Mg-Sr-Nd Isotopic Constraints On The Genesis Of The Jinchuan Cu-Ni-(PGE) Sulfide Deposit

The Jinchuan Cu-Ni-?PGE?sulfide deposit is the largest magmatic Cu-Ni-?PGE?sulfide deposit in China,and also the world's largest single magmatic Cu-Ni-?PGE?sulfide deposit,with the third largest Ni reserves in the world.It is a very typical example of metallogenesis of super-large Cu-Ni-?PGE?sulfide deposit,which is characterized by the small size of the host intrusion and high sulfide/silicate ratio in the intrusion.The genesis and metallogenic model of the deposit have been studied for a long time,and also has attracted much attention of deposit geologists at home and abroad.The Jinchuan Cu-Ni-?PGE?sulfide deposit occurs in Neoproterozoic mafic-ultramafic intrusions.The assimilation of carbonate country rocks by the Jinchuan parent magma may be the main mechanism for the formation of the Jinchuan Cu-Ni-?PGE?sulfide deposit,but the extent of contamination of carbonate country rocks remains unclear.Based on the analysis and study of petrography,major and trace elements and Mg-Sr-Nd isotopic compositions of the Jinchuan intrusion and marble country rocks,the origin and evolution characteristics of magma in the Jinchuan Cu-Ni-?PGE?sulfide deposit are discussed,and the contamination of crust-source components and the contribution ratio of marble country rocks in the process of magma differentiation and evolution are simulated and calculated.Several important conclusions can be made from this study.They are summarized below:1.The composition of the Jinchuan lherzolites is mainly controlled by olivine,clinopyroxene and orthopyroxene,as well as a small amount of hornblende and biotite.SiO₂,CaO and total alkali?Na₂O+K₂O?were negatively correlated with MgO,while total iron(FeO_Total)was positively correlated with MgO.2.The Jinchuan lherzolites shows moderate LREE enrichment and Nb negative anomalies;the marbles shows strong LREE enrichment,positive U,La and Sr anomalies and significant Nb negative anomalies.The Nb/U ratio in the lherzolites ranges from 11.57 to 24.04,significantly lower than E-MORB,slightly lower than the lower crust,but significantly higher than the marbles,indicating that the Jinchuan parental magma has undergone a certain degree of crustal contamination.3.The Jinchuan intrusion is characterized by light Mg isotopes.The?²⁶Mg values range from-0.40‰to-0.26‰,with a weighted mean value of-0.31±0.06‰?2SD?,which is slightly lighter than MORB?-0.25±0.06‰?and upper mantle?-0.25±0.07‰?.The Mg isotopic compositions of olivine and pyroxene are almost the same.The?²⁶Mg values of the marbles range from-2.00‰to-1.17‰,with a weighted mean value of-1.53±0.02‰?2SD?.The light Mg isotope characteristics of the Jinchuan deposit may be due to the assimilation of carbonate country rocks by magma during evolution and emplacement period.4.The (⁸⁷Sr/⁸⁶Sr)i ratios and?_Nd values of the Jinchuan lherzolite are ranging from 0.706615 to 0.714612 and from-8.29 to-6.13,respectively.The marbles have relatively high(⁸⁷Sr/⁸⁶Sr)i ratios?0.713662⁰.740069?and lower?_Nd values?-25.43^-12.41?.Mg-Sr-Nd isotopic simulation calculation shows that the magma source of the Jinchuan intrusion is relatively enriched.Mantle-derived magma underwent 10²0 wt.%bulk contamination involving the lower continental crust,followed by<10wt.%bulk contamination by marbles.5.The results of Mg-Sr-Nd isotopic study show that the contamination of the crust plays a crucial role in the metallogenesis of the Jinchuan intrusion.The contamination of the lower crust reduced the sulfur solubility;and the contamination of marble country rocks made magma more oxidized and hence decreased the sulfur solubility further.These two processes are efficient ways to produce sulfide saturation in magmas.6.Mg isotopes play an important role in tracing carbonate contamination in mafic magma.The marbles assimilation by basaltic magma is an important mechanism for forming magmatic Cu-Ni-?PGE?sulfide deposits.Mafic-ultramafic intrusions with similar ages and country rocks with similar lithogeochemical characteristics to those of the Jinchuan intrusion are the key to the exploration of additional Jinchuan-type deposits.