Age of Cu(-Fe)-Au mineralization and thermal evolution of the Punta del Cobre district, Chile

Abstract

The Punta del Cobre belt is located 15 km south of Copiapo, northern Chile. It comprises several Cu(-Fe)-Au deposits in the Punta del Cobre and Ladrillos districts, east of the Copiapo river, and the Ojancos Nuevo district, with the new Candelaria mine, and Las Pintadas district, west of the river. The mineralization in the Punta del Cobre belt is characterized by a simple hypogene mineral assemblage of chalcopyrite, pyrite, magnetite, and hematite. Average ore grades are 1.1 to 2% Cu, 0.2 to 0.6 g/t Au, and 2 to 8 g/t Ag. Massive magnetite occurs as veins and irregularly shaped bodies. The ore is spatially associated with alkali metasomatism and in particular with potassic alteration. The Cu(-Fe)-Au deposits are hosted mainly in volcanic rocks of the Punta del Cobre Formation (pre-upper Valanginian) that underlie Neocomian limestones of the Chanarcillo Group. This region experienced backarc basin formation in the Neocomian, uplift and granitoid intrusions in the middle Cretaceous, and eastward migration of the magmatic front of about 30 km between middle Cretaceous and Paleocene. To determine the timing of ore deposition and to reconstruct parts of the thermal history of the Punta del Cobre district, in the eastern part of the belt, we have obtained 40Ar/39Ar incremental-heating and Rb-Sr analyses of mineral and whole-rock samples. An 40Ar/39Ar incremental-heating experiment on hydrothermal biotite, formed synchronous with the Cu(-Fe)-Au mineralization, yielded an inverse isochron age of 114.9 ± 1.0 Ma (all errors reported at ±2σ), consistent with a Rb-Sr isochron of 116.8 ± 2.7 Ma calculated from 7 whole-rock samples. These data are interpreted to represent the age of potassic alteration that accompanies mineralization. Ore formation temperatures of 400 °C to 500 °C were previously estimated based on paragenetic relationships. Shearing at the Candelaria deposit occurred after ore deposition and before the main stage of batholith emplacement. Published K-Ar ages for the middle Cretaceous batholith near the Punta del Cobre belt range from 119 to 97 Ma. Our data suggest that the mineralization is related to the earlier stages of batholith emplacement. The biotite age spectrum indicates that the Punta del Cobre district was not affected by temperatures above ∼300 °C–350 °C, the closure temperature for argon in biotite, during the contact metamorphic overprint produced by later emplaced batholithic intrusions. Whole-rock 40Ar/39Ar ages are considerably younger; incremental-heating experiments yielded an inverse isochron age of 90.7 ± 1.2 Ma and weighted mean plateau ages of 89.8 ± 0.6 Ma and 89.5 ± 0.6 Ma. These samples are dominantly K-feldspar, for which we assume an argon closure temperature of ∼150 °C, thus they give the age of cooling below ∼150 °C–200 °C.