Amphibole-driven redox evolution and magmatic fertility at the Oyut Cu–Mo Deposit, Mongolia

The formation of porphyry Cu–Mo deposits in continental crust frequently encounters the "Cu paradox", where magmas exhibiting the strongest indicators of ore potential (e.g., high Sr/Y) possess the lowest bulk copper concentrations due to early sulfide saturation. This study investigates the magmatic evolution of the Permian–Triassic Oyut Cu–Mo deposit in Central Mongolia to clarify the mechanisms driving magmatic fertility and metal enrichment. Zircon U–Pb geochronology identifies two distinct magmatic stages: a pre-ore barren stage (256–240 Ma) and a subsequent fertile stage (240–227 Ma). Whole-rock data from the ore-bearing suite display typical “fertile magma” signatures, including high Sr/Y ratios and spoon-shaped REE patterns with depleted heavy REEs (HREE). However, Zircon trace element chemistry records a significant redox change: pre-ore suites were more reduced (≈ FMQ buffer), while strong positive Ce anomalies in zircon reflect elevated oxidation state during the emplacement of fertile magmas (log fO₂ ≈ NNO buffer). Zircon ε_Hf(t) values (+0.1 to +10) indicate constant addition of juvenile source, suggesting that high fO₂ was attained during differentiation rather than inherited. Moreover, the high content of HREE suggests that deep crustal garnet fractionation was not the primary driver. Instead, elevated ΣMREE/ΣHREE ratios through time confirm that differentiation was dominated by amphibole fractionation. We propose that water-saturated conditions promoted highly oxidizing conditions and extensive hornblende crystallization, depleting Fe from the melt, lowering sulfide saturation capacity, and triggering early sulfide sequestration as well as apparent Cu depletion. In contrast to the pre-ore reduced magmas, this amphibole-mediated pathway and oxidizing conditions maintained metals in high-solubility sulfate complexes, concentrating the volatiles and chalcophile elements necessary for large-scale Cu–Mo mineralization. These findings highlight hydrous magma and an amphibole fractionation as a key discriminator between barren and fertile magmas in the Central Asian Orogenic Belt.