The Sediment-Hosted Stratiform Copper Ore System

Abstract

Sediment-hosted stratiform copper deposits comprise disseminated to veinlet Cu and Cu-Fe sulfides in siliciclastic or dolomitic sedimentary rocks. Sediment-hosted stratiform copper deposits are extremely common though economically significant deposits are rare. They account for approximately 23 percent of the world's Cu production and known reserves in addition to being significant sources of Co and Ag. Three sedimentary basins (the Paleoproterozoic Kodaro-Udokan in Siberia, the Neoproterozoic Katangan in central Africa, and the Permian basin of central Europe) contain supergiant (>24 million metric tons (Mt) contained Cu) deposits. Sediment-hosted stratiform copper deposits are the products of evolving basin-scale fluid-flow systems that include source(s) of metal and S, source(s) of metal- and S-transporting fluids, the transport paths of these fluids, a thermal and/or hydraulic pump to collect and drive the fluids, and the chemical and physical processes which result in precipitation of the sulfides. Metal sources are undoubtedly red-bed sedimentary rocks containing Fe oxyhydroxides capable of weakly binding metals. Sulfur may be derived from marine or lacustrine evaporites, reduced seawater, or hydrogen sulfide-bearing petroleum. Metals appear to have been transported at low to moderate temperatures in moderately to highly saline aqueous fluids, with the temperature of the fluid largely dependent on the time of fluid migration in the basin's burial history. These basinal fluids were focused to potential metal precipitation sites by thinning of the red-bed sequence at basin margins, by faults, by differentially permeable sedimentary units, by paleotopography within the basin, or along the margins of salt diapirs. Fluid movement produced widespread, basin-scale alteration that has commonly been overlooked but can form an important exploration guide. Sulfide precipitation occurred due to reduction, typically caused by reaction with carbonaceous rocks or petroleum. The amount of sulfides present at any deposit may be either metals or sulfide limited or could have been controlled by the amount of available reductant (e.g., petroleum). While understanding of sediment-hosted stratiform copper ore genesis at the deposit scale is relatively robust, there are still significant questions in regards its position in terms of basin evolution. A wide variety of basin architectures and processes can lead to the formation of sediment-hosted stratiform copper deposits. Despite general agreement that sulfides postdate sedimentation, the absolute age of mineralization in many deposits has been difficult to document and the available evidence suggests that deposits can form throughout a basin's evolution from early diagenesis of ore host sediments to basin inversion and metamorphism. Supergiant and giant deposits formed in basins which underwent prolonged periods of fluid flow and in which unique conditions allowed for the accumulation of large amounts of metal-bearing fluid, sufficient reduced S, and large amounts of reductants.