Porphyry and Hydrothermal Cu-Au Deposits: A Global Perspective

The Agua Rica deposit consists of two porphyry copper centres (Quebrada Seca-Trampeadero and Melcho) associated with several porphyritic stocks and hydrothermal breccias of late Miocene-early Pliocene age that intrude a basement of ?Precambrian to early Palaeozoic meta-sedimentary rocks and Palaeozoic granitoids. Hypogene Cu-Au mineralisation is present in porphyry-style and epithermal assemblages. Main stage Cu-Au mineralisation is of epithermal, high sulphidation type and associated with pyrite rich associations of covellite, chalcopyrite, bornite and enargite emplaced in advanced argillic alteration. The alteration is dominated by sericite, pyrophyllite, zunyite, dickite, kaolinite, alunite, vuggy silica and silicification. Supergene Cu-Au mineralisation is present as an immature chalcocite blanket that consists of an upper part rich in chalcocite, underlain by a covellite-bearing zone. Hypogene Cu-Au mineralisation at Agua Rica formed over a period of 1.4 m.y., between 6.29 and 4.88 Ma. Syn-mineralisation uplift and erosion, induced by regional tectonism, caused the superposition of main stage Cu-Au mineralisation over higher temperature, deep seated K-silicate alteration-mineralisation assemblages. Supergene chalcocite formation, dated at 3.9 Ma, was favoured by the pyrite rich, main stage Cu-Au mineralisation and advanced argillic alteration associated with hydrothermal telescoping, and the pluvial regime of the elevated topography of the area. Twenty nine years elapsed between when the existence of porphyry copper mineralisation was first recognised in the area and when the real dimensions of the system were realised. Geological mapping, coupled with rock chip geochemistry and relict sulphide studies, resulted in the siting of the discovery holes. The commitment and tenacity of the exploration team and the backing of company management at a time when results were discouraging played an important role. This major porphyry copper-gold deposit is to the east of the Baja de la Alumbrera mine, and contains 750 Mt @ 0.62% Cu, 0.037% Mo, 0.23 g/t Au, with a further 90 Mt @ 0.4 g/t Au in the leached cap. The project is currently under evaluation and is a joint venture between BHP Minerals (70%)and Northern Orion Explorations Ltd (30%).