Genesis of the Hardat Tolgoi Ag-Pb-Zn deposit, Inner Mongolia, northeast China: Constraints from geology, fluid inclusions, and C-O-S-Pb isotope systematics

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The Hardat Tolgoi Ag-Pb-Zn deposit (4.3 Mt at 3.15% Pb + Zn and 26.4 g/t Ag) is located in the Erenhot-East Ujimqin polymetallic metallogenic belt, on the western part of the Great Xing’an Range, Inner Mongolia, northeast China. The Ag-Pb-Zn mineralization mainly occurs as vein type ores hosted in argillaceous siltstone, feldspar quartz sandstone with silty slate interlayers of the Aobaotinghundi Formation of Lower Devonian. The hydrothermal ore-forming process can be divided into three stages, i.e., quartz–pyrite–arsenopyrite stage (I), quartz–polymetallic sulfides stage (II), and quartz–calcite–fluorite stage (III). Four types of fluid inclusions have been identified in the quartz-bearing veins of each stage, i.e., liquid-rich two-phase (L-type), single liquid-phase (PW-type), vapor-rich two-phase (V-type), and pure vapor phase (PV-type) inclusions. The fluid inclusions of stage I to III yield homogenization temperatures of 285–407 °C, 202–317 °C, and 144–239 °C, with salinities of 1.2–9.2 wt% NaCl equiv., 0.9–5.0 wt% NaCl equiv., and 0.2–2.6 wt% NaCl equiv., respectively. The ore-forming fluids at Hardat Tolgoi are characterized by moderate temperatures, low salinities, and low densities, and dominated by the H2O–NaCl system. Carbon and oxygen isotope compositions of calcite demonstrate that the ore-forming fluids come from a primary magmatic-hydrothermal system, with subsequent involvement of meteoric water at a later stage. Sulfur isotope analyses of sulfides yield δ34SV-CDT values of 1.8 to 8.7‰, with calculated δ34SH2S values of 1.4 to 9.4‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of sulfides range from 18.141 to 18.468, 15.521 to 15.617, and 38.040 to 38.377, respectively, similar to those of granites from the Hardat Tolgoi deposit. Both sulfur and lead isotope data indicate that the ore-forming metals were most likely derived from the magmatic reservoirs and sedimentary country rocks. The available fluid inclusion results and isotope studies indicate that the pahse separation, temperature decrease, and fluid mixing resulted in large-scale Ag-Pb-Zn ore formation at Hardat Tolgoi.