Age and metal source of orogenic gold deposits in Southeast Guizhou Province, China: Constraints from Re-Os and He-Ar isotopic evidence

The orogenic gold deposits in Southeast Guizhou are an important component of the Xuefeng polymetallic ore belt and have significant exploration potential, but geochronology research on these gold deposits is scarce. Therefore, the ore genetic models are poorly constrained and remain unclear. In the present study, two important deposits (Pingqiu and Jinjing) are investigated, including combined Re-Os dating and the He-Ar isotope study of auriferous arsenopyrites. It is found that the arsenopyrites from the Pingqiu gold deposit yielded an isochron age of 400±24 Ma, with an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 1.24±0.57 (MSWD=0.96). An identical isochron age of 400±11 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 1.55±0.14 (MSWD=0.34) was obtained from the Jinjing deposit. These ages correspond to the regional Caledonian orogeny and are interpreted to represent the age of the main stage ore. Both initial ¹⁸⁷Os ratios suggest that the Os was derived from crustal rocks. Combined with previous rare earth element (REE), trace elements, Nd-Sr-S-Pb isotope studies on scheelite, inclusion fluids with other residues of gangue quartz, and sulfides from other gold deposits in the region, it is suggested that the ore metals from Pingqiu and Jinjing were sourced from the Xiajiang Group. The He and Ar isotopes of arsenopyrites are characterized by ³He/⁴He ratios ranging from 5.3×10^-4 Ra to 2.5×10^-2 Ra (Ra=1.4×10^-6, the ³He/⁴He ratio of air), ⁴⁰Ar^*/⁴He ratios from 0.64×10^-2 to 15.39×10^-2, and ⁴⁰Ar/³⁶Ar ratios from 633.2 to 6582.0. Those noble gas isotopic compositions of fluid inclusions also support a crustal source origin, evidenced by the Os isotope. Meanwhile, recent noble gas studies suggest that the amount of in situ radiogenic ⁴He generated should not be ignored, even when Th and U are present at levels of only a few ppm in host minerals.