Geochronology And Geochemistry Of Magmatic Rock In Sandaowanzi,Heilongjiang Province

Northern part of Heilongjiang is one of the most significant epithermal gold deposit regions in China,including the Sandaowanzi Gold Deposit,Wulaga Gold Deposit,Dongan Gold Deposit and Gaosongshan Gold Deposit.They all belong to epithermal gold deposit,lying in Cretaceous volcanic rocks or correlated closely with volcanic rocks in the early Cretaceous.Sandawanzi gold deposit is the only telluride-type gold deposit with the independent mineralization of telluride.This article give a systematic research on the granite and volcanic rocks which outcrop in the Sandaowanzi gold deposit from zircon U-Pb geochronology,zircon Hf isotope,trace elements of zircon,whole-rock major and trace elements and Sr-Nd isotope,and investigate the genesis,geodynamic setting and the relationship with mineralization.The zircon U-Pb geochronology and Hf isotope analysis of granite and early Cretaceous Longjiang formation and Guanghua formation indicate that:(1)the Sandaowanzi monzonitic granite forming at 193.9~183.0Ma is the result of early Jurassic magmatic activity which happened before mineralization.The granitic magma activity in Sandaowanzi areas are segmented into three episodes,including 360~380Ma,183 Ma and115~124Ma,with the combination of other scholars' research.(2)the formation age of Sandaowanzi Longjiang formation volcanic rocks and Guanghua formation volcanic rocks is 120.4~122.9Ma and 119.9~122.4Ma,and the Sandaowanzi gold deposit hosted in Longjiang formation volcanic rocks.According to the U-Pb ages of wall rocks surrounding the ore veins and diabase dikes penetrating the ore veins,the mineralization time is 122.9~116.4Ma.(3)the zircon Hf isotopes with depleted characteristic in Early cretaceous(?Hf(t)= +5.8 ~ +11.5),reveal that these granites are I-type granites,formed by partial melting of young lower crust.(4)the volcanic rocks in this area all possess the positive ?Hf(t)with the lager Hf model ages than the formative ages,which imply they derive from the enriched mantle source,or the exist of contamination of enrichment of components on the uplifted way.Sandaowanzi Longjiang formation and Guanghua formation volcanic rocks display slightly negative Eu anomalies,with gentle fractionation between light rare earth elementsand heavy rare earth elements and enrichment of large ion lithophile elements(Rb?Ba?Th?U)and depletion of high field strength elements(Nb?Ta?Ti).Then,trace elements shows low Y and Yb,but high Sr,which is consistent with the features of adakitic rock.The geochemical composition demonstrates that granites in early Jurassic have characteristics with enrichment of light rare earth elements,depletion of heavy rare earth elements,enrichment of large ion lithophile elements(Rb,U)and depletion of high field strength elements,relatively,shown on the the primitive mantle-normalized diagram.The low initial Sr ratio(0.70473~0.70506)and positive ?Nd(t)(+0.34 ~ +1.8)of Sandaowanzi early Cretaceous Longjiang formation volcanic rocks suggest that the volcanic rocks have mantle derived feature.The research on petrogenesis and tectonic setting shows that,the early Jurassic granites were formed in compression tectonic environment,influenced by the Izanagi Plate subduction.The tectonic setting which changed from compression to extension,leads to asthenosphere upwelling and regional extension in continental plates when the subduction direction of Izanagi Plate changed from late Jurassic to early Cretaceous.Simultaneously,the partial melting which resulted from the extension between lower crust and lithosphere mantle,caused the mantle-crust interaction,representing that lithospheric thinning took place in a large scale during the middle and late early Cretaceous.The metallogenesis is closely related to the sub-alkaline volcanic rocks during the extension period in early Cretaceous,and it happened to be the period of strong eruption of the Daxing'anling volcano.In early Cretaceous with large-scale lithosphere extension,Sandaowanzi ore-host rocks and the majority of epithermal gold deposits in China northeast were formed during the same period.Research shows that it might be a advantageous site near the early Cretaceous volcanic crater to form epithermal gold deposits.