Geology and in-situ S-Pb isotopes of the Luoboshan carbonate-hosted Pb-Zn deposit in Yunnan Province, SW China

Abstract

The Luoboshan Pb-Zn deposit (0.2 Mt Pb + Zn metal reserves, @ 0.94 % Pb and 5.33 % Zn) is located in the Lanping-Simao Basin in the southern part of the Sanjiang Tethys metallogenic domain in Yunnan Province, SW China. The deposit comprises three ore bodies hosted in the lower Permian bioclastic and micritic limestone. Ore body I is stratiform (mineralization is small in scale and ores have been mined out), with average grades of 0.02–1.44 % Pb and 1.5–14.88 % Zn. Ore bodies II and III are lens-like, the average grades of them are 0.10–5.07 % Pb and 1.68–12.39 % Zn, and 0.10–1.71 % Pb and 1.14–11.79 % Zn, respectively. Two hydrothermal stages were identified in the Luoboshan deposit: Pyrite-I-sphalerite-galena-calcite (stage I) and Pyrite-II-calcite (stage II). The in-situ δ34S values of pyrite and sphalerite reveals a medium range of −8.80 ‰ to +10.1 ‰. Furthermore, Py-I and sphalerite exhibit low δ34S values from −8.80 ‰ to −3.04 ‰, but Py-II displays high δ34S values (+2.37 ‰ to +10.0 ‰). These results suggest distinct formation mechanism for reduced sulfur during different stages of Pb-Zn mineralization. A reasonable explanation is that a transition from bacterial sulfate reduction (BSR) in the early stage to thermochemical sulfate reduction (TSR) in the late stage as the likely mechanism for the formation of reduced sulfur. In addition, the in-situ Pb isotopes exhibit a narrow range (206Pb/204Pb = 18.660–18.672, 207Pb/204Pb = 15.665–15.677 and 208Pb/204Pb = 38.940–38.980), indicating that the metals originated from either a single source or a uniform mixture of different sources. Comparison of geological charateristics and isotopic geochemical data (S and Pb) indicates that the basin sedimentary strata (wall rocks) is the only and most probable source. Hence, we propose that the formation of the Luoboshan deposit involved biogenic in-situ acid generation of marine sulfates, which subsequently mixed with Pb-and-Zn-rich hydrothermal fluids, initiating TSR and resulting in Pb-Zn mineralization.