Copper and molybdenum isotope signatures along a vertical section through the Valeriano epithermal-porphyry Cu-Au system, El Indio-Maricunga belt, northern Chile

Abstract The Valeriano composite epithermal-porphyry Cu‒Au system in the El Indio-Maricunga metallogenic belt, Vicuña segment, consists of a shallow silicic lithocap with low-grade Au‒Ag ± Cu mineralization underlain by a large porphyry-Cu (± Mo, ± Au) deposit. A deep drillhole (VALD-D13-14) allows the systematic evaluation of bulk-rock Cu‒Mo isotope compositions over a vertical extent of ~ 1,900 m through the hydrothermal porphyry-epithermal system. Copper isotope values in the lower 1,000 m of the mineralized potassic and phyllic altered porphyry zone increase slightly upwards in the range of +0.2 to +0.4‰, and scatter in the upper phyllic and advanced argillic zone (δ 65 Cu: ‒3.7‰ to +2.7‰). The near-surface low-Cu leached zone has very negative δ 65 Cu values down to ‒6.5‰ and there are also some distinctly negative excursions in the deep potassic zone with subordinate advanced argillic alteration. Molybdenum isotope values are slightly negative in the potassic porphyry zone (δ 98 Mo: ‒0.3 to ‒0.2‰), but increase over the phyllic zone to positive values of 0.2‒0.3‰, and strongly scatter at the transition to the high-sulfidation epithermal system. The Cu and Mo isotope patterns can be mainly attributed to Rayleigh fractionation between vapor and brine in the magmatic-hydrothermal porphyry system and to redox-controlled isotope shifts during supergene alteration in the epithermal system. Deviations from the general trend in some deep drillcore intercepts relate to short intervals of advanced argillic alteration and corresponding redox fluctuations with Cu enrichment.