Mesozoic porphyry Cu–Au mineralization and associated adakite-like magmatism in the Philippines: insights from the giant Atlas deposit

Abstract

The Atlas porphyry Cu–Au deposit is located in central Cebu Island in the Philippines, with a proven mineral reserve of 1420 Mt at 0.45% Cu, 0.24 g/t Au, 0.018 g/t Mo, and 1.8 g/t Ag. It is associated with the Lutopan quartz diorite porphyry stock that was emplaced into the Cretaceous volcano-sedimentary sequences of the Cansi Formation. Two stages of hypogene alteration have been identified at Atlas: stage I quartz–magnetite ± chalcopyrite ± pyrite veins associated with potassic and propylitic alteration and stage II anhydrite–pyrite–chalcopyrite ± specularite veins associated with phyllic alteration. Supergene gypsum veins are also present. Mineralization is mainly associated with the stage I and stage II veins. The Lutopan quartz diorite porphyry yielded a zircon U–Pb age of 108.5 ± 1.6 Ma and is characterized by high Sr/Y (54–69), with the corresponding magma having high oxygen fugacity (avg. ΔFMQ +2.7, avg. Ce4+/Ce3+ of zircon = 439) and H2O concentrations (presence of amphibole and biotite phenocrysts) and low magmatic temperatures (avg. Ti-in-zircon thermometry = 706 °C). The relatively low La/Yb ratios (9–11), listric-shaped normalized REE patterns with weak or absent Eu anomalies, together with zircon eHf(t) values of 3.4 to 10.0, whole-rock ISr values of 0.70373–0.70382, eNd(t) values of 2.4–3.4 and Mg numbers of 49–53 suggest that the Lutopan quartz diorite porphyry was likely derived from partial melting of young lower continental crust or interaction between asthenospheric melts and lower crust melts, that evolved to adakite-like trace element compositions through amphibole fractionation. Stage I veins yielded a pyrite Re–Os isochron age of 110.2 ± 5.6 Ma, making Atlas the only Mesozoic porphyry deposit discovered in the Philippines.