Zinc and lead isotope signatures of the Zhaxikang Pb--Zn deposit, South Tibet: Implications for the source of the ore-forming metals

Stable Zn isotopes may be applied to trace the source of ore-forming metals in various types of Pb-Zn deposits. To test this application, Zn and Pb isotope systematics for sulfides and associated basement rocks as well as Fe-Mn carbonates (gangue) from the Zhaxikang Pb-Zn deposit in South Tibet have been analyzed. The basement in this region includes metamorphosed mafic to felsic rocks (dolerite, quartz diorite, rhyolite porphyry, pyroclastics and porphyritic monzogranite). These rocks have similar delta Zn-66 values of 0.33 to 037 parts per thousand, with an average value of 036 +/- 0.03 parts per thousand (2 sigma), except for the more evolved porphyritic monzogranite that has a heavier value of 0.49 parts per thousand. Fe-Mn carbonates are present as hydrothermal veins and were probably precipitated from magmatic fluids. They have an average delta Zn-66 value of 0.27 +/- 0.05 parts per thousand, which is slightly lighter than the basement rocks, possibly representing delta Zn-66 isotopic compositions of the hydrothermal fluids. Sphalerite and galena have similar Zn isotopic compositions with delta Zn-66 ranging from 0.03 to 0.26 parts per thousand and 0.21 to 0.28 parts per thousand, respectively. Considering the Zn isotope fractionation factor between sphalerite and fluids of -0.2 parts per thousand at similar to 300 degrees C as reported in literature, hydrothermal fluids from which these sulfides precipitated will have delta Zn-66 values of ca. 039 +/- 0.10 parts per thousand, which are consistent with the values of basement rocks and the Fe-Mn carbonates. This similarity supports a magmatic-hydrothermal origin of the Zhaxikang Pb-Zn deposit. Both Pb and S isotopes in these sphalerite and galena show large variations and are consistent with being derived from a mixture of basement and sedimentary rocks in various proportions. Zn isotopic compositions of the sulfides significantly extend the range of regional basement rocks, suggesting that sedimentary rocks (e.g., shales) are also a significant source of Zn. However, the Zn isotopic compositions of sphalerite and galena differ from those of marine carbonates and those of typical SEDEX-type deposits (e.g. Kelley et al., 2009), confirming a magmatic-hydrothermal model. Combined with regional geological observations and the age constraints of similar to 20 Ma (Zheng et al., 2012, 2014), the results of our investigation indicate that the Zhaxikang Pb-Zn deposit is most likely a magmatic-hydrothermal deposit. (C) 2016 Elsevier B.V. All rights reserved.