Genesis of the Gangcha gold deposit, West Qinling Orogen, China: Constraints from Rb-Sr geochronology, in-situ sulfur isotopes and trace element geochemistry of pyrite

The Gangcha gold deposit, in the western zone of the Xiahe-Hezuo region within the West Qinling orogen of Central China, is a newly discovered medium-scale epithermal gold deposit with 19 tons of identified gold reserves. Mainly hosted in the Permian metasedimentary rocks and Triassic altered volcaniclastic rocks, and gold mineralization is closely associated with pyrite. Here we present results from field observation and geochemical studies of pyrite using electron probe microanalysis (EPMA), in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and femtosecond laser ablation coupled multi-collector inductively coupled plasma mass spectrometry (fsLA-MC-ICP-MS) analyses. Based on the results, we identified five types of pyrite in the ores and wall rocks (namely Py1, Py2, Py3, Py4 and Py5). Among these, the pre-ore Py1, which is enriched in As, shows markedly negative δ34S values of −30.99 to −17.70‰ that are assumed to be the reflection of abiotic thermochemical reduction of sulfate (TSR) related to extremely oxidized magmas that have assimilated carbonaceous sedimentary rocks. Py2, occurring in the pyrite veins, displays internal rhythmic As- and Au-enriched growth zoning, and the δ34S values of Py2 progressively increase from the cores to the outer rims that are interpreted to result from mixing between the pyrite-forming fluid and magmatic vapor that invaded the main hydrothermal system episodically. The intermediate ore stage pyrite (Py3), represented by the quartz-pyrite (±arsenopyrite) veins, is enriched in As, Sb and Au. The comparable δ34S values of Py3 (−4.59 to + 7.05‰, average + 4.46‰) and Py4 (−5.66 to + 6.50‰, average + 1.80‰) could indicate magmatic sulfur. Compared with pyrites of the earlier stages, the late ore stage pyrite (Py5) is distinctly more enriched in δ34S, and is interpreted to have formed by mixing between a magmatic fluid and meteoric water. Additionally, some of the pyrites (Py5) from Stage V, occurring as the rim of pyrite grain, are interpreted to have formed by replacement via a dissolution-reprecipitation process of the earlier-formed Py4. The Rb-Sr data of pyrites from veins of different mineralization stages yield a mean isochron age of 225.3 ± 2.6 Ma, interpreted as the age of gold mineralization. Based on a combination of geochronological and geochemical date of pyrite, we propose that the ore-forming fluids of the Gangcha gold deposit are closely associated with coeval magmatic activity during the region collisional orogenic event.