Ore Genesis of Yawan Antimony Deposit in Western Qinling, China: Constraints from Fluid Inclusions and H-O-S-Pb Stable Isotopes

The Yawan deposit is a representative large-scale antimony deposit in western Qinling, and Sb mineralization is dominantly hosted in Triassic sedimentary rocks and controlled by faults and fractures. However, its genesis is unclear. An integrated study of fluid inclusions, hydrogen, oxygen and sulfur–lead isotopes combined with deposit geology was conducted to elucidate the metallogenic processes of the Yawan deposit. Three ore-forming stages are identified in the Yawan antimony deposit: (1) pre-ore sedimentary diagenesis, possibly accompanied by metal pre-enrichment, (2) medium- to low-temperature hydrothermal mineralization and (3) post-ore supergene oxidation stages. Fluid inclusion microthermometry reveals that the Yawan Sb deposit belongs to a medium- to low-temperature (~140–340 °C), low-salinity (average salinities of 1.1–5.4 wt.% NaCl equivalent) hydrothermal system. H and O isotopes (δD = −93.9%–−68.7%, δ18O = −6.5%–+5.1%) suggest a significant magmatic fluid contribution during the early stage, with increasing involvement of meteoric water in the late stage. The Yawan deposit exhibits varied δ34S values in different ore-forming stages, with an average of +15.1‰ of euhedral pyrite and −12.8‰ of framboidal pyrite in pre-ore sedimentary carbonates, −3.2‰ of stibnite in the early mineralization stage and −8.8‰ of stibnite in the late stage. Furthermore, it has similar Pb isotopic compositions of sulfides between ores and pre-ore sedimentary strata, with 206Pb/204Pb = 17.13–19.18, 207Pb/204Pb = 15.48–15.76, 208Pb/204Pb = 37.63–38.93. These isotopic signatures indicate that ore-forming materials are dominantly derived from upper crustal sources involving minor mantle components. Our study demonstrates that the Yawan Sb deposit is best interpreted as a sedimentary–hydrothermal composite system, possibly related to deep granitic intrusions. The ore-forming model is proposed in this study to expound ore genesis of the Yawan Sb deposit, providing predictive exploration criteria for analogous Sb deposits in the West Qinling Orogenic Belt.