Geological and isotopic evidence for magmatic-hydrothermal origin of the Ag–Pb–Zn deposits in the Lengshuikeng District, east-central China

Abstract

The Lengshuikeng ore district in east-central China has an ore reserve of ∼43 Mt with an average grade of 204.53 g/t Ag and 4.63% Pb + Zn. Based on contrasting geological characteristics, the mineralization in the Lengshuikeng ore district can be divided into porphyry-hosted and stratabound types. The porphyry-hosted mineralization is distributed in and around the Lengshuikeng granite porphyry and shows a distinct alteration zoning including minor chloritization and sericitization in the proximal zone; sericitization, silicification, and carbonatization in the peripheral zone; and sericitization and carbonatization in the distal zone. The stratabound mineralization occurs in volcano-sedimentary rocks at ∼100–400 m depth without obvious zoning of alterations and ore minerals. Porphyry-hosted and stratabound mineralization are both characterized by early-stage pyrite–chalcopyrite–sphalerite, middle-stage acanthite–native silver–galena–sphalerite, and late-stage pyrite–quartz–calcite. The δ³⁴S values of pyrite, sphalerite, and galena in the ores range from −3.8 to +6.9‰ with an average of +2.0‰. The C–O isotope values of siderite, calcite, and dolomite range from −7.2 to −1.5‰ with an average of −4.4‰ (V-PDB) and from +10.9 to +19.5‰ with an average of +14.8‰ (V-SMOW), respectively. Hydrogen, oxygen, and carbon isotopes indicate that the hydrothermal fluids were derived mainly from meteoric water, with addition of minor amounts of magmatic water. Geochronology employing LA–ICP–MS analyses of zircons from a quartz syenite porphyry yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 136.3 ± 0.8 Ma considered as the emplacement age of the porphyry. Rb–Sr dating of sphalerite from the main ore stage yielded an age of 126.9 ± 7.1 Ma, marking the time of mineralization. The Lengshuikeng mineralization classifies as an epithermal Ag–Pb–Zn deposit.