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. Sulfides occur as bedding-parallel laminae, disseminations, metamorphic segregations and crosscutting veins. Sulfide δ34S ratios range –14.5 to +17.3‰ in bedded and disseminated sulfides (n=34). This broad range favours biogenic reduction of seawater sulfate as a major source of sulfur, although thermochemical reduction processes are not precluded. The narrower δ34S range of –6 to +12.2‰ in vein and segregation sulfides (n=21) suggests localised 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 sulfate coincident with profuse basaltic volcanism and ocean oxidation.
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- Sediment-hosted copper sulfide deposit
- biogenic sulfate reduction
- thermochemical sulfate reduction
- stratabound replacement