Role of organics and methane in sulfide ore formation, exemplified by Kupferschiefer Cu–Ag deposits, Poland

Abstract

Organic material has been involved in the formation of the Kupferschiefer Cu–Ag-sulfide deposits in Poland in several different ways and at several stages in their development. During sedimentation, a mixture of continental humic material and marine algal material was concentrated in the euxinic, peri-marine Kupferschiefer black shale environment (∼6% C org). Having the black shale directly on the continental redbeds below effected a strong redox gradient across which the ore is located. Early diagenetic pyrite-(marcasite) framboids and nodules were formed through bacteriogenic reduction of sulfate from seawater-derived pore waters. Metal zones and variations in organic composition indicate that upwelling oxidized ore fluids were reduced by organic material and Fe-sulfides in the shale, inducing the precipitation of Cu-sulfides. A network of natural hydrofractures, formed by generation of hydrocarbon-related gases in the shale, allowed the ore fluids to penetrate the otherwise impermeable caprock and concentrate sulfides. Natural gases also migrated up from Carboniferous coal source beds and accumulated in the C org-free eolian sandstone below the Kupferschiefer shale. Geologic and cathode luminescence evidence indicates that the CH 4 reacted with anhydrite cement to form distinctive calcite cements and H 2S which precipitated Cu-sulfide cements, although δ 13C- and δ 34S-values are inconclusive. Quartz replacement concomitant with sulfide formation in the Kupferschiefer and Proterozoic Nonesuch ores (Michigan, U.S.A.), and in Athabasca oil sands (Canada), provides field support that silica-organic complexing is a mechanism that accelerates dissolution and increases solubility of quartz at low temperature and neutral pH.