Evidence of hydrocarbon and metalliferous fluid migration in the Palaeoproterozoic Earaheedy Basin of Western Australia

Abstract

Dolomitized limestones of the Earaheedy Basin contain solid bitumen and metal‐sulphides, sulpharsenides and arsenides within a network of fractures which acted as pathways for migrating fluids. The solid bitumen formed from residual oil that was thermally altered within those fractures. Immobilization of oil was postdated by precipitation of fracture‐filling calcite and later replacive dolomite and quartz cements, as well as of magnesian chlorite. Solid bitumen has high concentrations of Fe‐, Zn‐, Pb‐, Ni‐ and FeCu‐bearing sulphides, sulpharsenides and arsenides in minute inclusions that are randomly distributed throughout its groundmass. Galena and pyrite also fill cracks in the bitumen, whereas pyrite, arsenopyrite, galena, gersdorffite, enargite? and chalcopyrite fill contraction vugs. Small crystals of cassiterite are associated with cavity‐filling magnesian chlorite. Multiple phases of base‐metal precipitation are apparent. The first, indicated by the inclusions, probably formed during mixing of hydrocarbon and metal‐rich fluids. Later phases resulted from the filling of bitumen contraction structures that formed during and after thermal alteration and degassing processes, and which imply the migration of metal‐rich fluids after oil immobilization. The association of metal‐rich minerals and fracture‐filling bitumen, in this non‐ore setting, is a guide to the possible existence of ore‐grade mineralization elsewhere in the basin.