The Metallogenic Age And Material Source Of Boka Gold Deposit In Dongchuan

Boka gold deposit is located in the middle and southern section of"Kang-Yunnan Axis"in the western margin of the Yangtze River.It is very different from Dongchuan copper deposit in min-Tangdan area in the Proterozoic"Dongchuan type"copper mineralization and its age.The surrounding rocks of the deposit are black carbonate-clastic rock strata of the Dongchuan Group.Au is produced in quartz-dolomite sulfide veins,which has obvious diabase magmatic extraction type Au alteration characteristics,different from ductile shear zone type Au deposit.In recent years,a lot of researches have been done on boka gold deposit,but there is still a lack of research on the ore-forming age of the deposit,the source of ore-forming materials,and the age attribution and tectonic attribute of the magmatic rocks in the mining area have not given a clear conclusion.Therefore,this paper takes the natrified diabase-albitite,surrounding rock carbonaceous slate and mineralization-related minerals as the research objects,and dissects their petrology,tectonic geology and tectonic-mineralization-alteration contact relationship.A comprehensive study of petrography,in-situ LA-ICP-MS trace elements,Re-OS and Rb-Sr isotope geochemistry has been carried out in the laboratory.Finally,the ore-forming age of boka gold deposit has been accurately determined,the source of ore-forming materials and the genesis of the deposit have been discussed,and the genesis and source properties of the natrified pyrochlorite-albitite of Boka gold deposit have been analyzed.And discussed the formation of tectonic environment.Finally,I gained the following understanding:（1）The Dongchuan Boka gold ore body is mainly produced in vein-like,lenticular and thin-skinned sac-like shapes,with a small amount of quasi-layered production,distributed on the side and upper parts of the albite-sodium diabase dykes,and the surrounding rocks on the contact zone or in the fracture zone,its spatial distribution,grade and scale are closely related to the albite dikes.From the core to the edge,there are obvious gold-bearing dolomitized sodium diabase-albite→massive or infested pyrite-chalcopyrite gold-bearing dolomite-quartz belt→star-shaped pyrite-characteristics of the alteration zoning of chalcopyrite gold-bearing dolomite.It is different from the ductile shear zone type Au deposit and has obvious alteration characteristics of the diabase magma extraction type Au deposit.（2）The Re-Os isotope and trace element studies of the gold-bearing pyrite and the surrounding rock carbonaceous slate in the Xinshan-Majiagou ore section of the Boka gold deposit have obtained the Re-Os isochron age of the pyrite as（779±14）Ma（MSWD=11.1）,indicating that the metallogenic age of the Boka gold deposit was Neoproterozoic.The initial ¹⁸⁷Os/¹⁸⁸Os value of the Re-Os isotopic system of pyrite is3.03±0.42,which is close to the average continental crust ¹⁸⁷Os/¹⁸⁸Os value of 3.63,indicating that the metallogenic minerals of the Boka gold deposit are mainly derived from the crust（the surrounding rock formation）rather than the crust.Mantle-derived diabase magma.This conclusion is supported by the evidence of trace element characteristics of Re,ordinary Os,¹⁸⁷Os,Au,and Cu contents in carbonaceous slate.（3）The rutile associated with the albite and albite mineralization alteration in the Dongchuan Boka gold deposit was formed at 344.89～1131.98°C,with an average of 563.64°C,and most of the grain temperatures ranged from 400 to 650°C.These temperatures indicate that The albite and sodium diabase of the Dongchuan Boka gold deposit were formed in a medium-high temperature magmatic environment.The rare earth element distribution map shows a nearly flat curve with a slight loss of Eu,and the difference between light and heavy rare earths is not obvious.The averageδEu is0.93,and the averageδCe is 0.96,showing a slight negative anomaly.The trace element spider diagram shows that the rock is obviously enriched with Rb and Ba large ions Lithophilic elements,active incompatible elements U and Th;relatively depleted high-field-strength elements Nb and Sr;Sr initial value(I_Sr value of0.71002-0.73204)changes greatly.The trace elements and Sr isotopes of the rocks show that the magma of sodium diabase and albite formed an intraplate extensional environment,which is the product of low differentiation and partial melting of enriched mantle,and has the characteristics of crust mantle source mixing.（4）Based on the study of Sr isotopes and trace elements of the sodalite diabase,albite,carbonaceous slate and dolomite in the Boka gold deposit,the content of rare earth elements in dolomite closely related to mineralization and alteration(average64.4×10^-6)Between the rare earth content of albite and sodium diabase(average85.3×10^-6)and the rare earth content of the surrounding rock carbonaceous slate(average 105×10^-6),and the Compared with the nearly flat rare earth distribution patterns of the two,dolomite shows the characteristics of weak LREE depletion and weak HREE enrichment,showing a slowly rising near-left-dip curve.The negativeδEu anomaly of dolomite（average 0.87）is lower than that of albite and sodium diabase（average 0.93）,but higher than that of host rock carbonaceous slate（average0.75）;albite,sodium diabase Both the rock and surrounding rock carbonaceous slate are obviously enriched in Rb and Ba large ion lithophile elements,active incompatible elements U and Th,and relatively depleted in high field strength elements Nb and Sr;dolomite is enriched in large ion lithophile elements Rb,Ba,active incompatible elements U and Th,while strongly enriched in Sr,relatively depleted of Nb,Ti and Zr high field strength elements.The high I_Sr values（0.73608,0.73831）of the dolomite during the metallogenic period are mainly provided by the surrounding rock carbonaceous slate(I_Sr values are 0.72357,0.69425),indicating that the diabase magma flows through the overlying carbonaceous slate strata,and has Dolomitization was carried out,and the resulting ore-forming fluids resulted in the precipitated dolomite with a strontium isotopic composition much higher than that of the aridite,albite and surrounding rock strata.The trace elements and Sr isotopes of dolomite show that the ore-forming fluid is mainly provided by carbonaceous slate,and a small amount of magmatic hydrothermal fluid relatively enriched in REE is also added.（5）To sum up,the Dongchuan Boka gold deposit is the metallogenic response of the global tectonic cracking-magmatic-metallogenic event of the Rodinia supercontinent in the Neoproterozoic period on the Kang-Dian axis in the western margin of the Yangtze,that is,the intraplate extensional environmental crust-The mantle-mixed porphyry basaltic diabase magma upwells,and metasomatism occurs in contact with the surrounding rock strata,discoloring and altering into albite,and at the same time extracting the pre-enriched gold,copper and other favorable mineralization in the surrounding rock strata Elements are enriched in a good metallogenic space to form the Boka gold deposit.