Study On Metallogenesis Of Gaosongshan Epithermal Gold Deposit In The Northern Lesser Xing'an Range

The Gaosongshan gold deposit is one of the significant epithermal gold deposits in the Northern Lesser Xing'an Range,Geologists pay more attentions to its metallogenesis,ore genesis and prospect since it was discovered as a medium size at the end of the last century and is expected to be a large epithermal gold deposit.On the basis of previous studies,this paper has carried out a systematic studies of geological,fluid geology,chronology,element and isotope geochemistry.The results show that:On the basis of ore deposit Geological research,the exposed geological bodies are volcanic rocks of Banzifang Formation of the early-Early Cretaceous?basaltic andesite,andesite,trachyandesite and neutral volcano clastic rocks?,rhyolites of Fu minhe Formation of the late-Early Cretaceous and minor hypabyssal rhyolite porphy ry.The main structures are composed of Northeast tensional Shaaqihe Fault and E W volcanic apparatus.The deposit is consisted of 11 ore bodies,which are mainly hosted in volcanic rocks of Banzifang Formation.They are mainly vein in shape which is controlled by the fracture of the volcano apparatus.The ore minerals are mainly pyrite,chalcopyrite and hematite,secondly sphalerite,galena and natural gol d.The wall-rock alterations are illicization,sericitization,silicification,adularization,chloritization and carbonation and the alteration zoning from the ore bodys to wal l rocks is silicification zone,silicification-adularization zone,illicization-sericitization zone and sericitization-propylitization zone,respectively.The process of mineraliz ation can be classified into five stages:???microcrystalline quartz-hematite±magnet ite stage;???beresitization stage;???milky white pyromineralized quartz-adularia st-age;???gray quartz-polymetallic sulphides stage;and???quartz-carbonate stage.The study of chronology reveals that zircon U-Pb dating results from trachyan desite of Banzifang Formation and rhyolite of Fuminhe Formation are 121¹29Ma,105.6+1.7Ma?n=13,MSWD=1.2?.It represents two stages volcanic events of E arly Cretaceous in the region.Rhyolite porphyry,which contains exsolved fluid,its zircon U-Pb dating resul-ts from the magmatic zircons and the hydrothermal zircon s are respectively 104.6±1.0Ma,n=23?MSWD=0.37?and 100.8±2.1Ma?n=4,MSWD=0.13?.This shows that the metallogenesis is closely related with the rhyolitic mag mtism,formed in later period of volcanism.The study of inclusion reveals that the pyritization spherical quartz aggregates?exsolved fluid?in the rhyolite porphyry developed massive vapor-rich inclusions?RV type?,pure vapor inclusions?V type?,liquid-rich inclusions?RL type?and tiny amounts of melt inclusions and daughter-minerals bearing three-phase inclusions.Homogenization temperature and salinity of the liquid inclusions from early exsolved fluid are respectively245⁴13?and 4.79⁸.27%?NaCleqv.?.The early stage hematization quartz???developed RL type inclusions and a small amount of RV type inclusions;The main stage quartz??,??developed massive RL type inclusions,and tiny amounts of RV type inclusions and V type inclusions;The late stage quartz???developed massive RL type inclusions and L type inclusions.The homogeneous temperature of the fluid inclusions from early stage to late stage are 243³70?,165²61?and 156¹98?,respectively;And the salinities of fluid inclusions are 3.53⁹.73%NaCleqv,1.22⁴.48%NaCleqv and 1.05².89%NaCleqv,respectively.The gas composition of inclusions in the exsolved fluid is mainly H₂O and little CO₂,whereas only H₂O in the main stage and late stage.This indicates that the ore-forming process gradually evolved from water and salt system containing CO₂ into water and salt system with the decrease of temperature.The studys of H-O isotope and S-Pb isotope reveal that the initial ore-forming fluid may be residual magmatic water with faintly acid and oxidizing properties.With the proportion of mixed meteoric water increaseing with the metallogenesis,the ore-forming fluid in the main and late stages are mainly meteoric water with near neutral and reducing properties.Deep magma?rhyolitic magma?is the source of ore-forming materials.The study of geochemical reveals that the intermediate volcanic lavas of Banzifang Formation belong to high-K calc-alkaline-shoshonite series,charactered by enrichment of large ion lithophile elements and light rare-earth elements,relatively depletion of high field strength elements.Geochemical characteristics of the intermediate volcanic lavas show that the magma originated from mantle,characterized by fluid metasomatism,and is strongly contaminated by the crust materials?Early Jurassic intermediate acid intrusions?during the process of rising and evolution.The rhyolite of Fuminhe Formation and rhyolite porphyry belong to potassium-high potassium series,charactered by enrichment of large ion lithophile elements?such as Rb,Ba,K?and light rare-earth elements,relatively depletion of high field strength elements?Nb,Ta,Ti,P?,are consistent with A-type rhyolite with the crust source property.Combining with the positive?Hf?t?of zircons and the ratios of related elements?Nb/Ta,Rb/Sr,Th/Yb?,rhyolitic magma were derived from partial melting of the Neoproterozoic crust that glowed from a depleted mantle and mixed with subducted sediment melt.The comparison of the comprehensive study with the typical low-sulfidation epithermal hydrothermal gold deposits at home and abroad reveals that Gaosongshan gold deposit is a typical low-sulfidation epithermal hydrothermal gold deposit,whose forming liquid and material were both derived from exsolution of rhyolitic magma,and large-scale fluid exsolution occured after the rhyolitic magma eruption and rhyolite porphyry emplacement?100¹02Ma?.With the crust-mantle interaction of extended stage of the Pacific Plate subduction in the early Cretaceous,the geological processes of diagenesis and mineralization have experienced respectively the eruption and underplating of basaltic magma,eruption and emplacement of rhyolitic magma,exsolution of liquid,ore-forming fluid boiling resulting in cryptoexplosion breccia and oxidizing ore bodies,the fluid boiling and mixing of the ore-forming fluid resulting in the typical low-sulfidation hydrothermal alterations,polymetallic sulfide quartz veins and quartz carbonate veins.