Fluid evolution and genesis of the Yidinan granitoid-hosted orogenic gold deposit (China)

Abstract The metamorphic model explaining orogenic gold ore formation has become widely accepted. However, there has been extensive debate regarding whether a magmatic-hydrothermal system contributes fluids or metals in the source of orogenic gold deposits. The Yidinan gold deposit is hosted by Triassic quartz diorite in the West Qinling Orogen, China, which is controlled by NNE-trending high-angle brittle-ductile faults. The gold mineralization is characterized by vein and disseminated type ores comprising auriferous pyrite and arsenopyrite. Magmatic apatite U-Pb and magmatic biotite Ar-Ar dating pinpoint the emplacement and the cooling of ore-hosting quartz diorite at 241.8 ± 2.8 Ma and 241.7 ± 0.32 Ma, respectively. In situ U-Pb dating of hydrothermal monazite yield an age of 234.6 ± 2.8 Ma for the gold mineralization. Systematic fluid inclusion investigation suggests that the ore-forming fluids belong to a NaCl-H2O-CO2 ± CH4 system with low salinity (5.76–10.09 wt% NaCl equiv.) and medium temperatures (253–395 °C). During fluid evolution, phase separation occurred, with CO2 and other gases preferentially fractionating into the vapor phase. The sulfur isotope data range from 5.50‰ to 7.85‰ and are higher than those from the nearby magmatic-hydrothermal deposits. Such results support that the gold-bearing fluids were sourced from devolatilization of underlying sedimentary rocks during regional metamorphism. Fluid immiscibility caused by fault-valve processes might be the critical mechanism for the gold deposition. Although the geological and geochronological evidence suggested gold mineralization was spatially and temporally associated with the quartz diorite, the ore-forming fluids are not consistent with a magmatic source; therefore, the Yidinan gold deposit is of an orogenic type. This study reveals that despite orogenic gold mineralization and magmatic activities showing a broad temporal or spatial overlap during orogenesis processes, there is no genetic link between gold mineralization and granitic magmatism in many hydrothermal gold deposits. The low-salinity auriferous metamorphic fluid was released from underlying metasedimentary sequences during orogenesis. The rapid cooling of the granitoid after emplacement further prevented it from contributing to gold-bearing fluid formation or creating the necessary pressure-temperature conditions for gold deposition.