Genesis of the Lehong large zinc–lead deposit in northeastern Yunnan, China: Evidences from geological characteristics and C–H–O–S–Pb isotopic compositions

Abstract

The Lehong large Zn–Pb deposit is located in the Sichuan–Yunnan–Guizhou (SYG) Zn–Pb–Ag (Ge) polymetallic metallogenic region on the southwestern margin of the Yangtze Block, China. The accumulated Zn–Pb resources account for approximately 2.4 million tons, with the average ore grade of Zn and Pb as 20 wt%, and the Ag grade as 148 × 10-6. The orebodies occur primarily in the fine-grained siliceous dolomite of the Ediacaran Denging Formation and are distributed along the NW-trending F2 fault (Lehong Fault) in the form of lenses, veins, and irregularities. The ore is predominantly massive, disseminated, stockwork, veined and bedded, and was formed during the Late Indosinian (ca. 200.9 Ma) orogeny. The associated minerals include pyrite, sphalerite, galena, dolomite, and calcite with granular, metasomatic, dissolved, filled, and included textures. According to the mineral assemblage distribution and the wall-rock alteration, mineralization zones can be subdivided into the (I) hydrothermal dolomite–calcite–pyrite (minor) zone, (II) dolomite–pyrite–quartz–barite (minor) zone, and (III) strongly pyrite–galena–sphalerite–hydrothermal dolomite zone. The hydrothermal period can be subdivided into four stages: barite, dolomite–pyrite–quartz, sphalerite–galena–pyrite, and calcite. It was revealed that the C in the ore-forming fluids originated from marine carbonate rocks and basement rocks. The original ore-forming fluids could have predominantly originated basement rocks, as indicated by the C–H–O isotope data from the gangue and ore minerals, with values of δ13CPDB from –6.4‰ to +0.5‰ and δ18OSMOW from +12.2‰ to +19.8‰, calculated values of δ18Ofluid from +2.2‰ to +10.0‰ and δ18OH2O from +1.1‰ to +18.1‰, and δDH2O from –47.7‰ to –114.6‰. It was also determined that the δ34S values of sulfides were fairly scattered, varying from 1.6‰ to 28.1‰, with sulfide and sulfate Pb isotope compositions of 208Pb/204Pb from 38.109 to 40.154, 207Pb/204Pb from 15.643 to 15.954 and 206Pb/204Pb from 18.158 to 20.567. These results indicate that the S in the ore-forming fluid may originate from marine sulfate rocks and could be the product of thermochemical sulfate reduction (TSR), with the metal elements in the ore-forming fluid having a mixed source of basement strata and sedimentary caprocks. In summary, the large Lehong deposit is a Mississippi Valley-type-like, also be called Huize-style, epigenetic hydrothermal carbonate-hosted Zn–Pb deposit, and the NW-trending tensile or tenso-shear Lehong Fault (F2) controls the spatial distribution of the orebodies and mineralization-alteration zones in the Lehong deposit.