New constraints on the origin of the Maozu carbonate-hosted epigenetic Zn-Pb deposit in NE Yunnan Province, SW China

Abstract

Micro-scale isotopic compositions determined by micro-beam in situ analysis techniques are expected to provide new information for the understanding of ore genesis. Here we use the Maozu (∼20 Mt of sulfide ore, grading 7.25 wt% Zn and 4.15 wt% Pb) Zn-Pb deposit in the Upper Yangtze Zn-Pb metallogenic province as a case study, utilizing NanoSIMS in situ S, femtosecond LA-MC-ICPMS in situ Pb and conventional bulk Sr isotopes, as well as field data, aimed to reveal its origin and ore formation process. NanoSIMS in situ δ34S values of sulfide minerals from the Maozu deposit have a wider range (+5.9–+20.7‰) than that of bulk δ34S values (+9.9–+19.9‰). Such S isotope signatures reflect the enrichment of heavy S isotopes in ore-forming fluids, and further indicate the source of S is multiple reservoirs (e.g. marine evaporites, coeval seawater, and metamorphic rocks/fluids). Pb isotopic ratios of galena determined by LA-MC-ICPMS in situ techniques have a narrower range (206Pb/204Pb = 18.20–18.21, 207Pb/204Pb = 15.69–15.71 and 208Pb/204Pb = 38.40–38.43) than that of bulk Pb isotopic ratios (206Pb/204Pb = 18.13–18.38, 207Pb/204Pb = 15.64–15.69 and 208Pb/204Pb = 38.22–38.58). This implies that in situ Pb isotopes can truly reflect the source of Pb, which is due to impurities enclosed by sulfide crystals and pollutants introduced by chemical analysis could significantly affect the quality of the bulk Pb isotopic data. Compared with previously reported Pb isotopic data of potential source rocks (i.e. late Permian Emeishan basalts, Paleozoic ore-bearing sedimentary rocks and Proterozoic basement metamorphic rocks), the in situ Pb isotopic data are significantly different from those of basalts, but are similar to those of both basements and sediments. This means that metal-rich fluids were mainly derived from and/or flowed through basements and sediments. Sphalerite separates have initial Sr isotopic ratios (0.711–0.719) that are distinctly different from those of ore formation age-corrected basalts (87Sr/86Sr200 Ma = 0.704–0.708) and basements (87Sr/86Sr200 Ma = 0.724–0.729), whereas partly overlap with those of age-corrected sediments (87Sr/86Sr200 Ma = 0.707–0.714). Such Sr isotope signatures suggest that hydrothermal fluids were predominantly derived from and/or flowed through basements and sediments with a certain influence from basalts. Hence, the Maozu deposit has complex source rocks that are related to the local geological setting. The mixing of metal-rich fluids (related to basements and sediments, and even basalts) and S-bearing fluids (associated with multiple S reservoirs) plays a key role in the formation of sulfide ores. Therefore, the Maozu deposit, formed during Early Mesozoic (∼196 Ma), is an epigenetic Zn-Pb deposit that hosts in carbonate rocks, occurs along fault-fold structures, and has a mixed source of mineralizing elements and associated fluids.