Geology, fluid inclusion and H-O-S-Pb isotope study of the Ganhegou epithermal gold deposit in Yanbian, NE China: Implications for ore genesis and mineralization process

Abstract

• Ganhegou deposite is a typical low-sulfidation epithermal gold deposit. • The ore-forming fluids were derived from intermediate-acid magmatic water. • The ore-forming materials were derived from Neoproterozoic juvenile lower crust . • Twice boiling are the main mechanism for the precipitation of precious elements. The Yanbian mining district is located at the eastern end of the Central Asian orogenic belt and northeast of the western Pacific continental margin metallogenic belt. It is a porphyry-epithermal Cu-Au mining district related to Mesozoic continental volcanic eruptions and hypabyssal magmatism. To explore the genesis and mineralization process of epithermal gold deposits, we conducted a systematic study of the geology, fluid inclusions and H-O-S-Pb isotopes of the Ganhegou gold deposit, which is expected to be revealed as a medium-sized deposit in the latest exploration. We found the following: (1) The deposit occurs in Early Cretaceous continental andesites of the Jingouling Formation and granodiorite porphyry. The ore belts are controlled by NW-trending volcanic faults. The mineralization process can be divided into the quartz-hematite-arsenopyrite-pyrite stage (I), grey-white quartz-pyrite stage (II), smoky quartz-polymetallic sulfide stage (III), milky quartz-adularia stage (IV) and carbonate stage (V). Gold precipitated in stage III. (2) Vapour-rich-, liquid-rich-, pure-liquid- and pure-vapour-type FIs developed in quartz in stages II, III and IV, and the average homogenization temperatures in these stages are 325 ℃, 273 ℃ and 229 ℃, respectively. The ore-forming fluids belonged to the H 2 O-NaCl ± CO 2 system. (3) The δ 18 O H2O and δD H2O values of stage II-IV range from −9.9 to −5.2‰ and −121.9 to −119.8‰, respectively, indicating that the ore-forming fluids were driven by intermediate-felsic magmatic water and mixed with meteoric water during mineralization. (4) The δ 34 S values of pyrite in stages II and III range from −5.8 to −3.4‰, whereas the values of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb are 15.529–15.551, 18.239–18.257 and 38.075–38.153, respectively, implying that the ore-forming materials were derived from Neoproterozoic lower crust. These characteristics indicate that Ganhegou is a low-sulfidation epithermal gold deposit. Mineralization occurred via several main processes: (i) The ore-forming fluids migrated along volcanic faults to suitable positions driven by intermediate-felsic magmatic water. (ii) In stage I, the quartz-arsenopyrite-hematite assemblage formed under relatively high oxidation conditions. (iii) In stage II, the pyrite-hessite-galena-quartz veins formed in the near-neutral environment, which was slightly reduced relative to that of stage I due to the fluid boiling. (iv) In stage III, the gold-bearing quartz veins formed due to the fluid boiling and mixing. (v) In stage IV, the precipitation of quartz-adularia-pyrite veins occurred under near-neutral conditions (pH≈5). (iv) Small-scale carbonate veinlets formed in stage V, signalling the end of mineralization.