Abstract:

The West Qinling Orogen, formed as a result of Late Triassic collision between the North and South China Blocks, has undergone complex geological evolution and multi-episodic magmatic activities. It has abundant ore resources, including over 1200 tons of gold by estimation. The genesis of the gold deposits is still debatable, with possibilities including Carlin-type, orogenic, magmatic hydrothermal, and intrusion-related. The Wanken gold deposit in the southwest contains ~3.5 tons of proven gold, hosted in metasedimentary rock of the Lower Triassic Longwuhe Formation. Typical of the region, the gold deposit is structurally controlled by EW and NW-SE trending faults, and mainly exhibits disseminated vein mineralization styles. This study investigates the gold occurrences in the Wanken gold deposit and its genesis through petrological and mineralogical analyses, combined with geochemical analysis of gold-bearing sulfides. Three mineralization stages are delineated: pyrite (Py1)-quartz-sericite-chlorite (Stage I), pyrite (Py2)-arsenopyrite-quartz-sericite-chlorite (Stage II), and quartz-calcite-sphalerite-galena-stibnite (Stage III). According to electron microprobe analysis, the gold content (wt%) is mostly below detection limits in pyrite (0.11%-0.24%) and arsenopyrite (0.11%-0.28%), but relatively high in sphalerite (0.16%-0.37%), with 70% sites above 0.15%, indicating a heterogeneous distribution of Au in sulfides. The Au/As ratio of pyrite (0.007-0.20) is generally greater than 0.004, and there is no evidence of native gold, indicating gold mainly occurs as Au nanoparticles or nanocrystals. A negative correlation between Fe and As or between S and As in As-rich pyrite and arsenopyrite reveals that Au enters sulfide lattice through substitution into octahedral Fe(II) sites, or through chemical adsorption. The Fe (45.41%-46.26%) and S (51.48%-52.79%) contents in pyrite correspond to a Fe/S ratio of greater than 0.875, indicating a metamorphic-hydrothermal origin. Arsenopyrite from the main metallogenic epoch contains 42.80%-46.52% arsenic, and late sphalerite contains 1.77%-2.57% Fe and 63.75%-64.82% Zn. Furthermore, using arsenopyrite/sphalerite geothermometers, the temperatures and sulfur fugacity (lgf(S₂)) in the main mineralization stage are constrained at 385±40 ℃ and -7.55±1.45, respectively, and the temperatures in the late mineralization stage are constrained at 251±7 ℃. These observations suggest that the Wanken gold deposit is derived from medium- to high-temperature metamorphic hydrothermal fluids. Combining the mineralogical and geochemical characteristics of gold-bearing sulfides in the Wanken gold deposit with previous thermodynamic modeling results, it is evidenced that the dominant Au-S complexes in the Wanken deposit are $\mathrm{Au}(\mathrm{HS})_{2}^{-}$ and Au(HS). Extensive water-rock interaction and high degree of wallrock sulfidation causes HS^- to decrease in the hydrothermal fluids, which results in the decrease of gold solubility and destabilization of Au-S complexes, whereby driving the formation of the gold-bearing sulfides. Therefore, sulfidation is likely the dominant mechanism of gold precipitation in the Wanken gold deposit. In summary, it is considered that the Wanken gold deposit belongs to orogenic gold deposit.

Key words: mineral geochemistry, electron microprobe analysis, gold occurrence, Wanken gold deposit, West Qinling Orogen