Zircon U–Pb geochronology and geochemistry of the Cerro Colorado porphyry copper deposit, northern Chile

Abstract Porphyry Copper Deposits (PCDs) dominantly form in convergent plate margins genetically related to volcanic arcs. However, their temporal relationships with active volcanism are less clear. Active volcanic activity is often considered to impede the formation of PCDs and some workers have concluded that PCD formation must take place at times of volcanic quiescence or at a waning stage of volcanism. However, several studies have documented evidence suggesting syn-eruption PCD formation. A combination of radiometric dating, geochemistry studies and observations of rock cores from the Cerro Colorado PCD, a BHP Billiton property located in N. Chile, provides evidence compatible with syn-volcanic PCD formation within a volcanic edifice. Textural observations at both hand specimen and microscopic scales combined with new LA-ICPMS and SHRIMP dating of zircons suggest the intrusive history of the Cerro Colorado area can be separated into two main units: a porphyritic tonalite ca. 53.5 Ma and a porphyritic quartz monzonite ca. 50 Ma. There is also evidence for crystallisation at ca. 60 Ma, but a corresponding distinct lithology was not identified. A quartz porphyry unit previously identified as the youngest intrusive body is reinterpreted as an altered equivalent of the monzonite. The youngest intrusive units are represented by porphyritic granodiorite with an age 51–50 Ma. These units only show minor metal mineralisation suggesting that the main phase of mineralisation in the Cerro Colorado mine occurred over a few million years, bracketed by the two main pulses of igneous intrusion, which are consistent with documented Ar–Ar ages. Re–Os ages reported by previous studies suggest an earlier phase of mineralisation at ca. 56–54 Ma. U–Pb dating of zircon from a volcanic-related breccia that cuts the host rocks of the Cerro Colorado Mine reveals a phase of volcanic activity at ca. 57 Ma, after formation of the regional Cretaceous Cerro Empexa Formation. This age range overlaps the ages derived for the intrusive porphyry units suggesting a close temporal relationship between volcanism and the ore-forming subvolcanic intrusions in Cerro Colorado. A link between active volcanism and mineralisation is reinforced by the ultrafine-grained matrix and the presence of strong brecciation associated with the monzonite-quartz porphyry stock suggesting decompression and volatile release have taken place during emplacement.