Mineralization and geochemistry of sediment-hosted strata-bound copper horizons in the Garedu Red Bed Formation of the Khormo–Gazak Mining District, Tabas Block, Iran

The eastern part of Ravar town in the southeastern Tabas Block of Iran hosts several sediment-hosted strata-bound copper-type deposits. These deposits occur within the Upper Jurassic Garedu Red Bed Formation, which includes red sandstone, conglomerate, siltstone and reduced siliciclastic layers with evaporite interbeds. We studied five major ore bodies/occurrences (Khormo, Mianrood, Dehoj, Godar-Rigi and Gazak), collectively known as the Khormo–Gazak Mining District. The copper mineralization in this district is primarily contained in woody fragments and the palaeo-permeability resulting from intragranular spaces and evaporite beds. The palaeo-permeability has been improved by the dissolution of diagenetic carbonate cements in bleaching zones. The spatial distribution of the woody fragments played a fundamental part in the copper mineralizing system. Two different stages of mineralization were distinguished based on the mineralogical and textural relationships and the morphology of the minerals studied: (1) reddening, bleaching and the formation of framboidal pyrites in the early diagenesis stage; and (2) the replacement of woody fragments, framboidal pyrites and cements with copper sulfides in the late diagenesis stage. The red, bleaching and ore zones are indicative of the main ore-forming event and are volumetrically the most important zones. The major ore minerals found in the studied deposits are, in order of abundance, chalcocite, bornite, chalcopyrite, digenite, yarrowite, djurleite, roxbyite, native copper, native silver, pyrite, atacamite, covellite, malachite and azurite, accompanied by minor amounts of chrysocolla, neotocite and hematite. The geochemistry of the ore-bearing ore horizons in the Khormo–Gazak Mining District deposits indicates a distinct variation in copper and nickel concentrations during the formation of different ore zones. On the basis of the mineral chemistry, the chalcocite in the Khormo deposit is enriched in silver, in contrast with the chalcocite at Gazak. Isotopically very light sulfur indicates that the chalcocite was supplied from diagenetic framboidal pyrite derived from the destruction of woody fragments. The destruction of organic matter leads to the creation of reducing environmental conditions and reducing geochemical traps for the deposition of sulfides. The Khormo–Gazak Mining District deposits are considered as a typical example of sediment-hosted strata-bound copper-type mineralization in which the minerals were deposited when oxidized brines flushed through the basin successions. The liberated copper was transported to units containing carbon-rich horizons, where the metals were deposited.