Hydrothermal alteration of footwall basalts to the Archaean Bulyanhulu gold deposit, Tanzania

摘要

Regionally extensive carbonate and/or chlorite alteration is found distal to the mineralised shears and is progressively overprinted by successive syn-tectonic hydrothermal alteration zones spatially associated Reef 1 mineralization at Bulyanhulu. The typical alteration zonation about the mineralized shears is characterized by carbonatisation in the footwall basalts, with limited sericitisation. There is a gradual increase in the intensity of deformation with proximity to the main ore zones, often with the development of mineral segregation banding parallel to the principal shear fabric. Mass transfer calculations indicate that chemical changes during alteration of the footwall basalts include enrichment of LOI, CaO, Fe2O3,K2O, Au, Ag, Ba, Bi, Cu, S and V, and depletion of Na. Whilst quartz occurs as veins, there is no visible silica addition or depletion in the intensely altered wallrocks. During the hydrothermal evolution at Bulyanhulu, carbonate addition occurred proximal to the shear-hosted Reef 1 vein.Distal to Reef 1, the carbonate isotopic composition is primarily controlled by isotope exchange with the wall rocks.