The unusual Imou porphyry Au-Cu deposit, Western Highlands PNG

Ireland, Timothy1; Federico Cernuschi2, Robert Sievwright1, Hannah Goswell1, Stefanova, Elitsa3, and John Dobe4.

1First Quantum Minerals Ltd, Canada, 2Eclectic Rock Ltd,  Punta del Este, Uruguay, 3Bulgarian Academy of Science – Geological Institute, Bulgaria, 4Footprint Resources Pty Ltd

Imou (4.944˚S; 142.806˚E) is a porphyry gold-copper deposit located western Papua New Guinea along structural strike between the deposits at Frieda River and Yandera. The area was highlighted during systematic geochemical exploration in the mid-1970s, when five porphyry centres were identified in a camp covering ~30 x 30 km. The Imou target was delineated in the 1990s as a stream sediment and soil Cu anomaly of >500 ppm coincident with Au anomalism >0.2 ppm and associated with a magnetic polyphase porphyritic intrusive complex. Two holes drilled in 1999 discovered the deposit, and a dozen recent holes permit this first attempt at geological description.

Magmatic rocks comprise equigranular and lesser porphyritic diorites that belong to the Nekiei intrusive suite, which was emplaced into a thrust duplex of obducted siltstone and oceanic crust. New zircon-U-Pb geochronology records at least 700 ky. but less than 2.1 my. of Late Miocene magmatic evolution. The intrusions are low to mid-K calc-alkaline series rocks composed primarily of calcic plagioclase and hornblende with rare biotite and quartz, and accessory titanomagnetite. Whole rock chemical proxies for porphyry fertility are comparable to causative suites in other deposits (Sr/Y up to 65 and V/Sc up to 16) but zircon in these rocks has relatively low concentrations of U, Th, LREE and Hf .The intrusions manifest in airborne magnetic surveys as discrete high amplitude magnetic ‘bullseye’ features, but no cogenetic large plutons are observed at surface, nor can be inferred from the magnetic response.

The outcropping manifestation of the porphyry deposit at Imou is in many ways typical: within a geochemical footprint of ~2 km2 there are domains of A-family quartz veins that sometimes contain Cu-Fe-sulphides and/or magnetite, the distribution of which broadly coincides with the occurrence of hydrothermal magnetite, feldspar, anhydrite and muscovite. These quartz vein domains reach 50 vol% quartz in the vicinity of the magnetite-bearing assemblage. There are structurally controlled domains of late D-veins and phyllic alteration, followed by post-mineral porphyritic dykes. However, there are numerous differences with economic PCD described elsewhere. Cu-Au grade is not associated with the abundance of A-family quartz veins. Further, there is no petrographic nor bulk chemical evidence for large volumes of potassic alteration. Observed feldspars are albite and sporadic hydrothermal biotite represents only remobilisation of local potassium. There is little disseminated sulphide associated with this alteration, instead, grade development is associated with chalcopyrite-pyrite-(magnetite) fracture paints, and sulphide-anhydrite veins that cross-cut the quartz vein stages. These paragenetically late brittle veins are widely distributed at low frequency, but are associated with best Cu-Au grades. These late hydrothermal features are not systematically associated with a particular wallrock alteration assemblage, although chlorite-montmorillonite is the dominant alteration assemblage outboard of localised sericite- and albite-bearing assemblages.

We advance several working hypotheses for why Imou differs from other porphyries, and especially why metals did not precipitate efficiently with the early high temperature alteration and quartz veining. We speculate that many barren or subeconomic porphyries may have inferior Cu-Au grade development for similar reasons.


I’m a jack-of-all-trades explorer with experience in porphyry-epithermal systems, carlin-type deposits, sed-hosted Cu and sed-hosted Zn. I’m currently the Principal Exploration Geologist – and informal resident geochemist – at FQM. I did my postgrad research at CODES, and have spent much of my career working in E Europe and the central African copperbelt.  

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