Neoproterozoic copper-gold mineralization in the Amani area, southwestern Tanzania

Abstract The Amani area in southwestern Tanzania contains several prospects where primary Cu-Au mineralization occurs at surface, the most noteworthy of these being the Amani Hills and Mpanga Hill prospects. At the Amani Hills prospect, structurally-controlled gold mineralization is hosted in quartz-muscovite schists of the Amani Group and minor high-grade gneisses belonging to the Kabelege Group. Here, quartz-carbonate veins contain pyrite-pyrrhotite-chalcopyrite, with free-milling gold occurring predominantly in quartz and along sulphide grain boundaries. These veins are associated with sericite-epidote-albite alteration of the wallrocks. At the Mpanga Hill prospect, Cu ± Au mineralization is hosted within impure micaceous marbles of the Rafiki Group. At this prospect, both the sulphidised carbonate host rocks and quartz-carbonate veins contain chalcopyrite, bornite and pyrite assemblages, with accessory phases of Au ± Pb ± Zn ± Ag ± Bi ± Se. Fluid inclusion microthermometry indicate that auriferous veins at both prospects record a bimodal occurrence of early low-temperature and low-salinity (≤23.2 wt.% NaCl eq.) fluids that are overprinted by later, high temperature and hypersaline fluids (32.8 – 54 wt.% NaCl eq.). These ore-forming fluids contain H2O-NaCl±CO2±CH4 compositions and were trapped at near-lithostatic to supralithostatic conditions of 330 – 400 °C and 1.7 – 6.9 kbar. These hydrothermal fluids were likely variably sourced and were focussed along kilometre-scale brittle-ductile reverse shear zones in the Amani area, where fluid-rock interactions led to Cu-Au mineralization. 40Ar/39Ar dating of muscovite constrained the timing of copper – gold mineralization in the Amani area to between c. 550 – 600 Ma. Mineralization is therefore related to the Pan-African aged Kuunga orogeny, which resulted from the collision between the Tanzania Craton and Bangweulu Block during the assembly of supercontinent Gondwana. The new findings presented in this study highlight the exploration potential of copper – gold mineralization along large-scale Proterozoic shear zones in the underexplored and overlooked Upangwa terrane of southwestern Tanzania.