Genesis of the Jiagantan gold deposit in West Qinling, China: Constraints from Re-Os dating, in-situ trace elements and S isotope of gold bearing minerals

The Jiagantan gold deposit in the West Qinling Orogeny is the largest gold deposit in the Xiahe-Hezuo region. However, its genesis remains controversial, mainly regarding to its metamorphic or magmatic origin. One main conflict stem from the lack of records on its mineralization time. Through field and microscopic observations, two types of arsenopyrites and two types of pyrites have been identified: Apy1 (common and disseminated in the ore), Apy2 (rare and occurred around Py1), Py1 (disseminated pyrite) and Py2 (pyrite veins in the ore). In this study, the Re-Os test on arsenopyrite of Apy1 is taken to determine the metallogenic time, from which the isochron age of 243.1 +/- 8.7Ma was obtained, corresponding to magmatism in the Xiahe-Hezuo region (232 similar to 249Ma). LA-ICP-MS and LA-MC-ICP-MS were performed on the ore stages of arsenopyrite and pyrite to determine the trace element contents and sulfur isotope values, respectively. Trace element content shows the invisible gold in the sulfide occurs as solid solution in Jiagantan gold deposit. The invisible gold content in Apy2 was 521x10(-6) at most, while the gold content in other sulfides was up to 119x10(-6); the grade of Apy2 is 2 similar to 3g/t, indicating that additional gold of magmatic origin would have been required even if reactivation occurred. The in-situ sulfur isotope test showed that the delta S-34 values of Py1, Py2, Apy1 and Apy2 ranged from -14.3 parts per thousand to -7.70 parts per thousand (-10.0 parts per thousand on average), -14.5 parts per thousand to -9.60 parts per thousand (-12.5 parts per thousand on average), -13.0 parts per thousand to -6.40 parts per thousand (-8.54 parts per thousand on average), and -11.2 parts per thousand to -9.00 parts per thousand (-10.1 parts per thousand on average), respectively, which shows the fractionation of sulfur isotopes. Comparing the sulfur isotope compositions of syngenetic pyrite and magmatic pyrite, the sulfur isotope values of ore stage sulfides indicate a magmatic hydrothermal origin. The results suggest that the Jiagantan gold deposit should be a magmatic hydrothermal gold deposit.