Metallogeny of the Late Jurassic Qiucun epithermal gold deposit in southeastern China: Constraints from geochronology, fluid inclusions, and H-O-C-Pb isotopes

Abstract

On the basis of zircon and apatite U-Pb dating, H-O-C-Pb isotopes, and fluid inclusion data, a metallogenic model is established for the Qiucun gold deposit (gold resource > 10 t, average grade: 4.9 g/t) in the Dehua goldfield in the Mesozoic Coastal Volcanic Belt (southeastern China). Three hydrothermal stages can be distinguished within the Late Jurassic volcanic host rocks: Stage I, dominated by quartz-pyrite, Stage II with a quartz-sulfide assemblage, and Stage III reflected by quartz-calcite. The ore-hosting volcanic rocks were formed at 170.2 ± 0.9 Ma. Distinctly younger U-Pb ages of 162.5 ± 1.0 Ma and 160.5 ± 6.0 Ma obtained on syn-ore hydrothermal zircon and apatite, respectively, clearly indicate that gold mineralization postdates the volcanic host rocks. Fluid inclusion data reveal that the ores were precipitated from low-temperature (275 to 170 °C), low-pressure (<55 bar), and low-salinity (<5 wt% NaCl equiv.) solutions, with a systematic decrease in temperature and pressure from stages I to III. The δ34SV-CDT ratios for gold ores vary between −3.2 and 0.6 ‰. Sulfide separates yielded 206Pb/204Pb ratios of 18.368 to 18.504, 207Pb/204Pb ratios of 15.685 to 15.718, and 208Pb/204Pb ratios of 38.792 to 39.063. These data suggest S and Pb derivation from the host volcanic rocks and perhaps deeper-seated crustal materials. Calcite from Stage III has δ13CV-PDB values of −2.9 to −2.7 ‰ and δ18OV-SMOW values of 8.2 to 10.8 ‰. Calculated δ18Ofluid (−7.6 to −2.7‰) and δDfluid (−119 to −49‰) values document a largely meteoric nature of the mineralizing fluid. All data together point at a Late Jurassic low-sulfidation epithermal gold mineralization with metal remobilization from host volcanic rocks and basement.