The Antamina deposit, Peru: U-Pb and Re-Os age constraints on magmatic-hydrothermal activity

摘要: Antamina (Peru) is the largest skarn in the world with resources of~ 2,968 Mt averaging 0.89% Cu, 0.77% Zn, 11 g/t Ag and 0.02% Mo as of 2015. From the highest elevation outcrop to the deepest drill core intersection, the skarn extends> 2.2 km vertically and remains open at depth. The deposit is hosted in structurally stacked limestones and marls of the Cretaceous Jumasha and Celendín Formations. Skarn and mineralisation occur in and around a multiphase porphyry complex that consists of 3 major intrusive phases (P1= early, P2= inter-mineral, and P3= late inter-mineral) emplaced between 10.96±0.03 Ma and 10.23±0.07 Ma (U-Pb zircon CA-TIMS). At least 11 sub-phases (P2a, P2b, etc.) are observed through cross-cutting relationships. Porphyry compositions range from quartz diorite to quartz monzonite. Only P1 porphyries are related to skarn formation (ie, contain endoskarn alteration). Inter-mineral and late inter-mineral porphyries (P2, P3) cross-cut earlier skarns and do not contain skarn alteration. Molybdenite-quartz-sericite veins occur in all units, and two distinct pulses of Mo mineralisation are resolved through Re-Os geochronology: an early stage (10.58±0.07 Ma to 10.39±0.05 Ma) occurs in endoskarn and exoskarn, while a late stage (10.05±0.06 Ma to 9.68±0.05 Ma) occurs in P2 and P3 porphyries. Ages of porphyry emplacement and Mo mineralisation decrease from NE to SW along the axis of the deposit (Fig. 1). Large scale mineralisation at Antamina is related to structurally-focussed magmatic-hydrothermal activity over approximately 1.5 million years.