The Escondida Porphyry Copper Deposit, Northern Chile: Discovery, Setting, Geology, Hypogene Mineralisation and Supergene Ore - A Review

The Escondida porphyry copper deposit and its satellites are the source of ore for the world's current largest copper mine, with an installed capacity of 1.2 Mt of fine copper per annum. The published ore reserve + mineral resource at Escondida and the satellite Escondida Norte deposits at the end of 2004 totalled 2.88 Gt @ 1.13% Cu, or 10.11 Gt @ 0.70% Cu when lower grade leach and oxide ores are included. Escondida was discovered in 1981 as the culmination of an exploration program initiated in 1978. This program, the Atacama Project, was specifically targeted at locating supergene enriched porphyry copper ore within the 500 km interval between Chuquicamata and El Salvador, in the established porphyry copper belt of northern Chile.

The supergene sulphide enrichment blanket at Escondida is entirely concealed below the remnant of a 'superleached' capping which is from a few to 350 m in thickness and contained < 100 to 600 ppm Cu and 10 to 480 ppm Mo. The enriched blanket covers an area of some 4.5 x 1 km and varies from a few metres to 500 m in thickness.' A zone of oxide ore occurs on one margin of the overlying leached capping. Supergene enrichment largely took place between 18.0 and 14.7 Ma.

Hypogene mineralisation is associated with the emplacement of the elliptical, 4.5 x 2.5 km, 38 Ma Escondida stock, a composite granodiorite-porphyry intrusive within a country rock largely composed of Paleocene to early Eocene andesites. Subsequent intrusions of 36 to 34 Ma rhyolite intrusive rocks, cutting the Escondida stock, post dated the first two of three hydrothermal stages related to the emplacement of ore, while the latest intrusives are 31 Ma barren rhyodacite dykes.

Three stages of hydrothermal alteration and mineralisation are recognised. The first, stage A, was potassic, comprising mainly K feldspar in the Escondida stock, and biotite in the surrounding andesites, passing out into a broad propylitic zone. Stage B is characterised by phyllic alteration, comprising an earlier chlorite-sericite tquartz and a later quartz-sericite, with the latter predominating in the core of the deposit and the former in the surrounding andesites. Stage C hydrothermal activity is the first to post date the intrusion of the rhyolites and is predominantly a multipulse, acid-sulphate, advanced argillic event that persisted from 36 to 34 Ma. Hypogene copper grades associated with stage A potassic alteration alone, are typically <0.3% Cu, while in areas of intense chlorite-sericite and quartz-sericite overprint, grades are, on average, between 0.4 and 0.6% Cu. Locally, where stage C veins cut earlier mineralisation, the hypogene grades may reach >1% Cu. Mineralisation is predominantly contained within stage B quartz-sulphide stockworks, with associated stage A disseminations and stage C polymetallic sulphide veining.

In addition to the main Escondida orebody, there are three other significant deposits within the Escondida District, namely: Escondida Norte, Zaldivar and Pinta Verde, all of which fall within a broad zone of propylitisation covering an area of some 25 x 15 km. The Escondida district lies within the Domeyko Fault Zone, a roughly 30 to 50 km wide, orogen parallel belt that stretches over a length of at least 1000 km in northern Chile, and embraces the giant porphyry copper deposits at Chuquicamata, Collahuasi and El Salvador and other smaller examples. During the late Eocene to early Oligocene, the Domeyko Fault Zone was the focus of, i). a major pulse of the eastward migrating magmatic arc related to the subduction zone to the west of the current coast, and ii). strike slip faulting arising from the NE directed convergence of the oceanic Nazca plate with the South American continent. The intrusions hosting the major deposits, including Escondida, appear to have been emplaced during a change in the sense of deformation in the Domeyko Fault Zone, from transpression to transtension, and focussed at points of intersection between this orogen parallel fault zone, and a series of long lived cross orogen lineaments.