THE SEDIMENT-HOSTED STRATIFORM COPPER ORE SYSTEM

Sediment-hosted stratiform copper (SSC) deposits comprise disseminated to veinlet Cu- and Cu-Fe-sulfides in siliciclastic or dolomitic sedimentary rocks. Economically significant deposits are few, but account for ~23 percent of the world's Cu production and known reserves and are significant sources of Co and Ag. SSC 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 typically oxidized siliciclastic rocks (red beds) containing Fe oxyhydroxides capable of weakly binding metals. Sulfur may be derived from marine or lacustrine evaporites, reduced seawater, hydrogen sulfide-bearing hydrocarbons, and/or precursor pyrite. Metals appear to have been transported at low to moderate temperatures in moderately to highly saline basinal fluids, with the fluid temperature mainly dependent on the timing of fluid migration in the basin's burial history. Migrating 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, hydrocarbons, or H2S. The amount of sulfides present at any deposit may be metals-, sulfur-, and/or reductant-limited. Although SSC deposits range in age from Paleoproterozoic (i.e. post-oxyatmoversion) to Holocene, giant and supergiant SSC deposits occur in rocks of late Proterozoic and late Paleozoic age, temporally associated with periods of continental breakup. Mineralization in these provinces appears to have been multi-stage and broadly associated with successive periods of basin evolution, including hydrocarbon maturation and basin inversion.