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

摘要

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 minerali zation 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 periph eral 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 δ~(34)S 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 iso topes indicate that the hydrothermal fluids were derived main ly 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 ~(206)Pb/~(238)U 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.