Genesis of the Aqishan skarn Zn-Pb deposit in the Eastern Tianshan, NW China: Constraints from geology, geochronology and Hf-S-Pb isotopic geochemistry

The Aqishan deposit, located in the Eastern Tianshan, Xinjiang, China, is a newly discovered large-scale Zn-Pb deposit with a proven reserve of 1.94 Mt Pb and Zn metal at an average grade of 1.05%. The Zn-Pb orebodies are tabular and lentoid in shape, developed within skarnized carbonates and calcareous volcanoclastic rocks of the Yamansu Formation. The skarn mineral assemblages are characterized by abundant garnet, epidote, chlorite, actinolite, calcite and quartz, with minor amounts of pyroxene. Metallic minerals, comprising sphalerite, galena, chalcopyrite, pyrite, pyrrhotite and arsenopyrite, mainly occur as disseminations and irregular patches within the skarn. Re-Os dating of five syn-ore stage pyrite samples yielded an isochron age of 301 +/- 13 Ma, which is in agreement with the 306 +/- 2.8 Ma age obtained from the granodiorite porphyry occurring at southeast part of the Aqishan Zn-Pb deposit. The epsilon(Hf)(t) values of zircon from the granodiorite porphyry vary from + 9.62 to + 14.69, suggesting a juvenile crust source. The delta S-34(V-CDT) values of 22 sulfide samples range from - 11.6 parts per thousand to - 2.76 parts per thousand, with an average of - 5.08 parts per thousand. The Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 ratios of 9 sulfide samples range from 18.102 to 18.336, 15.539 to 15.655, and 37.929 to 38.479, respectively, which are similar to the Carboniferous grantoids of the Aqishan area and partly overlap with the volcanoclastic rocks of the Yamansu Formation. Based on these data, we propose that the Aqishan deposit formed from magmatic-hydrothermal fluids that emanated from the granodiorite porphyry. Much of the metals were derived from the magmatic intrusions with minor contributions from the volcanoclastic rocks, whereas the sulfur may have been mainly extracted from the country rocks with slight contributions from a magmatic source. The close genetic relationships between the Carboniferous magmatism and skarn type Zn-Pb mineralization in the Aqishan area as well as other deposits in the region suggests a great potential of finding more analogous deposits in the Eastern Tianshan, where carbonates or calcareous volcanoclastic rocks and plutons that related to the subduction of the Northern Tianshan Oceanic slab beneath the Yili-Central Tianshan Terrane during Late Paleozoic are well-developed.