並列タイトル等ボツワナ北西部カラハリ銅鉱化帯コエマカウゾーン5鉱床の銅・銀鉱化作用
一般注記The Zone 5 Cu-Ag deposit in northwest Botswana extends for 4.2 km in length and contains a resource of 100 Mt grading 2% Cu and 21g/t Ag. Zone 5 is the main deposit in the Khoemacau district of the Kalahari Copperbelt. Despite the recent studies of mineralization in the Kalahari Copperbelt in Botswana, the fundamental question on the origin of the Zone 5 deposit remains contentious. This thesis presents a documentation of Cu-Ag mineralization across the width of the Zone 5 and contributes to a better understanding of the origin of the Cu-Ag deposit and the development of improved exploration models. At Zone 5, the redox-buffered metal-zoned ore body (~10 m average thickness) is preferentially hosted by chemically reduced metasedimentary rocks overlying oxidized, hematite-bearing arkosic sandstone. Locally, the ores are concentrated along bedding and foliation planes and within structures such shear fabrics, veins, and metamorphic cleavage. A combination of detailed microscopic, SEM-EDS, EPMA analyses, indicate that the Cu-Ag deposit is controlled by both lithologic and structural parameters and formed from a multi-stage mineralization history that includes both diagenetic and epigenetic events. The early, diagenetic mineralizing event is characterized by fine-grained stratiform pyrite, including recrystallized framboidal pyrite, intergrown with diagenetic mineral assemblages in the host-rock. Diagenetic pyrite is in textural equilibrium with chalcopyrite, sphalerite, galena, and (Fe-Co-Ni) sulfarsenide. These minerals were subsequently overprinted by a more intense, multi-stage, structurally controlled hydrothermal Cu-Ag mineralization event related to the Damaran orogeny (~600-480 Ma). Mineral chemistry results reveal an apparent overlap in trace metal associations (Cu, Fe, As, Zn, Pb, Ni, Co) between the two mineralizing events, which can be explained by remobilization of precursor sulfides. Microscopic and micro-XRF data indicate that the major Ag-carriers in the ore are chalcocite, covellite, and bornite. The main hydrothermal Cu-Ag mineralization precipitated from hot (P corrected Th = ~236-265oC), high salinity (19-24.6 wt. % NaCl equiv.) hydrothermal ore fluids. The δ34S values of diagenetic pyrite range from -35.8 to +11.4 ‰, whereas those of hydrothermal epigenetic sulfides, including pyrite, range from -28.0 to +3.0 ‰. These results suggest that hydrothermal sulfides acquired some bacterially-reduced sulfur from earlierformed minerals. The δ18O and δ13C values of quartz and calcite associated with the hydrothermal mineralization are typical of Neoproterozoic sediment-hosted Cu-Ag deposits. However, the δ18O isotopic values of the calcite gangue are anomalously depleted, likely due to recrystallization under metamorphic conditions.
コレクション（個別）国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
受理日（W3CDTF）2022-07-05T02:30:21+09:00
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