Mineralogy,mineral chemistry and metallogenic significance of spinels from the Poyi mafic-ultramafic intrusion in Beishan area,Xinjiang Uyghur Autonomous Region

The Poyi mafic-ultramafic intrusion,located in the western segment of the Beishan Rift,Xinjiang,hosts widespread spinel.Based on petrographic observations and electron microprobe analyses,these spinels can be classified into three types:included（Type Ⅰ）,interstitial（Type Ⅱ）,and green spinel（Type Ⅲ）.Type Ⅱ spinel is further subdivided into homogeneous（Ⅱa） and zoned（Ⅱb）subtypes.The zoned spinel consists of an inner chromian-spinel core（Ⅱ b-C）,intermediate Cr-rich（Ⅱ b-M1） and Fe-rich chromite zones（Ⅱ b-M2）,and an outer magnetite rim（Ⅱ b-R）.The chemical compositions of these spinels types show significant variations.The Cr^#values of Type Ⅰ,Ⅱ a and Ⅱ b-C spinels range from 0.45 to 0.60,0.43 to 0.64,and 0.43 to 0.63,respectively,with corresponding Mg^#values of 0.25 to 0.57,0.24 to 0.55,and 0.21 to 0.37,classifying them as chromian spinels.In contrast,the Ⅱ b-M1 and Ⅱ b-M2 zones have Cr^#values of 0.68 to 0.79 and 0.67 to 0.92,and Mg^#values of 0.19 to 0.29 and 0.07 to 0.18,respectively,all of which are identified as chromite.Type Ⅱspinel is characterized by Cr^#≤0.001 and Mg^#of 0.67 to 0.77,corresponding to aluminous spinel.Magmatic Types I and Ⅱa spinels exhibit a spatial compositional gradient,transitioning from Fe-enriched compositions in the east to Mg-Al-enriched compositions in the west.This pattern implies a higher degree of magmatic evolution and later formation timing in the eastern part,suggesting magma migration from east to west.In comparison,the secondary zoned（Type Ⅱb） and green（Type Ⅲ） spinels record a complex post-magmatic evolutionary history involving three stages of metasomatism or metamorphism.These stages were initially driven by post-magmatic melts or hydrothermal fluids,followed by a late-stage modification related to the infiltration of regional Al-rich fluid.