Paleozoic Magma Evolution And Superimposed Mineralization In The Jinchuan Cu-Ni Mining Area Of Gansu Province

The world-class Neoproterozoic Jinchuan Cu-Ni sulfide deposit is located in the southwestern edge of the North China Craton.A large number of mafic-felsic dykes are exposed in the Jinchuan deposit area,such as the predominant dolerite dykes and lamprophyre and rare granite porphyries.The geological background and petrogenesis of these dykes are still unclear,and whether they have superimposed contribution to mineralization also needs research to confirm.This comprehensive study focuses on dolerite,lamprophyre,granite porphyry veins crosscutting ore-bearing intrusions of the Jinchuan deposit area and volcanic rocks exposed in the mining area.Zircon U-Pb dating of this study indicates that granite porphyry,lamprophyre and volcanic rocks formed at 367±2 Ma,425±3 Ma and 404±3 Ma.Combined with the zircon U-Pb age of 424±1 Ma of dolerite measured by predecessors,preliminarily suggesting the formation age of various types of dykes in the mining being Paleozoic.In addition,the age of the lamprophyre titanite Zircon U-Pb dating is 353±6 Ma,which is significantly later than the formation era of the lamprophyre.It is judged that the formation of the lamprophyre titanite is due to metamorphic titanite formed during the later orogeny.It is judged that the later age is the deterioration of the metamorphic titanite formed in the later orogenic stage.Petrochemistry and isotope geochemistry analyses show that dolerite dykes and lamprophyre haveε_Nd（t）values ranging from-4.59 to-1.58 and-2.97 to-2.03,with corresponding(⁸⁷Sr/⁸⁶Sr)_ivalues from 0.7056 to 0.7077 and 0.7083 to 0.7085,respectively,indicating that the magma source of the mafic dykes derived form an enriched lithosphere mantle.The zirconε_Hf（t）values of granite porphyries rang from 5.11 to 12.84,combined with the data analysis of predecessors,it is the product of the partial melting of juvenile crust mixed with a few ancient crust.The volcanic rock source area is part of the melting of the mixed rock circle mantle in the crust.Combined with previous studies on regional Paleozoic magmatic activities and this study,the authors put forward an integrated tectonic-magmatic evolution model.The background analysis of regional Paleozoic magmatic activity indicates that the Qilian Ocean closed at～445 Ma.and then the continental collision between the Qilian-Qaidam Block and the Alxa Block caused partial melting of the crust to form a large number of intrusive granite bodies during 440 Ma,which were distributed in the southern part of the mining area.Due to the influence of subduction-collision,the subduction fluid partially melts the lithospheric mantle,forming the dolerite dykes,lamprophyre and volcanic rock at 400-425 Ma in the Jinchuan mining area.After the subduction-collision,the swelling up of asthenosphere due to the retreat of the plate contributed to partial melting of the juvenile crust to form the calc-alkaline granite porphyry in the ca.367 Ma.The relationship between dykes（quartz veins,dolerite,lamprophyre,granite porphyry）and mineralization in the mining area is studied,and it is found that the Paleozoic dykes are closely related to the structure,and the structure and dykes activity has a certain transformation effect on the ore body,which is manifested in the large migration of chalcopyrite,the local formation of copper-rich ore bodies or massive ores,the dykes and tectonic development areas could be used as a sign of superimposed mineralization.