Mineralization Of Orogenic Gold Deposits In The Indus-Yarlung Tsangpo Suture Zone Of Tibetan Plateau

Research has proven that orogenic gold deposits were not originally thought to occur only in accretionary orogenic tectonic settings,but also continental collisional environment.The gold deposits controlled by the Indus-Yarlung Tsangpo suture(IYS)zone in the southern Tibetan collisional orogen were formed during the main collisional stage of Indo-Asian collision(65–41 Ma)which includes Nianzha,Mayum,Bangbu,Zhemulang,Xingxia,Dejilin.This indicates that the IYS zone has great potential to generate large orogenic gold deposits and an orogenic gold belt with its length of about 1300 km from the West to the East exists along the IYS.This paper will focus on the orogenic gold deposits along the IYS zone,especially on the newly discovered Nianzha gold deposit,also the Bangbu and Mayum orogenic gold deposits as comprehensive study objects,expound their ore geological characters,find the ore-controlling elements,determine the metallogenic ages,trace the origins of mineralization fluid and material,and finally build the metallogenic model of the orogenic gold deposits along the IYS zone.The Nianzha gold deposit,located in the central section of the IYS zone in southern Tibet,is a large gold deposit(Au reserves of 25 tons with average grade of 3.08 g/t)controlled by a E–W striking fault that developed during the late period of the main stage of Indo-Asian collision(43.66Ma).It occurs in a fracture zone bordered by Cretaceous diorite in the hanging wall to the north and the Renbu tectonic mélange in the footwall to the south.High-grade mineralization(altered-rock type)occurs in a fracture zone between diorite and ultramafic rock in the Renbu tectonic mélange.The wall-rock alteration is characterized by silicification in the fracture zone,serpentinization and the formation of talc and magnesite in the ultramafic unit,and chloritization and the formation of epidote and calcite in diorite.Quartz veins associated with Au mineralization can be divided into three stages.Fluid inclusion data indicate that the deposit formed from H2O–Na Cl–organic gas fluid that homogenize at temperatures of 203°C–347°C and have salinities of 0.35–17.17 wt.% Na Cl equivalent.The quartz veins yield ?18Ofluid values of 0.15‰–10.45‰,low ?DV-SMOW values(–173‰ to –96‰),and the ?13C values of –17.6‰ to –4.7‰,indicating the ore-forming fluids were a mix of metamorphic and sedimentary orogenic fluids with the addition of some meteoric and mantle-derived fluids.The pyrite within the diorite has ?34SV-CDT values of-2.9‰–1.9‰(average-1.1‰),206Pb/204 Pb values of 18.47–18.64,207Pb/204 Pb values of 15.64–15.74,and 208Pb/204 Pb values of 38.71–39.27,all of which are indicative of the derivation of S and other ore-forming elements from deep in the mantle.Three types of gold mineralization are found within the IYS,namely Bangbu-type accretionary,Mayum-type microcontinent,and Nianzha-type ophiolite-associated orogenic Au deposits.The three types formed at different depths in an accretionary orogenic tectonic setting.The Bangbu type was formed at the deepest level and the Nianzha type at the shallowest.The origin of the orogenic gold mineralization system along the IYS is closely related to the main Indo–Asian collision.The break-off and roll-back of the subducted Neo-Tethyan oceanic slab triggered crustal thickening and upwelling of mantle fluid,which contributed to the differential metamorphism of the lower crust in the collision area.These deposits formed from CO2 metamorphic fluids that contain minor amounts of mantle,sedimentary organic,and meteoric fluids.The metals in the deposits were derived either from the deep lithosphere or from local sediments.The orogenic gold deposits along the IYS are structurally controlled and formed as a result of the precipitation of gold from mineralizing fluids within open spaces in fault zones.The orogenic gold systems within the IYS consist of shallow deposits in altered rocks and deep deposits with Au hosted in quartz veins.