Mesozoic Orogenic Gold Mineralization In The Eastern Qinling Orogen

The East Qinling Orogen is an important part of the Qinling Orogen.Mesozoic orgenic Au deposits are widely distributed in the Eastern Qinling Orogen.The Xiaoshan,Dongchuang and Huaishuping gold deposits are selected to investigate into the sources of ore-forming fluids and metals,the timing and characteristics of gold mineralization,and the geodynamic settings of gold deposits,in order to construct the genetic model of Mesozoic gold deposits in East Qinling Orogen.The main conclusions are shown below.?1?Detailed studies on the geological signatures of typical gold deposits suggest that their ore-forming processes can be divided into three stages,including quartz-pyrite stage,quartz-polymetallic sulfide stage and quartz-carbonate stage.Gold occurred as fissure gold,intergranular gold and inclusion gold within the pores or fissures of pyrite.Gold mineralization was mainly controlled by structures.?2?Fluid inclusions studies show that the fluid inclusions are dominated by H₂O-CO₂,pure CO₂ and pure H₂O types.The ore-forming fluids are characterized by enriched CO₂,low salinity and low density,belonging to metamorphic-origin fluid,consistent with the typical orogenic gold deposits.The precipitation of metals is mainly controlled by the fluid immiscibility.?3?The C-H-O-S isotopic compositions indicate that the ore-forming materials of Dongchuang and Xiaoshan gold deposits are derived from the deep mantle,with variable input of crust-derived materials.While,the ore-forming materials of Huaishuping gold deposit are mainly derived from the metamorphic dehydration of wall-rock strata,contaminated by minor magmatic-hydrothermal fluids.?4?Pyrites from the Dongchuang gold deposit yield a Re-Os isochron age of150±21Ma,indicating that the gold mineralization developed in the Late Jurassic to the Early cretaceous.A summary of the mineralization ages in the Eastern Qinling suggest that the Mesozoic gold mineralization can be divided into two stages,including the Triassic?250-200 Ma?and Late Jurassic to Early Cretaceous?160-100Ma?.The former metallogenic event generated under the tensional-extensional setting caused by the continent-continent collision between North China Craton and Yangtze Craton.The latter metllogenic event formed under the tensional setting caused by the continuous subduction of Paleo-Pacific Ocean underneath the southern margin of North China Craton.?5?The mineralization of the orogenic gold deposits in the East Qinling Orogeny is different between the two metallogenic epoch of the Triassic and Late Jurassic to Early Cretaceous.The Late Jurassic to Early Cretaceous ore-forming fluids are mainly metamorphic water,and their ore-forming materials are mainly derived from mantle,with the addition of crust-derived materials.The Triassic orogenic gold mineralization is divided by the closure timing of East Qinling Ocean?ca.215 Ma?.The orogenic deposits formed under syn-collisional setting are characterized by metamorphic water,and their ore-forming materials are mainly derived from the wall-rock.While,under the post-collision extensional setting,the ore-forming materials are mainly derived from the metamorphic dehydration of subducted oceanic slab,with the minor addition of magmatic-hydrothermal fluids from the oxidized magmatic system generated by the partial melting of thickened lower crust underneath the North China Craton.