Fluid Evolution Characteristics and Coupled Pyrite Microscale Elemental Variations of the Baifeng Gold Deposit, Western Guangxi

The Youjiang Basin in western Guangxi hosts a major Carlin-type gold ore district and constitutes an essential component of the Yunnan-Guizhou-Guangxi “Golden Triangle”. The Baifeng deposit represents a large-scale gold deposit in western Guangxi, yet the source of gold ore-forming materials for this deposit has not been previously investigated. Based on detailed field geological surveys, vein samples from different fault structures within the deposit were collected for integrated analyses, including apatite geochemistry, fluid inclusion microthermometry, laser Raman compositional analysis, and in situ trace element and sulfur isotope testing of pyrite. The weighted mean U-Pb age of apatite is 306.9 ± 6.5 Ma, inherited precursors captured by the hydrothermal system rather than syn-ore hydrothermal apatite; this age therefore records the formation age of the apatite source rock, not the timing of gold mineralization. Geochemical discrimination diagrams indicate that the apatite is not of sedimentary origin, and its parent source belongs to medium- to high-magnesium magma dominated by melted subducted sediment components. This suggests that the ore-forming fluids/melts and the reservoir of ore-forming metals such as Au were derived from terrigenous sediments subducted during the Late Paleozoic. In the main mineralization stage, pyrite mainly occurs as fine-grained As-rich aggregates with low automorphism. Fluid inclusions yield homogenization temperatures of 230–291 °C (average 266.2 °C) and salinities of 9.6–16.2 wt% NaCl eqv. (average 12.0 wt% NaCl eqv.), characteristic of a medium-temperature and medium-salinity hydrothermal system. Pyrite from this stage contains the highest Au concentration among the three generations and yields tightly clustered δ34S values around −7.55‰ (range −7.43 to −7.65‰), corresponding to the primary stage of Au precipitation and enrichment in the deposit. During the post-main mineralization stage, pyrite mostly occurs as regular polygonal grains with markedly higher automorphism relative to the main mineralization stage. Fluid inclusion homogenization temperatures range from 189 to 276 °C (average 223.1 °C), with salinities of 7.6–17.2 wt% NaCl eqv. (average 11.6 wt% NaCl eqv.). Pyrite Au content decreases while Sb increases. Pyrite δ34S values (−8.32 to −6.88‰) largely overlap with the main-stage range but show greater scatter, indicating weakening of the deep fluid signal and increasing wall-rock interaction. In the late mineralization stage, reactivation of regional faults triggered superimposed modification by late hydrothermal fluids. Pyrite grains in this stage are generally fine. Fluid inclusion homogenization temperatures vary from 131 to 222 °C (average 165 °C), and salinities range from 8.1 to 14.5 wt% NaCl eqv. (average 10.6 wt% NaCl eqv.). F Pyrite Au content reaches its minimum while Sb peaks. Pyrite δ34S shifts markedly to less negative values (−6.06 to −4.29‰, mean −5.42‰), recording a significant change in sulfur source toward oxidized, shallow fluids or wall-rock-derived sulfur. The overall evolution—decreasing temperature, progressive oxidation of the gas phase, and a systematic shift of pyrite δ34S toward less negative values—is consistent with a hydrothermal system evolving from a deep, reducing crustal fluid toward shallower, more oxidizing conditions, with progressive Sb enrichment in residual fluids. This is consistent with the regional Au–Sb association and late stibnite overprinting characteristic of Carlin-type gold deposits.