Genesis of the Ancun epithermal gold deposit, southeast China: Evidence from fluid inclusion and stable isotope data

The Ancun gold deposit is located in the Dehua-Youxi-Yongtai ore region of Fujian Province, southeast China. The orebodies are mainly hosted in early Paleozoic quartz diorite and structurally controlled alteration zones along NW- and NE-trending faults. Based on the characteristics of mineralization, alteration mineral assemblages, fluid inclusions, and H-O-S isotope data, the Ancun gold deposit can be classified as an intermediate-sulfidation epithermal deposit The primary metallic minerals are gold, electrum, pyrite, sphalerite, chalcopyrite, and galena, and the principal gangue minerals are quartz, calcite, sericite, and illite. The main types of wall rock alteration include sericitization, argillization, silicification, beresitization, and carbonation. Fluid inclusion studies were carried out on quartz and sphalerite from Au-bearing silicified massive ores and auriferous poly metallic-sulfide-bearing quartz veins from the main ore stage. Microthermometric measurements of fluid inclusions yielded homogenization temperatures (T-h) of 152-255 degrees C and salinities of 0.9-5.3 wt% NaCl equivalent in quartz, and T-h of 175-236 degrees C and salinities of 1.6-5.0 wt% NaCl equivalent in sphalerite. The low to moderate temperatures and salinities imply that the ore-forming fluids formed in epithermal environments. Gold deposition at Ancun is inferred to have been caused mainly by mixing of magmatic and diluted meteoric waters. The H and O isotopic compositions (delta D-H2O = -76.4 parts per thousand to -61.2 parts per thousand; delta O-18(H2O) = -6.1 parts per thousand to 4.0 parts per thousand) of inclusion water in quartz imply that ore-forming fluids were derived mainly from magmatic sources. Sulfur isotopic values of sulfide minerals vary over a narrow range from -3.9 parts per thousand to -0.6 parts per thousand, suggesting that S was derived from a homogeneous magmatic source. Based on the close temporal and spatial relationship between intermediate-sulfidation epithermal deposits and porphyry deposits, along with the alteration and mineralization characteristics of the Ancun gold deposit, potential exists in this region to drill into deeper-seated porphyry deposits.