GEOCHRONOLOGY OF THE DOS AMIGOS AND TRICOLOR PORPHYRY COPPER SYSTEMS OF THE DOMEYKO ALTERATION ZONE, NORTHERN CHILE

At Domeyko, some 40 km south of Vallenar city in northen Chile (28°59'S-70°53'W); two Cu-Au porphyry centers occur within a N-S elongated alteration zone of 5.5 x 1.5 km; these are related to tonalite to granodiorite porphyry stocks with potassic alteration (biotite-magnetite) surrounded by quartz-sericite±clays alteration that affects mostly Early Cretaceous andesitic volcanic rocks and varies eastward to propylitic alteration; advanced argillic alteration (quartz-sericite-alunite-pyrophyllite ± andalusite) occurs in the highest ground of the altered zone. The western boundary of the alteration zone is a sharp N-S trending contact with an unaltered granodioritic to dioritic batholith. LA-ICPMS zircon U-Pb ages for Dos Amigos are of 106.1±3.5 and 104.0±3.5 Ma, and of 108.5±3.4 for Tricolor porphyry copper. The biotite 40Ar/39Ar age of 104.1±1.9 Ma for a biotitized rock of Tricolor is consistent with potassic alteration being related to the porphyry intrusions, and the biotite 40Ar/39Ar age of 105.4±3.3 Ma for mylonitized porphyry at Tricolor indicate concurrent active sinistral shearing of the Dos Amigos fault zone that appears to have controlled porphyry emplacement. Two biotite and two sericite 40Ar/39Ar ages obtained for Tricolor and Dos Amigos range from 96.0±0.7 to 97.1±0.9Ma and are thought to be reset ages, because they are younger than the crystallization ages of the porphyries and are in agreement with the radiometric ages obtained for the neighboring unaltered batholith (U-Pb 99.6±1.8, 99.1±1.9, and 96.2±1.1 Ma; biotite 40Ar/39Ar 96.9±0.9 and 94.8±1.0 Ma). A sericite 40Ar/39Ar age of 85.8±1.2 Ma for a tourmaline breccia (Marisol) at Dos Amigos suggests a later hydrothermal event, though possible Ar loss cannot be discarded. An apatite fission track age of 60 Ma was obtained for Dos Amigos, and tree apatite fission track ages from 90 to 84 Ma for the batholith, suggesting that the alteration zone represent a deeper exhumed level relative to the western batholith.