AGE OF CU(-FE)-AU MINERALIZATION AND THERMAL EVOLUTION OF THE PUNTA-DEL-COBRE DISTRICT, CHILE

The Punta del Cobre belt is located 15 km south of Copiapo, northern C hile. It comprises several Cu(-Fe)-Au deposits in the Punta del Cobre and Ladrillos districts, east of the Copiapo river, and the Ojancos Nu evo 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 vei ns and irregularly shaped bodies. The ore is spatially associated with alkali metasomatism and in particular with potassic alteration. The C u(-Fe)-Au deposits are hosted mainly in volcanic rocks of the Punta de l Cobre Formation (pre-upper Valanginian) that underlie Neocomian lime stones of the Chanarcillo Group. This region experienced backarc basin formation in the Neocomian, uplift and granitoid intrusions in the mi ddle Cretaceous, and eastward migration of the magmatic front of about 30 km between middle Cretaceous and Paleocene. To determine the timin g 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 hav e obtained Ar-40/Ar-39 incremental-heating and Rb-Sr analyses of miner al and whole-rock samples. An Ar-40/Ar-39 incremental-heating experime nt on hydrothermal biotite, formed synchronous with the Cu(-Fe)-Au min eralization, yielded an inverse isochron age of 114.9 +/- 1.0 Ma (all errors reported at +/-2 sigma), consistent with a Rb-Sr isochron of 11 6.8 +/- 2.7 Ma calculated from 7 whole-rock samples. These data are in terpreted to represent the age of potassic alteration that accompanies mineralization. Ore formation temperatures of 400 degrees C to 500 de grees C were previously estimated based on paragenetic relationships. Shearing at the Candelaria deposit occurred after ore deposition and b efore the main stage of batholith emplacement. Published K-Ar ages for the middle Cretaceous batholith near the Punta del Cobre belt range f rom 119 to 97 Ma. Our data suggest that the mineralization is related to the earlier stages of batholith emplacement. The biotite age spectr um indicates that the Punta del Cobre district was not affected by tem peratures above similar to 300 degrees C-350 degrees C, the closure te mperature for argon in biotite, during the contact metamorphic overpri nt produced by later emplaced batholithic intrusions. Whole-rock Ar-40 /Ar-39 ages are considerably younger; incremental-heating experiments yielded an inverse isochron age of 90.7 +/- 1.2 Ma and weighted mean p lateau 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 temperatur e of similar to 150 degrees C, thus they give the age of cooling below similar to 150 degrees C-200 degrees C.