Epithermal deposits in South China: Geology, geochemistry, geochronology and tectonic setting

Abstract

South China Block (SCB) is the broad area including the Yangtze Craton in the northwest and Huanan Orogen in the southeast. It is an important epithermal metallogenic province in China, containing at least 1 high-sulfidation (HS) and 42 low-sulfidation (LS) Au-Ag±Cu±Pb-Zn±Sb epithermal deposits. Porphyry-type mineralization was recognized in four of the LS deposits, and thus they were regarded as LS–P type. These 43 deposits are mainly located in: (1) the Lower Yangtze River Belt and (2) the Northeastern Jiangnan Orogenic Belt in the Yangtze Craton, (3) the Wuyi-Yunkai Orogenic Belt and (4) the Southeast Coastal Volcanic Belt in the Huanan Orogen. They are mostly located in Mesozoic volcanic basins, especially where the regional faults and their subsidiaries occurred. The host rocks include Jurassic–Cretaceous volcanic-sedimentary rocks, coeval or slightly older subvolcanic, granitoids and breccias, and metamorphic basement rocks. The alteration of the HS epithermal deposit (Zijinshan Cu-Au) zoned from silicic (vuggy quartz), through alunite, to dickite and phyllic alteration zones, from the ore veins outwards. The alteration of the LS deposits is zoned from adularia-chalcedony-bladed calcite (or quartz pseudomorphs after bladed calcite) in ore veins to distal illite-sericite-chlorite-kaolinite assemblages. For those LS–P systems, besides the dominated LS alteration assemblages, phyllic and potassium silicate alteration related to porphyry mineralization were identified. Acid leaching textures and vein, stockwork and breccia structures are common in HS deposit, while the LS epithermal deposits are characterized by open-space filling, crustifications, colloform banding and comb structures. The ore-forming fluids are low-temperature, low-salinity meteoric water-dominated in most epithermal deposits in SCB, with variable input of magmatic water. The ore components were derived from both the deep magma and host rocks, and transported upwards or laterally and precipitated in the fracture systems by fluid boiling, mixing and cooling. Most of the epithermal deposits are formed at depth of <1.5km and <300°C, with few exceptions containing porphyry-type mineralization, such as the Zhilingtou, Yinshan and Longtoushan deposits. Deep drilling is suggested in these deposits as more epithermal and/or porphyry mineralization could be expected. The mineral systems were formed in Early Yanshanian (180–130Ma) and Late Yanshanian (120–90Ma) periods. The Early Yanshanian epithermal ore systems are mainly located in a series of E–W-trending metallogenic belts to the west of the Lishui–Haifeng Fault, which were formed in a syn- or post-collision tectonic setting by the collision between the SCB and its surrounding plates. The Late Yanshanian epithermal deposits are mainly located in Southeast Coastal Volcanic Belt, genetically related to the westward subduction of the paleo-Pacific plate.