The Formation Process and Metallogenic Implications of Carbonate Minerals in the Axi Low-Sulfidation Epithermal Gold Deposit in the Western Tianshan, Xinjiang

The Axi gold deposit in the western Tianshan of Xinjiang is a low-sulfidation epithermal gold deposit hosted in continental volcanic rocks. The ore-forming process of this deposit can be divided into five stages: quartz-sericite-pyrite (I), quartz-pyrite (II), quartz-polymetallic sulfide-carbonate (III), carbonate-quartz (IV), and carbonate (V). Carbonate minerals, apart from quartz, are the most important non-metallic minerals in gold ores. Their textural and compositional characteristics record information about the source of ore-forming materials and the evolution of ore-forming fluids. In this paper, petrographic observations, cathodoluminescence photography, electron probe analysis, and C-O isotope analysis were carried out on the euhedral coarse-grained dolomite (Dol-I) in the stage II crustiform quartz, the bladed dolomite (Dol-II) in stage III, the vein dolomite (Dol-III) in stage IV, and the vein calcite (Cal-IV) in stage V. The results show that Dol-I is composed of dolomite (Dol-Ia) and ankerite (Dol-Ib), with significant differences in FeO content (0.31%–0.68% and 14.17%–14.66%). The FeO contents of Dol-II and Dol-III (0.63%–1.48% and 1.57%–3.89%) are similar, and both belong to iron-bearing dolomite. The average δ¹³C_V-PDB values of Dol-III and Cal-IV are 3.05‰ and 2.48‰ respectively, which are similar to the carbon isotope composition of marine carbonates, indicating that the carbon in the fluid may be derived from the limestone in the basement of the mining area. The average δ¹⁸O_SMOW values of the two are 15.72‰ and 15.68‰, showing a negative shift, which may be due to the extraction of the volcanic rocks of the Dahalajunshan Formation hosting the ore by circulating meteoric water. In stage II, the crustiform ore formed parallel quartz micro-bands containing gold-bearing sulfides, comb quartz, colloform “spherical” quartz, and Dol-I from the vein wall to the center, indicating that it is a product of alkaline conditions after the ore-forming fluid experienced multiple episodes of fluid boiling, loss of acidic gases, and massive sulfide precipitation. In stage III, Dol-II developed as bladed crystals in smoky gray cryptocrystalline quartz, indicating that it is a product of direct precipitation from a non-equilibrium supersaturated hydrothermal system during the initial boiling of the fluid. In stage IV, Dol-III occurs as euhedral coarse-grained dolomite veins cutting through early-stage veins, formed by slow crystallization under stable shallow-surface mineralization conditions. In stage V, Cal-IV occurs as euhedral coarse-grained crystals distributed at the edges of the mineralization system, formed by the combination of CO₃²⁻ and Ca²⁺ generated by the dissociation of HCO³⁻ due to the loss of CO₂ and H₂S from the hydrothermal fluid under decreasing temperature and pressure conditions. Based on the comprehensive analysis of the textures, major elements, and isotopic geochemistry of carbonate minerals, this study concludes that fluid boiling is a key mechanism for the enrichment and precipitation of ore-forming materials in stages II and III of the Axi gold deposit.