Hypogene and supergene evolution of the Cerro Colorado porphyry copper (-molybdenum) deposit, Northern Chile

The 51.8 Ma Cerro Colorado porphyry Cu (-Mo) deposit, situated at 20° 2′41″ S; 69° 15 ′ 35″ W on the Pacific slope of the Central Andean Cordillera Occidental, comprises a supergene orebody with reserves of 228 Mt at 1.0 percent Cu, overlying a hypogene protore zone with Cu grades of 0.4–0.5 percent and a resource approximately double that of the supergene ore.; Initial hypogene alteration of Upper Cretaceous andesitic host-rocks generated an 8 km2, pervasive, biotite-albite-magnetite blanket. The non-boiling fluids at this stage, with moderate temperatures (≤380°C) and low salinities (≤8 wt percent NaCl equiv.), caused significant K-Na-Fe +2 metasomatism but deposited no sulfides. Subsequent Main Stage alteration-mineralization, which, with decreasing depth, comprises quartz-albite, sericite-chlorite-clay, quartz-sericite-clay and andalusite-diaspore-pyrophyllite facies, was responsible for over 70 percent of the chalcopyrite in the deposit. As they rose, the hot (≤544°C), boiling, ore-forming fluids, with salinities of ≤52 wt percent NaCl equiv., cooled (to ≤320°C), were diluted (to ≤37 wt percent NaCl equiv.), became more acid and generated quasi-contemporaneous intermediate- and advanced argillic alteration facies. Thus, the hypogene protore was the product of an unusual hybrid environment, in which a cool, barren hydrothermal system, similar in all salient aspects to those represented by non-explosive geothermal fields, abruptly evolved into one more characteristic of porphyry deposits. Whereas this transition may have been stimulated by melt incursion, such early geothermal stages and prograde histories may be inherent features of the initiation of magmatic-hydrothermal systems.; Dating of alunite-group minerals from various ore facies and elevations by laser 40Ar/39Ar incremental-heating reveals a supergene history extending over at least 20 m.y. The hypogene protore was plausibly unroofed at the onset of the Incaic orogeny at ca. 42 Ma, initiating the development of a thick chalcocite blanket, the Upper Supergene Ore, during the Oligocene. In the Early Miocene, at ca. 21.5 Ma, major Pehuenchean uplift resulted in the formation of a lower leached zone within the preexisting chalcocite blanket, which was thickened and, at shallow levels, oxidized. Normal supergene processes were interrupted at 19.25 Ma by a thick ignimbrite which partially covered the deposit, but the imposition of lateral groundwater flow reactivated oxidation and generated incipient exotic (chrysocolla) mineralization. Significant supergene activity had terminated by ca. 14.6 Ma as a result of climatic desiccation.; The establishment of a “proto-Humboldt Current” and the onset of the Incaic orogeny in the late Eocene provided a climatic and physiographic environment favorable for supergene enrichment of Cu deposits undergoing exhumation in the rain-shadow of an uplifting terrain. Because the Upper Supergene Ore zone at Cerro Colorado formed during the Oligocene, following the Incaic Orogeny, a single Cenozoic supergene metallogenetic epoch is proposed for northern Chile, ca. 20, or even 30, m.y. in duration, but attaining its greatest efficacy in the Early Miocene.