Geochemical Characteristic And Mineralization Process Of The Yangshan Gold Deposit,Gansu Province

The Yangshan gold deposit is a superlarge gold deposit discovered in the West Qinling Orogen.The systematic study of the geological and geochemical features shows the Yangshan gold deposit had undergone complex tectonic-magmatic events and occurred two mineralizations.With a wide range yet weak intensity,the first mineralization formed the alterated-rock orebody.The first mineralizition process was controlled by the ductile-brittle ductile deformation under the thrust nappe tectonics system.Due to the constantly tectonic activities,the occurrence of first phase orebodies had been transformed into varied scales and shapes.The gold mineralization is closely associated with disseminated pyritization and arsenopyritization while the alteration is sericitization and silicification mainly.Gold is typically invisible.Relatively,the second mineralization controlled by ductile brittle-intracontiental orogenic tectonic system,had small range and high intensity which formed the quartz-vein orebody.The orebody had a stable scale and shape,with no obvious alteration,only the development of stibnite-quartz veins.Different from the first type orebody,gold in the form of nature gold is concentrated in the quartz and stibnite.According to the research on the geochemistry and isotopic tracer,the first mineralizition combined materials from the Devonian srata and Bikou Group,meanwhile late Indosinian acid dikes also provided some materials.The ore-forming fluid-system is mainly the metamorphic fluid,and occurred fluid mixed effect.The material source of the second mineralization is comparatively simple,which is mainly composed of magmatic hydrothermal solution or mixed fluid from magmatic hydrothermal solution and metamorphic fluid.The late period of ore-forming fluid evolution had meteoric water.Through the metallogenic geochronology,the altered mineral muscovite 40Ar-39 Ar ages imply the first mineralization begins with the late Indosinian magmatic activity,and the hydrothermal zircons U-Pb ages indicate the second mineralization occurred between 128~122Ma,corresponding to the early Cretaceous latent magmatic activity.