Mercury and sulfur isotopic composition of sulfides from sediment-hosted lead-zinc deposits in Lanping basin, Southwestern China

Abstract The Lanping basin is an important Pb-Zn-Cu-Ag polymetallic province in southwestern (SW) China. Mineralization types in the basin include mainly MVT Pb-Zn deposits, vein-type Cu deposits, and minor quartz vein-type Pb-Zn deposits. In this study, we analyzed the total Hg (THg) content and Hg-S isotopic compositions of sulfides from two types of Pb-Zn deposits in the basin, and investigate the Hg isotope fractionation mechanism and its major controlling factors in the Pb-Zn ore-forming process. At the Shangnuluo quartz vein-type Pb-Zn-Sb deposit, sulfides (especially sphalerite) show a wide δ202Hg range (−0.40 to 2.71‰), suggesting that Hg0 evaporation from hydrothermal fluids occurred during the ore formation. Negative THg vs. δ202Hg correlation in the sphalerites suggests a mixing of Hg from the vapor phase (low δ202Hg, high THg) and the residual aqueous phase (high δ202Hg, low THg). The negative ∆199Hg (−0.42 to −0.08‰) suggests that the Hg was sourced from the metamorphic basement, whereas the negative δ34S values (−13.0 to −2.7‰) suggests that the sulfur was mainly sourced from sedimentary sequences, possibly related to the organic matter decomposition and thermochemical sulfate reduction (TSR). For the Liziping and Fulongchang MVT Pb-Zn deposits, sulfides have relatively narrow δ202Hg range (−1.46 to 0.25‰), suggesting limited Hg0 evaporation during the mineralization. Lower δ202Hg values were observed in the early-stage sulfides from these two deposits, probably due to the preferential precipitation of lighter Hg isotopes in these early-stage sulfides. Sulfides from both deposits have negative ∆199Hg values (−0.24 to 0.01‰), suggesting that the Hg was also derived from the metamorphic basement. Sulfide δ34S values from the two deposits are positive, which also suggests that the sulfur was sourced from sedimentary sequences, but mainly related to TSR under relatively low-temperatures. Sulfides from different Pb-Zn deposit types in the Sanjiang Tethyans Metallogenic Belt fall into distinct fields in the δ34S vs. δ202Hg (or ∆199Hg) diagrams, indicating that the integration of S and Hg isotopes is useful to discriminate different types of Pb-Zn deposits.