Hydrothermal evolution and ore genesis of the Zhaiping Ag-Pb-Zn deposit in Fujian Province of Southeast China: Evidence from stable isotopes (H, O, C, S) and fluid inclusions

Abstract

The Zhaiping Ag-Pb-Zn deposit (204 t Ag, average grade: 221.8 g/t) is located in the coastal area of the Cathaysia Block, Southeast China. Vein-type Ag-Pb-Zn orebodies are mainly hosted in the early Cretaceous volcanic and subvolcanic rocks, and structurally controlled by a group of NW-trending extensional faults. Ore-related hydrothermal alteration is well developed on both sides of the veins, dominated by silicic, phyllic, propylitic, and carbonate alteration. Mineralization can be divided into three stages: (1) the pre-ore quartz-pyrite stage, (2) syn-ore quartz-Ag-base metal stage, and (3) post-ore quartz-calcite stage. Microthermometric measurements of fluid inclusion assemblages in quartz, fluorite, sphalerite, and calcite from various hydrothermal stages reveal that from the pre-ore, syn-ore to post-ore stages, the homogenization temperatures range from 365 to 319 °C, from 317 to 222 °C, and from 233 to 172 °C, respectively. Fluid salinities range from 1.7 to 10.8 wt% NaCl equivalent. The microthermometric data indicate that the fluid cooling and fluid-rock interaction are two important mechanisms for ore precipitation. The δ34SV-CDT values of sulfide minerals (pyrite, sphalerite, galena, pyrrhotite and tetrahedrite) range from −1.9 to 6.2‰, and mostly between 1.0 and 4.5‰, consistent with a deep-seated magmatic sulfur source. Hydrogen and oxygen isotopic compositions of quartz indicate a primarily magmatic origin for the ore-forming fluids, and the proportion of meteoric water increased during the ore-forming processes. Post-ore stage calcite has δ13CV-PDB values of −2.8 to −0.8‰ and δ18OV-SMOW values of 4.2–6.4‰, corresponding to calculated values of fluids of −3.2 to −1.1‰, and −4.9 to −2.8‰, respectively. The carbon isotopes indicate a magmatic source for carbon but the oxygen isotopes indicate a significant contribution of meteoric water during calcite precipitation. We therefore proposed that the Zhaiping deposit is a typical mesothermal deposit that formed in an extensional environment related to the early Cretaceous subduction of the paleo-Pacific plate.