Formation of the giant Aynak copper deposit, Afghanistan: evidence from mineralogy, lithogeochemistry and sulphur isotopes

Aynak is the largest known copper deposit in Afghanistan, with indicated resources of 240 Mt grading 2.3% Cu placing it in the 'giant' category. Host rocks are Neoproterozoic metasediments comprising dolomitic marble, carbonaceous quartz schist and quartz-biotite-dolomite schist containing garnet, scapolite and apatite. Chalcopyrite and bornite dominate the hypogene ore with lesser pyrite, pyrrhotite, cobaltite and chalcocite, and rare sphalerite, molybdenite, uraninite and barite. Sulphides occur as bedding-parallel laminae, disseminations, metamorphic segregations and crosscutting veins. Sulphide delta S-34 ratios range -14.5 to +17.3 parts per thousand in bedded and disseminated sulphides (n = 34). This broad range favours biogenic reduction of seawater sulphate as a major source of sulphur, although thermochemical reduction processes are not precluded. The narrower delta S-34 range of -6 to +12.2 parts per thousand in vein and segregation sulphides (n = 21) suggests localized redistribution and partial homogenization during metamorphism. Geochemical associations suggest that Al, P, Ca, Ti and Fe were primary sedimentary constituents whereas Cu, Mg, S, Se, As, Co and Bi were introduced subsequently. We infer that Aynak originated as a shale- and carbonate-hosted stratabound replacement deposit, resembling orebodies of the Central African Copperbelt, although underlying red-beds are absent at Aynak and mafic volcanics were the probable copper source. These giant deposits formed worldwide in the Cryogenian probably due to marine enrichment in copper, magnesium and sulphate coincident with profuse basaltic volcanism and ocean oxidation.

Automated summary

Aynak is the largest known copper deposit in Afghanistan, formed in Neoproterozoic metasediments, with chalcopyrite and bornite as the dominant minerals. The sulphides occur in various forms, indicating localized redistribution and partial homogenization during metamorphism.