Hydrothermal mineralization at the Dahu Au-Mo deposit in the Xiaoqinling gold field, Qinling Orogen, central China

摘要

The Xiaoqinling gold field in central China is the second largest orogenic Au province in China. In the Xiaoqinling area, the Dahu Au-Mo deposit is typical because it is one of the five early-discovered large gold deposits, but also unique for its northernmost location and Mo-association. This study shows that the deposit is a fault-controlled lode system formed by a three-stage hydrothermal process. The early-stage pyrite-quartz veins are structurally deformed and broken. The middle-stage molybdenite-pyrite-quartz stockworks (mostly coaxial) infill the cracks of the early-stage veins and minerals. The late-stage open-space filling quartz-carbonate veinlets show a comb-like texture. The early- and late-stage quartz only contains the fluid inclusions of CO_2-H_2O and H_2O-NaCl types, respectively, suggesting an evolution from CO_2-rich to CO_2-poor; whereas the middle-stage quartz contains fluid inclusions of CO_2-H_2O, H_2O-NaCl, pure CO_2 and daughter mineralbearing types, supporting a boiling fluid system, because they can occur in a microscopic domain of a single crystal, homogenize divergently at similar temperatures clustering between 293 and 410 °C, and show contrasting salinities of <13.6wt.% and >26.3wt.% NaCl equiv. The inclusions in early-stage quartz are homogenized at 402-503 °C, with salinities of 4.5-10.4wt.% NaCl equiv.; and those in late-stage quartz are homogenized at temperatures of <251 °C, with salinities of 14.4-14.8 wt.% NaCl equiv. The trapping pressures of inclusions are estimated to be 138-331MPa in the early stage and 78-237MPa in the middle stage, implying that the fluid system alternately fluctuated from lithostatic to hydrostatic, which was controlled by a fault-valve mechanism at depths of >11 km and ~7.8 km, respectively. Hydrogen and oxygen isotope signatures indicate that the fluids originated from metamorphic devolatilization in the early stage, then mixed with meteoric water in the middle stage, and finally replaced by meteoric water in the late stage. Hence, it is concluded, that metal precipitation at the Dahu deposit was mainly caused by fluid boiling, followed by fluid mixing, along with a trans-compressing at the transitional zone from a magmatic arc to back-arc basin, which resulted from the Triassic northward subduction of the Mian-Lue oceanic slab.