Origin And Genetic Mechanism Of Magmatic Ni-Cu Sulfide Deposits In The Western Part Of Eastern Tianshan Region,Xinjiang,China

Magmatic Ni-Cu sulfide deposits are the main source of nickel,copper,cobalt and platinum group elements in China.The East Tianshan region in Xinjiang is one of the most important producing areas of magmatic Ni-Cu sulfide deposits,in which a series of deposits occur.The Huangshan,Huangshandong and Tulaergen large deposits,Hulu,Xiangshan,and Tudun medium-sized deposits make up the Tudun-Huangshan-Tulaergen Ni-Cu metallogenic belt.In recent years,the Baixintan,Lubei and Yueyawan deposits were successively found in the western part of the East Tianshan,making the Ni-Cu metallogenic belt in the East Tianshan region further extend to the west.Many achievements have been made on magmatic Ni-Cu sulfide deposits in the Eastern part of the East Tianshan region.However,the research on the geological characteristics and metallogentic mechanisms of the new-found deposits,such as Baixintan and Lubei in the western part of the East Tianshan region are relatively weak.There is a lack of systematic comparison between Ni-Cu sulfide deposits in the eastern and in the western part of the East Tianshan region.Their similarities and differences in the key magmatic processes,such as crystallization differentiation,crustal contamination,sulfide segregation,and enrichment of ore-forming elements,are important basis to enrich the Ni-Cu mineralization and prospecting theory in this region.In addition,the tectonic setting of the magmatic Ni-Cu sulfide deposits in this region remains controversial.To address these problems,this thesis mainly focuses on the Baixintan and Lubei deposits in the western part of the East Tianshan and the Baishiquan and Tianyu deposits in the Middle Tianshan Terrane in the eastern part of the East Tianshan region,systematically studied their chronology,petrology,mineralogy,geochemistry,and isotopic characteristics.The ages of the deposits and the nature of their parental magmas were clarified,the magma sources,the magma evolution processes,diagenetic and metallogenic mechanism were discussed.The main insights gained are as follows:1.Zircon SHRIMP U-Pb dating results show that the Baixintan and Lubei Ni-Cu magmatic sulfide deposits in the western part of the East Tianshan region generated at 276 Ma and 285 Ma,respectively.The Baishiquan and Tianyu deposits in the Middle Tianshan Terrane have a zircon U-Pb age of 279 Ma and283 Ma,respectively.They are all the products of magmatic activity in the Early Permian,similar to the deposits in the eastern part of the East Tianshan region,such as the Huangshan and Huangshandong deposits.2.The calculations of olivine composition,the analyses of the pyroxene components and platinum group elements show that the Baixintan and Lubei deposits have high-Mg tholeitic basaltic parental magma,same with the Huangshan and Huangshandong deposits.The composition of their parental magmas are close to N-MORB and P-MORB.However,the parental magma of the Baishiquan deposit is characterized by low-Mg content.The relationships between the Fo and Ni contents of olivine crystals from these deposits indicate distinct sulfide segregation of their parental magmas.3.The trace elements shows the parental magmas of Baixintan,Lubei,Baishiquan and Tianyu deposits experienced a certain degree of crustal contamination during evolution.Nevertheless,sulfur isotope characteristics indicate that the adding of crustal S is not necessary for minerlization in the East Tianshan region.Modeling results of olivine composition indicate that the crystallization of olivine may be the main reason for sulfide segregation and mineralization.The compositional variations in single olivine at different depth from the Lubei deposit show that the sulfide segregation degree is higher in the deeper position than that in the shallower position.The different olivine Ca contents between the two ore districts of the Lubei deposit indicate that the Yunhai district in the north crystalize faster at a shallower depth compared with the Lubei district in the south.4.The modeling results show that the primary magmas of the Baixintan,Lubei,Baishiquan and Tianyu deposits had experienced early sulfide segregation,and the proportions of early sulfide loss were0.1 wt.%,0.02 wt.%,0.05 wt.% and 0.2wt.%,respectively.The comparison of platinum group elements shows that low PGE contents possibly due to their PGE-depleted parental magmas caused by early sulfide segregation.The relatively high Cu/Pd ratios,low PGE contents and low R factor during sulfide segregation indicate that these deposits represent the products of the latter stage of mantle derived magma evolution in the East Tianshan region.5.The geochemical characteristics of trace elements imply that the magma sources of magmatic Ni-Cu sulfide deposits in the East Tianshan have been metasomatized by subduction slab-derived fluids’ components.The analysis and comparison of Sr-Nd isotopes show that the magma sources of Ni-Cu sulfide deposits in both East Tianshan and Beishan are basically depleted mantle.However,the depletion extent of magma sources varies from one tectonic unit to another.Generally,the magma sources of deposits seem to become increasingly depleted from south to north,and the Baixintan and Lubei deposits in the western part show more enriched mantle sources than the deposits in the eastern part of the East Tianshan.The Ni/Cu-Ni and Ni/Cu-Cu correlations demonstrate that the deposits’ Ni/Cu ratios are mainly controled by Cu contents,and are related to the depletion extent of their sources.A depleted magma source tend to generate ores with high Ni/Cu ratios and low Cu contents,while a enriched magma source tend to generate ores with low Ni/Cu ratios and high Cu contents.Compared with worldwide typical huge Ni-Cu sulfide deposits which almost have enriched mantle sources,the much more depleted magma sources of deposits in the East Tianshan region suggest that the potential of forming large deposits in the region is limited.Even then,the Baixintan and Lubei deposits located in the western part of the East Tianshan have less depleted sources than deposits from the eastern part,implying a relatively large Ni-Cu mineralization potential in the western part.6.The comparison of petrology,mineralogy and geochemistry between the ore-bearing mafic-ultramafic intrusions in the East Tianshan and Alaskan type intrusion formed in island arc setting shows that the intrusions in the East Tianshan are markedly different from the Alaskan type.Their negative Nb,Ta and Ti anomalies are likely to be caused by the influence of subduction slab fluids’ components.The high initial crystallization temperatures（1300℃ ～ 1400℃）of the intrusions show thermal anomalies in their magma sources.The intrusions have geochemical characteristics similar to E-MORB and Sr-Nd isotopic characteristics similar to OIB,indicating that the intrusions were emplaced in a plume-related environment,which may be the result of the combined mechanisms of plate subduction and plume.