Ore-Forming Mechanism of the Sellaite-Bearing Orogenic Gold Deposits in Pianyanzi, Kangding, West Sichuan: Constraints from Gold Fineness, Fluid Inclusions and Thermodynamic Modeling

Objective‌ The Pianyanzi gold deposit, situated on the western margin of the Yangtze Craton within the Daduhe gold belt in western Sichuan Province, represents a rare sellaite-bearing orogenic gold deposit.However, key aspects of its ore-forming processes, particularly regarding the nature of ore-forming fluids and the mechanisms governing gold enrichment and precipitation, remain poorly constrained.

Methods This study integrates field observations with microthermometric analysis of fluid inclusions, electron probe microanalysis (EPMA) of individual minerals, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in-situ microanalysis of individual fluid inclusions. Based on these experimental results, thermodynamic simulations were conducted to comprehensively investigate the conditions conducive to gold enrichment and its precipitation mechanisms.

Results Based on the ore vein cross-cutting relationships and mineral assemblages, the mineralization process can be divided into four distinct stages: the pyrite-quartz stage (I), the pyrite- sellaite-gold-quartz-dolomite stage (II), the quartz-polymetallic sulfide stage (III), and the hematite- limonite-gold stage (IV). EPMA analyses reveal that the gold fineness values in Stage II and Stage IV range from 976.4 to 986.9 and from 960.9 to 992.9, respectively, indicating a consistently high gold content. Fluid inclusions are predominantly CO₂-bearing and H₂O-bearing, with homogenization temperatures ranging from 135 to 357.1 ℃, peaking between 220 and 300 ℃, and salinities varying from 0.4% NaCl_eqv to 14.4% NaCl_eqv, with a peak concentration between 6% NaCl_eqv and 12%NaCl_eqv. LA-ICP-MS analyses of individual fluid inclusions reveal that during the evolution from the early to late stages of mineralization, both the total elemental content and the concentrations of trace elements such as Na and K exhibit a progressively decreasing trend. Moreover, the Rb/Na, Cs/Na, Sr/Na, and K/Na ratios observed in these fluid inclusions are consistent with the geochemical signatures of fluids associated with orogenic gold deposit.

Conclusion Thermodynamic simulations and integrated studies indicate that gold is primarily transported in the form of AuHS⁰ complexes. The decrease in temperature during stages II, along with an increase in oxygen fugacity specifically in stage IV, may serve as the primary factors contributing to gold precipitation and variations in gold fineness.