Comparative studies of the structural and alteration controls on gold mineralization in the Wa-Lawra region, NW Ghana.

Abstract

The objective of this thesis was to perform a detailed structural, mineralization and associated alteration studies of three gold deposits located in northwest Ghana, on the eastern margin of the West African Craton. Thus, the Kunche and Bepkong deposits, located in the Wa-Lawra belt and Julie deposit located in the Julie belt. These three deposits have multiple differences in terms of geological, structural, tectonic and geochemical characteristics but gold mineralization in these deposits are all associated with a green schist facies metamorphism.In Julie, the gold mineralization is hosted in granitoids with its composition akin to tonalite-granodiorite-trondhjemite (TTG) where as in the Kunche and Bepkong deposits, the gold mineralization are localized within volcaniclastic sedimentary formations and strongly silicified graphitic schists. The gold mineralization is associated with a network of quartz veins often boudinaged and formed in connection with a shear zones, oriented E-W in the Julie deposit and N to NNW at Kunche and Bepkong deposits.The dominant alteration in the mineralized zone is sericite, quartz, carbonate and sulphides with influences from the host rock. For example, the granitoid is influenced by tourmaline and chlorite in the schists or in the metavolcanic rocks. At Julie, gold is closely associated with pyrite whereas Kunche and Bepkong gold is associated with arsenopyrite. Two generations of gold are distinguished; the first corresponds to the invisible gold associated with primary growth areas in the pyrite crystals in Julie and arsenopyrite in Kunche and Bepkong, and late visible gold inclusions which are frequently found in fracture fillings.In Julie, the mineralizing fluid is rich in CO2, and has low to moderate salinity (NaCl-H2OCO2 system), trapped in P / T conditions around 220 ° C and <1 kbar; whilst in Bepkong, the mineralizing fluid is associated with quartz are rich in CH4, with a low salinity (CH4-CO2-SO2-H2O