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Multi-stage Cu Mineralization of the Mowana Copper Deposit, Matsitama Schist Belt, NE Botswana
https://doi.org/10.20569/00005716
https://doi.org/10.20569/00005716c706863f-eee8-45ac-9545-25c570acff3a
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内容要旨及び審査結果要旨 (168.6 kB)
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本文 (5.5 MB)
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| Item type | 学位論文 / Thesis or Dissertation(1) | |||||
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| 公開日 | 2021-06-03 | |||||
| タイトル | ||||||
| タイトル | Multi-stage Cu Mineralization of the Mowana Copper Deposit, Matsitama Schist Belt, NE Botswana | |||||
| 言語 | ||||||
| 言語 | eng | |||||
| 資源タイプ | ||||||
| 資源タイプ識別子 | http://purl.org/coar/resource_type/c_db06 | |||||
| 資源タイプ | doctoral thesis | |||||
| ID登録 | ||||||
| ID登録 | 10.20569/00005716 | |||||
| ID登録タイプ | JaLC | |||||
| アクセス権 | ||||||
| アクセス権 | open access | |||||
| アクセス権URI | http://purl.org/coar/access_right/c_abf2 | |||||
| 別タイトル | ||||||
| その他のタイトル | ボツワナ北東部、Matsitama片岩帯、Mowana銅鉱床の多段階Cu鉱化作用 | |||||
| 作成者 |
KOOGANNE, AMOGELANG
× KOOGANNE, AMOGELANG |
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| 内容記述（抄録） | ||||||
| 内容記述タイプ | Other | |||||
| 内容記述 | The Mowana hydrothermal Cu deposit is located within the Matsitama-Motloutse Complex in the southwestern part of the Zimbabwe Craton in the northeastern part of Botswana. The deposit is in the northern-most flank of the Matsitama Schist Belt, which makes one of the four granite-greenstone belts of north-eastern Botswana. It is poorly understood and has received less documentation since its discovery, by comparison to other ore deposits in the same area. Mowana deposit is unique from other Archean greenstone belt vein deposits in the sense that instead of Au mineralization it is rather dominated by Cu mineralization. There is a large, inferred time gap between rock formation and mineralization. The deposit is hosted by the NNE-striking and nearly vertically dipping (70-80°) Bushman Lineament (shear zone) within the graphitic schist lenses in the carbonaceous and argillaceous metasedimentary rocks of the Neoarchean to Paleoproterozoic Matsitama Sedimentary Group. Within the Matsitama Sedimentary Group, the Mowana deposit is hosted by rocks belonging to the Bushman Mine group, a sedimentary fining upward sequence. The Matsitama Sedimentary Group is enclosed within the foliated granitoids of the Matsitama-Motloutse Complex. The ca. 0.2 Ga Phanerozoic Karoo Supergroup overlies these units. The host metasedimentary rocks: limestone, argillite and graphitic schist, as well as the surrounding granites, suffered from hydrothermal alteration. The hydrothermal alteration is characterized by silicification, chloritization, epidotization, sericitization, hematite, and carbonate alterations. Based on the alteration mineral assemblage, the main mineralization stage is attributed to near-neutral pH fluids at temperatures between ~200 °C and ~>340 °C. The temperatures obtained from the chlorite II geothermometry of the Type 1 veins coincides with the above temperature in the range 340-400 °C and a modal temperature of 370-380 °C. The base metal mineralization of the Mowana deposit evolved in at least two hydrothermal vein types. The first mineralization type, represented by the quartz+calcite±K-feldspar veins and breccias, is characterized by the precipitation of principal chalcopyrite with pyrite, minor bornite and trace amounts of galena. The Type 2, represented by quartz+calcite±fluorite veins, hosts appreciable amounts of galena. The supergene mineralization is widely distributed in the shallow levels (up to 70 m below the surface) of the deposit. Supergene enrichment is manifested by the significant presence of chalcocite, bornite, covellite and anglesite. Oxidation is characterized by Fe-oxides and Cu carbonates: malachite, goethite, and hematite. The ore mineral assemblage of pyrite+chalcopyrite+bornite paired with the temperature obtained from the chlorite geothermometry from the Type 1 veins was used to determine the sulfidation state of the Mowana deposit. The ore mineral assemblage of the Type 1 veins lies on the pyrite+chalcopyrite+bornite buffer line at a temperature of 340 to 400 °C indicating that the Type 1 veins evolved under a transition between intermediate and high sulfidation state. Chalcopyrite is the principal Cu-bearing mineral, which also contains an average 67.4 ppm of Ag which may be due to incorporation of Ag in the Cu site. Galena from the Type 2 veins has high contents of Se with an average of 1231 ppm, possibly incorporated as PbSe. The average Ag content in galena from the Type 2 veins is 58.9 ppm. The average Co, Ni, and As contents in pyrite are 143 ppm, 33.6 ppm, and 107 ppm, respectively.III Fluid Inclusion analysis was also conducted. Two-phase (liquid and gas) fluid inclusions were observed in the mineralized quartz veins. However, the fluid inclusions were too small to perform microthermometry. The carbon isotopic composition of graphite from graphitic schist are from -21.0 to -29.8 ‰ (average 26 ‰) indicating a biogenic origin, possibly from cyanobacteria living in shallow water. The 13C values of the metacarbonates range from 6.05 to 6.89 ‰ which is higher than the global trend at the time of deposition and rarely observed in Archean carbonates. These heavier values may be explained by diagenetic carbonate precipitation during which dissolved inorganic C is enriched in 13C by methanogenic processes, or alternatively by burial of reduced carbon in an isolated basin at that time, with the residual dissolved carbon being enriched in 13C. The carbon isotopic compositions of the calcite of Type 1 veins ranging from 0.46 to 0.53 ‰ could indicate derivation from a mixed source including C from both host rock types. The 18O of the quartz is in the range 9.25 to 10.30 ‰ relative to Standard Mean Water Ocean Water (SMOW). The 18O values of calcite range from -12.42 to -22.34 ‰ with respect to Vienna Pee Dee Belemnite (VPDB). The calculated temperature from the co-existing mineral pairs of quartz and calcite in equilibrium range between 250 to 360 °C. Sulfur isotopic ratios determined on the sulfides from Type 1, Type 2 and diagenetic pyrite help to understand the evolution of the hydrothermal system. The sulfides, chalcopyrite, chalcocite and galena from Type 1 and Type 2 veins, recorded isotopic compositions 34S in the range -5 ‰ to +4 ‰. These values may indicate a magmatic sulfur source of S and/or leaching of the host metasedimentary rocks. However, 34S of diagenetic pyrite disseminated in silicified limestone and pyrite from IV a quartz+calcite±K-feldspar vein recorded sulfur isotopic values of +15.4 ‰ and +18.0 ‰. The values can be explained by closed system reduction of seawater sulfate, followed by remobilization of the 34S rich diagenetic pyrite during brecciation as the possible sources of S. | |||||
| 著者版フラグ | ||||||
| 出版タイプ | VoR | |||||
| 出版タイプResource | http://purl.org/coar/version/c_970fb48d4fbd8a85 | |||||
| 書誌情報 | 発行日 2021-03-22 | |||||
| 出版者 | ||||||
| 出版者 | 秋田大学 | |||||
| 学位名 | ||||||
| 学位名 | 博士（資源学） | |||||
| 学位授与機関 | ||||||
| 学位授与機関識別子Scheme | kakenhi | |||||
| 学位授与機関識別子 | 11401 | |||||
| 学位授与機関名 | 秋田大学 | |||||
| 学位授与年月日 | ||||||
| 学位授与年月日 | 2021-03-22 | |||||
| 学位授与番号 | ||||||
| 学位授与番号 | 甲第1368号 | |||||
