PETITHERMAL AND MICROPORPHYRY DEPOSITS: A NEW CLASS OF LINKED CU-AU DEPOSIT

Recent exploration work in the Tethyan orogen of Iran has revealed two new types of linked epithermal and porphyry deposits, characterized by very large tonnages but low grades. Microporphyry deposits feature textures that range from equigranular to microporphyritic, with weak to absent potassic alteration, sparse quartz veining, and grades up to 200 ppm Cu and 5 ppb Au. The deposits can be very large, up to 10 billion tonnes (cutoff at 50 ppm Cu, 2 ppb Au). Where shallower levels of the systems are preserved, low grade but large tonnage epithermal Au mineralization can be found (here termed “petithermal”), again characterized by cryptic alteration envelopes, sparse quartz veining, and gold grades of up to 10 ppb Au. At a cutoff of 1 ppb Au, these deposits can have tonnages of tens of million tonnes of ore.

These deposit types represent previously unrecognized targets for exploration, and have been found to occur in association with almost any intermediate-to-felsic plutonism, regardless of geodynamic setting. This means that wide swathes of intrusive rocks previously thought to be unprospective due to their interpreted tectonic setting and lack of obvious alteration and mineralization may in fact be potential targets for exploration for these deposit types.