Gold bearing characteristics and fluid evolution of Arsenopyrite in Huanglong gold deposit, South Qinling Orogenic Belt

To investigate the genesis of the Huanglong gold deposit, techniques such as electron probe microanalysis (EPMA) were employed to observe and analyze samples, with the aim of elucidating the occurrence state and formation environment of Au in the arsenopyrite—the principal gold-bearing mineral—and further exploring the genetic mechanisms. The birite porphyroblasts in the ore are categorized into fine, medium and coarse-grained types, corresponding to arsenopyrite crystal habits ranging from euhedral to anhedral. EPMA data indicate stable major element compositions in arsenopyrite, with Fe content ranging from 29.19% to 34.58%, S from 19.43% to 20.70%, and As from 45.13% to 46.21%. The empirical formula of arsenopyrite is derived as As 32.33 ~ 35.1, Fe 29.86 ~ 33.39, S 33.32 ~ 35.31. Scanning electron microscopy (SEM) observations revealed no native gold particles, suggesting that Au predominantly occurs as "invisible gold" within the arsenopyrite structure, likely through isomorphic substitution for S or As. The crystallization temperature of arsenopyrite is constrained between 360℃ and 475℃, indicating a high-temperature regime. The calculated sulfur fugacity (lgf(S2)) values range from − 10.5 to -6.6. The porous and fractured texture of arsenopyrite, coupled with the negative correlation between Au and S, points to characteristics of decompressive boiling. This process led to a decrease in sulfur fugacity within the hydrothermal system, triggering the decomposition and subsequent precipitation of Au complexes.