Pyrite textural, trace elemental and sulfur isotope signatures of the Badu gold deposit, Youjiang basin (SW China): Implications for ore-fluid source and Au precipitation mechanism

Abstract

The Badu Au deposit in the central Youjiang basin (SW China) is hosted by Devonian-Permian siltstone and Permian diabase. Gold at Badu occurs mainly as structurally bound Au+ in the disseminated pyrite. Based on textural and chemical analyses, the disseminated pyrite can be further divided into three generations (Py1, Py2, Py3a, and Py3b). Py1 corresponds to the porous pyrite core in the altered siltstone, featured by high mean contents of Cu (138 ppm), Zn (73.3 ppm), Co (351 ppm), Ni (1,861 ppm), and Se (33.7 ppm). It has negative δ34S values (mostly −21.64 to −19.1 ‰), indicating sulfur derivation by bacterial sulfate reduction (BSR) during sedimentation and diagenesis. Py2 is the pyrite core developed in the altered diabase, with pores filled by sericite. Compared to Py1, Py2 has higher mean contents of Au (1.46 ppm) and As (7,955 ppm), and has near-zero δ34S values (mostly −2.22 to + 4.74 ‰), which is consistent with previous HO isotope studies of fluid inclusions in quartz (δ18O H2O = +9.0 to + 11.2 ‰, δD = −83 to −55 ‰) and Sr isotope study of apatite (87Sr/86Sr = 0.709489 to 0.711787), indicating that Py2 was likely formed by a metamorphic fluid during the early ore hydrothermal stage. Py3a rims around Py1 and has high Au (up to 40.4 ppm) and As (up to 37,518 ppm) contents, with δ34S values (mean + 16.58 ‰) similar to the sedimentary host rocks, indicating a dominantly basinal fluid for the siltstone-hosted ore formation at Badu. Py3b overgrows on Py2, and has also high Au (up to 227 ppm) and As (up to 93,201 ppm) contents, with mean δ34S value of + 12.74 ‰. The increasing δ34S values from Py2 to Py3b indicate a basinal fluid input (44–70 %) in the diabase. At Badu, siltstone-hosted Au-rich arsenian pyrite was precipitated by the sulfidation of Fe-carbonates, whereas diabase-hosted Au-rich arsenian pyrite was precipitated by the sulfidation of clinopyroxene and ilmenite.