Superimposed Metallogenic Model Of The Yangshan Gold Belt, Western Qinling Orogen, Gansu Province

Located in the Mianlue tectonic belt of West Qinling, Yangshan gold metallogenic belt experienced complated tectonic evolution, including plate breakup and oceanic basin-spreading, oceanic plate subduction-accretionary orogeny, plate suturing-collisional orogeny and intra-continent orogeny. Long-term strong and complicated geological process induced multistage metallogenic events and superaccumulation of metals and formed many superlarge and large gold deposits, among which the Yangshan super large gold deposit is the most represntative one.It is concluded that the Yangshan gold metallogenic belt occurred in the strata composed of hydrothermal sedimentary and epimetamorphic rocks and was controlled by large-scale thrust nappe structure and strong ductile-brittle shear deformation during the subduction-collision stage, which was superimposed by strong brittle deformation during the subsequent intra-continent orogeny. In terms of composite orogenic and metallogenic evolution, episodic regional metallogenic processes occurred in this region, such as the Precambrian volcanic-sedimentary-metamorphic process, late Triassic-early Jurassic large scale structural deformation and metamorphic fluid activities and late Jurassic-early Cretaceous concealed magmatic activities. By means of deposit geology, geochemical features and ore-forming geochronology research, it is determined that the Yangshan gold metallogenic was the superimposed and reformed products of two major metallogenic events. The first metallogenic event occurred during early Jurassic(195Ma±), which was related to large scale regional structural deformation and metamorphic fluid during the transition from compression to extension of collisional orogeny. The metallogenic process was controlled by regional thrust nappe structure and ductile-brittle shear deformation and formed fine-disseminated mineralization and altered rock-type orebodies. The main alteration include silicification, pyritization, arsenopyritization and sericitization. The mineral assemblage is mainly composed of medium-low temperature minerals and the ore-forming fluid is CO2-rich metamorphic fluid. The second metallogenic event occurred during early Cretaceous(115-144Ma), which was related to the concealed magmatic activities. The metallogenic process was controlled by superimposed brittle structures and formed vein-type mineralization and quartz-sulfide type and altered rock-type orebodies. The main alteration include silicification, polymetallic sulfidization and sericitization. The mineral assemblage is characterized by native gold and medium-high temperature minerals. The ore-forming fluid is mainly magmatic water and has less CO2.The Yangshan gold metallogenic belt has the following regional metallogenic regularities: the metallogenic process is mainly controlled by two stage tectonic evolution, regional collision- extension and intra-continent orogeny, from the aspect of time and thrust nappe structure, ductile-brittle shear deformation and superimposed brittle structure from the aspect of space. As for mineralization type, the early metallogenic event formed fine-disseminated mineralization, which is controlled by ductile-brittle structure, and the late metallogenic event formed quartz-polymetallic sulfide vein type, which is controlled by brittle structure. Mineralization types in different parts depend on particular lithology, the development degree of brittle structure and characteristics of magmatic activities. In general, mineralization intensity of the first stage gradually increased while the mineralization intensity of the second stage weakened from the east to the west. The mineralization enrichment regularity has been summarized. In the end, multi-stage and-genesis superimposed model for the Yangshan gold metallogenic belt have been established and the further prospecting direction is also pointed out to provide theoretical support.