Metamorphic fluid superimposition of the Changba–Lijiagou Pb–Zn deposit, West Qinling Orogen, central China

Abstract

AbstractSeveral large to giant Pb–Zn deposits in the West Qinling Orogen in central China are argued to be of SEDEX (sedimentary exhalative) type or of epigenetic hydrothermal type. Additionally, the nature of the mineralizing fluids is poorly known. Our observations suggest that early stage primary marine sedimentary mineralization is characterized by laminated or disseminated fine-grained massive sulphide ores, and late stage metamorphic superimposition is represented by coarser equigranular annealed textures and the disruption of thinly laminated structures. Three coexisting types of fluid inclusions were recognized: H2O–NaCl (type I); H2O–NaCl–CH4–CO2 (type II); and CH4–CO2 (type III). The coexisting type I and II inclusions show similar homogenization temperature values but different salinities, indicating that fluid immiscibility occurred. Formation pressures calculated using type III inclusions are high (72.5–174.5 MPa). The lead isotopes of the sulphides and calcites show a narrow range. The primary sedimentary ore textures plus the similar lead isotopes between the ores and the wall rocks suggest a SEDEX origin, but the annealed recrystallization textures, the immiscible carbonic fluid inclusion assemblages and higher formation pressures suggest a strong late-stage metamorphic superimposition on the original SEDEX-type ores.