Geology, mineralogy, and geochemistry of fault-controlled hydrothermal Cu-Au mineralization in the Shanmen Volcanic Basin, SE China

摘要

The Yukeng-Banling deposit is a typical fault-controlled hydrothermal Cu-Au deposit in the Shanmen Volcanic Basin (SVB), SE China. Ore bodies commonly occur as lodes, lenses and disconnected pods dipping SW with vertical zonation of ore minerals. Ore-related hydrothermal alteration is well developed on both sides of the veins, dominated by silicification, sericitization, chloritization and argillation with a banded alteration zonation. The mineralization can be divided into three stages (stages I, II and III). Native gold is present as veinlets in fractures of fine-grained pyrite from stage II.Zircon U-Pb and Rb-Sr isochron ages indicate that the Cu-Au mineralization is coeval with the Caomen alkaline granite and Xiaokeng quartz-diorite, both emplaced at ca. 102 Ma. Microthermometric measurements of fluid inclusions in quartz and sphalerite from stage II veins indicate that the Yukeng-Banling deposit is an epithermal deposit. Six ore-related quartz grains have δD_(H2O) values of - 69 to - 43‰, and δ~(18)O_(H2O) values calculated using total homogenization temperatures that range from -2.0 to 0.7‰. All samples plot in an area between the magmatic field and the meteoric line, suggesting that the ore-forming fluids are derived from a mixed source of magmatic and meteoric waters. δ~(34)S values for eight pyrite separates range from -2.1 to +4.1‰ with an average of 4-1.7‰, and δ~(34)S values for galena and sphalerite are 2.3‰ and 2.2‰, similar to magmatic sulfur. Four alkaline granite samples have Pb isotopic ratios (~(206)Pb/~(204)Pb)_t = 18.175-18.411, (~(207)Pb/~(204)Pb)_t = 15.652-15.672 and (~(208)Pb/~(204)Pb)_t = 38.343-38.800. Three quartz-diorite samples have ratios (~(206)Pb/~(204)Pb)_t, (~(207)Pb/~(204)Pb)_t and (~(208)Pb/~(204)Pb)_t of 18.277-18.451, 15.654-15.693 and 38.673-38.846, respectively. These age-calculated lead isotopic data for alkaline granite are similar to those for the analyzed sulfides. Co/Ni ratios for stage II pyrites range from 1.42 to 5.10, indicating that the Yukeng-Banling deposit records the past involvement of magmatic hydrothermal fluids. The isotope data, together with geological, mineralogical and geochronological evidence, favor a primary magmatic source for sulfur and metals in the ore fluids. Mixing of the Cu- and Au-rich fluids with meteoric water led to precipitation of the Cu-Au veins along NW-trending faults. The Yukeng-Banling deposit, the contemporaneous Caomen alkaline granite and Xiaokeng quartz-diorite in the SVB formed under an extensional setting, due to high-angle subduction of the paleo-Pacific plate. The extensional setting facilitated the formation of Cu- and Au-rich magmas which was derived from enriched mantle and lower crust