Insights Into the Magma Source and Evolution of the Taca Taca Bajo Porphyry Deposit: Implications for the Metallogeny and Cu Fertility of the Central Andean Retro Arc

The magmatic processes that lead to porphyry Cu ore formation in continental retro-arc environments are not well understood. As a result, the uncertainty of predictive exploration in these tectonic settings is elevated, and new case studies are needed to enhance the probability of success in target identification. The Taca Taca Bajo porphyry Cu-Mo-Au deposit is a well-mineralized (11.7 Mt contained Cu), retro-arc expression of the Middle Eocene to Early Oligocene metallogenic belt in the central Andes and represents a key location for investigating continental retro-arc magmatic processes that culminate in the formation of porphyry Cu deposits. Mineralization at Taca Taca Bajo is spatially and temporally correlated with a NE-SW–trending rhyodacitic porphyry dike swarm. Six samples of the mineralized Taca Taca Bajo rhyodacite porphyry and one of the barren west rhyodacite porphyry were analyzed for whole-rock geochemistry and zircon petrochronology (U-Pb geochronology, O isotopes, Lu-Hf isotopes, trace element geochemistry). The U-Pb SHRIMP analyses of zircons from the Taca Taca rhyodacite porphyry intrusions reveal ages ranging from 30.3 ± 0.5 Ma to 29.1 ± 0.3 Ma (95% confidence interval). The barren west rhyodacite porphyry sample yields an overlapping zircon crystallization age of 30.4 ± 0.4 Ma (U-Pb SHRIMP; 95% confidence interval) with mineralized samples. Whole-rock geochemistry reveals a subducted slab component to the magma, with enrichments in Ba and Th as well as a marked negative Nb and Ta anomaly. The mean zircon δ18O of both the barren and mineralized Oligocene intrusions ranges from 5.6 ± 0.5 to 5.8 ± 0.2‰ (2 standard deviations [SD]), and zircon εHf from 5.3 ± 2.4 to 7.6 ± 0.7 (2 SD). These similarities suggest that the mineralized and barren intrusions may have a relatively juvenile mantle-derived source with minor assimilation of older crust. Inherited zircons yield U-Pb ages of 48 to 1063 Ma, with a cluster at 230 to 280 Ma, indicative of interaction with older arc magmatic rocks of the lower Choiyoi Igneous Complex. The Taca Taca Oligocene intrusions are moderately hydrous (mean zircon Eu/Eu* = 0.25–0.34) and oxidized (mean ΔFMQ = 0.2–1.0 [FMQ = fayalite-magnetite-quartz]) as estimated from zircon-based proxies. However, they also have lower inferred H2O contents and fO2 than other Cu-porphyry deposits of the Eocene to Early Oligocene metallogenic belt (e.g., Escondida, Chuquicamata, El Salvador). Based on these data, it is possible to conclude that the Taca Taca Bajo deposit, despite being a large porphyry Cu deposit, may have failed to reach the scale of some others in the metallogenic belt because it experienced a relatively isolated, short (1–1.5 m.y.) magmatic pulse, and did not undergo the multimillion year build-up of magmatism characteristic of these major deposits.