Chimborazo Copper Deposit, Region II, Chile; Exploration and Geology
C.R. Petersen, S.L. Rivera, M.A. Peri, 1998. "Chimborazo Copper Deposit, Region II, Chile; Exploration and Geology", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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Since the early 1950s, several companies and individuals have devoted considerable effort to exploration for precious metal and/or copper mineralization in the Chimborazo area, which is located in the Andes of northern Chile, about 180 km southwest from the port of Antofagasta. Minera Orion Chile, the predecessor of Minera Cyprus Chile, conducted exploration at Chimborazo, initially for precious metals and later for copper. The exploration for gold and silver had limited success, but that for copper lead to the discovery and delineation of a supergene enrichment blanket containing about 180 million metric tons averaging 0.8 percent Cu.
The deposit occurs within volcanic rocks of andesitic to dacitic composition, cut by an early Tertiary intrusive complex composed of diorite and granodiorite. The main stages of alteration and mineralization comprise an early, late-magmatic episode, overprinted by a high sulfidation epithermal event. The early episode is represented by K-silicate and propylitic alteration accompanied by weak chalcopyrite mineralization plus minor pyrite and bornite. This episode was followed by emplacement of a series of hydrothermal breccias along a northeasterly structural trend. Advanced argillic alteration, associated with silicification and sericitization, is part of this later event. The later mineralization is complex and includes open space fillings, stockworks, and cementation of breccia fragments by pyrite, enargite, and variable, but smaller, amounts of chalcocite, chalcopyrite, bornite, and tennantite. Lead-zinc-silver mineralization, with some gold, was emplaced as peripheral veins during a final stage. Supergene activity created a profile consisting of about 100 m of leached capping, which contains some copper oxide remnants, underlain by an enrichement blanket containing chalcocite and minor covellite. The blanket is up to 200 m thick, elongated to give the shape of a canoe, and with its axis roughly coincident with the northeasterly trend of the hydrothermal breccias. In the western part of the deposit, chalcocite replaced pyrite in a sericitized intrusion, whereas toward the southeast and northeast, the supergene enrichment is superimposed directly over on breccias. Later northwesterly faulting, and diorite porphyry intrusion along the northeasterly fractures distorted the primary morphology of the system.
Chimborazo is believed to represent the high-level portion of a porphyry copper system, preserved as the result of the special morphologic evolution of the Atacama Desert. The deposit illustrates the link between a porphyry copper deposit and a high level acid-sulfate epithermal system.