17:15 〜 18:45
[HRE13-P04] Hydrothermal alteration and gold mineralization in the Martial Lode deposit, Golden Eagle mine, Tati Greenstone Belt, northeastern Botswana
The highly prospective 1200km2 Tati Greenstone Belt, located in the northeastern part of Botswana hosts numerous gold deposits including the Golden Eagle Deposit with measured and indicated gold resources of 1007000 t and an average grade of 1.23g/t Au. Mineralization at the Golden Eagle deposit occurs in three ore shoots namely the Bateleur, Martial, and Falcon Lodes hosted in mafic schists. The host rock at the Martial Lode which is the focus of this study is biotite schist. Hydrothermal alteration of the host rock is characterized dominantly by distal fine-grained biotite + quartz, intermediate coarse-grained biotite + quartz + rutile + ilmenite + pyrrhotite, and proximal chlorite + carbonate + quartz + arsenopyrite zones. Gold occurs as electrum in equilibrium with arsenopyrite and within arsenopyrite in the proximal alteration zone. Mass balance calculations suggest that hydrothermal alteration of biotite schist involved enrichment of Fe, As, Pb, P, and depletion of Si, K, Al, Ni, Co, Cr, and V. Chlorite geothermometer on the proximal alteration zone suggest temperatures of 124-290 °C for the hydrothermal alteration in the Martial Lode deposit. The arsenopyrite geothermometer on arsenopyrite associated with gold mineralization yields temperatures between 350 and 510 °C. Metamorphic assemblages in the host rock at the Martial Lode deposit comprise dominantly biotite, quartz, and pyrrhotite indicating greenschist metamorphic conditions. Textural evidence such as biotite overprinting by ore-related chlorite suggests that hydrothermal alteration minerals in the proximal alteration zone formed as replacement products of earlier metamorphic minerals. The close association between chlorite and fine-grained arsenopyrite grains suggests wall rock sulfidation in the proximal zone. At the Martial Lode deposit circulation of hydrothermal fluids caused distinct fluid-rock interactions characterized by sulfidation reactions associated with arsenopyrite and gold mineralization at higher temperatures suggested by arsenopyrite geothermometer and later overprinting of biotite by chlorite at lower temperatures (124-290 °C).