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

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 delta D-H2O values of -69 to -43 parts per thousand, and delta O-18(H2O) values calculated using total homogenization temperatures that range from -2.0 to 0.7 parts per thousand. 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. delta S-34 values for eight pyrite separates range from -2.1 to +4.1 parts per thousand with an average of + 1.7 parts per thousand, and delta S-34 values for galena and sphalerite are 2.3 parts per thousand and 2.2 parts per thousand, similar to magmatic sulfur. Four alkaline granite samples have Pb isotopic ratios (Pb-206/Pb-204)(t) = 18.175-18.411, (Pb-207/Pb-204)(t) = 15.652-15.672 and (Pb-208/Pb-204)(t) = 38.343-38.800. Three quartz-diorite samples have ratios (Pb-206/Pb-204)t, (Pb-207/Pb-204)(t) and (Pb-208/Pb-204)(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. (C) 2014 Elsevier B.V. All rights reserved.