Part I: Exploration & Discovery, Part II: Geology Part II: Geology - J Perello, N Rojas, C Devaux, L Fava, E Etchart, P Harman, BHP World Minerals, Chile, USA & Australia.
 in - Porter, T.M. (Ed), 1998 - Porphyry and Hydrothermal Copper and Gold Deposits - A Global Perspective; PGC Publishing, Adelaide, pp 111-132.
ABSTRACT
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 my, 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%).
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