Genesis of the Xiaoyuzan epithermal Au deposit in the Yelimodun volcanic basin, Western Tianshan, China: Constraints from geology, geochronology, isotopes and mineralogical compositions

The Tulasu and Yelimodun volcanic basins are two important Au districts in the northwestern Chinese Western Tianshan. Two large Au deposits have been discovered in the Tulasu basin, whereas only small-sized Au deposits and prospects, including the Xiaoyuzan deposit, were found in the Yelimodun basin. The purpose of this study is to examine the genesis of the Xiaoyuzan deposit and its implications for further exploration through geological, geochronological, geochemical and mineralogical studies of the host rocks and ores. The orebodies are mineralized veins and veinlets and are structurally controlled by NW- and NNW-trending faults within the Lower Carboniferous Dahalajunshan Formation volcanic rocks (DF). Sulfides are abundant (10–20 vol%) in the ores and consist of pyrite, sphalerite, galena, chalcopyrite and minor tetrahedrite. Electron microprobe analysis (EMPA) indicates that pyrite and sphalerite are both auriferous. Three ore-forming and alteration stages were distinguished: (I) quartz-pyrite stage with silicification and propylitization, (II) quartz-polysulfide stage with silicification and phyllic alteration, and (III) quartz-carbonate stage with carbonatization. The mineralization age is constrained to 357–331 Ma by a LA-ICP-MS U-Pb age of 357.3 ± 4.0 Ma for zircon from the ore-hosting tuff and the unconformably overlying Lower Carboniferous Aqialehe Formation containing fossils of the Visean period. The zircons from the tuff have highly positive εHf(t) values ranging from 5.9 to 10.1, indicating a magma source of depleted mantle with limited crustal contribution. The values of δ18OH2O-VSMOW calculated from δ18Oquartz and δDH2O-VSMOW measured from fluid inclusions of stage II quartz range from −4.0‰ to −2.9‰ and −117‰ to −109‰, respectively. The values of δ13CCO2 calculated from δ13Ccal and δ18OH2O calculated from δ18Ocal range from −1.6‰ to −1.4‰ and −9.8‰ to −2.7‰, respectively. The in-situ δ34S values range from 2.6‰ to 3.7‰ for pyrite and 2.7‰ to 4.1‰ for sphalerite, respectively. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of pyrite vary from 18.196 to 18.253, 15.564 to 15.642, and 38.063 to 38.300, respectively, similar to those of ore-hosting DF and granites from the Tulasu-Yelimodun volcanic belt. The above geochemical characteristics suggest that the ore-forming fluids were mixtures of magmatic and meteoric waters, and the metals and sulfur may have been derived partly from magmatic fluids generated from the hidden causative intrusions and partly from the hosting volcanic rocks. These, together with high sulfide abundance and development of sulfide assemblages representing intermediate-sulfidation state, suggest that Xiaoyuzan is an intermediate-sulfidation epithermal Au deposit. Considering the limited erosion for the Au mineralization within the interior of Yelimodun basin and close spatio-temporal relationship between intermediate-sulfidation and porphyry mineralization, it is inferred that there may be concealed porphyry Cu-Au mineralization underneath the Xiaoyuzan deposit and its vicinity. © 2020 Elsevier B.V.