Hydrothermal apatite record of ore-forming processes in the Hatu orogenic gold deposit, West Junggar, Northwest China

The textures and compositions of hydrothermal apatite grains have been reported in many iron oxide-apatite (IOA), porphyry, and rare-earth elements (REE) polymetallic deposits. However, such information for apatite in hydrothermal gold deposit is not available. In this study, we present integrated textural, elemental, and in situ oxygen isotopic data of hydrothermal apatite grains from the Hatu gold deposit (56t Au, average grade: 5 g/t) in the West Junggar, Northwest China, to investigate the detailed mineralization processes and nature of ore-forming fluids. Hydrothermal apatite is present within the quartz-sulfide ores at the Hatu gold deposit. Apatite grains of the Hatu gold ores contain abundant fluid inclusions and fine sulfide crystals, indicating that they are closely related to gold mineralization. Three types of hydrothermal fluorapatite (Ap1, Ap2, and Ap3) were revealed by Cathodoluminescence (CL) imaging. The Ap1 and Ap2 formed in the auriferous ore-forming stage, whereas the Ap3 formed in the post-ore stage. The three types of apatite have distinct total REE concentrations (216-751 ppm for Ap1, 14-120 ppm for Ap2, and 914-1422 ppm for Ap3) as well as chondrite-normalized REE distribution patterns. The textures and compositions of different apatites are inconsistent with dissolution-re-precipitation processes but instead point to three episodes of apatite formation in the Hatu hydrothermal system. The Ap3 has different REE and lower Eu/Eu* values (0.97-1.72) than Ap1 (1.34-4.50) and Ap2 (1.69-4.95), indicating a change of fluid compositions and a decrease of oxygen fugacity from the auriferous ore-forming stage to post-ore stage. All three types of apatite (Ap1, Ap2, and Ap3) show relatively high and consistent oxygen isotope compositions ranging from 14.2 +/- 0.3 parts per thousand to 15.2 +/- 0.2 parts per thousand, from 14.5 +/- 0.2 parts per thousand to 15.0 +/- 0.3 parts per thousand, and from 13.8 +/- 0.3 parts per thousand to 15.4 +/- 0.2 parts per thousand, respectively. Their calculated delta O-18(fluid) values are higher than magmatic-hydrothermal fluids but consistent with those from metamorphic fluids derived from meta-basalts. The near-zero delta S-34 values of hydrothermal pyrite grains in the gold ores are much higher than those of pyrite framboid grains in the ore-hosted sedimentary rocks (- 41.1 to - 24.3 parts per thousand), suggesting the sulfur may be derived from meta-basalts rather than sedimentary rocks in the mining area. Taken together, we suggest that Hatu is an orogenic gold deposit formed by fluids and metals derived from metamorphic de-volatilization of meta-basalts at depth. Our study highlights that a combination of CL imaging, in situ trace elements, and oxygen isotope compositions of hydrothermal apatite can be a novel and powerful tool to trace the nature of ore-forming fluids in the hydrothermal gold deposits.