and fluid compositions that occurred during the multiple hydrothermal events in the Mankayan District.. |
| 4. | Zhekai Zhou, Kotaro Yonezu, Akira Imai, Thomas Tindell, Huan Li, Jillian Aira Gabo-Ratio, Trace elements mineral chemistry of sulfides from the Woxi Au-Sb-W deposit, southern China, Resource Geology, 70, 2022.01. |
| 5. | Cirineo, A. V. L., Imai, A., Takahashi, R., Baluda, R. P., Oliveros, N., Maglambayan, V. B., Luis, R. R., Faustino, M. L. M. and Almadin, J., Overprinting porphyry-type veinlets on the intrusive rocks and phreatomagmatic breccias in the Southwest prospect, southwestern Sto. Tomas II (Philex), Baguio District, Philippines, Resource Geology, DOI: 10.1111/rge12242, 71, 1, 1-40, 71 (1), 1-40. , 2021.01. |
| 6. | Jacob Kaavera, Akira Imai, Kotaro Yonezu, Thomas David Tindell, Kenzo Sanematsu, Koichiro Watanabe, Controls on the disseminated Ni-Cu-PGE sulfide mineralization at the Tubane section, northern Molopo Farms Complex, Botswana Implications for the formation of conduit style magmatic sulfide ores, Ore Geology Reviews, 10.1016/j.oregeorev.2020.103731, 126, 2020.11, The Molopo Farms Complex (MFC) is located near the western margin of the Archean Kaapvaal Craton, in southern Botswana. The complex hosts low grade disseminated Ni-Cu and platinum group elements (PGE) mineralization restricted to the lower ultramafic cumulates, notably in the feldspathic orthopyroxenites. The mineralized feldspathic orthopyroxenites contain up to 0.47 ppm Pt + Pd + Au manifested in the form of platinum group mineral (PGM) assemblages dominated by PGE bismuthotellurides, asernides and minor Au and Ag alloys. The PGM encountered are froodite (PdBi2), moncheite (PtTe2), michenerite (PdBiTe), sperrylite (PtAs2) largely associated with altered silicates and sulfide minerals. The association of PGM with sulfides suggests that they were originally concentrated by sulfide melt, while late stage reaction of sulfides with hydrothermal fluids resulted in the observed spatial proximity of PGM with altered silicates. The Cu/Pd ratios of most of the rocks fall within the primitive mantle range. However, the Cu/Pd ratios of the feldspathic orthopyroxenites are more variable, with few samples falling above and below the primitive mantle range suggesting the prior sulfide extraction from the magmas and the presence of cumulus sulfides. This is supported by the Cu/Zr ratios of these rocks which are also variable above and below unity. Trace element ratios on the (Th/Yb) PM versus (Nb/Th) PM plot indicate strong crustal contamination across all units including those not in proximity to the country rocks. The δ34S of the magmatic rocks are variable between −6.3‰ and −11.4‰. In contrast, the δ34S of dolomitic siltstones of the Transvaal Supergroup which constitute the immediate country rocks vary from −4.1‰ to +0.6‰. These data indicate that crustal contamination largely occurred prior to magma emplacement. It is inferred that the most important contaminant and source of crustal S were the Ramotswa and Ditlhojana Formations that include sulfidic shales at depth. We consider that magnesian basaltic magmas of B1 Bushveld magma composition underwent crustal contamination, assimilation of crustal S and segregation of sulfide melt in the feeder conduit or staging-chamber prior to final magma emplacement. The feeder conduit or staging-chamber was subsequently invaded by further B1 magma. This magma entrained some of the sulfides lodged in the plumbing system and flushed them to the final MFC chamber studied here. After the staging-chamber had already partially solidified, it was invaded by yet another influx of magma, possibly even more primitive than the B1 Bushveld magma. Being a crystal mush, this magma assimilated and entrained some of the previously formed orthopyroxenites manifested by fragments of orthopyroxenite in the harzburgites of the final MFC chamber. The presence of centimeter-sized globular sulfides and fragments of orthopyroxenite within harzburgites suggests that the complex was emplaced as separate pulses of magmas. Therefore, it is considered that sulfide accumulations may remain undiscovered in the feeder zones or the staging-chamber.. |
| 7. | Pearlyn C. Manalo, Akira Imai, Ryohei Takahashi, Hinako Sato, Leo Subang, Mervin de los Santos, Lithogeochemistry of hydrothermally-altered host rocks by multiple mineralizations in the Mankayan Mineral District, Philippines, Journal of Geochemical Exploration, 10.1016/j.gexplo.2020.106612, 218, 2020.11, The overprinting episodes of porphyry-type and epithermal-type mineralization in the Mankayan District, northern Luzon, Philippines allow an investigation on variation of geochemical signatures with different alteration assemblages. Due to the multiple hydrothermal activities in Mankayan, some older porphyry-type deposits have been overprinted by acid-sulfate alteration that is commonly associated with high-sulfidation epithermal mineralization. In this study, we analyzed metavolcanic basement rocks and dioritic rocks that host porphyry-type mineralization in the Far Southeast deposit, Honeycomb prospect and Christine prospect, which all have near-neutral pH alteration assemblage of illite/muscovite + chlorite ± quartz. We compared their geochemical data with those of the Fatima porphyry deposit, which was overprinted by quartz + alunite ± kaolinite/dickite alteration. Furthermore, we also compared the geochemical data of the metavolcanic and dioritic rocks in the Carmen and Florence prospects hosting quartz-pyrite-gold and enargite veins which overprinted earlier porphyry-type mineralization. N-MORB-normalized multi-element diagrams show that the host rocks preserve negative anomalies of high field strength elements (HFSEs) that are typical of magmas generated in a supra-subduction zone setting. The non-mineralized metavolcanic and dioritic rocks show variable signature of the large ion lithophiles (LILEs), while the altered metavolcanic and dioritic rocks show a more consistent signature. Negative anomaly of Rb is associated with acid-sulfate alteration, while positive anomaly of Rb was observed in the dioritic rocks that have been altered by near-neutral pH fluids. Furthermore, acid-sulfate alteration significantly added LREEs and depleted HREEs compared to the near-neutral pH alteration. Mass changes in altered rocks were determined using the isocon technique. Mass changes are generally more pronounced in the acid-sulfate altered rocks compared to the near-neutral pH altered rocks. The metavolcanic rocks altered to quartz + alunite in Carmen and Florence significantly gained SiO2, Al2O3 and S, and lost Na2O, CaO, MgO and Fe2O3. The dioritic and metavolcanic rocks altered to chlorite + illite in the Far Southeast, Honeycomb, Christine and Fatima porphyry deposits lost SiO2 and Al2O3, while the concentrations of other elements remain relatively unchanged. The pH-dependence of the geochemical behavior of Rb in hydrothermal conditions was utilized to construct new molar element ratios that could differentiate the rocks that underwent K-metasomatism either under near-neutral pH or acidic conditions. The recognition of this behavior could be useful in a routine lithogeochemical analysis that is being used in exploration.. |
| 8. | Pearlyn C. Manalo, Leo L. Subang, Akira Imai, Mervin C. de los Santos, Ryohei Takahashi, Nigel J.F. Blamey, Geochemistry and Fluid Inclusions Analysis of Vein Quartz in the Multiple Hydrothermal Systems of Mankayan Mineral District, Philippines, Resource Geology, 10.1111/rge.12214, 70, 1, 1-27, 2020.01, Several high-sulfidation epithermal gold orebodies in the Mankayan Mineral District were formed in an environment that has been already affected by earlier porphyry-type mineralization. This study reports the geologic and geochemical characteristics of the Carmen and Florence epithermal orebodies, which are located in the south of the Lepanto main enargite–gold orebody. The gold-bearing epithermal quartz veins in the Carmen and Florence areas are of two types: (i) the enargite-rich veins and (ii) the quartz–pyrite–gold (QPG) veins. The two types of veins are mainly hosted by the Cretaceous Lepanto Metavolcanics basement rocks, with minor veins cutting the Pleistocene Imbanguila Dacite Pyroclastics. The mineral assemblages and homogenization temperatures of fluid inclusions indicate that the Carmen and Florence orebodies were deposited by fluids varying from high to very high sulfidation state. The enargite and QPG epithermal veins of Carmen and Florence cut porphyry-type quartz veinlet stockworks and veins that host polyphase hypersaline fluid inclusions that did not homogenize at or below 400°C. These high-temperature quartz exhibits distinctly different mineral chemistry from the quartz of the QPG and enargite-rich epithermal veins. In particular, the Ti content of quartz of the porphyry-type veinlet stockwork is elevated (>100 ppm), whereas the Ti concentration of the epithermal vein quartz crystals are below detection limits. The Fe concentration of quartz is high in epithermal vein quartz (>300 ppm), whereas nearly undetected in the porphyry-type stockwork veinlet quartz. Multiple generations of quartz with different mineral chemistry, fluid inclusions morphology, temperature, salinity and bulk gas compositions, and stable isotopic ratios indicate the variable hydrothermal conditions throughout the mineralization history of the Mankayan District. The temperature, pH, sulfidation state, oxidation state, and fluid composition vary among the orebodies in Carmen and Florence areas. Furthermore, the characteristics of earlier alteration affected the apparent characteristics of subsequent mineralization.. |
| 9. | Tomy Alvin Rivai, Kotaro Yonezu, Syafrizal, Kenzo Sanematsu, Damar Kusumanto, Akira Imai, Koichiro Watanabe, A Low-Sulfidation Epithermal Mineralization in the River Reef Zone, the Poboya Prospect, Central Sulawesi, Indonesia Vein Textures, Ore Mineralogy, and Fluid Inclusions, Resource Geology, 10.1111/rge.12206, 69, 4, 385-401, 2019.10, The Poboya Prospect lies along the North Northwest - South Southeast Palu-Koro Fault Zone in the central part of the West Sulawesi Arc. The geology of the area consists of the Palu Metamorphic Complex overlain by the Paleogene-Neogene Tinombo Formation of volcanosedimentary rocks, the Celebes Molasse sediment, and Late Cenozoic granitic rocks. Petrography, scanning electron microscope with energy-dispersive spectrometry (SEM-EDS), electron probe microanalyzer (EPMA), and fluid inclusion microthermometry were carried out to examine vein textures, ore mineralogy, and characteristics of the ore-forming fluid responsible for mineralization in the River Reef Zone, the Poboya Prospect. Textures of quartz-carbonate veins in the River Reef Zone include massive micro-comb, moss, colloform, crustiform, mosaic, feathery, flamboyant, lattice bladed, ghost bladed, parallel bladed, and saccharoidal textures representing primary growth, recrystallization, and replacement. The homogenization temperature and fluid salinity are 240–250°C and 0.3–0.7 wt% NaCl eq., respectively. Ore minerals precipitated in the early stage consist of electrum, naumannite-aguilarite, chalcopyrite, pyrite, marcasite, sphalerite, and pyrrhotite. Apart from pyrrhotite, these ore minerals were also precipitated in the late stage along with selenopolybasite, freibergite, argyrodite, pyrargyrite, and galena. Selenium more preferably occurs as the crystallographic replacement of sulfur in naumannite-aguilarite, argyrodite, pyrargyrite, selenopolybasite, and freibergite instead of as independent selenide minerals. The low-sulfidation epithermal deposit in the River Reef Zone, the Poboya Prospect, illustrates the potential of the West Sulawesi Arc, particularly along the Palu-Koro Fault Zone, to host epithermal gold mineralization.. |
| 10. | Thomas T. Sorulen, Ryohei Takahashi, So Tanaka, Kana Suzuki, Akira Imai, Yasushi Watanabe, Shingo Kikuchi, Mineralogical and Geochemical Characteristics of the Utanobori Gold Deposit in Northern Hokkaido, Japan, Resource Geology, 10.1111/rge.12211, 69, 4, 402-429, 2019.10, The Utanobori gold deposit is a low-sulfidation, epithermal vein-type deposit located in northern Hokkaido, Japan. The deposit is hosted by conglomerate, sandstone, and tuff of the Middle to Late Miocene Esashi Formation. These rocks were hydrothermally altered. Silica sinters and quartz-adularia veins are common in the deposit. The quartz-adularia veins either contain a ginguro band, which corresponds to the main gold-bearing vein (Type 1 Veins), or do not contain a ginguro band but contain minor adularia (Type 2 Veins). Type 1 Veins are divided into three stages with 12–14 substages. Ore minerals identified include electrum, naumannite, chlorargyrite, bromargyrite, an unidentified Fe-Sb mineral, and an Fe-(Sb)-As mineral. These ore minerals formed in the main mineralization stages I (bands I-b and I-d) and II (band II-a). Scanning electron microscopy with cathodoluminescence images show that cathodoluminescence-dark microcrystalline quartz exhibiting colloform (ghost-sphere) texture is closely associated with ore minerals in the Type 1 Vein and Type 2 Vein, and the Al and K contents of such quartz are commonly '1000 ppm. This indicates that the ore minerals were crystallized from alkaline, silica-saturated fluids at temperatures '200°C, which initially deposited amorphous silica that was recrystallized to microcrystalline quartz. The average Au content of electrum is 52.5 at% Au (n = 10), 65.7 at% Au (n = 20), and 55.5 at% Au (n = 5) in bands I-b, I-d, and II-a, respectively, of Type 1 Veins. The δ34SCDT values of two fine-grained disseminated pyrites in the altered conglomerate and bedded tuff in the argillic altered zone are −4.3 and −4.2‰. Ar-Ar dating on adularia yielded 13.6 ± 0.06 Ma, 13.6 ± 0.07 Ma, and 13.6 ± 0.06 Ma for the stages I, II, and III of the Type 1 Vein, respectively. K-Ar ages determined on adularia in the silica sinter and on whole-rock of glassy rhyolite of the Esashi Formation are 15.0 ± 0.4 Ma and 14.6 ± 0.4 Ma, respectively. These radiometric ages indicate that silica sinter associated with the rhyolitic volcanic rocks formed prior to the main gold mineralization.. |
| 11. | Sofia Marah P. Frias, Akira Imai, Ryohei Takahashi, Ma Ines Rosana Balangue-Tarriela, Carlo Arcilla, Nigel Blamey, Geology, Alteration, and Mineralization of the Kay Tanda Epithermal Gold Deposit, Lobo, Batangas, Philippines, Resource Geology, 10.1111/rge.12205, 2019.07, The Kay Tanda epithermal Au deposit in Lobo, Batangas is one of the Au deposits situated in the Batangas Mineral District in southern Luzon, Philippines. This study aims to document the geological, alteration, and mineralization characteristics and to determine the age of the mineralization, the mechanism of ore deposition, and the hydrothermal fluid characteristics of the Kay Tanda deposit. The geology of Kay Tanda consists of (i) the Talahib Volcanic Sequence, a Middle Miocene dacitic to andesitic volcaniclastic sequence that served as the host rock of the mineralization; (ii) the Balibago Diorite Complex, a cogenetic intrusive complex intruding the Talahib Volcanic Sequence; (iii) the Calatagan Formation, a Late Miocene to Early Pliocene volcanosedimentary formation unconformably overlying the Talahib Volcanic Sequence; (iv) the Dacite Porphyry Intrusives, which intruded the older lithological units; and (v) the Balibago Andesite, a Pliocene postmineralization volcaniclastic unit. K-Ar dating on illite collected from the alteration haloes around quartz veins demonstrated that the age of mineralization is around 5.9 ± 0.2 to 5.5 ± 0.2 Ma (Late Miocene). Two main styles of mineralization are identified in Kay Tanda. The first style is an early-stage extensive epithermal mineralization characterized by stratabound Au-Ag-bearing quartz stockworks hosted at the shallower levels of the Talahib Volcanic Sequence. The second style is a late-stage base metal (Zn, Pb, and Cu) epithermal mineralization with local bonanza-grade Au mineralization hosted in veins and hydrothermal breccias that are intersected at deeper levels of the Talahib Volcanic Sequence and at the shallower levels of the Balibago Intrusive Complex. Paragenetic studies on the mineralization in Kay Tanda defined six stages of mineralization; the first two belong to the first mineralization style, while the last four belong to the second mineralization style. Stage 1 is composed of quartz ± pyrophyllite ± dickite/kaolinite ± diaspore alteration, which is cut by quartz veins. Stage 2 is composed of Au-Ag-bearing quartz stockworks associated with pervasive illite ± quartz ± smectite ± kaolinite alteration. Stage 3 is composed of carbonate veins with minor base metal sulfides. Stage 4 is composed of quartz ± adularia ± calcite veins and hydrothermal breccias, hosting the main base metal and bonanza-grade Au mineralization, and is associated with chlorite-illite-quartz alteration. Stage 5 is composed of epidote-carbonate veins associated with epidote-calcite-chlorite alteration. Stage 6 is composed of anhydrite-gypsum veins with minor base metal mineralization. The alteration assemblage of the deposit evolved from an acidic mineral assemblage caused by the condensation of magmatic volatiles from the Balibago Intrusive Complex into the groundwater to a slightly acidic mineral assemblage caused by the interaction of the host rocks and the circulating hydrothermal waters being heated up by the Dacite Porphyry Intrusives to a near-neutral pH toward the later parts of the mineralization. Fluid inclusion microthermometry indicates that the temperature of the system started to increase during Stage 1 (T = 220–250°C) and remained at high temperatures (T = 250–290°C) toward Stage 6 due to the continuous intrusion of Dacite Porphyry Intrusives at depth. Salinity slightly decreased toward the later stages due to the contribution of more meteoric waters into the hydrothermal system. Boiling is considered the main mechanism of ore deposition based on the occurrence of rhombic adularia, the heterogeneous trapping of fluid inclusions of variable liquid–vapor ratios, the distribution of homogenization temperatures, and the gas ratios obtained from the quantitative fluid inclusion gas analysis of quartz. Ore mineral assemblage and sulfur fugacity determined from the FeS content of sphalerite at temperatures estimated by fluid inclusion microthermometry indicate that the base metal mineralization at Kay Tanda evolved from a high sulfidation to an intermediate sulfidation condition.. |
| 12. | Muhammad Zain Tuakia, Ryohei Takahashi, Akira Imai, Geological and Geochemical Characteristics of Gold Mineralization in the Salu Bulo Prospect, Sulawesi, Indonesia, Resource Geology, 10.1111/rge.12193, 69, 2, 176-192, 2019.04, The Salu Bulo prospect is one of the gold prospects in the Awak Mas project in the central part of the western province, Sulawesi, Indonesia. The gold mineralization is hosted by the meta-sedimentary rocks intercalated with the meta-volcanic and volcaniclastic rocks of the Latimojong Metamorphic Complex. The ores are approximately three meters thick, consisting of veins, stockwork, and breccias. The veins can be classified into three stages, namely, early, main, and late stages, and gold mineralization is related to the main stage. The mineral assemblage of the matrix of breccia and the veins are both composed of quartz, carbonate (mainly ankerite), and albite. High-grade gold ores in the Salu Bulo prospect are accompanied by intense alteration, such as carbonatization, albitization, silicification, and sulfidation along the main stage veins and breccia. Alteration mineral assemblage includes ankerite ± calcite, quartz, albite, and pyrite along with minor sericite. Pyrite is the most abundant sulfide mineral that is spatially related to native gold and electrum ( 2 and N 2 gases are detected in the fluid inclusions by Laser Raman microspectrometry. Fluid boiling probably occurred when the fluid was trapped at approximately 120–190 m below the paleo water table. δ 18 O SMOW values of fluid, +5.8 and +7.6‰, calculated from δ 18 O SMOW of quartz from the main stage vein indicate oxygen isotopic exchange with wall rocks during deep circulation. δ 34 S CDT of pyrite narrowly ranges from −2.0 to +3.4‰, suggesting a single source of sulfur. Gold mineralization in the Salu Bulo prospect occurred in an epithermal condition, after the metamorphism of the host rocks. It formed at a relatively shallow depth from fluids with low to moderate salinity (3.0–8.5 wt% NaCl equiv.). The temperature and pressure of ore formation range from 190 to 210°C and 1.2 to 1.9 MPa, respectively.. |
| 13. | Nyein N. Sint, Kotaro Yonezu, Thomas Tindell, May T. Aye, Htay Win, Akira Imai, Koichiro Watanabe, Geology and Skarn Cu–Bi–Au Mineralization at Shwe Min Bon Area, Kalaw Township, Southern Shan State, Myanmar, Resource Geology, 10.1111/rge.12187, 69, 1, 85-106, 2019.01, The Shwe Min Bon Cu–Au skarn deposit lies within one of the largest Au–Cu belts in Myanmar. The deposit is situated along the Shan scarp zone, which marks the boundary between the Myanmar central basin to the west and the Shan plateau to the east. The Shwe Min Bon deposit comprises skarn-type metasomatic alteration, and the Cu–Au mineralization occurs along the contact face between the Nwabangyi Dolomite and Shweminbon Formation and the Cretaceous dioritic rocks. The metasomatic process resulted in pro- and retrograde mineral assemblages in exoskarn. Hydrothermal activities in the Shwe Min Bon deposit are classified into prograde, retrograde stage I, and retrograde stage II. The prograde skarn is classified into a proximal garnet skarn with minor clinopyroxene and a distal wollastonite skarn. Chlorite, epidote, and tremolite–actinolite were formed during the retrograde stage I. Cu–Au mineralization mainly occurred in retrograde stage I, which was characterized by moderate temperatures (260–320 °C) and fluid with a moderate salinity (5.0–6.0% NaCl equiv.). Low temperature (180–200 °C) and low salinity (2.0–3.0% NaCl equiv.) were responsible for retrograde stage II. Au mineralization is mainly associated with chalcopyrite and tennantite in retrograde stage I and with tellurobismuthite in retrograde stage II.. |
| 14. | Pearlyn C. Manalo, Akira Imai, Leo L. Subang, Mervin C. De los Santos, Kyoka Yanagi, Ryohei Takahashi, Nigel J.F. Blamey, Mineralization of the Northwest quartz-pyrite-gold veins Implications for multiple mineralization events at lepanto, mankayan mineral district, Northern Luzon, Philippines, Economic Geology, 10.5382/econgeo.2018.4606, 113, 7, 1609-1626, 2018.11, The Northwest quartz-pyrite-gold veins are situated 500 m east of the Lepanto fault in Mankayan, Luzon, Philippines. Most vein mineralization is hosted by the Lepanto metavolcanic basement rocks at an elevation between 700 and 1,050 m. The earliest stage, stage 1, is characterized by sphalerite + chalcopyrite + pyrite ± magnetite veins cutting the host rocks that were altered to chlorite + illite + epidote. Precious metal deposition started in stage 2 as electrum, native gold, and gold-silver tellurides deposited with pyrite, quartz, and carbonate. Deposition of gold and silver tellurides continued in stage 3a with abundant pyrite and tennantite-tetrahedrite solid solution intergrown with chalcopyrite, bornite, and minor sphalerite. The stage 3a veins and host-rock alteration are characterized by abundant muscovite and quartz, while the stage 3b veins and alteration consist of quartz, pyrophyllite, alunite, and dickite. Enargite and luzonite are the dominant sulfide minerals in stage 4, which are either disseminated in silicified host rock or within wide quartz veins. Lesser amounts of quartz and abundant pyrite with inclusions of enargite and luzonite were precipitated in stage 5. Microthermometry on fluid inclusions in quartz of stages 2, 3a, and 4 indicates boiling of the hydrothermal fluids. Bulk gas analysis on fluid inclusions in quartz shows that the stage 2 and 4 fluids had components derived from basaltic and andesitic magma, respectively. Fluids that formed stage 4 quartz were more diluted by meteoric water than the fluids that formed the stage 2 veins. Radiometric 40Ar/39Ar dating on alunite separated from the stage 3b advanced argillic alteration zone yielded 2.2 ± 0.1 Ma. Sulfur isotope compositions of the Northwest quartz-pyrite-gold deposit reveal a bulk δ34S of approximately 5, similar to the calculated value for the adjacent Far Southeast porphyry deposit. Calculated oxygen and hydrogen isotope ratios of the fluids of the Northwest quartz-pyrite-gold deposit stage 3b dickites are similar to those that formed the illite alteration in the Far Southeast porphyry deposit but are higher in δ18O when compared to the fluids that formed the kandites in the Lepanto enargite deposit. The northward cooling of mineralizing fluids previously reported in the Lepanto enargite deposit is not consistent with the mineralogic indications in the Northwest quartz-pyrite-gold deposit. These data indicate multiple mineralization events in the Mankayan district, which is one of the largest mineral districts in the western Pacific.. |
| 15. | Thomas Tindell, Koichiro Watanabe, Akira Imai, Ryohei Takahashi, Adrian J. Boyce, Kotaro Yonezu, Anders Schersten, Laurence Page, Takeyuki Ogata, The Kago low-sulfidation gold and silver deposit A peripheral mineralisation to the Nansatsu high-sulfidation system, southern Kyushu, Japan, Ore Geology Reviews, 10.1016/j.oregeorev.2017.10.027, 102, 951-966, 2018.11, The Kago deposit is a small deposit located at the southern tip of the Satsuma Peninsula of Southern Kyushu, Japan. It lies proximal to the well-known Nansatsu-type mineralisation province dominated by high-sulfidation type epithermal deposits. The deposit was heavily mined in the 18 th Century, largely for its relatively higher gold compared to that of surrounding and regional deposits. The Kago deposit is a typical low-sulfidation deposit, characterised by adularia-quartz veins, composed of electrum, Ag-tetrahedrite, polybasite, chalcopyrite and pyrite. Based on mine records, the grade ranged from 4.1 to 13.3 g/t Au and 2.6–6.6 g/t Ag. Alteration grades from low to high temperature argillic into a propylitic zone at the extremes of the vein exposures. Carbonate is absent. Fluid inclusion microthermometry reveals a typical temperature range of 220–240 °C with salinity of 0.7–2.6 NaCl eq. wt%. Electrum from high-grade ore ranges from 66 to 69 wt% Au. 40Ar/39Ar age dating of adularia bearing colloform/crustiform and brecciated veins, suggests a mineralisation event from 4.23 to 4.0 Ma. δ18O of veined and silicified quartz ranges from +4.0 to +18.4‰. δ18OH2O of fluids in equilibrium with quartz, in the dominant range of measured fluid inclusion temperatures, ranges from −6.5‰ to −0.2‰. δ34S of pyrite has a narrow range from −1.8 to 2.7‰. The deposit lies at the northern extent of the classic Nansatsu high-sulfidation epithermal area, in which a number of silicified bodies punctuate the region in a roughly semi-circular shape. The Kago deposit lies within the principle mineralisation age range of the high-sulfidation deposits, which range from 5.5 to 3.7 Ma. The structural displacement of the Kago deposit from the Nansatsu mineralisation and the differing host rocks has greatly influenced alteration, ore and rock-water interaction of the ore depositing fluids. Here we seek to establish the relationship that this extended mineral province has between the differing styles of mineralisation.. |
| 16. | Patchawee Nualkhao, Ryohei Takahashi, Akira Imai, Punya Charusiri, Petrochemistry of Granitoids Along the Loei Fold Belt, Northeastern Thailand, Resource Geology, 10.1111/rge.12176, 68, 4, 395-424, 2018.10, Petrochemical characteristics of Permo-Triassic granitoids from five regions (i) Mung Loei, (ii) Phu Thap Fah – Phu Thep, (iii) Phetchabun, (iv) Nakon Sawan – Lobburi, and (v) Rayong – Chantaburi along the Loei Fold Belt (LFB), northeastern Thailand were studied. The LFB is a north–south trending 800 km fold belt that hosts several gold and base-metal deposits. The granitoids consist of monzogranite, granodiorite, monzodiorite, tonalite, quartz-syenite, and quartz-rich granitoids. These are composed of quartz, plagioclase, and K-feldspar with mafic minerals such as hornblende and biotite. Accessory minerals, such as titanite, zircon, magnetite, ilmenite, apatite, garnet, rutile, and allanite are also present. Magnetic susceptibilities in the SI unit of granitoids vary from 6.5 × 10 −3 to 15.2 × 10 −3 in Muang Loei, from 0.1 × 10 −3 to 29.4 × 10 −3 in Phu Thap Fah – Phu Thep, from 2.7 × 10 −3 to 34.6 × 10 −3 in Petchabun, from 2.4 × 10 −3 to 14.1 × 10 −3 in Nakon Sawan – Lobburi, and from 0.03 × 10 −3 to 2.8 × 10 −3 in Rayong – Chantaburi. Concentration of major elements suggests that these intermediate to felsic plutonic rocks have calc-alkaline affinities. Concentration of REE of the granitoids normalized to chondrite displays moderately elevated light REE (LREE) and relatively flat heavy (HREE) patterns, with distinct depletion of Eu. Rb versus Y/Nb and Nb/Y tectonic discrimination diagrams illustrate that the granitoids from Muang Loei, Phu Thap Fah – Phu Thep, Phetchabun, Nakon Sawan – Lobburi, and Rayong – Chantaburi formed in continental volcanic-arc setting. New age data from radiometric K-Ar dating on K-feldspar from granodiorite in Loei and Nakhon Sawan areas yielded 171 ± 3 and 221 ± 5 Ma, respectively. K-Ar dating on hornblende separated from diorite in Lobburi yielded 219 ± 8 Ma. These ages suggest that magmatism of Muang Loei occurred in the Middle Jurassic, and Nakon Sawan – Lobburi occurred in Late Triassic. Both Nb versus Y and Rb versus (Y + Nb) diagrams and age data indicate that Nakon Sawan – Lobburi granitoids intruded in Late Triassic at Nong Bua, Nakon Sawan province and Khao Wong Phra Jun, Lobburi province in volcanic arc setting. Muang Loei granitoids at the Loei province formed later in Middle Jurassic also in volcanic arc setting. The negative δ 34 S CDT values of ore minerals from the skarn deposit suggest that the I-type magma has been influenced by light biogenic sulfur from local country rocks. The Au-Cu-Fe-Sb deposits correlate with the magnetite-series granitoids in Phetchabun, Nakon Sawan – Lobburi and Rayong – Chantaburi areas. Metallogeny of the Au and Cu-Au skarn deposits and the epithermal Au deposit is related to adakitic rocks of magnetite-series granitoids from Phetchabun and Nakon Sawan areas. All mineralizations along the LFB are generated in the volcanic arc related to the subduction of Paleo-Tethys. The total Al ( T Al) content of biotite of granitoids increases in the following order: granitoids associated with Fe and Au deposit Mg of biotite in granitoids in Muang Loei indicates the crystallization of biotite in magnetite-series granitoids under high oxygen fugacity conditions. On the other hand, low X Mg ( Mg of biotite were formed by reduction of initially oxidizing magnetite-series granitic magma by interaction with reducing sedimentary country rocks as suggested by negative δ 34 S CDT values.. |
| 17. | Muhammad Tahir, Akira Imai, Ryohei Takahashi, Shujiro Yano, Ore Genesis and Geochemical Characteristics of Carbonate-Hosted Talc Deposits in Nangarhar Province, Afghanistan, Resource Geology, 10.1111/rge.12174, 68, 4, 352-372, 2018.10, Talc deposits in Nangarhar Province, are hosted by Paleoproterozoic carbonate rocks, metamorphosed to amphibolite facies in the east–west (E–W) trending Spinghar Fault Block. Many deposits in this province have potential economic importance. However, detailed geologic and petrological studies on ore genesis are still lacking. In this study, eight talc deposits and two prospects of the Spinghar Fault Block were investigated. Talc is mainly formed by alteration of the host dolomite marble, magnesite rocks, and tremolitite. Talc ore bodies occur parallel to subparallel to the beddings of the host carbonate rocks. Dolerite occur as dikes and sills and are mostly metamorphosed to amphibolite. Although the amphibolite occurs mostly parallel to subparallel to the beddings of the host carbonate rocks, and talc orebodies, it partly crosscuts the host rocks. Massive layers of tremolitite were observed with most of the talc ore bodies. Quartz veins occur along the gneissosity of gneiss all over the study area. SiO2 and MgO content in talc rocks from all deposits ranged from 49.1 to 65.1 wt% and from 26.1 to 32.9 wt%, respectively. CaO content in talc rocks and magnesite rocks are less than 1 wt%. ƩREE content in talc rocks ranged from 0.1 to 8.9 ppm. Chemical compositions of host carbonate rocks are close to the ideal composition. Concentrations of Al, Ta, Hf, Zr, Th, Cr, Ni, Co, and ƩREE in talc ores and host carbonate rocks were very low and inconsistent with mafic and ultramafic rocks protolith. Therefore, the metamorphosed sedimentary carbonate rocks were likely to be the protolith of the talc ores. The occurrence of parallel to subparallel quartz veins to the gneissosity of gneiss, as well as the presence of hydrous minerals in host carbonate rocks, suggested that hydrothermal fluids were most probably supplied through the gneiss.. |
| 18. | Benedict J. Williamson, Matthew Hodgkinson, Akira Imai, Ryohei Takahashi, Robin N. Armstrong, Richard J. Herrington, Testing the Plagioclase Discriminator on the GEOROC Database to Identify Porphyry-Fertile Magmatic Systems in Japan, Resource Geology, 10.1111/rge.12162, 68, 2, 138-143, 2018.04, Despite showing favorable geology, there are no known porphyry-type deposits in Japan. This, therefore, provides an ideal porphyry-“barren” case area on which to test the recently developed plagioclase-based discriminator for porphyry-“fertile” calc-alkaline systems. The discriminator was applied to plagioclase data for Japan in the online GEOROC database for convergent margin settings. Of the 3933 data points available, and considered reliable, 91% fall in the “barren” field, which compares with 73% for equivalent data worldwide. “Fertile” signatures in certain parts of Kyushu and central Honshu should be further investigated, as well as areas where there are magnetite-series magmatic systems with elevated whole-rock Sr/Y but for which there are currently little or no plagioclase data (SW-, N-central-, and NE-Honshu). Why the Japanese arcs appear to show anomalously poor potential for porphyry deposits compared with other arcs worldwide, including Western Luzon, is as yet unclear but possibly relates to tectonic factors or the nature of the magmatic source or because melt PH2O in many of the magmatic systems was too low, as suggested from low average excess Al in plagioclase.. |
| 19. | Adi Maulana, Asri Jaya, Akira Imai, Study on Gold and base metal occurrence in Uluwai Prospect, Western Latimojong Mountain, South Sulawesi, 2nd International Conference on Nuclear Technologies and Sciences, ICoNETS 2017 Journal of Physics: Conference Series, 10.1088/1742-6596/962/1/012011, 962, 1, 2018.03, Uluwai Prospect is located in the northern part of South Arm of Sulawesi along the eastern part of the Kalosi Fold Belt and Latimojong Mountain. The area is generally characterized by moderate to rugged topography area with elevation in the range of 700 to 1400 m above sea level in the mountainous complex called Latimojong Mountain Complex. The mineralization is characterized by a relatively simple sulphide ore mineral assemblage consists of pyrite, sphalerite and chalcopyrite. Samples were collected in areas showing abundant sulphide minerals where younger faults cut the bedding and foliation of country rocks. A number of silicified zones have been observed, as well as float material containing disseminated pyrite, chalcopyrite, and sphalerite with hematite, goethite and limonite. Some alteration types have been observed including sericitization, albitization, carbonatization and silisification. The samples collected indicated that the mineralisation is contained within metasedimentary (sandstone to mudstone) and greenschist. Geochemical analyses from 16 samples including 5 stream sediment samples indicated that the most promising mineralization occur in the prospect area are copper (Cu) and zinc (Zn). This is also supported by the abundance of chalcopyrite and sphalerite in some highly altered samples. Assaying of the collected samples revealed most of samples contain relatively low gold (Au) concentration. However, two samples contain 0.007 and 0.01 ppm of Au. In the mineralized area, Zn concentrations are up to 134 ppm, Cu up to 120 ppm and Pb up to 18 ppm and As up to 70 ppm. There is no clear relationship that exists between Au and the base metals except that one of the samples with highest Au values tend to have high Zn and As. This unclear pattern also shown by Cu, Pb and Zn. Base metal concentration in stream sediment samples show a relatively stable pattern than in rock samples. Arsenic tends to be elevated in base metal rich samples. Sb and Mo are relatively low in all sample type. However, Mo values will be high in the samples which contain highest Cu and Zn.. |
| 20. | Padrones, Jenielyn T., Imai, Akira, Takahashi, Ryohei, Geochemical Behavior of Rare Earth Elements in Weathered Granitic Rocks in Northern Palawan, Philippines, RESOURCE GEOLOGY, 10.1111/rge.12123, 67, 3, 231-253, 2017.07. |
| 21. | Setiawan, I., Takahashi, R., Imai, A., Geology and geochemistry of granitoids associated with REE enrichment at Sibolga and its surroundings, North Sumatra, Indonesia, Resource Geology, 67, 3, 254-278, 2017.07. |
| 22. | Takahashi, Ryohei, Tagiri, Ryo, Blamey, Nigel J. F., Imai, Akira, Watanabe, Yasushi, Takeuchi, Akito, Characteristics and Behavior of Hydrothermal Fluids for Gold Mineralization at the Hishikari Deposits, Kyushu, Japan, RESOURCE GEOLOGY, 10.1111/rge.12136, 67, 3, 279-299, 2017.07. |
| 23. | Padrones, Jenielyn T., Tani, Kenichiro, Tsutsumi, Yukiyasu, Imai, Akira, Imprints of Late Mesozoic tectono-magmatic events on Palawan Continental Block in northern Palawan, Philippines, JOURNAL OF ASIAN EARTH SCIENCES, 10.1016/j.jseaes.2017.01.027, 142, 56-76, 2017.07. |
| 24. | Iwan Setiawan, Ryohei Takahashi, Akira Imai, Petrochemistry of Granitoids in Sibolga and its Surrounding Areas, North Sumatra, Indonesia, Resource Geology, 10.1111/rge.12132, 67, 3, 254-278, 2017.07, Petrochemical studies were conducted on granitoids in Sibolga and surrounding areas in North Sumatra, Indonesia. The granitoids presented the characteristics of A- and I-type ilmenite series. The syenogranites from Sarudik, the syenogranite, quartz syenite, and quartz alkali feldspar syenite from Sibuluhan Sihaporas, and the monzogranites from Sibolga Julu are A-type granitoids that have high SiO2, CaO, Al2O3, Na2O + K2O, and Rb content, a high FeO/MgO ratio, and low Ba, Sr, and Zr content. The alkali feldspar syenite from Sarudik, the quartz alkali feldspar syenite from Tukka, and the quartz syenite from Adian Koting show I-type characteristics. I-type granitoids have lower SiO2, Na2O + K2O, Rb, Yb, and total REE (ΣREE) content and low FeO/MgO ratio. However, quartz syenite from the Sibolga Julu, which exhibits I-type characteristics, has high ΣREE content but a low FeO/MgO ratio. The granitoids consisted mainly of K-feldspar megacrysts, quartz, plagioclase, and biotite with hornblende in some cases. Accessory minerals were zircon, apatite, allanite, and titanite. Cordierite and corundum occurred as xenocrysts and inclusions in K-feldspar in syenogranite (Sarudik) and quartz syenite (Adian Koting). SiO2 and ΣREE content and the Rb/Sr ratio are positively correlated to each other. The REE are enriched in quartz alkali feldspar syenites from Sibolga Julu and Sarudik, quartz syenite from Tarutung, and quartz alkali feldspar syenite, syenogranite, and quartz syenite from Sibuluhan Sihaporas. These highly-differentiated granitoids were formed within the plate settings. In contrast, the ΣREE content of hornblende-bearing granitoids that were formed in volcanic arc settings is low. The coexistence of cordierite and corundum with biotite and hornblende indicates that the metaluminous magma was contaminated by metasedimentary basement rocks.. |
| 25. | Maulana, A., Imai, A., Koichiro Watanabe, KOTARO YONEZU, Nakano, T., Page, L., Schersten, A., Boyce, A., van Leewuen, T., Petrochemistry of Late Cenozoic granitic rocks in Sulawesi, Indonesia: Origin and geodynamic setting, Journal of Asian Earth Sciences., 124, 102-125, 2016.10. |
| 26. | Tanaka, Kosuke, Okawa, Hirokazu, Hashimoto, Keiichiro, Takahashi, Ryohei;, Imai, Akira, Sugawara, Katsuyasu, Effect of NO2 in exhaust gas from an oxyfuel combustion system on the cap rock of a proposed CO2 injection site, APPLIED GEOCHEMISTRY, 10.1016/j.apgeochem.2016.04.007, 70, 17-26, 2016.07, A laboratory geochemical study was conducted using a drill core sample of cap rock from the Surat Basin, Australia, to investigate the effect of NO2 contained in the CO2 gas exhausted from the oxyfuel combustion process (oxyfuel combustion CO2) on the cap rock. A gas (CO2 containing NO2) was prepared to simulate the exhaust gas produced from the oxyfuel combustion process. Two types of gases (pure CO2 and CO2 containing SO2) were also prepared as reference gases. The effect of NO2 on cap rock was studied experimentally using these gases. No differences in the amounts of leached ions and pH changes for CO2 containing NO2 (36 ppmv), pure CO2, and CO2 containing SO2 (35 ppmv) existed. The pH values decreased immediately after CO2 gas injection but increased with time as a result of mineral buffering. Leaching of Fe, Mg, Ca, and K was suggested to have occurred as the result of dissolution of Fe-chlorite, prehnite and illite-smectite mixed layer clay in the shale sample. The amounts of Ca, Fe, and Mg leached with CO2 containing NO2 (318 ppmv) were higher than those for pure CO2. For the mixture containing 318 ppmv NO2, the pH increased more than that for the other gas conditions immediately after the pH fall at the start of the experiment, because oxidation-reduction reactions occurred between Fe2+ and NO3-. Moreover, the results indicated that some of the leached Ca and Fe were deposited on the shale sample because of the pH increase. Therefore, we concluded that the effects of NO2 on mineral dissolution and pH changes of formation water are negligible when oxyfuel combustion CO2 containing about 30 ppmv of NO2 is injected into an underground aquifer. In addition, even if about 300 ppmv NO2 is accidentally injected into the underground aquifer, mineral dissolution is suppressed due to the buffering of pH decrease after gas injection.. |
| 27. | Adi Maulana, Akira Imai, Theo Van Leeuwen, Koichiro Watanabe, Kotaro Yonezu, Takanori Nakano, Adrian Boyce, Laurence Page, Anders Schersten, Origin and geodynamic setting of Late Cenozoic granitoids in Sulawesi, Indonesia, Journal of Asian Earth Sciences, 10.1016/j.jseaes.2016.04.018, 124, 102-125, 2016.07, Late Cenozoic granitoids are widespread in a 1600 km long belt forming the Western and Northern Sulawesi tectono-magmatic provinces. They can be divided into three rock series: shoshonitic (HK), high-K felsic calc-alkaline (CAK), and normal calc-alkaline to tholeiitic (CA-TH). Representative samples collected from eleven plutons, which were subjected to petrography, major element, trace element, Sr, Nd, Pb isotope and whole-rock δ18O analyses, are all I-type and metaluminous to weakly peraluminous. The occurrence of the two K-rich series is restricted to Western Sulawesi, where they formed in an extensional, post-subduction tectonic setting with astenospheric upwelling providing thermal perturbation and adiabatic decompression. Two parental magma sources are proposed: enriched mantle or lower crustal equivalent for HK magmas, and Triassic igneous rocks in a Gondwana-derived fragment thrust beneath the cental and northern parts of Western Sulawesi for CAK magmas. The latter interpretation is based on striking similarities in radiogenic isotope and trace element signatures. CA-TH granitoids are found mostly in Northern Sulawesi. Partial melting of lower-middle crust amphibolites in an active subduction environment is the proposed origin of these rocks. Fractional crystallization and crustal contamination have played a significant role in magma petrogenesis, particularly in the case of the HK and CAK series. Contamination by organic carbon-bearing sedimentary rocks of the HK and CAK granitoids in the central part of Western Sulawesi is suggested by their ilmenite-series (reduced) character. The CAK granitoids further to the north and CA-TH granitoids in Northern Sulawesi are typical magnetite-series (oxidized). This may explain differences in mineralization styles in the two regions.. |
| 28. | Saing, S., Takahashi, R., Imai, A., Fluid inclusion and stable isotopes study on the southeastern Martabe deposit: Purnama, Barani and Horas ore bodies, North Sumatra, Indonesia, Resource Geology, 66, 127-148, 2016.04. |
| 29. | SAING Stephanie, TAKAHASHI Ryohei, IMAI Akira, Fluid Inclusion and Stable Isotope Study at the Southeastern Martabe Deposit : Purnama, Barani and Horas Ore Bodies, North Sumatra, Indonesia, Resource geology : an official journal of the Society of Resource Geology, 10.1111/rge.12093, 66, 2, 127-148, 2016.04. |
| 30. | Harijoko, Agung, Uruma, Ryusuke, Wibowo, Haryo Edi, Setijadji, Lucas Doni, Imai, Akira, KOTARO YONEZU, Koichiro Watanabe, Geochronology and magmatic evolution of the Dieng Volcanic Complex, Central Java, Indonesia and their relationships to geothermal resources, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/j.jvolgeores.2015.12.010, 310, 209-224, 2016.01, We analyzed new radiometric dating and petrological data of DVC in an attempt to reconstruct volcanic history as groundwork to understand magmatic temporal and spatial evolution. The magma of DVC can be divided on the basis of mineral composition into three types: olivine bearing basalt-basaltic andesite, pyroxene basaltic andesite-andesite, and biotite andesite-dacite, which coincide with three volcanic episodes of DVC: pre-caldera, second, and youngest episode, respectively. The pre-caldera episode was active no later than 1Ma, the second episode occurred between 0.3 and 0.4Ma, and the youngest occurred after 0.27Ma. Plots of CaO, K2O, Al2O3, and Rb/Sr against FeO*/MgO and/or MgO suggest that each volcanic episode has distinct differentiation trends, indicating the presence of multiple shallow magma chambers. The close spatial relationship between the geothermal manifestation, geophysical anomalies, geothermal production zones and volcanic edifices supports the presence of multiple shallow magma chambers beneath DVC, which act as a heat source for the existing geothermal system.. |
| 31. | Sanematsu, Kenzo, Kon, Yoshiaki, Imai, Akira, Influence of phosphate on mobility and adsorption of REEs during weathering of granites in Thailand, JOURNAL OF ASIAN EARTH SCIENCES, 10.1016/j.jseaes.2015.05.018, 111, 14-30, 2015.11, The Permo-Jurassic North Thai (NT) Granites and the Late Cretaceous to Paleogene Western Province (WP) Granites in Thailand are contrasting in terms of tectonic settings and chemical compositions. The NT Granites, which are dominated by S-type features, are characterized by lower SiO |
| 32. | Gabo, J.A.S., Dimalanta, C.B., Yumul, G.P., Jr., Faustino-Eslava, D.V., Imai, A., Geophysical signatures of a terrane boundary in Northwest Panay, Philippines: Results from the gravity, seismic refraction and electrical resistivity investigations, Terrestrial, Atmospheric, and Oceanic Science, 10.3319/TAO.2015.05.11.03(TC), 26, 6, 663-678, 2015.10, Northwest Panay consists of two terranes that form part of the Central Philippine collision zone: Buruanga Peninsula and Antique Range. The Buruanga Peninsula consists of a Jurassic chert-clastic-limestone sequence, typical of oceanic plate stratigraphy of the Palawan Micro-continental Block. The Antique Range is characterized by Antique Ophiolite Complex peridotites and Miocene volcanic and clastic rocks, representing obducted oceanic crust that serves as the oceanic leading edge of the collision with the Philippine Mobile Belt. The Nabas Fault is identified as the boundary between the two terranes. This study employed the gravity method to characterize the Northwest Panay subsurface structure. Results indicate higher Bouguer anomaly values for Buruanga Peninsula than those for Antique Range, separated by a sudden decrease in gravity values toward the east-southeast (ESE) direction. Forward gravity data modeling indicates the presence of an underlying basaltic subducted slab in the Buruanga Peninsula. Furthermore, the Nabas Fault is characterized as an east-dipping thrust structure formed by Buruanga Peninsula basement leading edge subduction beneath Antique Range. Additional geophysical constraints were provided by shallow seismic refraction and electrical resistivity surveys. Results from both methods delineated the shallow subsurface signature of the Nabas Fault buried beneath alluvium deposits. The gravity, seismic refraction and electrical resistivity methods were consistent in identifying the Nabas Fault as the terrane boundary between the Buruanga Peninsula and the Antique Range. The three geophysical methods helped constrain the subsurface configuration in Northwest Panay.. |
| 33. | ERDENEBAYAR Jamsran, OGATA Takeyuki, IMAI Akira, Textural and Chemical Evolution of Unidirectional Solidification Textures in Highly Differentiated Granitic Rocks at Kharaatyagaan, Central Mongolia, Resource geology : an official journal of the Society of Resource Geology, 10.1111/rge.12050, 64, 4, 283-300, 2014.10. |
| 34. | Ryohei Takahashi, Yuki Shingo, Akira Imai, Koichiro Watanabe, Agung Harijoko, I. Wayan Warmada, Arifudin Idrus, Lucas Donny Setijadji, Phouvang Phoumephone, Anders Scherstén, Laurence Page, Epithermal gold mineralization in the Trenggalek District, East Java, Indonesia, Resource Geology, 10.1111/rge.12034, 64, 2, 149-166, 2014.04, Gold-mineralized quartz veins at the Trenggalek district of the Southern Mountains Range in East Java, Indonesia, are hosted by Oligo-Miocene volcaniclastic and volcanic rocks, and are distributed close to andesitic plugs in the northern prospects (Dalangturu, Suruh, Jati, Gregah, Jombok, Salak, and Kojan) and the southern prospects (Sentul and Buluroto). The plugs are subalkaline tholeiitic basaltic-andesite to calc-alkaline andesite in composition. 40Ar-39Ar dating of a quartz-adularia vein at the Dalangturu prospect yielded an age of 16.29 ± 0.56 Ma (2σ), and a crystal tuff of a limestone-pyroclastic rock sequence at the southwest of the Dalangturu prospect was determined as 15.6 ± 0.5 Ma (2σ). Statistic overlap of ages suggests that the gold mineralization in the northern prospects took place in a shallow marine to subaerial transitional environment. Hydrothermal alteration of the host rocks is characterized by the replacement of quartz, illite and adularia. Quartz veins in surface outcrops are up to 50cm wide in the northern prospects and up to 3m wide in the southern prospects, showing a banded or brecciated texture, and are composed of quartz, adularia, carbonates with pyrite, electrum, sphalerite, galena, and polybasite. Gold contents of quartz veins are positively correlated with Ag, Zn, Pb, and Cu contents in both the northern and southern prospects. The quartz veins at the Jati, Gregah, and Sentul prospects have relatively lower gold-silver ratios (Ag/Au = 23.2) compared to those at the Kojan, Dalangturu, Salak, and Suruh prospects (Ag/Au = 66.8). The quartz veins at the Dalangturu prospect are relatively rich in base metal sulfides. Ag/(Au+Ag) ratios of electrum in the Dalangturu prospect range from 45.2 to 65.0 at%, and FeS contents of sphalerite range from 1.2 to 6.4 mol%. Fluid inclusion microthermometry indicates ore-forming temperatures of 190-200°C and 220-230°C at the Sentul and Kojan prospects, respectively. Widely variable vapor/liquid ratio of fluid inclusions indicates that fluid boiling took place within the hydrothermal system at the Sentul prospect. Salinities of ore-fluids range from 0 to 0.7 wt% (av. 0.4 wt% NaCl equiv.) and from 0.5 to 1.4 wt% (av. 0.9 wt%) for the Sentul and Kojan prospects, respectively. The boiling of hydrothermal fluid was one of the gold deposition mechanisms in the Sentul prospect.. |
| 35. | Maulana, Adi, Christy, Andrew G., Ellis, David J., Imai, Akira, Koichiro Watanabe, Geochemistry of eclogite- and blueschist-facies rocks from the Bantimala Complex, South Sulawesi, Indonesia: Protolith origin and tectonic setting, ISLAND ARC, 10.1111/iar.12037, 22, 4, 427-452, 2013.12, We present the first data on bulk-rock major and trace element compositions for a suite of eclogite- and blueschist-facies rocks from the Bantimala Complex, Indonesia, with the aim of better constraining the protolith origins and nature of the subducted crust. The eclogites can be classified into two groups: glaucophane-rich eclogite and glaucophane-free eclogite, whereas the blueschists are divided into albite-epidote glaucophanite and quartz-glaucophane schists. SiO2 contents of the eclogites are 43.3-49.6 wt%, with Na2O+K2O contents 3.7-4.7 wt%. The blueschists show a wider range of compositions, with SiO2=40.7-63.8 wt% and Na2O+K2O=2.7-4.5 wt%. Trace element data suggest that the eclogite protoliths include both enriched and normal mid-oceanic ridge basalt (E-MORB and N-MORB) and also gabbroic cumulates. The blueschists show more variation in protoliths, which include N-MORB, Oceanic Island Basalt (OIB) and Island Arc Basalt (IAB). Plots of element concentrations against the immobile Zr show considerable mobility of large ion lithophiles but not of high field-strength elements during high-pressure metamorphism, and indicate that the high SiO2 content of some blueschists is probably due to metasomatism by a LILE-rich siliceous aqueous fluid. Strong correlations between K, Rb, Ba and Cs suggests that enrichment of these elements occurred by a single process. All the protoliths were subducted, metamorphosed to blueschist/eclogite-facies and subsequently exhumed. It is noteworthy that the samples deduced to have come from thicker-crust environments (OIB, IAB) were subducted to shallower depths (blueschist-facies) than MORB-derived samples, all except one of which reached eclogite-facies conditions. The geochemical data of this study demonstrate the variety of ocean floor types that were subducted under the southeast margin of Sundaland in the late Jurassic period. © 2013 Wiley Publishing Asia Pty Ltd.. |
| 36. | Takahashi, Ryohei, Matsueda, Hiroharu, Okrugin, Victor M., Shikazono, Naotatsu, Ono, Shuji, Imai, Akira, Andreeva, Elena D., Koichiro Watanabe, Ore-forming Ages and Sulfur Isotope Study of Hydrothermal Deposits in Kamchatka, Russia, RESOURCE GEOLOGY, 10.1111/rge.12005, 63, 2, 210-223, 2013.07, In Kamchatka, Central Koryak, Central Kamchatka and East Kamchatka metallogenic belts are distributed from northwest to southeast. K-Ar age, sulfur isotopic composition of sulfide minerals, and bulk chemical compositions of ores were analyzed for 13 ore deposits including hydrothermal gold-silver and base metal, in order to elucidate the geological time periods of ore formation, relationship to regional volcanic belts, type of mineralization, and origin of sulfur in sulfides. The dating yielded ore-forming ages of 41Ma for the Ametistovoe deposit in the Central Koryak, 17.1Ma for the Zolotoe deposit and 6.9Ma for the Aginskoe deposit in the Central Kamchatka, and 7.4Ma for the Porozhistoe deposit and 5.1Ma for the Vilyuchinskoe deposit in the East Kamchatka metallogenic belt. The data combined with previous data of ore-forming ages indicate that the time periods of ore formation in these metallogenic belts become young towards the southeast. The averaged δ34SCDT of sulfides are -2.8‰ for the Ametistovoe deposit in Central Koryak, -1.8‰ to +2.0‰ (av. -0.1‰) for the Zolotoe, Aginskoe, Baranievskoe and Ozernovskoe deposits in Central Kamchatka, and -0.7 to +3.8‰ (av. +1.7‰) for Bolshe-Bannoe, Kumroch, Vilyuchinskoe, Bystrinskoe, Asachinskoe, Rodnikovoe, and Mutnovskoe deposits in East Kamchatka. The negative δ34SCDT value from the Ametistovoe deposit in Central Koryak is ascribed to the contamination of 32S-enriched sedimentary sulfur in the Ukelayat-Lesnaya River trough of basement rock. Comparison of the sulfur isotope compositions of the mineral deposits shows similarity between the Central Koryak and Magadan metallogenic belts, and East Kamchatka and Kuril Islands belts. The Central Kamchatka belt is intermediate between these two groups in term of sulfur isotopic composition. © 2013 The Society of Resource Geology.. |
| 37. | Sanematsu, Kenzo, Kon, Yoshiaki, Imai, Akira, Koichiro Watanabe, Watanabe, Yasushi, Geochemical and mineralogical characteristics of ion-adsorption type REE mineralization in Phuket, Thailand, MINERALIUM DEPOSITA, 10.1007/s00126-011-0380-5, 48, 4, 437-451, 2013.04. |
| 38. | Dimalanta, Carla B., Yumul, Graciano P., Jr., Imai, Akira, Geodynamic evolution of the Baguio Mineral District: Unlocking the Cenozoic record from clastic rocks, JOURNAL OF ASIAN EARTH SCIENCES, 10.1016/j.jseaes.2012.09.026, 65, 118-130, 2013.03, Geochemical studies done on the Baguio Mineral District had dominantly looked at the igneous rocks. A major gap is the scarcity of studies dealing with the sedimentary rock geochemistry for this district which this paper intends to address. The petrographic data and major and trace element compositions of the siltstones-sandstones from the lower member of the Late Oligocene to Early Miocene Zigzag Formation show that they are generally quartz-rich and have high K2O, Th and La/Sc contents. The chondrite-normalized REE pattern exhibits a negative Eu anomaly, enrichment in LREEs and flat HREEs similar to typical post-Archean shales. Taken together, the data indicate derivation of the lower member of the Zigzag Formation from intermediate to acid igneous rock sources generated in an active margin setting. A likely candidate source is the Cordon Syenite Complex/Palali Formation in the Northern Sierra Madre - Caraballo Mountains. Derivation from mafic source rocks in an oceanic island arc setting is inferred for samples of the Middle to Late Miocene Klondyke, Late Miocene to Early Pliocene Amlang and Late Pliocene Cataguintingan Formations. These samples are characterized by low K2O, Th and La/Sc but high Cr/Th values. Unroofing of the Pugo Metavolcanics and the younger plutons in the Central Cordillera provided the materials which eventually produced the Klondyke and Amlang Formations. Continuous uplift and shallowing of the basin resulted in the deposition of the Cataguintingan Formation. The petrography, geochemistry and geological features of the sedimentary rocks clearly define the change in sediment provenance from quartz-rich during the Oligocene to quartz-deficit in the Miocene. This offers additional constraints in understanding the geological evolution of the mineral district. © 2012 Elsevier Ltd.. |
| 39. | Akira Imai, Kotaro Yonezu, Kenzo Sanematsu, Takashi Ikuno, Shingo Ishida, Koichiro Watanabe, Visut Pisutha-Arnond, Somchai Nakapadungrat, Jarupong Boosayasak, Rare Earth Elements in Hydrothermally Altered Granitic Rocks in the Ranong and Takua Pa Tin-Field, Southern Thailand, Resource Geology, 10.1111/j.1751-3928.2012.00212.x, 63, 1, 84-98, 2013.01, Geochemical studies were conducted on the hydrothermally altered granitic rocks in the Ranong and Takua Pa tin-fields in southern Thailand in order to investigate the mode of occurrence of REE (rare earth elements), with emphasis placed on a potential REE resource associated with granitic rocks in the Southeast Asian Tin Belt. The total REE (ΣREE) content of altered granitic rocks ranges from 130 to 350ppm at Haad Son Paen (which is presently mined for kaolin clay) in the Ranong tin-field, and that of altered granitic rocks and kaolinite veinlets reaches up to 424ppm and 872ppm, respectively, at Nok Hook in the Takua Pa tin-field. Rare earth elements in the altered granitic rocks and kaolinite veinlets show a relatively flat chondrite-normalized pattern, thus enriched in heavy REE compared with the original granitic rocks and their weathered crusts. At Nok Hook (Takua Pa), the ΣREE content of kaolinite separated from an altered granitic rock by elutriation is 1313ppm, a ΣREE amount about four times higher than that of whole-rock composition of the altered granitic rock. Chondrite-normalized REE patterns of the elutriated kaolinite and of the altered granite are relatively flat. Sequential extraction experiments suggest that 41 and 85 percent of REE are present as ion exchangeable-form in the altered granitic rock, and in the kaolinite veinlets, respectively. In addition, more than 90% of REE in the kaolinite veinlets are present as the acid-soluble state. On the other hand, the ΣREE content of kaolinite veinlets and of the kaolinite concentrated by elutriation from an altered granitic rock at Haad Som Paen (Ranong) is 70ppm and 75ppm, respectively, thus enrichment of REE in kaolinite was not confirmed. In addition, by the sequential extraction experiments, 23% and 4% of REE were extracted from the altered granitic rock and the kaolinite veinlets at Haad Som Paen. In the altered granitic rocks at Haad Som Paen, REE are present as refractory phases, and REE in the acid-soluble states had been leached by hydrothermal fluid.. |
| 40. | Concepcion, R. A. B., Dimalanta, C. B., Yumul, G. P., Jr., Faustino-Eslava, D. V., Queano, K. L., Tamayo, R. A., Jr., Imai, A., Petrography, geochemistry, and tectonics of a rifted fragment of Mainland Asia: evidence from the Lasala Formation, Mindoro Island, Philippines, INTERNATIONAL JOURNAL OF EARTH SCIENCES, 10.1007/s00531-011-0643-5, 101, 1, 273-290, 2012.01, Petrological and geochemical investigations of the sedimentary Lasala formation in northwest Mindoro, Philippines, offer new insights into the origin of this geologically contentious region. Mindoro island's position at the boundary between Sundaland and the Philippine Mobile Belt has led to variable suggestions as to how much of it is continent derived or not. The Eocene Lasala formation overlies the Jurassic Halcon metamorphics, a regionally metamorphosed suite generally thought to have formed as a result of arc-continent collision processes. The sedimentary formation consists mainly of sandstones and shales interbedded with mudstones, basalt flows, and subordinate limestones and conglomerates. Petrographic information on the Lasala clastic rocks demonstrates a uniform framework composition that is predominantly quartzose. Major oxide, trace element abundances, and various elemental ratios similarly impart a strongly felsic signature. These characteristics are taken to indicate a chiefly continental, passive margin derivation and deposition of the Lasala sediments during the Eocene. The weak indication of active margin influence is suggested to be an inherited signature, supported by paleogeographic models of the southeastern Asian margin area during the pre-Cenozoic. © 2011 Springer-Verlag.. |
| 41. | Sagapoa, C. V., Imai, A., Ogata, T., KOTARO YONEZU, Koichiro Watanabe, Laterization process of peridotites in Siruka, Choiseul, Solomon Islands, Jour. Southeast Asian Applied Geology,, 3, 76-92, 2011.10. |
| 42. | Harijoko, A., Htun, T. M., Saputra, R., Warmada, I W., Setijadji, L. D., Imai, A., Koichiro Watanabe, Mercury and arsenic contamination from small scale gold mining activities at Selogiri area, Central Java, Indonesia, Jour. Southeast Asian Applied Geology, 2, 56-64, 2010.10. |
| 43. | 田中 崇裕, 今井 亮, 渡邊 公一郎, フィリピン、ルソン島北部、ヴィクトリア金鉱床におけるルソン銅鉱-ファマチナ鉱系鉱物, 日本鉱物科学会年会講演要旨集, 10.14824/jakoka.2010.0.254.0, 2010, 259-259, 2010.09, ヴィクトリア金鉱床は低硫化型浅熱水性鉱床に分類された。しかしながら、本研究で高硫化型の指標鉱物（例えば、ルソン銅鉱、硫砒銅鉱そしてコルース鉱）がヴィクトリア金鉱床の深部から確認された。ルソン銅鉱グループ鉱物と黄銅鉱はしばしば元々存在した鉱脈構成鉱物を置き換えている。このタイプのルソン銅鉱グループ鉱物はしばしば四面銅鉱グループ鉱物を置き換えており、黄銅鉱と共にしばしばサブミクロンオーダーの小規模フレーク状連晶組織や縞状組織を示す。これらの組織は四面銅鉱グループ鉱物を置き換えた中間生成物の分解によって形成されると考えられる。また、末期の鉱化流体は高硫化型に相当する。. |
| 44. | Tanaka, T., Akira Imai, Egashira, S., Sakomoto, S., Yasunaga, K., Petrological and Geochemical Characteristics of Intrusive Rocks Related to Porphyry Copper Mineralization and the Implications for the Genesis of Deposits in the Namosi area, Viti Levu, Republic of the Fiji Islands, Resource Geology, 10.1111/j.1751-3928.2010.00113.x, 60, 1, 35-51, 2010.01, In the Namosi district, of Viti Levu Island, Republic of the Fiji Islands, we conducted chemical analysis on intrusive rocks and attempted to clarify the relationship between petrological characteristics and mineralization. Samples were collected from the Waisoi copper deposit, Waivaka corridor, Waisomo, Wailutelevu and Wainabama Southwest localities. The intrusive rocks in the district show the overall characteristics of calc-alkaline magnetite series. Using the normative composition diagram for granite classification, most of the intrusive rocks plot in the tonalite, granodiorite, quartz monzodiorite and quartz diorite fields. Rare earth element (REE) analysis revealed that at Waisoi the intrusive rocks tend towards an Eu positive anomaly, whereas most of those at Waisomo tend to have a negative anomaly. The (Cecn/Ybcn)/Ybcn ratio shows that the intrusive rocks in the Namosi district may be derived from a mantle origin magma possibly contaminated by crustal materials. Waisoi rocks have less contamination with continental crust and show higher gold content compared to those of the Waivaka corridor. The observed differences between Waisoi and Waivaka corridor in the Cu/Au ratio might also be caused by varying amounts of contamination by crustal materials. There is no clear correlation between the La/Yb ratio and K-Ar ages, however, the La/Yb ratio of the Waivaka corridor rocks has a wide range, suggesting a variable amount of crustal contamination of the magma from 7 to 5 Ma. The less crustal contamination and generation of relatively high silica contents under relatively oxidized conditions in the Waisoi magmas is correlated with Au-rich porphyry copper mineralization and in contrast more crustal contamination produced Au-poor porphyry copper mineralization at the Waivaka Corridor in the Namosi district. © 2010 The Authors. Journal compilation © 2010 The Society of Resource Geology.. |
| 45. | Jillian Aira S. Gabo, Carla B. Dimalanta, Mary Grace S. Asio, Karlo L. Queaño, Graciano P. Yumul, Akira Imai, Geology and geochemistry of the clastic sequences from Northwestern Panay (Philippines) Implications for provenance and geotectonic setting, Tectonophysics, 10.1016/j.tecto.2009.02.004, 479, 1-2, 111-119, 2009.12, [URL], Whole-rock geochemical analysis of samples from the clastic sequences in Northwestern Panay revealed different compositional ranges for the Buruanga Peninsula and Antique Range. The results provide additional evidence that Northwestern Panay consists of two distinct terranes as a result of the arc-continent collision between the Palawan Microcontinental Block and the Philippine Mobile Belt. Major element compositions of clastic rocks from the Buruanga Peninsula (Saboncogon Formation) revealed that they belong to a continental margin tectonic setting whereas the Antique Range sedimentary rocks (Lagdo Formation) show an affinity to an oceanic island arc setting. The Fragante Formation of the Antique Range shows some similarities in tectonic setting and provenance with Buruanga Peninsula clastic rocks, which suggests a Buruanga Peninsula sediment contribution during its time of formation. Furthermore, provenance analyses from combined major and trace element data indicate a more silicic source for the Buruanga Peninsula clastics and a more mafic derivation for the sedimentary rocks of the Antique Range. These findings are consistent with previous studies suggesting that the Buruanga Peninsula forms part of the Palawan Microcontinental Block and the Antique Range belongs to the Philippine Mobile Belt. The Palawan Microcontinental Block is believed to be a drifted continental fragment from the Mesozoic East Asian accretionary complex in the southern part of the Eurasian mainland, which represents the continental margin setting and felsic source of the Buruanga Peninsula clastic rocks. Meanwhile, the oceanic island setting and mafic source of the Antique Range clastic rocks represent the volcanic rocks of the Philippine Mobile Belt that were formed as a result of the subduction of the leading oceanic edge of the Palawan Microcontinental Block.. |
| 46. | Ryohei TAKAHASHI, Koichiro WATANABE, Akira IMAI, and Yuuki KOMATA, Applications of Digital Database and Geographical Information System to Earth Resources in Southeast Asia, Proceedings of International Symposium on Earth Science and Technology, 27-32, 2009.12. |
| 47. | Ryohei TAKAHASHI, Koichiro WATANABE, Akira IMAI, and Yuuki KOMATA, Applications of Digital Database and Geographical Information System to Earth Resources in Southeast Asia, Proceedings of International Symposium on Earth Science and Technology, 27-32, 2009.12. |
| 48. | Kotaro YONEZU, Takushi YOKOYAMA, Akira IMAI and Koichiro WATANABE, Contents of Rare Earth Elements of Quartz Veins in the Hishikari Epithermal Gold Deposit: Preliminary Investigation by Laser Ablation ICP-MS, Proceedings of International Symposium on Earth Science and Technology, 125-128, 2009.12. |
| 49. | May T. AYE, Akira IMAI, Subagyo PRAMUMIJOYO, Lucas D. SETIJADJI, Arifudin IDRUS and Johan ARIF, Characteristics of Calc-silicate Unit Related with Skarn Mineralization at Batu Hijau Porphyry Copper-gold Deposit, Sumbawa Island, Indonesia, Proceedings of International Symposium on Earth Science and Technology, 259-262, 2009.12. |
| 50. | Thomas TINDELL, Akira IMAI and Ryohei TAKAHASHI, Fluid Inclusion and Geochemical Behaviour of The Kasuga High-Sulphidation Gold Deposit and Basement Veins, Kagoshima, Japan, Proceedings of International Symposium on Earth Science and Technology, 263-268, 2009.12. |
| 51. | Harrison GEDIKILE, Christopher V. SAGAPOA and Akira IMAI, Mineralization Style of the Charivunga Prospect, Gold Ridge Epithermal Au Deposit, Guadalcanal, Solomon Islands, Proceedings of International Symposium on Earth Science and Technology, 269-274, 2009.12. |
| 52. | Takahiro TANAKA, Akira IMAI and Koichiro WATANABE, Mineralogical Characterization of Ore Minerals at the Victoria Low Sulfidation Epithermal Gold Deposit, Northern Luzon, Philippines, Proceedings of International Symposium on Earth Science and Technology, 275-278, 2009.12. |
| 53. | Yuki SHINGO, Akira IMAI, Ryohei TAKAHASHI, Koichiro WATANABE, Agung HARIJOKO, I Wayan WARMADA, Arifudin IDRUS, and Phouvang PHOUMEPHONE, Outline of Epithermal Gold Mineralization at Trenggalek Prospects, East Java, Indonesia, Proceedings of International Symposium on Earth Science and Technology, 279-283, 2009.12. |
| 54. | Yohei KAWABATA, Kotaro YONEZU, Koichiro WATANABE, Akira IMAI and Takushi YOKOYAMA, Removal of Boron by Magnesium Silicate from Synthetic Geothermal Water, Proceedings of International Symposium on Earth Science and Technology, 369-372, 2009.12. |
| 55. | Cecep Y. SUNARIE, Mega F. ROSANA, Koichiro WATANABE and Akira IMAI, Characteristics of Epithermal Gold Mineralization in Tanggeung Area, Cianjur West Java, Indonesia, Proceedings of International Symposium on Earth Science and Technology, 403-408, 2009.12. |
| 56. | Yusuke JUYANAGI and Akira IMAI, Fluid Inclusion Thermometry on Quartz Veins and Mineral Assemblages at Baturappe Area and Vicinity, Sulawesi Island, Indonesia, Proceedings of International Symposium on Earth Science and Technology, 409-412, 2009.12. |
| 57. | Arifudin IDRUS, Akira IMAI, Andi MAKKAWARU, KAMRULLAH, I Wayan WARMADA, Irzal NUR and Rohaya LANGKOKE, Preliminary Study on Orogenic Deposit Type as a Source of Placer Gold at Bombana, Southeast Sulawesi, Indonesia, Proceedings of International Symposium on Earth Science and Technology, 569-572, 2009.12. |
| 58. | Arifudin IDRUS, Akira IMAI, Andi MAKKAWARU, KAMRULLAH, I Wayan WARMADA, Irzal NUR and Rohaya LANGKOKE, Preliminary Study on Orogenic Deposit Type as a Source of Placer Gold at Bombana, Southeast Sulawesi, Indonesia, Proceedings of International Symposium on Earth Science and Technology, 569-572, 2009.12. |
| 59. | Kotaro YONEZU, Sachihiro SAKOMOTO, Koichiro WATANABE, Akira IMAI and Takushi YOKOYAMA, Removal of Silicic Acid as Calcium Silicate and Uptake Behavior of Trace Elements, Proceedings of the 4th International Symposium of Novel Carbon Resource Sciences, 27-30, 2009.12. |
| 60. | Huizi PIAO. Kotaro YONEZU, Yohei KAWABATA, Koichiro WATANABE, Akira IMAI and Takushi YOKOYAMA, Recovery of Lithium from Geothermal Water by Manganese Oxide Adsorbent: Approach to the effective Utilization of Geothermal Resources, Proceedings of the 4th International Symposium of Novel Carbon Resource Sciences, 38-41, 2009.12. |
| 61. | Jillian Aira S. Gabo, Carla B. Dimalanta, Mary Grace S. Asio, Karlo L. Queaño, Graciano P. Yumul, Akira Imai, Geology and geochemistry of the clastic sequences from Northwestern Panay (Philippines) Implications for provenance and geotectonic setting, Tectonophysics, 10.1016/j.tecto.2009.02.004, 479, 1-2, 111-119, 2009.12, Whole-rock geochemical analysis of samples from the clastic sequences in Northwestern Panay revealed different compositional ranges for the Buruanga Peninsula and Antique Range. The results provide additional evidence that Northwestern Panay consists of two distinct terranes as a result of the arc-continent collision between the Palawan Microcontinental Block and the Philippine Mobile Belt. Major element compositions of clastic rocks from the Buruanga Peninsula (Saboncogon Formation) revealed that they belong to a continental margin tectonic setting whereas the Antique Range sedimentary rocks (Lagdo Formation) show an affinity to an oceanic island arc setting. The Fragante Formation of the Antique Range shows some similarities in tectonic setting and provenance with Buruanga Peninsula clastic rocks, which suggests a Buruanga Peninsula sediment contribution during its time of formation. Furthermore, provenance analyses from combined major and trace element data indicate a more silicic source for the Buruanga Peninsula clastics and a more mafic derivation for the sedimentary rocks of the Antique Range. These findings are consistent with previous studies suggesting that the Buruanga Peninsula forms part of the Palawan Microcontinental Block and the Antique Range belongs to the Philippine Mobile Belt. The Palawan Microcontinental Block is believed to be a drifted continental fragment from the Mesozoic East Asian accretionary complex in the southern part of the Eurasian mainland, which represents the continental margin setting and felsic source of the Buruanga Peninsula clastic rocks. Meanwhile, the oceanic island setting and mafic source of the Antique Range clastic rocks represent the volcanic rocks of the Philippine Mobile Belt that were formed as a result of the subduction of the leading oceanic edge of the Palawan Microcontinental Block.. |
| 62. | Gabo, J. A. S., Dimalanta, C. B., Asio, M. G. S., Queano, K. L., Yumul, G. P., Jr. and Imai, A., Geology and geochemistry of the clastic sequences from northwestern Panay (Philippines): Implications for provenance and geotectonic setting, Tectonophysics, 2009.10. |
| 63. | Imai, A. and Nagai, Y., Fluid inclusion study and opaque mineral assemblage at the deep and shallow part of the Batu Hijau porphyry Cu-Au deposit, Sumbawa, Indonesia, Resour. Geol.,, 59, 231-243., 2009.09. |
| 64. | Suerte, L. O., Imai, A. and Nishihara, S., Geochemical characteristics of intrusive rocks, southeastern Mindanao, Philippines: Implication to metallogenesis of porphyry copper-gold deposits, Resour. Geol., 59, 244-262., 2009.09. |
| 65. | Imai, A., Suerte, L. O. and Nishihara, S., Origin of bornite pods in intrusive rocks at the Kingking porphyry Cu-Au deposit, southeastern Mindanao, Philippines, Resour. Geol., 59, 307-313., 2009.09. |
| 66. | Imai Akira, Nagai Yuki, Fluid Inclusion Study and Opaque Mineral Assemblage at the Deep and Shallow Part of the Batu Hijau Porphyry Copper-gold Deposit, Sumbawa, Indonesia, Resource geology, 10.1111/j.1751-3928.2009.00093.x, 59, 3, 231-243, 2009.09. |
| 67. | Suerte Leilanie Osano, Imai Akira, Nishihara Sho, Geochemical Characteristics of Intrusive Rocks, Southeastern Mindanao, Philippines : Implication to Metallogenesis of Porphyry Copper-gold Deposits, Resource geology, 10.1111/j.1751-3928.2009.00094.x, 59, 3, 244-262, 2009.09. |
| 68. | Imai Akira, Suerte Leilanie Osano, Nishihara Sho, Origin of Bornite Pods in Intrusive Rocks at the Kingking Porphyry Copper-gold Deposit, Southeastern Mindanao, Philippines, Resource geology, 10.1111/j.1751-3928.2009.00098.x, 59, 3, 307-313, 2009.09. |
| 69. | IMAI Akira, DIMALANTA Carla B., IDRUS Arifudin, WARMADA I. Wayan, HARIJOKO Agung, Themetic Issue : Collection of Achievements on Earth Resources from AUN/SEED-Net, Part III, Resource geology, 10.1111/j.1751-3928.2009.00091.x, 59, 3, 213-214, 2009.09. |
| 70. | Akira IMAI, Takashi IKUNO, Kenzo SANEMATSU, Takafumi SUEOKA, Shingo ISHIDA, Koichiro WATANABE, Kong SITHA, Lucas Donny SETIJADJI and Jarupon BOOSAYASAK , Rare Earth Elements in Weathered Crusts of Granitic Rocks in Southeast Asian Tin Belt (Northern Thailand, Southern Thailand (Ranong-Takua Pa-Phuket) and Bangka Island, Indonesia), Proceedings of the 2nd Regional Conference Interdisciplinary Research on Natural Resources and Materials Engineering , 45-52, 2009.08. |
| 71. | Lucas Donny SETIJADJI, I Wayan WARMADA, Akira IMAI and Kenzo SANEMATSU , Investigation on Rare Earth Elements Mineralization in Indonesia, Proceedings of the 2nd Regional Conference Interdisciplinary Research on Natural Resources and Materials Engineering , 53-58, 2009.08. |
| 72. | Kong SITHA, Lucas Donny SETIJADJI, Kenzo SANEMATSU, Takashi IKUNO, Akira IMAI, Arius DIMARA and Koichiro WATANABE , REE in Monzo-granites in Bangka Island, Indonesia , Proceedings of the 2nd Regional Conference Interdisciplinary Research on Natural Resources and Materials Engineering , 145-152, 2009.08. |
| 73. | May Thwe AYE, Subagyo PRAMUMIJOYO, Lucas Donny SETIJADJI, Arifudin IDURS and Akira IMAI , Porphyry Related Skarn Mineralization with Calc-silicate Rocks in Batu Hijau Deposit, Sumbawa Island, East Indonesia, Proceedings of the 2nd Regional Conference Interdisciplinary Research on Natural Resources and Materials Engineering , 153-160, 2009.08. |
| 74. | Phovane PHOUMEPHONE, Agung HARIJOKO, I Wayan WARMADA, Akira IMAI, Ryohei TAKAHASHI and Yuki SHINGO , Hydrothermal Alteration of Epithermal Gold Deposit at Dalangturu and Suruh Areas, Trenggalek Regency, East Java, Indonesia, Proceedings of the International Seminar on Geology of the Southern Mountains of Java, 169-178, 2009.08. |
| 75. | Irzal NUR, Arifudin IDRUS, Subagyo PRAMUMIJOYO, Agung HARIJOKO, Yusuke JUYANAGI and Akira IMAI, Characteristics of Epithermal Quartz Veins at Baturappe Area, Gowa, South Sulawesi: Implication to Base Metal Exploration , Proceedings of the International Seminar on Geology of the Southern Mountains of Java, 179-186, 2009.08. |
| 76. | I Wayan WARMADA, Agung HARIJOKO, Akira IMAI and Koichiro WATANABE, Landslide Triggering Factor in a Hydrothermally-altered Area, Case Study: Kasihan District, Pacitan Regency, East Java, Proceedings of Earth Science International Conference, 2009.08. |
| 77. | May T. AYE, Subagyo PRAMUMIJOYO, Lucas D. SETIJADJI, Arifudin IDRUS, Akira IMAI and Johan ARIF, Occurrence of Skarn Mineralization in Deep Drill Holes at Batu Hijau Porphyry Copper-gold Deposit, SUmbawa Island, Indonesia, Proceedings of Earth Science International Conference, 2009.08. |
| 78. | Phovang PHOUMEPHONE, Agung HARIJOKO, I Wayan WARMADA, Akira IMAI, Ryohei TAKAHASHI and Yuki SHINGO, Hydrothermal Alteration Associated with Epithermal Gold Mineralization at Dalangturu and Suruh Prospects in Trenggalek Regency, East Java Island, Indonesia, Proceedings of Earth Science International Conference, 2009.08. |
| 79. | Phovang PHOUMEPHONE, Agung HARIJOKO, I Wayan WARMADA, Akira IMAI, Ryohei TAKAHASHI and Yuki SHINGO, Hydrothermal Alteration Associated with Epithermal Gold Mineralization at Dalangturu and Suruh Prospects in Trenggalek Regency, East Java Island, Indonesia, Proceedings of Earth Science International Conference, 2009.08. |
| 80. | Akira IMAI, Takashi IKUNO, Kenzo SANEMATSU, Takafumi SUEOKA, Shingo ISHIDA, Koichiro WATANABE, Kong SITHA, Lucas Donny SETIJADJI and Jarupon BOOSAYASAK , Rare Earth Resource Potential in Weathered Crust of Granitic Rocks in Southeast Asian Tin Belt, Proceedings of Earth Science International Conference, 2009.08. |
| 81. | Kotaro YONEZU, Takushi YOKOYAMA, Akira IMAI and Koichiro WATANABE, Characteristics of Rare Earth Elements in Quartz Veins of the Hishikari Epithermal Gold Deposit, Japan: Comparison between Bulk and Local Analyses, Proceeding of the 3rd International Workshop and Conference on Earth Resources Technology, 49-58, 2009.05. |
| 82. | Chompunoot WIRASERANEE, Kotaro YONEZU, Takushi YOKOYAMA, Akira IMAI, Koichiro WATANABE, Quanchai LEEPAWPANTH and Dawan WIWATTANADATE, Coprecipitation of Au(III) Complex Ions with Mn(II) Hydroxide as Gold Nanoparticles and Its Implication to Selective Recovery from Electronic Wastes , Proceeding of the 3rd International Workshop and Conference on Earth Resources Technology, 224-231, 2009.05. |
| 83. | Ryohei Takahashi Koichiro Watanabe, Akira Imai, Hiroharu Matsueda, Victor M. Okurugin, Genesis and Formation of Ore Deposits in Kamchatka Peninsula, Far Eastern Russia, Proceedings of International Symposium on Earth Science and Technology 2008, 253-260ページ, 2008.12. |
| 84. | Kong Sitha, Lucas Donny Setijadji, Kenzo Sanematsu, Takashi Ikuno, Akira Imai, Arius Dimara, Koichiro Watanabe, Granitic Rocks of Bangka Island (Tin Island), Indonesia and Their Associated Mineralization: A Review and Potential of Unexplored Deposits, Proceedings of International Symposium on Earth Science and Technology 2008, 517-524ページ, 2008.12. |
| 85. | Saori Egashira, Akira Imai, Takayuki Tanaka, Kentaro Masaki, Kentaro Yasunaga, Katsuhiko Maeda, Fluid Inclusion and Sulfur Isotope Study on Porphyry Copper Deposit at the Waivaka Corridor Area, Namosi District, Viti Levu, Republic of Fiji Islands, Proceedings of International Symposium on Earth Science and Technology 2008, 533-540ページ, 2008.12. |
| 86. | Takayuki Tanaka, Akira Imai, Saori Egashira, Kentaro Yasunaga, Katsuhiko Maeda, Trace Element Composition of the Intrusive Rocks Related to Porphyry-type Copper Mineralization in Namosi District, Viti Levu, Republic of the Fiji Islands, Proceedings of International Symposium on Earth Science and Technology 2008, 549-556ページ, 2008.12. |
| 87. | Chumpunoot Wiraseranee, Kotaro Yonezu, Takushi Yokoyama, Dawan Witwattanadate, Quanchai Leepawpanth, Akira Imai, Koichiro Watanabe, Recovery of Precious Metals from Computer Microprocessor by Coprecipitation with Manganese Hydroxide, Proceedings of International Symposium on Earth Science and Technology 2008, 557-564ページ, 2008.12. |
| 88. | A. Imai, K. Sanematsu, S. Ishida, K. Watanabe, J. Boosayasak, Rare Earth Elements in Weathered Crust in Sn-bearing Granitic Rocks in Southern Thailand, Proceedings of the International Symposia on Geoscience Resources and Environments of Asian Terranes, Special Issue of Bulletin of Earth Sciences of Thailand, 232-237ページ, 2008.11. |
| 89. | K. Yamada, T. Fujii, A. Imai, Y. Kawabata, H. Matsushita, Alkali silica reactivity of various andesites in saturated NaCl solution, 4th International Structural Engineering and Construction Conference, ISEC-4 - Innovations in Structural Engineering and Construction Proceedings of the 4th International Structural Engineering and Construction Conference, ISEC-4 - Innovations in Structural Engineering and Construction, 525-531, 2008.11, Reactivities of various andesites and some other volcanic rocks were investigated by an accelerated mortar bar test in saturated NaCl solution (saturated NaCl method). In Japan, because of its low alkali content of Portland cement, total alkali amount in concrete can be controlled under 3.0 kg/m 3 for most cases. However, environmental alkalis such as deicing salt may add extra alkalis to concrete and ASR risks may increase. This risk is thought to be able to be checked by saturated NaCl method. The experiment shows that the most reactive andesites contain tridymite. Cristobalite is the second reactive phase. Glass seems non-reactive in the saturated NaCl method. Glassy andesite without silica minerals is judged as reactive by chemical method but innocuous by saturated NaCl method. Glass phase has been believed reactive in high alkali condition but there is a possibility that it is less reactive in specific environments such as deicing salt.. |
| 90. | Saadi, N. M., Watanabe, K., Imai, A., Saibi, H., Integrating potential field with remote sensing data for geological investigations in the Eljufra area of Libya, Earth, Planet, Space, 60巻, 539-547ページ, 2008.06. |
| 91. | Akira Imai, Kenzo Sanematsu, Shingo Ishida, Koichiro Watanabe, Jarupong Boosayasak, Enrichment of Rare Earth Elements Associated with Granitic Rocks, Southern Thailand, Proceedings of the 2nd International Workshop and Conference on Earth Resources Technology, 9-13ページ, 2008.04. |
| 92. | Takayuki Tanaka, Akira Imai, Saori Egashira, Sachihiro Sakomoto, Kentaro Yasunaga, Petrography and Petrochemical Characteristics of the Intrusive Rocks Related to Porphyry Copper Mineralization in Namosi District, Viti Levu, Republic of the Fiji Island, Proceedings of the 2nd International Workshop and Conference on Earth Resources Technology, 20-24ページ, 2008.04. |
| 93. | Saori Egashira, Akira Imai, Takayuki Tanaka, Kentaro Masaki, Kentaro Yasunaga, Katsuhiko Maeda, Study on Porphyry-type Copper Mineralization at the Waivaka Corridor Area, Namosi District, Viti Levu, Republic of the Fiji Islands, Proceedings of the 2nd International Workshop and Conference on Earth Resources Technology, 126-130ページ, 2008.04. |
| 94. | Kotaro Yonezu, Tomoaki Shibao, Takushi Yokoyama, Akira Imai, Koichiro Watanabe, Interaction Between Gold(I) Complexes and the Surface of Metal Sulfides (Fe2S and Ag2S), Proceedings of the 2nd International Workshop and Conference on Earth Resources Technology, 131-138ページ, 2008.04. |
| 95. | Nureddin Mohamed Saadi, Koichiro Watanabe, Akira Imai, Hakim Saibi, Integrating potential fields with remote sensing data for geological investigations in the Eljufra area of Libya, earth, planets and space, 10.1186/BF03353116, 60, 6, 539-547, 2008.01, In this study, we successfully integrated geological data, potential field data, and remote sensing data with the aim of investigating and improving our knowledge of the structural setting of the Eljufra area, northwestern Libya. SPOT-5 panchromatic band and digital elevation models (DEM) were used to construct surface shaded maps, and gravity and aeromagnetic surveys were used to provide information on subsurface structures and the nature of intrusive bodies. The results revealed that the prevailing trend of lineaments is NNW-SSE and NNE-SSW. A Bouguer anomaly map revealed that horst and graben blocks occur inside the large graben. An analytic signal method was applied to the aeromagnetic data to estimate locations and the minimum depths to the contact. A comparison of gravity and magnetic data provided information on hydrothermally altered basalt and sediment density. An anticlinal structure was detected in the western part of the study area. The effects of faults on basalt rocks indicated geological time for volcanic activity.. |
| 96. | Imai, Akira, Ohbuchi Yukiko, Tanaka Takayuki, Morit Seita, Yasunaga Kentaro, Characteristics of porphyry Cu mineralization at Waisoi (Namosi district), Viti Levu, Fiji, Resource Goelogy, 57巻4号374-385頁, 2007.12. |
| 97. | Yonezu Kotaro, Yokoyama Takushi, Okaue Oyoshihiro, Imai Akira, Watanabe Koichiro, Concentration of gold(I) thiosullfate complex ions on the surface of alumina gel and their change in chemical state: Preliminary experiment in the elucidation of the formation mechanism of epithermal gold deposits, Resource Geology, 2007.12. |
| 98. | Sakomoto Sachihiro, Yonezu Kotaro, Okaue Yoshihiro, Yokoyama Takushi, Imai Akira, Watanabe Koichiro, Uptake of boron and gold by calcium silicate hydrate (CSH): Application for effective utilization of geothermal energy at geothermal power plant and the recovery of are metals from geothermal water, Proceedings of 5the Internationas Symposium on Earth Science and Technology, 287-292, 2007.12. |
| 99. | Egashira Saori, Tanaka Takayuki, Masaki Kentaro, Imai Akira, Yasunaga Kentaro, Maeda Katsuhiko, Characteristics of intrusive rocks associated with copper mineralization at the Waivala Corridor area, Namosi district, Viti Levu, Republic of the Fiji Islands, Proceedings of 5the Internationas Symposium on Earth Science and Technology, 293-298, 2007.12. |
| 100. | Yonezu Kotaro, Okaue Yoshihiro, Yokoyama Takushi, Imai Akira, Watanabe Koichiro, Coprecipitation of Au(I) complex anions by aluminum hydroxide, Proceedings of 5the Internationas Symposium on Earth Science and Technology, 309-316, 2007.12. |
| 101. | Uruma Ryusuke, Kohno Yasuaki, Watanabe Koichiro, Imai Akira, Itaya Tetsumaru, Setijadji Lucas Donny, Harijoko Agung, Migration of subduction in central Java, Indonesia, Proceedings of 5the Internationas Symposium on Earth Science and Technology, 377-384, 2007.12. |
| 102. | Tanaka Takayuki, Imai Akira, Egashira Saori, Sakomoto Sachihiro, Yasunaga Kentaro, Petrography and chemical composition on the intrusive rocks related to porpghyry-type copper mineralization in Namosi district, Viti Levu, Republic of the Fiji Isalnds, Proceedings of 5the Internationas Symposium on Earth Science and Technology, 385-394, 2007.12. |
| 103. | Tohma Yuki, Imai Akira, Watanabe Koichiro, Sanematsu Kenzo, Characteristics of the Seisen veins, Hishikari gold deposits, Japan, Proceedings of 5the Internationas Symposium on Earth Science and Technology, 395-402, 2007.12. |
| 104. | Nagai Yuki, Imai Akira, Fluid inclusion study and opaque mineral assemblage in quartz veins at deep and shallow part of the Batu Hijau porphyry Cu-Au deposit, Sumbawa, Indonesia, Proceedings of 5the Internationas Symposium on Earth Science and Technology, 403-410, 2007.12. |
| 105. | Imai Akira, Ohbuchi Yukiko, Tanaka Takayuki, MORITA Seiya, YASUNAGA Kentaro, Characteristics of Porphyry Cu Mineralization at Waisoi (Namosi District), Viti Levu, Fiji, Resource geology, 10.1111/j.1751-3928.2007.00031.x, 57, 4, 374-385, 2007.12. |
| 106. | Yonezu Kotaro, Yokoyama Takushi, Okaue Yoshihiro, IMAI Akira, WATANABE Koichiro, Concentration of Gold(I) Thiosulfate Complex Ions on the Surface of Alumina Gel and their Change in Chemical State : Preliminary Experiment in the Elucidation of the Formation Mechanism of Epithermal Gold Deposits, Resource geology, 10.1111/j.1751-3928.2007.00035.x, 57, 4, 400-408, 2007.12. |
| 107. | Yonezu Kotaro, Yokoyama Takushi, Okaue Oyoshihiro, Imai Akira, Watanabe Koichiro, Concentration of gold(I) thiosullfate complex ions on the surface of alumina gel and their change in chemical state: Preliminary experiment in the elucidation of the formation mechanism of epithermal gold deposits, Resource Geology, 2007.12. |
| 108. | Egashira Saori, Masaki Kentaro, Imai Akira, Tanaka Takayuki, Yasunaga Kentaro, Maeda Katsuhiko, Porphyry-type copper mineralization at the Waivaka Corridor area, Namosi district, Viti Levu, Republic of the Fiji Islands, 4th International Symposium on Gondwana to Asia and 2007 IAGR Annual Convention, no4, 30-31, 2007.11. |
| 109. | Fukuda Kazuchika, Imai Akira, Warmada I Wayan, Titisari Anastasia Dewi, Idrus Arifudin, Tun Myo Min, Furqon Reza Al, Saldi Rizza Nurarfi, Study on hydrothermal alteration in Kasihan area, East Java, Indonesia, 4th International Symposium on Gondwana to Asia and 2007 IAGR Annual Convention, no4, 38-39, 2007.11. |
| 110. | Nagai Yuki, Imai Akira, Deposition model of copper sulfides and ore genesis of the Batu Hijau porphyry Cu-Au deposit, Sumbawa, Indonesia, 4th International Symposium on Gondwana to Asia and 2007 IAGR Annual Convention, no4, 110-111, 2007.11. |
| 111. | Setijadji Lucas donny, Watanabe Koichiro, Imai Akira, Wibowo Haryo Edi, Barianto Didit Hadi, Itaya Tetsumaru, Island arc growth, metallogeny, and volcanic hazard risks of western Sunda arc: clarifying the significance of Gondwana crust, 4th International Symposium on Gondwana to Asia and 2007 IAGR Annual Convention, no4, 139-140, 2007.11. |
| 112. | Tanaka Takayuki, Egashira Saori, Sakomoto Sachihiro, Imai Akira, Yasunaga Kentaro, Maeda Katsuhiko, Petrological study on the intrusive rocks related to porphyry-type copper mineralization in Namosi district, Viti Levu, Republic of the Fiji Islands, 4th International Symposium on Gondwana to Asia and 2007 IAGR Annual Convention, no4, 152-153, 2007.11. |
| 113. | Tohma Yuki, Watanabe Koichiro, Imai Akira, Sanematsu Kenzo, The Seisen vines, Hishikari epithermal gold deposits, Japan: microscopic observation and fluid inclusions, 4th International Symposium on Gondwana to Asia and 2007 IAGR Annual Convention, no4, 156-157, 2007.11. |
| 114. | Uruma Ryusuke, Kohno Yasuaki, Watanabe Koichiro, Imai Akira, Itaya Tetsumaru, Setijadji Lucas Donny, Harijoko Agung, Migration of subduction degrees in central Java, Indonesia, 4th International Symposium on Gondwana to Asia and 2007 IAGR Annual Convention, no4, 174-175, 2007.11. |
| 115. | Harijoko, A., Ohbuchi, Y., Motomura, Y., Imai, A. and Watanabe, K., Characteristics of the Cibaliung gold deposit: A Miocene low sulfidation type epithermal gold deposit at the western Java, Indonesia, Resource Geology, 57巻2号, 2007.06. |
| 116. | Syafrizal, Imai, A. and Watanabe, K., Origin of ore-forming fluids responsible for gold mineralization of the Pongkor Au-Ag deposit, West Java, Indonesia: Evidences from mineralogic, fluid inclusion microthermometry and stable isotope study of the Ciurug-Cikoret vein, Resource Geology, 57巻2号, 2007.06. |
| 117. | Suerte, L. O., Nishihara, S., Imai, A., Watabnabe, K., Yumul, G. P., Jr. and Maglambayan, V. B., The occurrences of ore minerals and fluid inclusion study on the Kingking porphyry copper-gold deposit, eastern Mindanao, Philippines, Resource Geology, 57巻2号, 2007.06. |
| 118. | Imai, A., Shinomiya, J., Soe, M. T., Setijadji, L. D., Watanabe, K. and Warmada, I W., Porphyry-type mineralization at Selogiri area, Wonogiri regency, central Java, Indonesia, Resource Geology, 57巻2号, 2007.06. |
| 119. | Imai A., Dimalanta C.B., Harijoko A., Special issue: achievements on earth resources from AUN/SEED-Net, Part 2, Resource geology, 10.1111/j.1751-3928.2007.00009.x, 57, 2, 99-101, 2007.06. |
| 120. | Harijoko Agung, Ohbuchi Yukiko, Motomura Yoshinobu, IMAI Akira, WATANABE Koichiro, Characteristics of the Cibaliung Gold Deposit : Miocene Low-Sulfidation-Type Epithermal Gold Deposit in Western Java, Indonesia, Resource geology, 10.1111/j.1751-3928.2007.00011.x, 57, 2, 114-123, 2007.06. |
| 121. | Leilanie O. Suerte, Sho Nishihara, Akira Imai, Koichiro Watanabe, Graciano P. Yumul, Victor B. Maglambayan, Occurrences of ore minerals and fluid inclusion study on the Kingking Porphyry copper-gold deposit, Eastern Mindanao, Philippines, Resource Geology, 10.1111/j.1751-3928.2007.00018.x, 57, 2, 219-229, 2007.06, The Kingking deposit is a gold-rich porphyry copper deposit and the southernmost deposit at the eastern Mindanao mineralized belt, Philippines. It is underlain by Cretaceous-Paleogene sedimentary and volcanic rocks that are intruded by mineralized Miocene diorite porphyries and by barren Miocene-Pliocene dacite and diorite porphyries. The main alteration zones in the deposit are the inner potassic zone and the outer propylitic zone. The biotite-bearing diorite and hornblende diorite porphyries are the primary host rocks of mineralization. Two dominant copper minerals, bornite and chalcopyrite, which usually occur as fracture fillings, are associated with fine crystalline quartz veinlet stockworks in the mineralized diorites. Minor secondary covellite, chalcocite and digenite are also observed. The primary Cu-Fe sulfide phases initially deposited from ore fluids consisted of bornite solid solution (bnss) and intermediate solid solution (iss), which decomposed to form the bornite and chalcopyrite. Peculiar bornite pods that are different from dissemination and are associated with volcanic rock xenoliths in biotite-bearing diorite porphyry are noted in a drill hole. These pods of bornite are not associated with quartz veinlet stockworks. Fluid inclusion analyses show three types of inclusions contained in Kingking samples: two-phase fluid-rich and vapor-rich inclusions and polyphase hypersaline inclusions from porphyry-type quartz veinlet stockworks. The liquid-vapor homogenization temperatures (TH) and the dissolution temperature of halite daughter crystals (TM) from the polyphase hypersaline inclusions predominantly range from 400°C up to >500°C. The wide range of TH and TM may be due to heterogeneous trapping of variable ratios of vapor and brine. For some inclusions, TH > TM and in some cases, TH M, indicating that some of the brine was supersaturated or saturated with NaCl at the time of entrapment. Calculated salinity of the polyphase hypersaline inclusions ranges from 40 to 60% NaCl equivalent. Temperature and vapor pressure of mineralized fluid were estimated to be 400°C and 16 MPa.. |
| 122. | Syafrizal, Akira Imai, Koichiro Watanabe, Origin of ore-forming fluids responsible for gold mineralization of the Pongkor Au-Ag deposit, West Java, Indonesia Evidence from mineralogic, fluid inclusion microthermometry and stable isotope study of the Ciurug-Cikoret veins, Resource Geology, 10.1111/j.1751-3928.2007.00013.x, 57, 2, 136-148, 2007.06, The Pongkor gold-silver mine is situated at the northeastern flank of the Bayah dome, which is a product of volcanism in the Sunda-Banda Arc. The hydrothermal alteration minerals in the Ciurug-Cikoret area are typical of those formed from acid to near-neutral pH thermal waters. On the surface, illite/smectite mixed layer mineral (I/Sm), smectite and kaolinite, and spotting illite, I/Sm and K-feldspar alteration occur at the top of the mineralized zone. Silicification, K-feldspar and I/Sm zones are commonly formed in the wall rock, and gradually grade outwards into a propylitic zone. The mineralization of precious metal ore zone is constrained by fluid temperatures between 180 and 220°C, and with low salinity (18O H2O). There is no specific trend in the data with respect to the mineralization stages and elevation, which suggests that the ore-forming fluids did not significantly change spatially during the vein formation. The stable isotope data indicate mixing between the hydrothermal fluids and meteoric water and interaction between the hydrothermal fluids and the host rock.. |
| 123. | Akira Imai, Jumpei Shinomiya, Maung Thiha Soe, Lucas Donny Setijadji, Koichiro Watanabe, I. Wayan Warmada, Porphyry-type mineralization at Selogiri Area, Wonogiri Regency, Central Java, Indonesia, Resource Geology, 10.1111/j.1751-3928.2007.00019.x, 57, 2, 230-240, 2007.06, The Selogiri area, situated in Wonogiri regency, Central Java, is one of several gold prospecting areas in the Southern areas Mountain Range in Java, Indonesia. Three types of dioritic-andesitic intrusive rocks occur in the Selogiri area, namely, hornblende andesite porphyry, hornblende diorite porphyry and hornblende diorite, exposed in a half-circular depression where volcanic breccia and tuff are widely distributed. The occurrence of stockwork quartz veinlets and associated with magnetite and malachite coating along the cracks in the diorite porphyry suggests porphyry type mineralization. This is also supported by the occurrence of polyphase hypersaline fluid inclusions in the stockwork veinlet quartz. Small-scale miners are mining NS-trending quartz veins for gold associated with base metal sulfides. These veins are probably epithermal-type mineralization that overprinted porphyry-type mineralization. The Neogene intermediate to silicic hydrous magmatism in Java could have formed the porphyry-type mineralization in Selogiri, as in the rest of the Sunda-Banda arc.. |
| 124. | Rare Earth Element Geochemistry on the Formation Environment of the Hishikari Epithermal Gold Deposit, Japan. |
| 125. | Setijadji Lucas Donny, Imai Akira, Itaya Tetsumaru, Watanabe Koichiro, Geology of metallic deposits in Java island (Indonesia) with a special reference to the island arc magmatism, Proceedings of the International Workshop on Earth Resources Technology, 33-36, 2007.04. |
| 126. | Yonezu Kotaro, Yokoyama Takushi, Motomura Yoshinobu, Imai Akira, Watanabe Koichiro, Study on the determination of trace elements in siliceous deposits formed from hydrothermal water by ICP-MS, Proceedings of the International Workshop on Earth Resources Technology, 77-80, 2007.04. |
| 127. | Suerte Leilanie Osano, Nishihara Sho, Imai Akira, Watanabe Koichiro, Diorites and their roles in the mineralization of southeastern MIndanao, Philippines, Proceedings of the International Workshop on Earth Resources Technology, 81-84, 2007.04. |
| 128. | Sakomoto Sachihiro, Yonezu Kotaro, Yokoyama Takushi, Okaue Yoshihiro, Imai Akira, Watanabe Koichiro, Adsorption behavior of gold(I)-thiosulfate complex anions onto the surface of manganese dioxide: approach the recovery of gold from geothermal waters, Proceedings of the International Workshop on Earth Resources Technology, 105-110, 2007.04. |
| 129. | Egashira Saori, Imai Akira, Tanaka Takayuki, Yasunaga Kentaro, Maeda Katsuhiko, Characteristics of the mineralization at the Wainaoulo-Southeast area, Namosi district, Viti Levu, Republic of the Fiji Islands, Proceedings of the International Workshop on Earth Resources Technology, 111-116, 2007.04. |
| 130. | Imai, A. Geshi, N., Shimano, T. and Nakada, S., Implication of sulfur isotopic variation to 2000 eruptions of Miyake-jima volcano, Izu arc, Island Arc, 16, 83-92., 2007.03. |
| 131. | IMAI AKIRA, GESHI NOBUO, SHIMANO TAKETO, NAKADA SETSUYA, Implication of the temporal sulphur isotopic variation during the 2000 eruption of Miyakejima Volcano, Japan, The Island arc, 10.1111/j.1440-1738.2007.00549.x, 16, 1, 83-92, 2007.03. |
| 132. | Lucas Donny Setijadji, Shigeo Kajino, Akira Imai, Koichiro Watanabe, Cenozoic island arc magmatism in Java Island (Sunda Arc, Indonesia) Clues on relationships between geodynamics of volcanic centers and ore mineralization, Resource Geology, 10.1111/j.1751-3928.2006.tb00284.x, 56, 3, 267-292, 2006.09, Java island, regarded as a classic example of island arcs, is built through multi events of Cenozoic arc magmatism produced by the subduction of Indian-Australian oceanic crusts along the southern margin of Eurasian plate. Regional crustal compositions, subducted slabs, and tectonics determined the spatial-geochemical evolution of arc magmatism and regional metallogeny. Tertiary geodynamics of island arc was dominated by backarc-ward migrations of volcanic centers. Only after the Miocene-Pliocene roll-back effects of retreating slab, slab detachment, and backarc magmatism took place in central Java. The source of arc magmas is mainly partial melting of mantle wedge, triggered by fluids released from dehydrated slabs. Increasing potassium contents of arc magmas towards the backarc-side and younger magmas is typical for all magmas, while alkali and incompatible trace elements ratios are characteristics for different settings of volcanic centers. The oceanic nature of crust and the likely presence of hot slab subducted beneath the eastern Java determine the occurrences of adakitic magmas. Backarc magmatism has a deeper mantle source with or without contributions from subduction-related materials. The domination of magnetite-series magmatism determines the sulfide mineralization for the whole island. District geology, geodynamics, and magma compositions in turn control particular styles and scales of precious metals concentrations. Deep-seated crustal faults have focused the locations of overlapping volcanic centers and metalliferous fluids into few major gold districts. Porphyry deposits are mostly concentrated within Lower Tertiary (early stage) volcanic centers in eastern Java which are not covered by younger volcanic centers, and whose sulfides are derived from partial melting of basaltic parental materials. On the other hand, high-grade low-sulfidation epithermal gold deposits formed in later stages of arc development and are spatially located within younger volcanic centers (Upper Miocene-Pliocene) that overlap the older ones. Gold in low-sulfidation epithermal system is likely to be derived from crustal materials. The overall interacting factors resulting in the petrochemical systematics that are applicable for exploration: 1) early-stage volcanic centers with high Sr/Y and Na2O/K2O ratios are more prospective for porphyry mineralization, while 2) later-stage volcanic centers with high K2O, total alkali, and K2O/Na2O ratios are more prospective for low-sulfidation epithermal mineralization.. |
| 133. | Akira Imai, Naotatsu Shikazono, Masaaki Shimizu, Hidehiko Shimazaki, Sulfur isotope study on Hg and Sb deposits in Japan, Resource Geology, 10.1111/j.1751-3928.2006.tb00266.x, 56, 1, 37-48, 2006.03, Sulfur isotope ratios of cinnabar from Hg deposits and stibnite, jamesonite and berthierite from Sb deposits in Japan are examined in order to understand metallogeneses of Hg and Sb deposits in Japanese island arcs. The studied Hg and Sb deposits include the Hg deposit at Yamato-suigin (Honshu) and the Sb deposit at Ichinokawa (Shikoku) in the Southwest Japan arc. In addition, Hg deposits including Itomuka and Ryushoden in central Hokkaido and Hg and Sb mineralizations in Northeast Japan arc are examined. The δ34S values of cinnabar from the Hidaka-Kitami district, central Hokkaido, including the Itomuka and Ryushoden deposits range widely, from -10 to +16 ‰, the highest values encountered at the Samani deposit. The δ34S values of cinnabar from other areas in Japan range from -12 to +5 ‰, having δ 34S values higher than +2 ‰ from southwestern Hokkaido (Meiji deposit), Shikoku (Suii deposit) and Kyushu (Hasami and Yamagano deposits). On the other hand, the δ34S values of stibnite from all areas in Japan range from -14 to +5 ‰, having positive δ34S values higher than +2 ‰ up to +5 ‰ from southwestern Hokkaido (Yakumo, Toyotomi and Teine deposits) and eastern-central Honshu (Hachiman and Daikoku deposits). The variation in δ34S values of Hg and Sb deposits may reflect the variation in δ34S values of country rocks or variation in mixing ratio of sulfur extracted from the country rocks, sulfur derived from seawater sulfate, and sulfur derived from magmatic emanations. The relatively high δ34S values of cinnabar and stibnite higher than +2 ‰ from southwestern Hokkaido, eastern-central Honshu and Kyushu are probably caused by contribution of volcanic emanation from arc magmas having positive Σδ 34S values, whereas the positive δ34S values of cinnabar higher than +2 ‰ from Suii deposit in Shikoku may be attributed to structurally substituted sulfate in limestone country rocks and/or sulfur derived from seawater sulfate. However, the wide range of the δ34S values of cinnabar from the Hidaka-Kitami district, central Hokkaido, is difficult to explain at this moment. Other relatively low, negative δ34S values of cinnabar and stibnite, berthierite from other areas in Japan may be attributed to 1) incorporation of isotopically light sedimentary sulfur or sulfur derived from ilmenite-series silicic magma, or 2) less contribution of volcanic emanation from arc magmas having positive Σδ34S values.. |
| 134. | Setijadji, L. D., Kajino, S., Imai, A. and Watanabe, K., Cenozoic island arc magmatism in Java Island (Sunda Arc, Indonesia): Clues on relationships between geodynamics of volcanic centers and ore mineralization, Resource Geol., 56, 267-292., 2006.01. |
| 135. | Ishihara, S., Ohmoto, H., Anhaeusser, C. L., Imai, A., and. Robb, L. J., Discovery of the oldest oxic granitoids in the Kaapvaal Craton and its impolication for the redox evolution of early Earth, Geol. Soc. Amer. Memoir, 198, 67-80., 2006.01. |
| 136. | Shunso Ishihara, Hiroshi Ohmoto, Carl R. Anhaeusser, Akira Imai, Laurence J. Robb, Discovery of the oldest oxic granitoids in the Kaapvaal Craton and its implications for the redox evolution of early Earth, Memoir of the Geological Society of America, 10.1130/2006.1198(04), 198, 67-80, 2006.01, Phanerozoic granitoids have been classified into magnetite and ilmenite series based on the abundance of magnetite, which is related to the Fe 2 O 3 /FeO ratio of the rock and the oxygen fugacity ( f O2 ) of its parent magma. We have examined the temporal and spatial distributions of both series in Archean granitoids from the Barberton region and the Johannesburg Dome of the Kaapvaal Craton, South Africa. The oldest syntectonic TTG (tonalite-trondhjemite-granodiorite) granitoids (ca. 3450 Ma in age) were found to be ilmenite series, whereas some intermediate-series granitoids occurred locally. Younger and larger syntectonic TTGs (e.g., the 3230 Ma Kaap Valley plutons) comprise nearly equal quantities of magnetite and ilmenite series. The major 3105 Ma calc-alkaline batholiths (e.g., Nelspruit batholith), emplaced during the late-tectonic stage, comprise mostly magnetite-series granitoids, suggesting that an oxidized continental crust already existed by this time. The rare earth element ratios and δ 18 O values, as well as the Fe 2 O 3 /FeO ratios, of the Archean magnetite-series granitoids suggest that their magmas were generated from the partial melting of subducted oceanic basalts that had been oxidized by interaction with seawater on mid-oceanic ridges; the processes of magma generation were much like those for Phanerozoic magnetite-series granitoids. This further suggests that the concentrations of oxidants (O 2 and/or SO 4 2- ) in the Archean oceans were similar to those in Phanerozoic oceans. Low concentrations of chlorine in the magmas, as well as deep levels of granite erosion, appear to explain the absence of major mineral deposits associated with the Kaapvaal granitoids.. |
| 137. | Syafrizal, 今井 亮, 本村 慶信, インドネシア,西ジャワ,ポンコール金銀鉱床のチウルグ脈の金鉱化作用の特徴, Resource Geology, 10.1111/j.1751-3928.2005.tb00244.x, 55, 3, 225-238, 2005.09. |
| 138. | IMAI A., Primary ore mineral assemblage and fluid inclusion study of the Batu Hijau porphyry Cu-Au deposit, Sumbawa, Indonesia, Resource Geology, 10.1111/j.1751-3928.2005.tb00245.x, 55, 3, 239-248, 2005.09. |
| 139. | IMAI A., Evolution of hydrothermal system at the Dizon porphyry Cu-Au deposit, Zambales, Philippines, Resour. Geol., 10.1111/j.1751-3928.2005.tb00230.x, 55, 2, 73-90, 2005.06. |
| 140. | MINAMI T., Using sulfur isotope to determine the sources of ver-million in ancient burial mounds in Japan, Geoarcheology, 10.1002/gea.20035, 20, 1, 79-84, 20, 79-84., 2005.01. |
| 141. | Imai, A., Evolution of hydrothermal system at the Dizon porphyry Cu-Au deposit, Zambales, Philippines, Resource Geol., 55, 73-90., 2005.01. |
| 142. | Syafrizal, Imai, A., Watanabe, K. and Motomura, Y., Mineralization at the Ciurug vein, Pongkor gold-silver deposit, Indonesia, Resource Geol., 55, 225-238., 2005.01. |
| 143. | Imai, A. and Ohno, S., Primary ore mineral assemblage and fluid inclusion study of the Batu Hijau porphyry Cu-Au deposit, Sumbawa, Indonesia, Resource Geol., 55, 239-248., 2005.01. |
| 144. | 実松 健造, Duncan Robert, 今井 亮, 40Ar/39Ar法による浅熱水性菱刈金鉱床の鉱化作用の地質年代学的な制約, Resource Geology, 10.1111/j.1751-3928.2005.tb00246.x, 55, 3, 249-266, 55, 249-266., 2005.01. |
| 145. | Aramaki, Y., Yokoyama, T., Okaue, Y., Imai, A. and Watanabe, K., A study on the formation of smectite in silica scale formed from geothermal water: The effect of magnesium, Resource Geol., 55, 281-284., 2005.01. |
| 146. | Yoshinobu Aramaki, Takushi Yokoyama, Yoshihiro Okaue, Akira Imai, Koichiro Watanabe, A study on the formation of smectite in silica scales precipitated from geothermal water The effect of magnesium, Resource Geology, 10.1111/j.1751-3928.2005.tb00248.x, 55, 3, 281-284, 2005.01, Silica scales containing large amounts of smectite were recently found in the pipelines for geothermal water at a geothermal power plant. To elucidate the mechanism of smectite formation, seven silica scale samples were characterized by powder X-ray diffraction, chemical analysis and 27Al MAS NMR. Smectite was present in samples with MgO levels above 10 wt% and Al2O3 levels below 10 wt%. In 27Al MAS NMR spectra, peaks assigned to both tetrahedrally and octahedrally coordinated aluminum (Al(4) and Al(6)) were observed for Mg-rich samples, whereas a peak due to Al(4) alone appeared in Mg-poor samples. From these observation and comparison between 27Al MAS NMR spectra for synthesized precipitates of Al2O3-SiO2 containing MgO and not containing MgO, it is concluded that magnesium plays an important role in the stabilization of Al(6), and results in the formation of smectite.. |
| 147. | Kenzo Sanematsu, Robert Duncan, Akira Imai, Koichiro Watanabe, Geochronological constraints using 40AR/39AR dating on the mineralization of the Hishikari epithermal gold deposit, Japan, Resource Geology, 10.1111/j.1751-3928.2005.tb00246.x, 55, 3, 249-266, 2005.01, Ages for thirty adularia samples collected from various veins were in the Hishikari gold deposit determined by 40Ar/39Ar dating to constrain the timing of adularia-quartz vein formation and to determine the temporal change in temperature of hydrothermal fluid. Plateau ages were obtained from all adularia samples, and significant excess 40Ar is not recognized from inverse isochrones. The duration of mineralization within individual veins was determined by adularia ages from the early and late stages of mineralization within the same vein. The durations of mineralization in the Daisen-1, Daisen-3, Hosen-2 and Keisen-3 veins in the Honko-Sanjin zone were 7,000, 140,000, 160,000 and 170,000 years, respectively. The durations of mineralization in the Seisen-2 and Yusen-1-2 veins in the Yamada zones were 360,000 and 320,000 years, respectively. Mineralization lasted for a relatively longer period in individual veins at the Yamada zone. Mineralization ages from the Honko-Sanjin zone range from 1.04 to 0.75 Ma, and most mineralization ages are concentrated in a short period from 1.01 to 0.88 Ma. In contrast, mineralization ages for the Yamada zone range from 1.21 to 0.64 Ma. These results indicate that fracturing and subsequent vein formation lasted for a longer period in the Yamada zone (about 570,000 years) compared with those events in the Honko-Sanjin zone (about 290,000 years). The homogenization temperatures of liquid-rich fluid inclusions in columnar adularia used for age determination were determined to be 223°C on average, and most of these temperatures range from 180 to 258°C. No significant temporal change in homogenization temperature is recognized in this study. However, adularia in the Keisen veins indicated higher homogenization temperatures compared with elsewhere in the deposit, suggesting that the principal ascent of mineralizing hydrothermal fluid was via the Keisen veins.. |
| 148. | Takeshi Minami, Akira Imai, Michiaki Bunno, Kunihiko Kawakami, Setsuo Imazu, Short contribution Using sulfur isotopes to determine the sources of vermillion in ancient burial mounds in Japan, Geoarchaeology, 10.1002/gea.20035, 20, 1, 79-84, 2005.01, This study represents an attempt to determine the sources of vermillion found in ancient Japanese burial mounds of the 1st-6th centuries A.D., by comparing their ratios of sulfur isotopes with those of cinnabar ore samples collected in Japan and China. Cinnabar ore samples were taken from three mines in central Japan (Niu in Mie, Yamato in Nara, and Sui in Tokushima prefectures), and from Wanshan in China, where mining activity has been recorded back to the 6th century A.D. and earlier. When the ratios of 34S and 32S were compared with the Canyon Diablo meteorite standard, a high δ34S value of + 22.6 ± 3.60‰ was found for the Wanshan mine, as opposed to low values ranging from -7.3 ± 1.9 to -2.1 ± 1.6‰ for the Japanese mines. The ratios of sulfur isotopes in vermillion collected from ancient Japanese burial mounds also divided into two groups. High ratios (+11.1 to +22.8‰) were found in 1st- and 2nd- century burials in the western regions of northern Kyushu and San'in, suggesting that local, powerful chiefs obtained vermillion through relations with China. Lower ratios (-8.4 to -2.0‰) were found in burials of the 2nd through 6th centuries in central Japan, where the ancient Yamato dynasty emerged as the first unified polity around the end of the 2nd century A.D. We, therefore, conclude that the Yamato dynasty exploited local sources of vermillion, rather than depending solely on China. The present study demonstrates the feasibility of determining sources of vermillion using sulfur isotope ratios, and the relevance of such findings for archaeological research.. |
| 149. | Akira Imai, Carla B. Dimalanta, Graciano P. Yumul, Lucas Donny Setijadji, Koichiro Watanabe, About the special issue Collection of achievements on Earth Resources from AUN/SEED-net, Resource Geology, 10.1111/j.1751-3928.2005.tb00236.x, 55, 3, 141-144, 2005. |
| 150. | IMAI A., Occurrence of halogen-rich phlogopite in Late Cenozoic volcanic rocks in the Japanese arcs, Resource Geol., 10.1111/j.1751-3928.2004.tb00196.x, 54, 2, 153-166, 2004.06. |
| 151. | Imai, A., Occurrence of halogen-rich phlogopite in Late Cenozoic volcanic rocks in the Japanese arcs, Resource Geol, 55, 225-238., 2004.01. |
| 152. | Imai, A., Variation of Cl and SO3 contents of microphenocrystic apatite in intermediate to silicic igneous rocks of Cenozoic Japanese Island Arcs: Implications for porphyry Cu metallogenesis in the Western Pacific Island Arcs, Resource Geol.,, 54, 357-372., 2004.01. |
| 153. | IMAI A., Variation of Cl and SO_3 Contents of Microphenocrystic Apatite in Intermediate to Silicic Igneous Rocks of Cenozoic Japanese Island Arcs : Implications for Porphyry Cu Metallogenesis in the Western Pacific Island Arcs, Resour. Geol., 10.1111/j.1751-3928.2004.tb00211.x, 54, 3, 357-372, 2004.01. |
| 154. | Imai, A. and Uto, T., Association of electrum and calcite and its significance to the genesis of the Hishikari gold deposits, southern Kyushu, Japan., Resource Geology, 52, 381-394, 2002.12. |
| 155. | Sajona, F. G., Izawa, E., Motomura, Y., Imai, A., Sakakibara, H. and Watanabe, K., Victoria carbonate-base metal gold deposit and its significance in the Mankayan mineral district, Luzon, Philippines., Resource Geology, 52, 315-328, 2002.12. |
| 156. | Akira Imai, Tadakazu Uto, Association of Electrum and Calcite and Its Significance to the Genesis of the Hishikari Gold Deposits, Southern Kyushu, Japan, Resource Geology, 10.1111/j.1751-3928.2002.tb00148.x, 52, 4, 381-394, 2002.12, Mineral assemblage, precipitation sequence and textures of the gold-bearing veins from the Hishikari epithermal vein-type deposits, southern Kyushu, Japan, were examined. In addition, fluid inclusion microthermometry and carbon and oxygen isotopic compositions of calcite were determined. Calcite, and that replaced by quartz, were commonly observed throughout the precipitation sequence of the veins. Thus, calcite must be a more common gangue constituent initially than observed presently. Association of calcite and electrum is observed immediately subsequent to columnar adularia in some vein samples. In addition, close association of electrum with pseudo-acicular quartz, and electrum with truscottite were observed. The initial coprecipitation of electrum and calcite might be a common phenomenon in the gold-bearing veins at the Hishikari deposits. The Th (homogenization temperature) data from the Honko-Sanjin deposits are generally higher than those from the Yamada deposit. Samples that show association of calcite and electrum yielded higher Th (206-217°C, average) than the Th data from calcite associated with low-grade Au ore or barren (180-204°C, average). The measured Tm (temperature of last melting point of ice) range from -0.4 to 0.0°C. The result suggests that the salinity of the hydrothermal solution was low during the precipitation both of calcite associated with Au mineralization and of barren calcite. Fluid inclusion evidence suggestive of boiling of hydrothermal solution for the precipitation of calcite was not recognized in the present work. The δ13C and δ18O values of calcite range from -10.8 to -4.7 ‰ and from +3.2 to +15.2 ‰, respectively. The δ13C value of H2CO3 and the δ18O value of H2O in the hydrothermal fluids calculated assuming isotopic equilibrium with calcite using the temperature obtained by fluid inclusion microthermometry, range from -14.4 to -9.1 ‰, and from -6.2 to +5.5 ‰, respectively. Thus, the calculated δ18O values of H2O for calcite further confirm the presence of the 18O-enriched ore fluids during the mineralization at the Hishikari deposits. The hydrothermal solution isotopically equilibrated with the sedimentary basement rocks was responsible for the gold mineralization associated with calcite.. |
| 157. | Fernando G. Sajona, Eiji Izawa, Yoshinobu Motomura, Akira Imai, Hiroyuki Sakakibara, Koichiro Watanabe, Victoria Carbonate-Base Metal Gold Deposit and Its Significance in the Mankayan Mineral District, Luzon, Philippines, Resource Geology, 10.1111/j.1751-3928.2002.tb00142.x, 52, 4, 315-328, 2002.12, The Victoria gold deposit is a low-sulfidation style epithermal carbonate-base metal gold deposit discovered in 1995 in the Mankayan mineral district, northern Luzon, Philippines. It occurs just south of a high-sulfidation copper-gold orebody (Lepanto enargite deposit) and a porphyry copper-gold orebody (Far Southeast (FSE) deposit). Radiometrie dating of the Victoria mineralization yields ages from 1.6 to 1.1 Ma, similar to ages of the Lepanto enargite deposit. Mineralogical, geochemical, fluid inclusion and sulfur isotope studies suggest that the Victoria deposit is not directly related to the late stage of hydrothermal activity that produced the FSE porphyry copper-gold and the Lepanto enargite deposits. The hydrothermal fluids from which Victoria was formed appear to have been derived from the south, and not from the FSE intrusive center to the northeast. This would suggest a regional south to north hydrothermal gradient, consistent with the similar flow within the FSE porphyry-Lepanto enargite system. On a district scale, structures similar to those of the Victoria deposit in trend and nature would be highly prospective, especially if they occur north of identified plutons and volcanic centers.. |
| 158. | Zeolite-bearing veins pervaded in the Shirahama Formation Upper Pliocene) of the Chikura Group at the southern end of Boso Peninsula, Chiba Prefecture. |
| 159. | S Ishihara, LJ Robb, CR Anhaeusser, A Imai, Granitoid series in terms of magnetic susceptibility: A case study from the Barberton region, South Africa, GONDWANA RESEARCH, 10.1016/S1342-937X(05)70630-4, 5, 3, 581-589, 2002.07, Magnetic susceptibilities were measured on a representative collection of Archaean granitoids of the Barberton region using a portable KT5 magnetic susceptibility meter. The studied granitoids comprise, (1) syn-tectonic tonalite-trondhjemite-granodiorite (TTG) granitoids (132 samples), (2) late-tectonic calc-alkaline granitoids (402 samples) and (3) post-tectonic low-Ca and high-Ca granitoids (12 samples). Most of the early-stage syn-tectonic granitoids (similar to3450 Ma) have low magnetic susceptibilities, less than 3 x 10(-3) SI units, and correspond to ilmenite-series granitoids. The late-stage Kaap Valley tonalite pluton (similar to3230 Ma) contains sporadically distributed higher magnetic susceptibility values (greater than 3 x 10(-3) SI units), which are less than one-third in magnetic susceptibility of typical magnetite-series TTG of the Japanese Island Arc and thus strictly belong to an intermediate series. The Barberton TTG suite is essentially derived from reduced amphibolitic lower crust that reflects the anoxic nature of the Earth surface during the Archaean Eon. The more oxidized nature of the Kaap Valley tonalite may be generated in an oxidized lower crust by fluids squeezed out of the subducting plate. Late-tectonic granodiorite - adamellite batholithic complexes (similar to3105 Ma) belong mostly to the magnetite series, and seem to suggest that relatively oxidized continental crust, reflecting oxic atmosphere and subduction mechanism operating, had evolved it by this time. Post-tectonic granitic plutons formed largely between circa 2900 Ma and 2700 Ma can be subdivided into low-Ca ilmenite series and high-Ca magnetite series.. |
| 160. | Ishihara, S., Robb, L. J., Anhaeusser, C. L. and Imai, A., Granitoid series in terms of magnetic susceptibility: A case study from the Archean Barberton Region, South Africa., Gondwana Research, 5, 581-589, 2002.06. |
| 161. | Imai, A., Metallogenesis of porphyry Cu-Au deposits of the western Luzon arc, Philippines: K-Ar ages, SO3 contents of microphenocrystic apatite and significance of intrusive rocks., Resource Geology, 52, 147-161, 2002.06. |
| 162. | Akira Imai, Metallogenesis of porphyry Cu deposits of the western Luzon arc, Philippines K-Ar ages, SO3 contents of microphenocrystic apatite and significance of intrusive rocks, Resource Geology, 10.1111/j.1751-3928.2002.tb00127.x, 52, 2, 147-161, 2002.01, K-Ar ages of the following porphyry Cu deposits in the western Luzon arc are determined: Lobo-Boneng (10.5±0.4 Ma), Santo Niño (9.5±0.3 Ma), Black Mountain (2.1±0.1 Ma), Dizon (2.5±0.2 Ma) and Taysan (7.3±0.2 Ma). Microphenocrystic apatite in the late Cenozoic intermediate to silicic intrusions associated with porphyry Cu deposits in the western Luzon arc contains sulfur as SO3 detectable by electron probe microanalyzer. Sulfur is supposed to have been accommodated dominantly as oxidized species in oxidizing hydrous magmas that generated porphyry Cu deposits. Likewise, such high SO3 contents in microphenocrystic apatite are common characteristics of the intermediate to silicic magmatism of the western Luzon arc, from tonalitic rocks of the Luzon Central Cordillera of about 15 Ma to an active magmatism at Mount Pinatubo. Thus, the western Luzon arc has been generating porphyry Cu mineralization associated with oxidizing hydrous intermediate to silicic magmatism related to eastward subduction, since Miocene to the present day. Intermediate to silicic rocks since 15 Ma to present-day western Luzon arc generally show high whole-rock Sr/Y ratio ranging from 20 to 184. However, porphyry Cu deposit is not necessarily related to the rocks that show higher Sr/Y ratios compared to the other barren rocks in the western Luzon arc. The characteristics of the intermediate to silicic magma associated with porphyry Cu deposit are not attributed to the composition of the source material of the magma, but to the properties defined by the high activity of oxidized species of sulfur in the fluid phase that is encountered during the generation of intermediate to silicic magmas.. |
| 163. | Genesis of Na-series rock alteration widespread in the southeastern area of Hachimantai geothermal field: Water-rock interactions driven by descending groundwater and fossil seawater.. |
| 164. | Imai, A., Ishizuka, O., Yamada, R. and Miyamoto, H., Further occurrence of brown ores in Kuroko-type deposits in Japan., Resource Geology, 51, 263-268, 2001.09. |
| 165. | Akira Imai, Osamu Ishizuka, Ryoichi Yamada, Hiroshi Miyamoto, Further occurrence of brown ores in Kuroko-type deposits in Japan, Resource Geology, 10.1111/j.1751-3928.2001.tb00098.x, 51, 3, 263-268, 2001.09, The occurrence of so-called brown ore from the Kuroko-type deposits in Japan was examined. Brown ore specimens from the Kannondo, Inarizawa, Matsumine, Fukazawa, Uchinotai, Kosaka (orebody unknown) and Nurukawa deposits have been found in the ore collection stored by Dowa Mining Co. Ltd. and the subsidiary companies. In addition, occurrences from the Fukazawa, Matsumine, Ezuri, Shakanai, and Ginzan deposits were previously reported. The brown ore is characterized by its color and by its higher Ag concentration (up to around 2,400 g/t) than ordinary black ores. This type of ore occurs commonly in the Kuroko-type deposits in Japan, whereas its extent is limited. The brown ore is a type of Au-rich massive sulfide ore formed in submarine hydrothermal system.. |
| 166. | Imai, A., Generation and evolution of ore fluids for porphyry Cu-Au mineralization at the Santo Tomas II (Philex) deposit, Philippines., Resource Geology, 51, 71-96, 2001.06. |
| 167. | Akira Imai, Generation and evolution of ore fluids for porphyry Cu-Au mineralization of the Santo Tomas II (Philex) deposit, Philippines, Resource Geology, 10.1111/j.1751-3928.2001.tb00083.x, 51, 2, 71-96, 2001.06, The Santo Tomas II (Philex) deposit is a porphyry Cu-Au deposit, located in the southern part of the Baguio mineral district, Benguet Province, northern Luzon, Philippines. The Santo Tomas II deposit is associated with an intrusive complex consisting of four rock types that are distinguished based on petrography. They are 1) post-ore clinopyroxene-bearing hornblende andesite porphyry, 2) ore-generating hornblende andesite porphyry, 3) hornblende quartz diorite porphyry and 4) porphyritic hornblende quartz diorite. K-Ar age of hydrothermal biotitization was estimated to be 1.5±0.4 Ma. A number of intrusive bodies having broadly similar petrography and K-Ar age occur in the vicinity of the Santo Tomas II deposit, such as at Clifton, Ligay (Binang), Bumolo (Waterhole) and Philex Main Camp areas. The intrusions at the Santo Tomas II deposit and in the vicinity are characterized by high XMg (Mg/[Mg+Fe] atomic ratio, about 0.7 or higher) of mafic silicate phenocrysts such as hornblende, and high sulfur contents (> 0.2 wt% as SO3) in accessory microphenocrystic apatite, suggesting a highly oxidizing condition. Sulfur is accommodated dominantly as oxidized species since the crystallization of phenocrysts. Sub-dendritic rim of tremolitic amphibole on hornblende phenocryst in the ore-generating andesite porphyry at the Santo Tomas II deposit suggests interaction of magma and aqueous fluid(s) exsolved due to decompression during intrusion. Dissemination of magnetite is associated with hydrothermal biotitization and is followed by sheeted and stockwork quartz veinlets having silician magnetite and rare titanohematite instead of Cu-Fe sulfides. The silician magnetite-rich quartz veinlet was formed at fO2 near the hematite-magnetite buffer at nearly magmatic temperature, where sulfur dominantly existed as oxidized species such as SO2. Chalcopyrite and bornite, which commonly exhibit micrographic texture often accompanying Pd telluride and native gold/Au-rich electrum, are associated with subsequent anhydrite (-quartz) veinlets and stringers. Both intermediate solid solution (iss) and bornite solid solution (bnss) are thought to have coprecipitated primarily at above 500°C based on fluid inclusion microthermometry and sulfur isotope thermometry applied for anhydrite and associated chalcopyrite and bornite. The initial iss is considered to have converted to chalcopyrite partly replacing bnss during cooling. The hypersaline polyphase fluid inclusions abundantly found in the sheeted and stockwork quartz as well as anhydrite veinlets with scarce gaseous inclusions suggest that they have been trapped in the two aqueous fluid immiscible region. The western Luzon arc associated with porphyry Cu mineralization is characterized by oxidized hydrous magmatism and shallow emplacement, and by the source of sulfur enriched in 34S.. |
| 168. | Horizontal strain rate in relation to vein formation of the Hishikari gold deposits, southern Kyushu, Japan.. |
| 169. | Tadakazu Uto, Akira Imai, Yu Yamato, Horizontal strain rate in relation to vein formation of the Hishikari gold deposits, Southern Kyushu, Japan, Resource Geology, 10.1111/j.1751-3928.2001.tb00077.x, 51, 1, 7-18, 2001.03, The Hishikari deposits comprise the Honko (Main), Yamada, and Sanjin deposits. The horizontal strain in the direction (approx. N40°W) normal to the general NE-SW strike of the Hishikari vein system was calculated, based on the measured total vein widths to the distance along three crosscuts. Veins were assumed to accompany no significant fault displacement in the calculation. Veins in the eastern and the middle parts of the Honko-Sanjin deposits spatially occupy 3.2% and 1.3%, respectively, and veins in the Yamada deposit occupy 6.7%. Significant local variation of strain is observed in some areas. Reported K-Ar ages on adularia-quartz veins indicate the duration of vein opening to be 2.6×105 yr in the Honko-Sanjin deposits and 5.9×105 yr in the Yamada deposit. Horizontal strain rates were calculated to be 5.0-12×10-8 yr-1 through the Hishikari deposits. The calculated strain rates at the Hishikari deposits are roughly comparable to the regional horizontal strain rate in the Recent. Widespread extensional movement in southern Kyushu seems to have been able to provide sufficient strain for the formation of the Hishikari deposits, rather than contribution of local movements.. |
| 170. | Imai, A. and Anan, S., Sulfur isotope study and re-examination of ore mineral assemblage of the Hol Kol and the Tul Mi Chung skarn-type copper-gold deposits of the Suan mining district, Korean Peninsula., Resource Geology, 50, 213-228, 2000.12. |
| 171. | Akira Imai, Shin'ichi Anan, Sulfur isotope study and re-examination of ore mineral assemblage of the Hol Kol and the Tul Mi Chung Skarn-type copper-gold deposits of the Suan Mining District, Korean Peninsula, Resource Geology, 10.1111/j.1751-3928.2000.tb00071.x, 50, 4, 213-228, 2000.12, Ore specimens collected by the late Professor Takeo Watanabe from the Hol Kol and the Tul Mi Chung deposits, Suan mining district, Korean peninsula, were examined. In addition, measurements of sulfur isotopic ratio of ores and preliminary fluid inclusion microthermometry were carried out. Ores from the New orebody of the Hol Kol deposit consist mainly of bornite, wittichenite and chalcopyrite presently, which exhibit lamellae intergrowth texture, associated with native bismuth and electrum. Bismuthian bornite solid solution is considered to be a principal initial phases, while native bismuth was nucleated as molten bismuth melt initially. The occurrence of cubanite, miharaite, carrollite, siegenite, hessite and geikielite are recognized from the New orebody. Ores from the Eastern orebody of the Hol Kol deposit consist chiefly of chalcopyrite, occasionally associated with trace amounts of pyrrhotite, pyrite, bismuthinite and rare tellurobismuthite, while an ore specimen from the Western orebody consists mainly of sphalerite associated with chalcopyrite, pyrite and galena. Ores from the Tul Mi Chung deposit consist mainly of chalcopyrite and pyrite, occasionally associated with magnetite, sphalerite, galena and rare molybdenite. Some portions of magnetite are revealed to be silician magnetite. Sulfur fugacity is supposed to be below the stability field of bismuthinite in the New orebody. A reducing condition is suggested by the occurrence of geikielite without Fe3+ content. The sulfur and oxygen fugacities for the Eastern and Western orebodies of the Hol Kol deposit and for the Tul Mi Chung deposit were higher than the New orebody of the Hol Kol deposit. On the other hand, the Suan granite (porphyritic granodiorite) and the Chil Sing Dai granite (biotite granite porphyry) from the Hol Kol area can be classified as weakly magnetic magnetite-series. Polyphase fluid inclusions are observed in gangue diopside associated with Cu ore of two specimens. The dissolution temperatures of daughter crystals are 394±26°C and 442±45°C, while the disappearing temperatures of vapor bubble were 475±25°C and >500°C. Highly saline fluids were responsible for the mineralization at the Hol Kol deposit. The δ34S values of ore sulfides of the Hol Kol and the Tul Mi Chung deposit range from +11.5 ‰ to +16.1 ‰, having anomalous lower values mainly from the Tul Mi Chung deposit. Such anomalous lower δ34S values can be caused by isotopic fractionation against oxidized sulfur species. The δ34S value of bulk sulfur in the ore solutions responsible for the Hol Kol and the Tul Mi Chung deposit is estimated to be +13.5±2.5 ‰.. |
| 172. | Ishihara, S. and Imai, A., Geneses of high chlorine and silver-lead-zinc-mineralized granitoids in Tsushima, Japan., Resource Geology, 50, 169-178, 2000.09. |
| 173. | Imai, A., Mineral paragenesis, fluid inclusions and sulfur isotope systematics of the Lepanto Far Southeast porphyry Cu-Au deposit, Mankayan, Benguet, Philippines., Resource Geology, 50, 151-168, 2000.09. |
| 174. | Shunso Ishihara, Akira Imai, Geneses of high chlorine and silver-lead-zinc-mineralized granitoids in Tsushima, Japan, Resource Geology, 10.1111/j.1751-3928.2000.tb00066.x, 50, 3, 169-178, 2000.09, Miocene granitoids of the Tsushima Islands have unique characteristics that cannot be seen in other major granitic plutons in the Japanese Islands as follows: (1) They are granitic in composition but contain synplutonic mafic dikes, abundant mafic enclaves, and intermediate facies between granite and mafic enclaves. (2) They are mixture of magnetite-bearing and -free facies, but generally magnetite-free in the marginal part. (3) They are high in K2O content (K65=3.1) and intermediate in normative corundum (C65=0.1) and δ18O value (+9 ‰ at SiO2 70 %), which may be comparable with those of the Miocene Outer Zone granitoids. (4) Yet the initial Sr ratio is low as 0.7037. (5) They are high in Cl and S, which occur in fluid inclusions and as pyrrhotite>pyrite, respectively. Two genetic models are considered for the source of the unique granitoid magmas: the continental crust or the upper mantle fertilized with Si, K and 18O. The latter may be the case for the Tsushima granitoids, because of the low initial Sr ratio. The age of the granitoids (16 Ma) indicates the magmatism related to the opening of the Sea of Japan. It is suggested that both basaltic and granitic magmas were generated in the continental lithosphere under an extensional tectonic setting; the two magmas could have been partly mingled. The mingled magma was originally an oxidized type, but reduced during the emplacement by repeated inflow of S and C-bearing gases from the pelitic wall rocks. Because of the reduction, SO3 sulfur is almost nil in the rock-forming apatite, and most of sulfur remained in fluid phase of the magma as reduced species. Cl content was high in the original magma and concentrated in the fluid phase of the residual system which dissolved silver, lead and zinc metals. Such a fluid migrated into the Taishu fracture systems, as the magma crystallized, and formed the silver-lead-zinc deposits.. |
| 175. | Akira Imai, Mineral paragenesis, fluid inclusions and sulfur isotope systematics of the Lepanto Far Southeast porphyry Cu-Au deposit, Mankayan, Philippines, Resource Geology, 10.1111/j.1751-3928.2000.tb00065.x, 50, 3, 151-168, 2000.09, The Lepanto Far Southeast porphyry Cu-Au deposit is located beneath and to the southeast of the Lepanto enargiteluzonite Cu-Au deposit in Mankayan, Benguet Province, Philippines. The principal orebody consists of potassic alteration subjected to partial retrograde chlorite alteration that rims stockwork of quartz-anhydrite veinlets. Fluid inclusions found in stockwork quartz and anhydrite in the biotitized orebody center are dominated by polyphase inclusions that homogenize at temperatures of >500°C. Sulfur isotopic thermometry applied to the sulfides-anhydrite pairs suggests around 500°C. The principal ore minerals associated with quartz-anhydrite stockworks are chalcopyrite and pyrite with minor bornite and Bi-Te-bearing tennantite, with trace of native gold. Rounded pyrite grains appear fractured and corroded and are interpreted as remnants of primary intermediate solid solution + pyrite assemblage. A breccia pipe truncates the deposit. Mineralization in the breccia pipe is brought by quartz-anhydrite veinlets and infilling in the interstices between clasts. Chalcopyrite-Au mineralization associated with molybdenite is recognized in the deeper zone in the breccia pipe. Fluid inclusion microthermometry on polyphase inclusions in veinlet quartz as well as sulfur isotope thermometry applied for the pair of anhydrite and sulfides suggests >450°C. Fluid inclusions in veinlet quartz and anhydrite in the fringe advanced argillic alteration are chiefly composed of coexisting liquid-rich inclusions and gas-rich inclusions, in addition to coexisting polyphase inclusions and gas-rich inclusions. These inclusions exhibit a wide range of homogenization temperatures, suggesting heterogeneous entrapping in the two-fluid unmixing region. Sulfur isotopes of aqueous sulfide and sulfate exhibit a general trend from the smallest fractionation pairs (about 11 ‰) in the biotitized orebody center to the largest fractionation (about 25 ‰) pairs in the fringe advanced argillic alteration, suggesting a simple evolution of hydrothermal system. The slopes of arbitrary regression lines in δ34S versus △34S[SO4=-H2S] diagram suggest that the abundance ratio of aqueous sulfate to sulfide in the hydrothermal fluid has been broadly constant at about 1:3 through temperature decrease. The intersection of these two regression lines at the δ34S axis indicates that the bulk δ34S is about +6 ‰. Thus, the Lepanto FSE deposit is a further example which confirms enrichment in 34S in the hydrous intermediate to silicic magmas and associated magmatic hydrothermal deposits in the western Luzon arc.. |
| 176. | Hamada, M. and Imai, A., Sulfur isotopic study of the Toyoha deposits, Hokkaido, Japan -Comparison between the earlier-stage and later-stage veins-., Resource Geology, 50, 113-122, 2000.06. |
| 177. | Imai, A., Genesis of the Mamut porphyry Cu deposit, Sabah, East Malaysia., Resource Geology, 50, 1-23, 2000.03. |
| 178. | Akira Imai, Genesis of the Mamut porphyry copper deposit, Sabah, East Malaysia, Resource Geology, 10.1111/j.1751-3928.2000.tb00052.x, 50, 1, 1-23, 2000.03, The Mamut deposit of Sabah, East Malaysia, is a porphyry type Cu-Au deposit genetically related to a quartz monzonite ("adamellite") porphyry stock associated with upper Miocene Mount Kinabalu plutonism. The genesis of the Mamut deposit is discussed based on petrology of the intrusives in the Mount Kinabalu area combined with ore-and alteration-petrography, fluid inclusion and sulfur isotope studies. Groundmass of the adamellite porphyry at Mamut is rich in K which suggests vapor transport of alkaline elements during the mineralizing magmatic process, while the groundmass of the post-ore "granodiorite" porphyry at Mamut contains small amounts of normative corundum suggesting depletion in alkaline elements at the root zone of the magma column. Subdendritic tremolitic amphibole rims on hornblende phenocrysts in the Mamut adamellite porphyry suggest interaction between the mineralizing magma and the exsolved fluids. Occurrences of clinopyroxene microphenocrysts and pseudomorphic aggregates of shredded biotite and clinopyroxene after hornblende phenocrysts in the barren intrusives imply lower water fugacity and decreasing in water fugacity, respectively. Compositional gap between the core of hornblende phenocrysts and the tremolitic amphibole rims and those in the groundmass of the Mamut adamellite porphyry suggests a decrease in pressure. Higher XMg (=Mg/(Mg+Fe) atomic ratio) in the tremolitic amphibole rims in the Mamut adamellite porphyry compared to those of the barren intrusions suggests high oxygen fugacity. High halogen contents of igneous hydrous minerals such as amphiboles, biotite and apatite in the Mamut adamellite porphyry suggest the existence of highly saline fluids during the intrusion and solidification of the mineralizing magma. Fluid inclusions found in quartz veinlet stockworks are characterized by abundant hypersaline polyphase inclusions associated with subordinate amounts of immiscible gaseous vapor Both Cu and Au are dispersed in disseminated and quartz stockwork ores. Chalcopyrite and pyrrhotite as well as magnetite are the principal ore minerals in the biotitized disseminated ores. Primary assemblage of intermediate solid solution (iss) and pyrrhotite converted to the present assemblage of chalcopyrite and pyrrhotite during cooling. Subsequent to biotitization, quartz veinlet stockworks formed associated with retrograde chlorite alteration. The Cu-Fe sulfides associated with stockwork quartz veinlet are chalcopyrite and pyrite. Overlapping Pb and Zn and subsequent Sb mineralizations were spatially controlled by NNE-trending fractures accompanying the phyllic and advanced argillic alteration envelope. Sulfur isotopic composition of ore sulfides are homogeneous (about +2‰) throughout the mineralization stages. These are identical to those of the magmatic sulfides of Mount Kinabalu adamellitic rocks.. |
| 179. | Morihisa Hamada, Akira Imai, Sulfur isotopic study of the Toyoha deposit, Hokkaido, Japan - Comparison between the earlier-stage and the later-stage veins -, Resource Geology, 10.1111/j.1751-3928.2000.tb00061.x, 50, 2, 113-122, 2000.01, The sulfur isotopic ratios (δ34S) of ore minerals from the Toyoha deposit, Hokkaido, one of the largest Pb-Zn-Ag polymetallic vein-type deposits in Japan, were studied. More than 90% of the δ34S values of the studied sulfide minerals collected from the Toyoha deposit range from +5 to +9‰, with an average of approximately +7‰ irrespective of the mineral species, veins, elevations, depth, mineralization stages and mineral assemblages. Relatively uniform δ34S values obtained in this study suggest that the reduced aqueous sulfur species predominated over the oxidized sulfur in the ore solution throughout the mineralization stages. Thus, the source of sulfur for the ore sulfides is postulated to be magmatic. The temperature obtained from the fluid inclusion study and the temperature of the sulfur isotopic geothermometer are not consistent. This suggests that each mineral precipitated under disequilibrated conditions with respect to the sulfur isotope. The change in redox conditions presumably encountered between the mineralization stages did not account for the isotopic fluctuations since the isotopic exchanges between the oxidized and reduced aqueous sulfurs are much slower than the rate of oxidation of the ore solution.. |
| 180. | Imai, A., Pyrite disease in luzonite from the Lepanto Cu-Au deposit, Mankayan, Philippines: Further example of disease texture and its origin., Resource Geology, 49, 163-168, 1999.09. |
| 181. | Akira Imai, Pyrite disease in Luzonite from the Lepanto Cu-Au deposit, Mankayan, Philippines: Further example of disease texture and its origin, Resource Geology, 10.1111/j.1751-3928.1999.tb00042.x, 49, 3, 163-168, 1999.09. |
| 182. | Spinifex texture of native sulfur: A cooling product of sulfur flow eruptions at Shiretoko-Iwozan volcano, Hokkaido, Japan. |
| 183. | Imai, A., Matsueda, H., Yamada, R. and Masuta, K., Polymetallic mineralization at the Shin-Ohtoyo deposit, Harukayama district, Hokkaido, Japan., Resource Geology, 49, 75-88, 1999.06. |
| 184. | Akira Imai, Hiroharu Matsueda, Ryoichi Yamada, Kenzo Masuta, Polymetallic mineralization at the Shin-Ohtoyo deposit, Harukayama district, Hokkaido, Japan, Resource Geology, 10.1111/j.1751-3928.1999.tb00033.x, 49, 2, 75-88, 1999.06, The Shin-Ohtoyo Cu-Au deposit is located in the Harukayama district, 20 km west of Sapporo, Hokkaido, Japan. Both acid-type disseminated and adularia-quartz-type vein Au mineralizations have been recognized within a small distance of less than 500 m in the district. Mineralogical characteristics of sulfide ores from the Shin-Ohtoyo deposit have been proved to be polymetallic. Ore minerals containing Sn, V, Bi and Te are recognized. Nine ore types are recognized in terms of characteristic mineral assemblage; (1) chalcedonic quartz veinlets in silicified zone around the deposit, (2) bismuthinite, emplectite, ±friedrichite and tetrahedrite, (3) an unnamed Cu-Sn-Fe-Zn sulfide, colusite-series minerals, ±stannoidite, emplectite and tetrahedrite, (4) bournonite, Se-bearing galena and tetrahedrite, (5) luzonite/famatinite and Ag-bearing tetrahedrite, (6) colusite-series minerals, emplectite, aikinite and tetrahedrite/goldfieldite, (7) luzonite/famatinite, colusite-series minerals, mawsonite and tetrahedrite/goldfieldite, (8) enargite, luzonite/famatinite and tetrahedrite, and (9) colusite-series minerals and tetrahedrite. The first occurrence of friedrichite and stibiocolusite from Japan are reported. The chemical formula of the unnamed phase corresponds to Cu6(Cu,Fe,Zn)Sn3S10. Sulfur isotopic ratios (δ34S) of sulfides from the stockpile range from -0.5 ‰ to + 1.9 ‰, and those from drill cores recovered by Metal Mining Agency of Japan (MMAJ) vary from -2.7 ‰ to +0.8 ‰. Sulfur isotopic ratio of barite in a cavity in the silicified tuff breccia collected from the stock pile yields +27.1 ‰, while that of barite collected from MMAJ core is +21.7 ‰. Sulfur isotopic thermometry applied for a pair of barite (+21.7 ‰) and associated pyrite (+1.8 ‰) indicates about 300°C. High-Te tetrahedrite composition from both the chalcedonic quartz vein in the silicified zone around the Shin-Ohtoyo deposit and the polymetallic sulfide ores from the adit of the deposit, suggests that the Au mineralization in the former is attributed to a hydrothermal system marginal to the polymetallic mineralization.. |
| 185. | Iizasa, K., Fiske, R. S., Ishizuka, O., Yuasa, M., Hashimoto, J., Ishibashi, J., Naka, J., Horii, Y., Fujiwara, Y., Imai, A. and Koyama, S., A Kuroko-type polymetallic sulfide deposit in a submarine silicic caldera., Science, 283, 975-977, 1999.02. |
| 186. | K Iizasa, RS Fiske, O Ishizuka, M Yuasa, J Hashimoto, J Ishibashi, J Naka, Y Horii, Y Fujiwara, A Imai, S Koyama, A Kuroko-type polymetallic sulfide deposit in a submarine silicic caldera, SCIENCE, 10.1126/science.283.5404.975, 283, 5404, 975-977, 1999.02, Manned submersible studies have delineated a large and actively growing Kuroko-type volcanogenic massive sulfide deposit 400 kilometers south of Tokyo in Myojin Knell submarine caldera, The sulfide body is Located on the caldera floor at a depth of 1210 to 1360 meters, has an area of 400 by 400 by 30 meters, and is notably rich in gold and silver. The discovery of a Large Kuroko-type polymetallic sulfide deposit in this are-front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits.. |
| 187. | Akira Imai, Nobuo Geshi, Spinifex texture of native sulfur A cooling product of sulfur flow eruptions at Shiretoko-Iwozan volcano, Hokkaido, Japan, Resource Geology, 10.1111/j.1751-3928.1999.tb00035.x, 49, 2, 99-104, 1999.01, Several native sulfur specimens, collected from Shiretoko-Iwozan volcano, eastern Hokkaido, Japan, exhibit spinifex texture, which appears to resemble that often observed in komatiite. The spinifex texture is exhibited by yellow-colored elongated skeletal native sulfur crystals up to 5 cm long settled in medium gray-colored fine-grained clayey matrix. One surface of a specimen is coated by layers of micro pillow lava of native sulfur. Such specimens were rarely found as clasts or fragments around the 1936 No. 1 crater that erupted native sulfur flows, together with the most common monomineralic native sulfur fragments of native sulfur flows having pahoehoe surface and of native sulfur dikes. The elongated spinifex native sulfur crystals presently consist of aggregated polygrains of orthorhombic sulfur crystals formed through crystallographic transition from the single crystal of monoclinic sulfur initially crystallized. The spinifex texture exhibited by elongated skeletal native sulfur crystals is a product of rapid cooling of sulfur melt. Many lithic fragments of altered country rocks are present in the specimens exhibiting native sulfur spinifex texture. This suggests that segregation of the sulfur melt from the mixture of lithic fragments and sulfur melt was incomplete because the mixture was chilled before the melt segregation. Elongated skeletal native sulfur crystals may have nucleated and crystallized directly from the molten sulfur liquid. Lithic fragments mixed in the melt are supposed to have acted as nuclei for the nucleation of the native sulfur crystals. On the other hand, the most of native sulfur flows consist of monomineralic massive native sulfur with very scarce lithic fragments. Such massive monomineralic native sulfur crystallized from the supercooled, solidified amorphous sulfur. Such supercooled amorphous state may have been attained due to the lack of nuclei because of the scarcity of lithic fragments. The unique structures exhibited by native sulfur lava flow, including pahoehoe surface and spinifex texture, are due to the characteristic physical property of molten sulfur liquid, that is, low viscosity.. |
| 188. | Akira Imai, Hidehiko Shimazaki, Tomoko Nishizawa, Hydrogen Isotope Study of Fluid Inclusions in Vein Quartz of the Hishikari Gold Deposits, Japan, Resource Geology, 10.1111/j.1751-3928.1998.tb00014.x, 48, 3, 159-170, 48, 159-170, 1998.09, The origin of mineralizing fluids responsible for the Hishikari vein-type epithermal Au deposits was studied on the basis of the hydrogen isotopic ratio (δD) of the inclusion fluid from vein quartz and adularia. The origin of hydrothermal fluids was estimated by combination of the present δD values and the oxygen isotopic ratios (δ18O) previously reported by Shikazono and Nagayama (1993). The water in the fluid inclusions was extracted by means of decrepitation of quartz at 500°C. Hydrogen was obtained by reduction of the collected water with Zn shot at 450°C. The δD values were determined by mass spectrometer. The δD values of inclusion fluid obtained from quartz range from -61 to -114 ‰. These are significantly lower than the δD value of the thermal water presently venting from the Hishikari deposits and that of local meteoric water. Hydrogen isotopic fractionation between water and amorphous silica, which might have initially precipitated from the hydrothermal fluids at least partly, is not a probable cause of this isotopic depletion, while some water might have been released from the initial hydrous amorphous silica during recrystallization to quartz observed presently. Thus, a part of ore fluids for the Hishikari deposits is supposed to have been originated from the water having anomalous δD values of lower than -100 ‰. Such D depletion cannot be caused by simple oxygen-shift of meteoric water or by contribution of magmatic volatiles. The δD values of water released from the shale samples of the Shimanto-Supergroup, a major host to the Hishikari veins range from -132 to -148 ‰. Therefore, the anomalous δD values of inclusion fluids from some vein quartz and adularia suggest that the water released from hydrous minerals of the sedimentary basement rocks by dehydration or the groundwater isotopically exchanged with sedimentary rocks at elevated temperatures during circulation, partly contributed to the hydrothermal fluids responsible for the Hishikari deposits.. |
| 189. | Akira Imai, Hidehiko Shimazaki, Tomoko Nishizawa, Hydrogen Isotope Study of Fluid Inclusions in Vein Quartz of the Hishikari Gold Deposits, Japan, Resource Geology, 10.1111/j.1751-3928.1998.tb00014.x, 48, 3, 159-170, 1998.09, The origin of mineralizing fluids responsible for the Hishikari vein-type epithermal Au deposits was studied on the basis of the hydrogen isotopic ratio (δD) of the inclusion fluid from vein quartz and adularia. The origin of hydrothermal fluids was estimated by combination of the present δD values and the oxygen isotopic ratios (δ18O) previously reported by Shikazono and Nagayama (1993). The water in the fluid inclusions was extracted by means of decrepitation of quartz at 500°C. Hydrogen was obtained by reduction of the collected water with Zn shot at 450°C. The δD values were determined by mass spectrometer. The δD values of inclusion fluid obtained from quartz range from -61 to -114 ‰. These are significantly lower than the δD value of the thermal water presently venting from the Hishikari deposits and that of local meteoric water. Hydrogen isotopic fractionation between water and amorphous silica, which might have initially precipitated from the hydrothermal fluids at least partly, is not a probable cause of this isotopic depletion, while some water might have been released from the initial hydrous amorphous silica during recrystallization to quartz observed presently. Thus, a part of ore fluids for the Hishikari deposits is supposed to have been originated from the water having anomalous δD values of lower than -100 ‰. Such D depletion cannot be caused by simple oxygen-shift of meteoric water or by contribution of magmatic volatiles. The δD values of water released from the shale samples of the Shimanto-Supergroup, a major host to the Hishikari veins range from -132 to -148 ‰. Therefore, the anomalous δD values of inclusion fluids from some vein quartz and adularia suggest that the water released from hydrous minerals of the sedimentary basement rocks by dehydration or the groundwater isotopically exchanged with sedimentary rocks at elevated temperatures during circulation, partly contributed to the hydrothermal fluids responsible for the Hishikari deposits.. |
| 190. | Maglambayan, V. B., Ishiyama,D., Mizuta, T., Imai, A. and Ishikawa, Y., Geology, alteration, and formation environment of the disseminated gold-silver telluride Bulawan deposit, Negros Occidental, Philippines., Resource Geology, 48, 87-104, 1998.06. |
| 191. | Ishizuka, O. and Imai, A., "Brown Ore" from the Fukasawa kuroko deposits, northeast Japan: Its characteristics and formation process., Resource Geology, 48, 53-73, 1998.06. |
| 192. | Osamu Ishizuka, Akira Imai, "Brown Ore" from the Fukasawa Kuroko Deposits, Northeast Japan Its Characteristics and Formation Process, Resource Geology, 10.1111/j.1751-3928.1998.tb00007.x, 48, 2, 53-73, 1998.06, Brown-colored sulfide ore (brown ore) occurs in the easternmost part of the Tsunokakezawa No. 1 orebody of the Fukasawa kuroko-type deposits, northern Honshu, Japan. As this type of ores also occur in the marginal or uppermost part of several other kuroko deposits in Japan, the formation of brown ore appears to be repeated in the process of kuroko formation. The brown ore is characterized by its higher Ag concentration (up to around 2000 g/t) than ordinary black ore (Zn-Pb ore) of volcanogenic massive sulfide deposits. The brown ore from the Fukasawa deposits can be divided into following three ore types based on its texture and mineral composition: pyritic brown ore, principal brown ore and "diseased" brown ore. Primary precipitation textures such as framboidal- and colloform-textures and compositional zoning within sulfide grains are significant in the brown ores. This seems to be due to lack of overprinting high temperature mineralization resulting in preservation of primary features. The Ag-Au mineralization is widely observed within the brown ores. Silver and gold are especially concentrated in the barite veinlets in the principal brown ore, which are supposed to be fillings of conduit of hydrothermal solution precipitated in the latest stage of hydrothermal activity. This mineralization seems to occur at waning stage of brown ore formation by ore solution at a lower temperature (around 250°C) than that of main part of brown ore (around 270°C). Relatively low fluid temperature and contribution of oxic ambient seawater may be responsible for the development of the Ag-Au mineralization in the brown ore. The occurrence of framboidal-rich pyritic brown ore having negative δ34S values (less than -10‰) and filamentous texture of sphalerite, seeming remnant of bacteria, indicate the presence of intensive microbial activity in the hydrothermal area for brown ore formation. Formation environment of each ore type of the brown ore is supposed to be as follows: Pyritic brown ore is likely to have formed on the sea-floor around redox boundary at temperature (around 240°C) lower than ordinary black ore. Principal brown ore seems to have been formed beneath the shell of the pyritic brown ore at temperature around 270°C. Footwall of the brown ore is disseminated tuff breccia corresponding to feeder zone of hydrothermal fluid. Overprinting chalcopyrite mineralization is not observed in the brown ore except in limited part of "diseased" ore, which occurs just above the disseminated tuff breccia. Based on the features distinct from the ordinary black ore, the brown ore can be regarded as a product in the marginal part of submarine hydrothermal system, where temperature and flow rate of hydrothermal solution was relatively low and microbial activity was intensive. The brown ore seems to well preserve its primary features after its deposition and might show the initial feature of some part of the ordinary stratiform black ore.. |
| 193. | Victor B. Maglambayan, Daizo Ishiyama, Toshio Mizuta, Akira Imai, Yohei Ishikawa, Geology, Mineralogy, and Formation Environment of the Disseminated Gold-Silver Telluride Bulawan Deposit, Negros Occidental, Philippines, Resource Geology, 10.1111/j.1751-3928.1998.tb00009.x, 48, 2, 87-104, 1998.06, The Bulawan deposit is located in the porphyry copper belt of southwest Negros island, Philippines. Propylitic, K-feldspar, sericitic, and carbonate alteration types can be distinguished in the deposit. Propylite alteration occurs mainly in Cretaceous-Eocene andesitic lavas and agglomerates while K-feldspar, sericite and carbonate alteration types occur mostly in the Middle Miocene dacite porphyry breccia pipes and stocks which were intruded into the andesites. K-feldspar zones occur in the inner parts of the sericitized zone. Sericite alteration overprinted the propylitized and K-feldspar alteration zones, at lower temperature than epidote and chlorite in the propylitized zone. Carbonate alteration is associated with the mineralization in the center of the breccia pipes and along faults. Mineralization consists of gold-silver telluride ores that are hosted by the carbonate- and sericite-altered dacite porphyry breccia pipes. The Bulawan ores occur mainly as disseminations, but unlike many epithermal gold deposits, lack classical epithermal colloform and crustiform quartz veins. The ore minerals are sphalerite, galena, chalcopyrite, pyrite and tetrahedite-tennantite with minor amounts of electrum, calaverite, petzite, sylvanite, hessite, tellurobismuthite, coloradoite, altaite, and rucklidgeite. Electrum and telluride minerals are associated mostly with calcite and dolomite-ankerite minerals. Fluid inclusions in quartz and calcite in clasts of propylitized andesite in the breccia pipes homogenize from about 300° to 400°C while fluid inclusions in quartz, calcite and sphalerite within the dacite porphyry breccia pipes homogenize between 300° to 310°C. The ores were formed around 300°C from hydrothermal solutions with salinity of about 6.6 wt % NaCl equivalent. The presence of sylvanite and calaverite as intergrowths with each other, and the Ag content of calaverite are consistent with the above temperature estimate. Based on paragenesis, the Bulawan deposit formed in a pyrite-stable environment, with pH between 3.4 and 5.5, fO2 between 10-32 to 10-30 atm, fS2 between 10-9.8 to 10-7.8 atm, fTe2 between 10-8.9 to 10-6.5 atm, and total sulfur content about 10-2.8 molal. The dominant reduced sulfur species in the ore solutions may have been H2S(aq), and the likely aqueous tellurium species were H2Te(aq) and H2TeO3(aq). The ore minerals in the Bulawan deposit were probably formed by mixing of slightly saline and low salinity fluids.. |
| 194. | Imai, A., Listanco, E. L. and Fujii, T., Highly oxidized and sulfur-rich magma of Mount Pinatubo: Implication for metallogenesis of porphyry copper mineralization in the western Luzon arc., Newhall, C. G. and Punongbayan, R. S., eds., Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines, 865-874, 1996.12. |
| 195. | Y KAJIWARA, S YAMAKITA, K ISHIDA, H ISHIGA, A IMAI, DEVELOPMENT OF A LARGELY ANOXIC STRATIFIED OCEAN AND ITS TEMPORARY MASSIVE MIXING AT THE PERMIAN TRIASSIC BOUNDARY SUPPORTED BY THE SULFUR ISOTOPIC RECORD, PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY, 10.1016/0031-0182(94)90072-8, 111, 3-4, 367-379, 111, 367-379, 1994.10, Systematic sulfur isotope data for whole-rock sulfides have been obtained from the chert-dominated, continuous, pelagic sedimentary sequences spanning the Permian/Triassic (P/Tr) boundary at Tenjinmaru in the Chichibu Terrane and at Sasayama in the Tanba Terrane in Japan. The P/Tr boundary is characterized by the occurrence of siliceous shales in association with a carbonaceous black mudstone which is similar in appearance to the worldwide distributed Cretaceous/Tertiary (K/T) boundary claystone. The observed data clearly demonstrate a significant bimodalism. The S-34/S-32 ratios with respect to CDT are generally low throughout the Middle Permian (ca.-39 to -25parts per thousand) and show a remarkable increase beginning in the lower Upper Permian and persisting into the Lower Triassic (ca. -20 to -2parts per thousand), with a temporary and drastic negative shift, down to roughly the same isotopic level as in the Middle Permian, just at the suspected P/Tr boundary (ca. -41 to -23parts per thousand). Interestingly, the mode of isotopic excursion across the P/Tr boundary is in striking contrast to that across the K/T boundary which was recently described at Kawaruppu in Hokkado, Japan. The apparent extent of fractionation, with respect to contemporaneous seawater sulfate, in the high S-34/S-32 group lies within the range of -25 +/- 10parts per thousand, which is quantitatively equivalent to the currently confirmed range of kinetic isotope effect during bacterial dissimilatory sulfate reduction, and that in the low S-34/S-32 group mostly exceeds this range, giving the values typically in the range of -45 +/- 10 parts per thousand, which is similar to what is generally observed in the present-day oceanic sediments. The present data would provide strong evidence for the development of a largely stagnant, anoxic, stratified ocean, which presumably began to form in the lower Upper Permian and persisted into the Lower Triassic, and for a brief episode of its temporary massive mixing just at the suspected P/Tr boundary. Such an oceanic oxic-anoxic history may account to some extent for the relatively high enrichments of chalcophile elements in basal Triassic sediments in the world and add a significant constraint to the current arguments on the cause and consequence of the terminal Permian mass extinction.. |
| 196. | Yoshimichi Kajiwara, Satoshi Yamakita, Kotaro Ishida, Hiroaki Ishiga, Akira Imai, Development of a largely anoxic stratified ocean and its temporary massive mixing at the Permian/Triassic boundary supported by the sulfur isotopic record, Palaeogeography, Palaeoclimatology, Palaeoecology, 10.1016/0031-0182(94)90072-8, 111, 3-4, 367-379, 1994.10, Systematic sulfur isotope data for whole-rock sulfides have been obtained from the chert-dominated, continuous, pelagic sedimentary sequences spanning the Permian/Triassic (P/Tr) boundary at Tenjinmaru in the Chichibu Terrane and at Sasayama in the Tanba Terrane in Japan. The P/Tr boundary is characterized by the occurrence of siliceous shales in association with a carbonaceous black mudstone which is similar in appearance to the worldwide distributed Cretaceous/Tertiary (K/T) boundary claystone. The observed data clearly demonstrate a significant bimodalism. The 34S/32S ratios with respect to CDT are generally low throughout the Middle Permian (ca.-39 to -25%0) and show a remarkable increase beginning in the lower Upper Permian and persisting into the Lower Triassic (ca.-20 to -2%0), with a temporary and drastic negative shift, down to roughly the same isotopic level as in the Middle Permian, just at the suspected P/Tr boundary (ca.-41 to -23%0). Interestingly, the mode of isotopic excursion across the P/Tr boundary is in striking contrast to that across the K/T boundary which was recently described at Kawaruppu in Hokkado, Japan. The apparent extent of fractionation, with respect to contemporaneous seawater sulfate, in the high 34S/32S group lies within the range of -25 ± 10%0, which is quantitatively equivalent to the currently confirmed range of kinetic isotope effect during bacterial dissimilatory sulfate reduction, and that in the low 34S/32S group mostly exceeds this range, giving the values typically in the range of -45 ± 10%0, which is similar to what is generally observed in the present-day oceanic sediments. The present data would provide strong evidence for the development of a largely stagnant, anoxic, stratified ocean, which presumably began to form in the lower Upper Permian and persisted into the Lower Triassic, and for a brief episode of its temporary massive mixing just at the suspected P/Tr boundary. Such an oceanic oxic-anoxic history may account to some extent for the relatively high enrichments of chalcophile elements in basal Triassic sediments in the world and add a significant constraint to the current arguments on the cause and consequence of the terminal Permian mass extinction.. |
| 197. | A IMAI, SULFIDE GLOBULES ASSOCIATED WITH A FELSITE INTRUSION IN THE MOUNT KINABALU QUARTZ MONZONITE, SABAH, EAST MALAYSIA - SULFIDE MELT IMMISCIBILITY IN A HIGHLY SILICIC MEAT, ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS, 10.2113/gsecongeo.89.1.181, 89, 1, 181-185, 89, 181-185, 1994.01, The occurrence of sulfide globules in highly silicic rocks associated with the Mount Kinabalu quartz monzonitic complex suggests that the unmixing of sulfide liquid from a silicic melt may be an important step in the ore-generating magmatic process. -from Author. |
| 198. | A. Imai, Sulfide globules associated with a felsite intrusion in the Mount Kinabalu quartz monzonite, Sabah, east Malaysia; sulfide melt immiscibility in a highly silicic melt, Economic Geology, 10.2113/gsecongeo.89.1.181, 89, 1, 181-185, 1994.01, The occurrence of sulfide globules in highly silicic rocks associated with the Mount Kinabalu quartz monzonitic complex suggests that the unmixing of sulfide liquid from a silicic melt may be an important step in the ore-generating magmatic process. -from Author. |
| 199. | A IMAI, EL LISTANCO, T FUJII, PETROLOGIC AND SULFUR ISOTOPIC SIGNIFICANCE OF HIGHLY OXIDIZED AND SULFUR-RICH MAGMA OF MT-PINATUBO, PHILIPPINES, GEOLOGY, 10.1130/0091-7613(1993)0212.3.CO;2, 21, 8, 699-702, 21, 699-702., 1993.08, Dacitic pumices from pyroclastic-flow deposits and air-fall tephra of the June 14-15, 1991, eruption of Mt. Pinatubo, Philippines, are characteristically rich in sulfur. The presence of microphenocrystic anhydrite suggests that sulfur existed mostly as oxidized species in the magma. This supposition is corroborated by unusually high sulfur contents (up to 0.78 wt% as SO3) in apatite microphenocrysts and apatite inclusions in other phenocrystic minerals and by the highly oxidized state of the magma, near or slightly below the hematite-magnetite buffer, on the basis of extrapolation from biotite compositions and the two-oxide method. This highly oxidized state possibly caused the extraordinarily high sulfur content of Mt. Pinatubo dacite through prohibition of sulfide fractionation and because of increasing solubility of sulfur as oxidized species with increasing oxygen fugacity. Hornblende geobarometry indicates a pressure of approximately 2 kbar for phenocryst formation. Among the two pumice types, cummingtonite-rimmed hornblende phenocrysts are typically found in type 1 (white and crystal-rich) pumice, whereas rare hypersthene has been observed as discrete microphenocrysts in type 2 (tan and crystal-poor) pumice. These observations indicate conditions at least near if not at vapor saturation in type 1 magma and undersaturation in type 2 magma prior to eruption. Vapor-saturated type 1 dacitic magma probably caused the explosive June 14-15, 1991, eruption. The highly oxidized condition and magmatic water saturation are similar to those of intrusions related to porphyry copper deposits.. |
| 200. | A. Imai, E. L. Listanco, T. Fujii, Petrologic and sulfur isotopic significance of highly oxidized and sulfur-rich magma of Mt. Pinatubo, Philippines, Geology, 10.1130/0091-7613(1993)0212.3.CO;2, 21, 8, 699-702, 1993.01, Dacitic pumices from pyroclastic-flow deposits and air-fall tephra of the June 14-15, 1991, eruption of Mt. Pinatubo, Philippines, are characteristically rich in sulfur. The presence of microphenocrystic anhydrite suggests that sulfur existed mostly as oxidized species in the magma. This supposition is corroborated by unusually high sulfur contents in apatite microphenocrysts and apatite inclusions in other phenocrystic minerals and by the highly oxidized state of the magma. This highly oxidized state possibly caused the extraordinarily high sulfur content of Mt. Pinatubo dacite through prohibition of sulfide fractionation and because of increasing solubility of sulfur as oxidized species with increasing oxygen fugacity. Among the two pumice types, cummingtonite-rimmed hornblende phenocrysts are typically found in type 1 pumice, whereas rare hypersthene has been observed as discrete microphenocrysts in type 2 pumice. These observations indicate conditions at least near if not at vapor saturation in type 1 magma and undersaturation in type 2 magma prior to eruption. -from Authors. |
| 201. | Y. Kajiwara, S. Yamakita, D. Kobayashi, A. Imai, Sulfur isotopic data from the Permian/Triassic boundary at Tenjinmaru in the Chichibu Terrane in eastern Shikoku, Japan, Annual Report - University of Tsukuba, Institute of Geoscience, 19, 59-65, 1993.01, A temporary and drastic decrease in 34S/32S ratio of whole rock sulfide has been recognized at the suspected Permian/Triassic (P/Tr) boundary at Tenjinmaru in the Chichibu Terrane in eastern Shikoku, Japan, which is characterized by the occurrence of a carbonaceous black mudstone. The observed isotopic profile in the P/Tr section is in striking contrast in mode of 34S/32S excursion to that at the K/T boundary at Kawaruppu in eastern Hokkaido, Japan, and adds to significant geochemical constraint to the current arguments on the palaeoenvironmental changes during the time of terminal Permian mass extinction. -from Authors. |
| 202. | Imai, A. and Ozawa, K., Tectonic implication of the hydrated garnet peridotites near Mt. Kinabalu, Sabah, East Malaysia, Jour. Southeast Asian Earth Sci., 6, 431-445., 1991.12. |
| 203. | Akira Imai, Kazuhito Ozawa, Tectonic implications of the hydrated garnet peridotites near Mt Kinabalu, Sabah, East Malaysia, Journal of Southeast Asian Earth Sciences, 10.1016/0743-9547(91)90086-D, 6, 3-4, 431-445, 1991.01, Hydrated garnetiferous peridotites form part of an ultramafic complex in the Mt Kinabalu area of Sabah, East Malaysia. Garnet peridotites are associated with abundant spinel lherzolites and rare plagioclase-bearing spinel lherzolites. The complex is in fault contact with the surrounding Tertiary strata. High-temperature peridotite mineral assemblages are overprinted by lower temperature hydrous assemblages characterized by abundant hornblende which is found filling interstices or thin veinlets. The small amounts of K, Na, Cl and F in the hornblende suggest that the metasomatic fluid may have been depleted in these elements. Carbon dioxide-rich, incompatible element-enriched, fluids are not thought to be the cause of the metasomatism since metasomatic phlogopite and carbonates, commonly reported from garnet peridotites and metasomatized alpine-type ultramafic rocks, are absent. The Mt Kinabalu garnet peridotites represent part of the sub-crustal mantle beneath Kalimantan which was metasomatized during ascent due to tectonism. The metasomatic fluid, poor in incompatible elements and H2O-rich, may have been introduced into a sub-continental mantle wedge after dehydration of a subducted oceanic slab.. |
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