16:15 〜 16:30
[HRE13-08] Mineralogical, geochemical, and fluid inclusion characteristics of the Neoarchean gold deposits in the Tati and Vumba Greenstone Belts of NE Botswana
キーワード：Tati greenstone belt, Vumba greenstone belt, native gold, Botswana
The Tati Greenstone Belt (TGB) and the Vumba Greenstone Belt (VGB) are Neoarchean terranes located in northeastern Botswana, along the southwestern margin of the Zimbabwe craton. Both the greenstone belts are composed of supracrustal rocks dominated by mafic and ultramafic meta-volcanic rocks, which are bounded by granitoids. These belts host numerous gold deposits, as well as minor Ni-Cu-PGE sulfide deposits. Despite early gold discoveries in the 1800s, limited studies have been conducted on the fluid evolution and origin of gold mineralization in the TGB and VGB. As such, this study aims to constrain the genesis of gold mineralization in the TGB and VGB through mineralogy, geochemistry, and alteration patterns. We studied the Mupane, and Golden Eagle gold deposits in the TGB, as well as the White Pidgeon and Somerset gold deposits in the VGB.
The gold-bearing sulfide mineralization in both the belts is mainly observed in quartz veins as well as strongly altered schists and amphibolites. The host rocks in the TGB are predominantly quartz-calcite-graphite-mica-chlorite schists that are typically silicified, with some amphibolites. The quartz veins in the TGB include vuggy, brecciated, and boudinaged types which contain sporadic pyrrhotite and pyrite, as well as massive pyrrhotite. In comparison, the host rocks in the VGB are dominated by quartz-calcite-mica schists and amphibolites which are cross-cut by sulfide-rich quartz veins.
Visible gold in the Mupane and Golden Eagle deposits of the TGB occurs mostly as inclusions of native gold within sulfides, but also within fractures and vugs of sulfides, particularly arsenopyrite and sphalerite. Similarly, visible gold in the Somerset and White Pidgeon deposits of the VGB occurs as inclusions within arsenopyrite but is also intergrown with other sulfides such as chalcopyrite and pyrrhotite. The δ34S values for pyrrhotite, arsenopyrite, and pyrite from the studied deposits in both the belts range from -0.7 to +4.0 ‰, indicating a possibility of magmatic origin of sulfur.
Fluid inclusions in quartz veins of the Golden Eagle and Somerset deposits are 2 to 20 µm in size and can be classified into four types at room temperature, i.e., in decreasing order of abundance carbonic (monophase (liquid CO2) and biphase (liquid CO2 + vapor CO2), aqueous (liquid H2O + vapor H2O), aqueous-carbonic (liquid H2O + liquid CO2 + vapor CO2), and multiphase inclusions (liquid H2O + liquid CO2 + vapor CO2 + solid). The four fluid inclusion types commonly coexist in the same quartz crystals suggesting coeval entrapment of likely immiscible fluids. The fluid inclusions in the quartz veins or quartz-calcite veins of the Mupane, Golden Eagle, and Somerset deposits have gas compositions with elevated CO2 (0.92-39.9 mol%) and CH4 (0.33-4.33 mol%) contents. Ar/He vs. N2/Ar and CO2/CH4 vs. N2/Ar of fluid inclusions in quartz show that the fluids of the Golden Eagle deposit have a magmatic component with a meteoric water contribution, while the fluids of the Mupane and Somerset deposits have meteoric water characteristics with a magmatic contribution. The CO2/CH4 vs. N2/Ar values of the fluid inclusions in quartz of the Somerset deposit show a crustal fluid signature.
The gold-bearing sulfide mineralization in both the belts is mainly observed in quartz veins as well as strongly altered schists and amphibolites. The host rocks in the TGB are predominantly quartz-calcite-graphite-mica-chlorite schists that are typically silicified, with some amphibolites. The quartz veins in the TGB include vuggy, brecciated, and boudinaged types which contain sporadic pyrrhotite and pyrite, as well as massive pyrrhotite. In comparison, the host rocks in the VGB are dominated by quartz-calcite-mica schists and amphibolites which are cross-cut by sulfide-rich quartz veins.
Visible gold in the Mupane and Golden Eagle deposits of the TGB occurs mostly as inclusions of native gold within sulfides, but also within fractures and vugs of sulfides, particularly arsenopyrite and sphalerite. Similarly, visible gold in the Somerset and White Pidgeon deposits of the VGB occurs as inclusions within arsenopyrite but is also intergrown with other sulfides such as chalcopyrite and pyrrhotite. The δ34S values for pyrrhotite, arsenopyrite, and pyrite from the studied deposits in both the belts range from -0.7 to +4.0 ‰, indicating a possibility of magmatic origin of sulfur.
Fluid inclusions in quartz veins of the Golden Eagle and Somerset deposits are 2 to 20 µm in size and can be classified into four types at room temperature, i.e., in decreasing order of abundance carbonic (monophase (liquid CO2) and biphase (liquid CO2 + vapor CO2), aqueous (liquid H2O + vapor H2O), aqueous-carbonic (liquid H2O + liquid CO2 + vapor CO2), and multiphase inclusions (liquid H2O + liquid CO2 + vapor CO2 + solid). The four fluid inclusion types commonly coexist in the same quartz crystals suggesting coeval entrapment of likely immiscible fluids. The fluid inclusions in the quartz veins or quartz-calcite veins of the Mupane, Golden Eagle, and Somerset deposits have gas compositions with elevated CO2 (0.92-39.9 mol%) and CH4 (0.33-4.33 mol%) contents. Ar/He vs. N2/Ar and CO2/CH4 vs. N2/Ar of fluid inclusions in quartz show that the fluids of the Golden Eagle deposit have a magmatic component with a meteoric water contribution, while the fluids of the Mupane and Somerset deposits have meteoric water characteristics with a magmatic contribution. The CO2/CH4 vs. N2/Ar values of the fluid inclusions in quartz of the Somerset deposit show a crustal fluid signature.