Textures, trace element geochemistry and in-situ sulfur isotopes of pyrite from the Xiaojiashan gold deposit, Jiangnan Orogen: Implications for ore genesis

The northeastern Hunan region in the Jiangnan Orogen (South China) is endowed with many important gold deposits, whose genesis remains controversial. The Xiaojiashan is a representative gold deposit (7.77 t Au @ 2.88 g/t) in NE Hunan, and is characterized by multi-stage pyrite formation. Its mineralization can be divided into 3 stages: (I) quartz-ankerite-pyrite-gold; (II) quartz-ankerite-polymetallic sulfide-gold; (III) quartz-ankerite stage. Three pyrite generations were identified: fine-grained porous Py1 and coarse-grained Py2 in stage I, and core-rim-textured Py3 in stage II. Py1 and Py2 are enriched in Co, Ni, Se and Te, but depleted in As and Au. Comparatively, Py3a and Py3b have higher As, Au, and Zn, but lower Co, Ni, Se and Te contents, indicating the compositional variations of ore-forming fluids from stage I to stage II. The Au- and As-poor Py1 and Py2 have similarly low delta 34S values (Py1: -33.30 to - 16.23 %o; and Py2: -25.41 to - 15.45 %o), indicating that the sulfur of Py1 and Py2 may have been derived from metamorphism of sedimentary strata, such as Neoproterozoic Lengjiaxi Group. Meanwhile, both the core (Py3a) and rim (Py3b) of the Au-As-rich Py3 have high delta 34S values (Py3a: -3.44 to - 1.72 %o; and Py3b: -1.00 to - 0.99 %o), resembling typical magmatic sulfur. The trace element compositions and sulfur isotopes of pyrite in stages I and II suggest a distinct shift of ore-forming fluids source from metamorphic (stage I) to magmatic (stage II). Therefore, we propose that the Xiaojiashan gold mineralization was likely associated with early metamorphism and late magmatic-hydrothermal overprinting, and its multistage and complex ore-forming process had recorded the complexity of gold mineralization in NE Hunan.