Geochemical Characteristics And Genesis Of Pyrrhotite And Magnetite Of The Jinchuan Cu-Ni Sulfide Deposit In Gansu Province

The Jinchuan Cu-Ni(PGE)magmatic sulfide deposit contains more than 500 million tons of sulphide ores(with nickel grade 1.1 wt% and copper grade 0.7 wt%)and is the third largest magmatic sulphide deposit in the world,and also the largest magmatic sulphide deposit in China.Many of the previous studies focus on deposit geology,petrography,geochemistry,geochronology,and metallogenic model of the Jinchuan Cu-Ni(PGE)magmatic sulfide deposit.However,the ore minerals like pyrrhotite and magnetite are not well study in Jinchuan.Detailed studies of ores minerals typomorphic characteristics,and minerals chemistry obtained by the means of LA-ICP-MS,could provide significant genetic clue to understand the metallogenic process.In previous work,Cao Yawen(1994)and Ding Ruiying(2012)present the occurrence and composition of pyrrhotite in the Jinchuan deposit,and conclude the structure type of pyrrhotite simply based on the composition characteristics.In this study,author presents the mode of occurrence,intergrowth and major element composition of pyrrhotite in various types of ores using magnetic colloid and micro probe analysis,as well as the mode of occurrence,major and trace element composition of magnetite using micro probe analysis and laser ablation inductively coupled plasma mass spectrometer analysis.Combined with recent research results,we discuss the genesis of main types of sulphide ore.This study was financially supported by the National Natural Science Foundation of China(41672064).We carry out systematic geological surveys of Jinchuan Cu-Ni sulfide deposits,identification of thin sections and polish sections of rock and ores,study the typomorphic characteristics of pyrrhotite in various types of ores and magnetite in ores and skarn in margin of mafic-ultramafic intrusion,by using the magnetic colloid under microscope,EMPA and LA-ICP-MS.The main results obtained are as follows:1.The magnetite widely distributed in all types of ore and country rocks,and the massive ores and Cu-rich sulfide ores contains more magnetite.Comprehensive analysis based on structure features and mineral assembles show that rounded magnetite in massive ores iscrystallized from sulfide melt,but the magnetite in Cu-rich sulfide ore produced by metasomatism.2.Magnetite crystallized by sulfide melt has high trace element content,enrich Ni,Co,Zr,Zn,Ti and V,but magnetite produced by metasomatism has low trace element content,relative depleted V,Ti,Zn and Co.3.Pyrrhotite in disseminated ores and net-texture ores show the similar chemical composition,with high M/S valus and low sulfur content.Crystal structures of pyrrhotite indicate that pyrrhotite from these two ore types formed in a slow cooling environment after crystallization in a high-temperature and low-S magma system.4.Massive ores often intrude in to disseminated ores,net-texture ores and country and trapped ultrabasic rocks breccia.Massive sulfide ores were formed by injection or flow of the immiscible sulfide melts and experienced a rapid reduction of temperature in a high-S magma system.