The role of fluid overpressure in Cu-Au porphyry mineralization: Evidence from the Oubulage deposit, Inner Mongolia, China

Fluids play a critical role in the formation of porphyry copper (Cu)‑gold (Au) deposits, although the mechanism of metal transport remains enigmatic. The presence of phengite in the Oubulage porphyry Cu-Au deposit in Inner Mongolia provides a unique opportunity to evaluate this problem. In this study, we present results from texture, elemental and isotopic composition of phengite and calcite in the Oubulage quartz porphyry, with a view to understand the role of fluids in the ore genesis. The quartz phenocrysts from this deposit show shock-induced textures, substantial fragmentation, and deformation lamellae in quartz porphyry. The phenocrysts are frequently intersected by veins which consist of phengite, K-feldspar, rutile, ankerite, apatite, and sulfide. Phengite, with Si contents peaking at 3.40 apfu indicates a maximum pressure of approximately 14.4 Kbar. The quartz porphyry shows vesicular and amygdaloidal structures, with vesicular volumes reaching a maximum of 25 %. The elevated Nb + Ta contents and a Nb/Ta ratio exceeding 17.7 in phengite, suggest the important role of supercritical fluids for the origin of phengite. According to the Zr thermometer for rutile, the temperature of these supercritical fluids is approximately 788.3 °C. In-situ carbon‑oxygen isotopic measurements in calcite associated with the sulfide mineralization (δ 13 C PDB ranging from −6.54 ‰ to −4.26 ‰ and δ 18 O VSMOW between 8.24 ‰ and 9.13 ‰)， and the strontium isotope measurements in apatite ( 87 Sr/ 86 Sr = 0.7056–0.7060) associated with sulfide mineralization, suggest a mantle source for the fluids. We propose that the injection of supercritical fluids rich in Si, Ti, Al, K, Fe, C and F into sulfide melt might have caused fluid overpressure in the magma chamber. This resulted in the fracturing of the phenocryst minerals and formation of high-Si phengite, as well as propelling the upward movement of the ore-bearing melt-fluid towards the upper crust. Our findings also have implications for the exploration of porphyry ore deposits.