High-Resolution Geochronology of the Coroccohuayco Porphyry-Skarn Deposit, Peru: A Rapid Product of the Incaic Orogeny

Abstract

Precise and accurate determination of the timing and duration of ore-forming processes in porphyry sys- tems is a fundamental step in understanding their genesis and placing them in a regional context. Here, we take advantage of the considerable improvements in the field of geochronology over the last decade to pro- vide a robust geochronologic framework for hydrothermal and magmatic events in the Eocene Coroccohuayco porphyry-skarn Cu deposit, and the first robust dating of an ore system in the emerging Andahuaylas-Yauri batholith and metallogenic belt, southern Peru. This batholith and associated porphyry systems were emplaced during the Incaic orogeny, in a context of slab flattening, compression, exhumation, uplift, and the initiation of the bending of the Bolivian orocline. High-precision ages from early skarn (U-Pb, hydrothermal titanite) and later-stage mineralization (Re-Os, molybdenite) in the Coroccohuayco deposit are indistinguishable from each other and from available high- precision U-Pb zircon ages of the porphyries. All together, they indicate that the deposit was formed in less than 100 k.y. between 35.7 and 35.6 Ma. We also highlight a previously unrecognized pre-ore high-temperature hydrothermal event (U-Pb, hydrothermal titanite) that corresponds to the emplacement of a precursor gab- brodiorite complex at ca. 40.2 Ma. A new 40Ar/39Ar age at 26.6 Ma of a post-ore alkali basalt is interpreted as recording the initiation of slab roll-back following the flat slab episode and is therefore not related to the magmatic-hydrothermal system at Coroccohuayco. These data, together with structural measurements at the Coroccohuayco deposit and available regional data, suggest that the Coroccohuayco deposit was formed toward the end of Eocene arc magmatism, in a context of transpressional stress, intense erosion, and exhumation associated with Incaic orogeny. At the scale of the Tintaya ore district (which hosts the Coroccohuayco, Tintaya, and Antapaccay deposits), available data and a new molybdenite Re-Os age obtained for the Tintaya deposit suggest that mineralizing events were spatially focused and episodic over several millions of years, while a single economic deposit may have been formed within less than 100 k.y.