The Petrogenesis And Metallogenesis Of The Yaojialing Polymetallic Zn-Au Deposit In Tongling City,Anhui Province

The Yaojialing polymetallic Zn-Au deposit is located in the northeastern margin?Shatanjiao orefield?of the Tongling ore district,Anhui Province.The large-scale enrichment of Zn and the orebody is located in the xenoliths and their contact zones in intrusive rocks are the prominent metallogenic features of the Tongling ore district,which is famous for skarn copper deposits.The genesis of the deposit and the Yaojialing granodiorite porphyry which is closely related to the metallogenesis,has been published by predecessors,but there are still some controversies.On the basis of extensive collection of predecessor's research data and achievements,taking Yaojialing intrusion as the breakthrough point,this work discusses the petrogenesis of intermediate-acid dioritic intrusive rocks,represented by quartz diorite and granodiorite,which are widely outcropped in the Tongling ore district.Through field geological survey,sampling,sample preparation and geochemical analysis,this paper identify the geological characteristicsof and discusses the metallogenesis of Yaojialing Zn-Au polymetallic deposit.The following main understandings have been obtained.?1?The formation age of Yaojialing granodiorite porphyry is the same as the Yaojialing deposit,which is the product of large-scale late Mesozoic diagenesis and mineralization in the Tongling ore district,and they have a close genesis relationship.?2?The Yaojialing granodiorite porphyry and intermediate-acid dioritic intrusive rocks represented by quartz diorite and granodiorite,which is widely outcropped in Tongling ore district,closely related to Cu-Au polymetallic deposits,are all the product of Yanshanian magmatic activity.Through collecting and sorting out the previous research results,it is found that quartz diorite and granodiorite have the same geochemical composition,Sr-Nd isotope composition and zircon Hf-O isotope composition,and the common characteristics of adakite-like rocks,indicating that they have the same magma source and petrogenesis.Combination with the age characteristics of inherited zircons from quartz diorite,we believe that the diagenetic magma is the result of crust-mantle mixing.It was formed by mixing of the adakitic-like magma formed by the melting of the juvenile crust which were formed after the collision of Neoproterozoic Cathaysia Block and Yangtze plate,and the enriched lithospheric mantle partial melting magma which were formed the fluid metasomatism of the Yangtze plate lithospheric mantle by the subduction of the Neoproterozoic Cathaysian Ocean.The late Mesozoic large-scale magmatism in the Tongling ore district which inside of the continental plate tectonic background is attributed to the upwelling and heating of enriched lithospheric mantle partial melting magma and the decompression melting under the compression and crustal thickening and redirection tension stress of the Paleo-Pacific plate.The Subduction of Paleo-Pacific Plate mainly provides a dynamic source rather than a material source.?3?Ore bodies of the Yaojialing polymetallic Zn-Au deposit are lenticular and vein-like which located in the cryptoexplosive breccia zone of granodiorite porphyry,the upper and lower contact zones and interlayer fracture zones of limestone xenoliths,and the fracture of granodiorite porphyry.Three mineralization-alteration types?Porphyry-type,skarn-type and vein-type?are developed in this deposit,and they have obvious zonation of metallic elements.According to the sequence of mineral formation,three mineralization periods and six mineralization stages can be divided.?4?The main element compositions of porphyry-type,skarn-type and vein-type ore bodies are basically the same,and Sphalerite in vein-type ore bodies has two types:iron-poor and iron-rich,which reflect the different forming conditions.The composition characteristics of trace elements of sphalerite and pyrite in three types of ore bodies are both common and different.The trace element composition of sphalerite shows the characteristics of enrichment of Mn,Co,Cd and poor in Ni,In,Ge and Ga.The sphalerite of skarn-type ore body is more enriched in Co,Se and In,but relatively poor in Cd,indicating that it may exist in the form of isomorphism,Cu and Pb exist in sphalerite as microinclusion of chalcopyrite and galena respectively.The trace elements of Co and Ni in pyrite are obviously enriched in skarn-type ore bodies,which are negatively correlated with Fe,indicating that Co and Ni exist in the lattice of pyrite in the form of isomorphism instead of Fe.The contents of Cu,Zn and Pb vary greatly,and exist in the form of microinclusion of chalcopyrite,sphalerite and galena respectively.Although there are some differences in trace element compositions among the three ore body types,the overall characteristics are basically the same,reflecting that although Yaojialing deposit has c mineralization-alteration types,it is still under the control of a unified magmatic hydrothermal system.?5?The research of fluid inclusions and stable isotope geochemistry of H-O,C-O,S and Pb in Yaojialing deposit,it is shown that the origin of metallogenic materials are closely related to the genesis of Yaojialing intrusion and have the characteristics of crust-mantle mixing.Based on the vein cut-through relationship,mineralization stages and the changes of fluid composition and physicochemical conditions,it is inferred that the ore-forming fluid of Yaojialing deposit was a simple NaCl-H₂O system when it first precipitated/exsolved from the deep magma chamber.When ore-forming fluids migrate upward to the contact zone between intrusion and surrounding rock?limestone xenoliths?,marmarization and hornfelization of surrounding rocks are caused by thermal metamorphism which caused by magmatic thermodynamics in the early stage,and cryptoexplosion occurs due to blocking gas and liquid,resulting in the formation of brecciation rocks.Minerals are subsequently precipitated in granodiorite porphyry and cryptoexplosive breccia,with copper?gold?mineralization as the main mineralization and a small amount of inner skarn minerals.At this time,the ore-forming fluids are characterized by high temperature and high salinity.When the ore-forming fluids moves up to the contact zone between intrusion and surrounding rock near the side of the surrounding rock,strong contact metasomatism takes place.The mineral are further precipitated,with?copper?zinc mineralization as the main mineral,and large-scale skarn minerals occur at this time.Because of the widely developed interlayer fissure structure in the limestone xenoliths,strong water-rock reaction occurs between hot ore-forming hydrothermal and cold carbonate strata in a large area.A large amount of zinc?gold?deposits occur at this time.At this time,the whole ore-forming system is relatively open.Low-temperature and low-salinity meteoric water mixes with medium-high temperature and high-salinity fluids and caused strong water-rock reaction with surrounding rocks which extracting minerals,and finally,the Yaojialing deposit was formed.?6?The Yaojialing polymetallic Zn-Au deposit is a porphyry-skarn-vein types complex deposit controlled by a unified magmatic hydrothermal system.It is an aggregate of various genetic types of polymetallic copper-gold deposits in Tongling ore district.