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

The Caixiashan Zn-Pb deposit is the largest carbonate-hosted Zn-Pb deposit in Eastern Tianshan, NW China. The deposit comprises the No. I, II, III and IV ore zones with a proven reserve of 131 Mt ore at 3.95% Pb + Zn. The orebodies generally occur as irregular lenses and pods in altered carbonate rocks of the Mesoproterozoic Kawabulake Group near faults. The mineralization is characterized by massive, disseminated and vein/veinlet sulfides including pyrite, pyrrhotite, sphalerite and galena with minor arsenopyrite and chalcopyrite, as well as sulfosalt minerals of Ag, As, Sb and Pb. The mineralization is associated with tremolite, chlorite, silica and carbonate alterations. Rb-Sr isotopic dating of sphalerite and pyrrhotite yields an isochron age of 337.2 +/- 5.7 Ma, which is interpreted as the mineralization age. Zircon U-Pb dating reveals that stocks of diorite, quartz diorite, K-feldspar granite and monzonitic granite that occur in the deposit area were emplaced at 348.2 +/- 3.7 Ma, 351.9 +/- 3.5 Ma, 333.6 +/- 3.6 Ma and 330.0 +/- 3.6 Ma, respectively. These age data suggest that the mineralization is coeval with Carboniferous granitic magmatism. The delta S-34(v-CDT) values of sulfides (excluding the syn-sedimentary pyrite) range from 2.42 parts per thousand to 19.1 parts per thousand, suggesting that the reduced sulfur was mainly derived from thermal sulfate reduction (TSR) of seawater-derived sulfates in the marine sedimentary basement rocks and minor contribution by replacement of syn-sedimentary pyrite, as well as a possible input of magmatic sulfur. The Pb isotopic compositions of sulfides, with Pb-206/Pb-204 ranging from 17.074 to 17.361, Pb-207/Pb-204 from 15.422 to 15.614, and Pb-208/Pb-204 from 36.685 to 37.303, partly overlap with those of basement rocks of the Mesoproterozoic Kawabulake Group and the Carboniferous intrusions, suggesting that the ore metals were derived from both the basement and Carboniferous magmatism. On the basis of the spatiotemporal relations between hydrothermal alterations/mineralization and Carboniferous magmatism, we conclude that the Caixiashan Zn-Pb deposit is a high-temperature carbonate replacement deposit related to concealed granitic intrusions, which were formed in an arc environment related to the subduction of the South Tianshan Ocean plate beneath the Central Tianshan massif during the Carboniferous time. The results of this study indicate that there is a great potential of finding more large-scale Zn-Pb deposits similar to Caixiashan in Eastern Tianshan.