Anatomy of a Moderately Telescoped Porphyry–High-Sulfidation System: Insights of a 2 km Vertical Profile of Alteration and Mineralization at Valeriano Cu-Au ± Mo Deposit, Chile

Abstract The Valeriano Cu-Au ± Mo deposit is located at the northern end of the El Indio metallogenic belt in the Chilean Central Andes, a region well-known for its prolific Au-Ag high-sulfidation epithermal deposits. In the past decade, deep drilling beneath the Valeriano lithocap resulted in the discovery of a concealed Cu-Au ± Mo porphyry system at depth, providing insights into a 2-km vertical profile of alteration from the shallow high-sulfidation epithermal to the porphyry environment. The Valeriano system is dominantly hosted in early Permian to Late Triassic felsic volcaniclastic rocks, and near the surface, by Middle Miocene intermediate composition volcanic rocks. Porphyry-style mineralization was temporally and spatially associated with the emplacement of a series of hornblende-biotite-bearing porphyries of granodioritic composition, and related magmatic-hydrothermal breccias between 10.39 ± 0.19 and 10.12 ± 0.18 Ma, based on zircon U-Pb ages. These intrusions and breccias at depth (>1,000 m below the present-day surface) are associated with Cu-Au mineralization of chalcopyrite and lesser bornite and molybdenite coupled with K-silicate alteration, which formed between 10.55 ± 0.05 and 10.1 ± 0.04 Ma, as determined by molybdenite Re-Os dating. K-silicate alteration is pervasively overprinted by muscovite-quartz ± chlorite alteration containing chalcopyrite and pyrite. This assemblage transitions upward over a distance of ~400 m into a 400- to 750-m-thick zone of muscovite-quartz ± pyrophyllite at the base of the lithocap that is intimately associated with low-grade, high-sulfidation state Cu sulfides and Cu-As sulfosalts dominated by bornite, digenite, chalcocite, and minor covellite. The upper part of the lithocap consists of a 100- to 200-m-thick, concentrically zoned, advanced argillic alteration—characterized by different proportions of quartz, alunite, kaolinite, and pyrophyllite—centered on stratiform or structurally controlled zones of massive and vuggy quartz. These zones host discrete Au-bearing enargite epithermal mineralization, which occurs as hydrothermal breccias, veins, and open-space fillings, which seem to be the remnants of a larger eroded high-sulfidation system. Muscovite and alunite Ar-Ar ages from zones of white mica and advanced argillic alteration, respectively, yielded ages between 10.151 ± 0.024 and 10.038 ± 0.014 that overlap with the age of porphyry intrusion and associated mineralization, suggesting that porphyry and epithermal mineralization at Valeriano formed over a short period of between 690 and 510 k.y. Advanced argillic alteration transitions outward and upward to intermediate argillic alteration over a distance of 200 to 500 m, which in turn transitions sharply into weakly altered to unaltered rocks in the uppermost lateral portion of the system, suggesting that the Valeriano lithocap was partially blind.