Crustal Magmatic Evolution of the Oyut Porphyry Cu–Mo Deposit, Northern Mongolia

Copper (Cu) is essential for modern technologies, with demand expected to increase significantly due to the global transition towards green energy. Porphyry Cu deposits, which supply ~75% of the world’s Cu, are primarily associated with subduction-related magmatism in calc-alkaline arcs. Despite the widespread presence of such magmas, large porphyry Cu deposits are rare and increasingly challenging to discover. This study investigates the Permian–Triassic magmatic evolution of the Oyut Cu–Mo deposit in northern Mongolia through zircon U–Pb geochronology, lithogeochemistry, and mineral thermobarometry. The deposit is hosted in porphyritic andesite and basaltic andesite, intruded by (mostly) barren quartz-diorite and granodiorite, as well as mineralized granodiorite and dacite porphyries exhibiting intense K-feldspar and sericite alteration. Zircon U–Pb ages range from 254–246 Ma for the barren intrusions to 240–237 Ma for the mineralized porphyries. All magmatic rocks are high-K calc-alkaline and peraluminous to metaluminous, with Nb and Ta depletion indicating a continental arc setting. REE patterns distinguish two magmatic episodes: older barren intrusions show high overall REE contents and weak negative Eu anomalies, suggesting evolved melts. In contrast, mineralized intrusions (240–230 Ma) exhibit lower REE contents and positive Eu anomalies, indicating plagioclase accumulation or retention and a more oxidized, water-rich magma, favorable for Cu–Mo mineralization. Thermobarometric estimates (635–843 °C, 0.2–4.6 kbar) and trace element signatures suggest derivation from an enriched, volatile-rich mantle source and crystallization at mid-crustal depths. These findings highlight the importance of crustal differentiation, the role of plagioclase accumulation, and fluid evolution in forming Cu–Mo porphyry systems in continental arcs.