Mineralogy and Geochemistry Studies of the Sorkheh Sediment-hosted Stratiform Copper (SSC) Deposit, NW Iran

Abstract

The Sorkheh deposit in northwestern Iran exhibits several readily visible general characteristics of sediment-hosted stratiform copper (SSC) mineralization. It consists of fine-grained disseminated base-metal sulfides within gray sandstones (gray beds, the basal whitish Miocene sandstone and shallow-water) that overlie a thick sequence of red beds (Miocene Upper Red Formation). The host gray beds are the basal sandstone and are intercalated with red bed sandstones, which are shown from textural studies to be carbonaceous and to have initially contained very fine-grained, disseminated, syndiagenetic pyrite. The sediment-hosted stratiform copper deposit of the Sorkheh area formed in a location where basinal fluids passed through a series of highly oxidized rocks and could obtained copper, which was then precipitated as the fluids encountered a reductant that destabilized the complexing ability of the fluid. According to the proposed model, the Sorkheh deposit appeared to have begun formation during diagenesis when mineralizing fluids became focused into constrained areas by stratigraphic and/or structural architecture. To form the Sorkheh sediment-hosted stratiform copper deposit, a number of major features must be present: abundant, highly oxidized metal source rocks, highly reduced strata in a position favoring interaction with significant amounts of fluid that previously passed through the oxidized strata package, and significant thicknesses of evaporates capping the reduced strata to serve as a hydrologic seal and a source of high-salinity (and possibly sulfur-rich) brines. <br />Ore-stage sulfides are zoned vertically and obliquely through the mineralized zones, from cupriferous sulfides at low stratigraphic levels to copper-rich mineralization above, with unreplaced pyrite remaining within the upper Miocene. The zoned sulfides and their replacement textures, configuration of the mineralized zones, and the position of ore stage mineralization adjacent to a stratigraphically defined redox transition from red beds upward into graybeds indicate an overprint of copper (and accompanying ore-stage metals) on originally pyritic gray beds. The influx of ore-stage metals, presumably in an oxidized low-temperature brine, terminated with a silicification event that effectively sealed the host sandstones. Consequently, these observations and the overall genetic interpretation are consistent with the general deposit-scale genetic model for early diagenetic SSC mineralization. The regional geologic context is also consistent with its classification as a SSC deposit and is hosted by sediments that were formed in association with evaporates at a low latitude in a Sabkha environment. Source of Sorkheh deposition has been indicated as a sedimentary arid type. Sandstones of the Sorkheh area tectonic settings are obtained in a passive continental margin.