JaLCDOI 10.18926/ESR/13948
FullText URL ESR_2_81.pdf
Author Osae, Shiloh| Kase, Katsuo| Yamamoto, Masahiro|
Abstract The Ashanti gold deposit at Obuasi occurs in the Early Proterozoic Birimian formation. The orebodies consist of gold-sulphides disseminated in metasediments and metavolcanics, and gold-bearing quartz veins. Gold occurs mainly as native gold normally with arsenopyrite, pyrrhotite and pyrite in the metasediments and metavolcanics. In the quartz veins, however, it is associated with tetrahedrite and sphalerite. Microprobe analysis of gold grains gave an Ag content of about 8 wt.%. Textural relationships among the various mineral associations suggest that pyrrhotite, pyrite, arse-nopyrite and possibly gold may have been formed esrlier in the paragenetic sequence. The precipitation of some amounts of gold may have occurred during the later stages of the mineralisation sequence. Average arsenopyrite composition determined from electron microprobe study is: Fe: 33.6 ± 0.5 at.%, As: 29.3 ± 1.2 at.%, S: 37.1 ± 1.1 at.%, This suggests an average temperature of formation of less than 300℃ for the sulphides and possibly the gold assuming equilibrium was attained between pyrite, pyrrhotite and arsenpopyrite. The δ34S values for sulphide mineral separates are in the range between -4.1 and -10.8%. The values were, however, constrained between -4.1 and -7.0%. The narrow range and the depleted nature of the δ34S values may suggest a sedimentary sourcc of sulphur which may have undergone high temperature homegenisation before deposition. Sulphides from quartz vein samples had the most depleted δ34S values. This may indicate a different mineralisation history of the sulohides in the quartz veins from those dissemi-nated in metasediments and metavolcanics.
Keywords Birimian Ashanti gold arsenopyrite
Publication Title Okayama University Earth Science Report
Published Date 1995-09-20
Volume volume2
Issue issue1
Start Page 81
End Page 90
ISSN 1340-7414
language 英語
Copyright Holders 岡山大学地球科学研究報告編集委員会
File Version publisher
NAID 120002310561
JaLCDOI 10.18926/ESR/13947
FullText URL ESR_2_71.pdf
Author Frank k, Nyame| Kase, Katsuo| Yamamoto, Masahiro|
Abstract X-ray diffraction, ore microscopy, and electron microprobe studies have been done on ore samples obtained from the mine site at Nsuta, Ghana. Manganese ore minerals identified include nsutite, cryptomelane, pyrolusite, and lithiophorite. Except for pyrolusite, all these minerals are poorly crystalline. Chemical compositions of the minerals indicate little contamination by other elements. Quartz, muscovite, and clay minerals feature prominently as gangue in most samples. Textures are varied, and modes of occurrence are prinicipally by replacements, cavity or fracture fills, leaching and precipitaion. Mineral associations seem rather complex, as a result of which definitive paragenetic relations could not be established. Supergene formation of the manganese oxide ore is shown by the abundance of manganese minerals in solution channels such as fractures and cavities within silicate gangue. Much of the ore, howver, appears to have been enriched by leaching of gangue minerals and the later addution of manganese.
Keywords Nsuta nsutite cryptomelane pyrolusite lithiophorite
Publication Title Okayama University Earth Science Report
Published Date 1995-09-20
Volume volume2
Issue issue1
Start Page 71
End Page 80
ISSN 1340-7414
language 日本語
Copyright Holders 岡山大学地球科学研究報告編集委員会
File Version publisher
NAID 120002310527
JaLCDOI 10.18926/ESR/13912
FullText URL ESR_5_17.pdf
Author Yamamoto, Masahiro| Kase, Katsuo| Harada, Kaori|
Abstract Sulfur isotope rations of sulfide minerals in the ore specimens from the deposit of the Tsuchikura mine are distrib-uted in a range from -0.8 to 5.4%, mostly from +0.5 to 3.0%, close to the previous data (from +1 to +3%) by Miyake and Sasaki (1980). The distribution range is also similar to the major Besshi-type deposites of the Sambagawa terrain. Sulfur isotope fractionation between coexisting pyrite and chalcopyrite is not uniform. This may be largely due to spatial tearrangament of the initial orebodies to the present form. Sulfide minerals mostly pyrite from host rocks (siltsones, sandstones, basaltic rocks) exhibit remarkably low δ34S values ranging from -49.3 to -2.6%, dis-tinctly different from the δ34S values for sulfide minerals of ores. The low δ34S values may indicate the bacteriogenic origin of host rock sulfides, although it seems peculiar that basaltic rocks also have extremely low δ34S values.
Keywords Sulfur isotope ratio Besshi-type deposit Tsuchikura mine Pyrite Chalcopyrite
Publication Title Okayama University Earth Science Report
Published Date 1998-09-20
Volume volume5
Issue issue1
Start Page 17
End Page 22
ISSN 1340-7414
language 英語
Copyright Holders 岡山大学地球科学研究報告編集委員会
File Version publisher
NAID 120002310378
JaLCDOI 10.18926/ESR/13904
FullText URL ESR_6_1.pdf
Author Inoue, Yoshinori| Kase, Katsuo|
Abstract Phlogopite and hornblende were found in amall amounts in the contact-metamorphosed dunite-harzburgite complex at Yanomine. Phlogopite occurs in interstices of silicate minerals in dunite and harzburgite located near the contact with granite. Some phlogopite grains also occur as inclusions in chromian spinel of chromitite bands. Hornblende is present associated with such metamorphic minerals as talc, olivine and orthopyroxene. Interstitial phlogopite is characterized by lower TiO2 and higher K/(K+Na) atomic ratio compared to that included in chromian spined. Hornblende in dunite chages its composition from edenite associated with olivine-talc through edenitic hornblende to Si-poorer magnesio-hastingsitic hornblende and magnesio-hastingsite with olivine or olivine-orhtopyroxene. The interstitial phlogopite is suggested to have been formed intimately connected with fluids generated in ralation to the intrusion of granite. On the other hand, included phlogopite is considered to have crystallized from the incompatible elements-enriched hydrous melt resulted from mantle-melt interaction. Hornblende should be a metamorphic mineral formed under high temperature conditions.
Keywords Phlogopite Hornblende Ultramafic complex Dunite Harzburgite Chromitite Contact metamor-phism Yanomine
Publication Title Okayama University Earth Science Report
Published Date 1999-09-20
Volume volume6
Issue issue1
Start Page 1
End Page 12
ISSN 1340-7414
language 英語
Copyright Holders 岡山大学地球科学研究報告編集委員会
File Version publisher
NAID 120002310269
JaLCDOI 10.18926/ESR/13871
FullText URL earth_science_reports_010_1_1.pdf
Author Tomiya, Hisashi| Inoue, Yoshinori| Kase, Katsuo|
Abstract In order to know the reason why the Cu/Zn-Pb ratios of skarn-type deposits related with the ilmenite-series granitic rocks are markedly higher than those related with the magnetite-series, comparative mineralogical studies were carried out for the two types of granitic rocks in the Chugoku district. An attention was focused on the be-havior of Zn during the crystallization of granitic magma, because Zn contents of mafic minerals can be obtained by non-destructive electron microprobe analyses. Microscopic observation indicats that no sphalerite occurs in the magnetite-series granitic rocks, which im-plies that the magmas corresponding to the granitic rocks were undersaturated in respect to ZnS. Electron micro-probe analyses revealed that the Fe/(Fe+Mg) ratios of mafic minerals such as biotite and hornblende in the ilmen-ite-series granitic rocks are remarkably higher than those in the magnetite-series, and the Zn contents are posi-tively correlated with the Fe/(Fe+Mg) ratios in the ilmenite-series granitic rocks. Also the Zn/Fe rations seem to be slightly higher in the ilmenite-series granitic rocks. In contrast, the correlation between Zn contents and Fe/(Fe+Mg) ratios is ambiguous in magnetite-series granitic rocks. With a progress of crystallization differentiation, therefore, Zn may be removed more effectively in the il-menite-series granitic magma, and the contents may be growing scarce in the fractionated ilmenite-series granitic magma. On the other hand, magnetite-series granitic magma probably increases the Zn-contents in the advanced stage of the crystallization. The hydrothermal fluids genetically related to the fractionated magnetite-series grani-tic magma are likely to be enriched in Zn, being favorable for the formation of Zn-rich deposits.
Keywords Magnetite-series Ilmentie-series Granitic rocks Zn contents Mafic minerals Ore deposit
Publication Title Okayama University Earth Science Report
Published Date 2003-12-20
Volume volume10
Issue issue1
Start Page 1
End Page 10
ISSN 1340-7414
language 英語
Copyright Holders 岡山大学地球科学研究報告編集委員会
File Version publisher
NAID 120002310429
JaLCDOI 10.18926/ESR/13850
Title Alternative 沖縄トラフ第四与那国海丘海底熱水系チムニーの錫を含む黄銅鉱と白金を含む輝蒼鉛鉱
FullText URL ESR_12_1.pdf
Author Gena, Kaul| Chiba, Hiroshi| Kase, Katsuo|
Abstract The active sulfide chimney ore sampled from the flank of the active Tiger chimney in the Yonaguni Knoll IV hydrothermal system, South Okinawa Trough, consists of anhydrite, pyrite, shalerite, galena, chalcopyrite and bismuthinite. Electron microprobe analyses indicated that the chalcopyrite and bismuthinite contain up to 2.4 wt. % Sn and 1.7 wt. % Pt, respectively. The high Sn-bearing chalcopyrite and Pt-bearing bismuthinite are the first occurrence of such minerals on the submarine hydrothermal systems so far reported. The results confirm that the Sn enters the chalcopyrite as a solid solution towards stannite by the coupled substitution of Sn 4+ Fe 2+ for Fe 3+ Fe 3+ while Pt enters the bismuthinite structure as a solid solution during rapid growth. The homogenization temperature of the fluid inclusions in anhydrite (220-310℃) and metasured end-member temperature of the vent fluids (325℃) indicate that the minerals are precipitated as metastable phases at temperature around 300℃. The Sn-bearing chalcopyrite and Pt-bearing bismuthinite expess the original composition of the minerals deposited from a hydrothermal fluid with temperatures of about 300℃.
Keywords Sn-bearing chalcopyrite Pt-bearing bismuthinite Active sulfide Chimmey Yonaguni Knoll IV Okinawa Trough seafloor hydrothermal system
Publication Title Okayama University Earth Science Report
Published Date 2005-12-31
Volume volume12
Issue issue1
Start Page 1
End Page 5
ISSN 1340-7414
language 英語
Copyright Holders 岡山大学地球科学研究報告編集委員会
File Version publisher
NAID 120002310444