Ore geology, fluid inclusions, and C–H–O–S–Pb isotopes of Nagengkangqieergou Ag‐polymetallic deposit, East Kunlun Orogen, NW China

Several large Ag‐polymetallic deposits have been newly discovered in the eastern segment of the East Kunlun Orogen, with the largest one being the high‐grade Nagengkangqieergou deposit (3,017 tons Ag at 310.04 g/t). The Ag ore veins are hosted in the Palaeoproterozoic Jinshuikou Group and Upper Triassic Elashan Formation, principally along NW‐trending faults and volcanogenic ring fractures. Alteration and mineralization processes occurred in two periods (J and E), which are hosted in two different ore‐bearing sequences (Jinshuikou Group and Elashan Formation respectively). These two periods can be subdivided into Stages J‐I, J‐II, J‐III, and J‐IV and Stages E‐I, E‐II, and E‐III. Fluid inclusions in the ore‐bearing quartz and calcite comprise predominantly of vapour‐rich two‐phase (vapour + liquid) type and minor pure‐liquid type. Fluid inclusions in Stage J quartz and calcite homogenized mainly at 270–330°C, 250–310°C, 250–290°C, and 170–250°C, with salinities of 5–6 wt.%, 4–6 wt.%, 4–5 wt.%, and 3–5 wt.% NaCl equiv., respectively. Fluid inclusions in Stage E quartz homogenized mainly at 210–250°C, 170–230°C, and 150–170°C, with salinities of 6–7 wt.%, 4–5 wt.%, and 1–2 wt.% NaCl equiv., respectively. δ 13 C and δ 18 O SMOW values of the calcite samples are of −6.0‰ to −2.8‰ and 5.2‰ to 20.5‰, respectively. The results indicate a mantle‐derived magmatic source and low temperature alteration. and δD V–SMOW values of the quartz and calcite from the Jinshuikou Group vary from −1.9‰ to 9.0‰ and −116.5‰ to −69.3‰, respectively, whilst those of δ 18 O H 2 Oquartz from the Elashan Formation vary from −11.6‰ to 7.8‰ and −99.7‰ to −86.7‰. The results indicate that the ore‐forming fluids are of medium‐low temperatures and low salinities and were originated from magmatic water to meteoric water. Sulfide δ 34 S V–CDT values of the Jinshuikou Group and the Elashan Formation are of −6.1‰ to 0.8‰ and −3.4‰ to 0.9‰, respectively, suggesting a mantle‐derived magmatic source. Sulfide 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb ratios of the J‐period ores (18.190–18.622, 15.598–15.725, and 38.383–39.103, respectively) and E‐period ores (18.408–18.420, 15.746–15.758, and 38.879–38.895, respectively) are similar to those of granodiorite porphyry and Elashan Formation (sub)‐volcanic rocks, respectively, indicative of genetic connections between them. Integrating new evidence from fluid inclusions and C–H–O–S–Pb isotopes, we suggest that (a) mantle‐derived materials and their mixture with crust‐derived ones were critical to the ore formation, (b) the Nagengkangqieergou deposit was formed in a magmatic system and a (sub)‐volcanic system under the syn‐ and post‐collisional tectonics of the East Kunlun Orogen, respectively.