| A series of Permian magnesia-ultramafic rocks have been developed in the Beishan area in Xinjiang,as an important copper-nickel reserve base in China.Numerous Ni-Cu deposits have been explored,such as Poyi large,Poshi medium and Luodong,Posan small Ni-Cu deposits.As several rounds of mineral exploration and diagenesis and metallogenesis have been completed,a large amount of basic geological,geophysical and geochemical data has been accumulated in the region,and the metallogenic models have been established.However,the comprehensive ore-seeking model of Ni-Cu deposits in this region is still blank,which restricts the development of next prospecting workconsiderately.Based on the detailed field geological survey and guided by the metallogenic theory of magmatic Cu-Ni sulfide deposits,this paper systematically analyzes the metallogenic controlling mechanism of the structure and magma for Cu-Ni deposits in three scales including the typical deposits,the Pobei forecasting area and the Xinjiang Beishan region.The results could contribute to the dissecting prospecting geological conditions,summarizing multi-scale prospecting indicators andbuilding the geological-geophysical-geochemical-remote sensing prospecting model.After that,a metallogenic prediction is quantified for the Pobei area and several other target areas,which could provide a scientific basis for theexploration work in the future.The main achievements of this study are as follows:The metallogenic model of Ni-Cu deposits in Pobei area is "deep melting + in-situ melting multi-stage pulsation metallogenic".Meanwhile,The ore-controlling factors of Ni-Cu deposits in Pobei area are tectonic and magmatic rock,and the former controls the ore formation indirectly by controlling the rock which is a direct factor.Affected by Metallogenic Specialization of the Mafic-ultramafic rocks Related to the Cu-Ni Deposits,the formation and distribution of Ni-Cu deposits in the Beishan area of Xinjiang are controlled by the late Carboniferous-Permian mafic-ultramafic magma.The magmatic ore-control is mainly manifested in five aspects:(1)The mineralization type of Ni-Cu deposits is controlled by magmatic composition,and the pyroxene peridotite is the best ore-bearing lithofacies.(2)The spatial distribution of the ore body is controlled by the magma channel,that is,the ore body is mainly locatedat the bottom of the ore-bearing ultramafic rock.(3)The mineralization period is controlled by magmatic activity stage,i.e.the ore-bearing facies are the most advanced intrusive ultramafic rock in the latest cycle of magmatism.(4)The physical and chemical conditions of magmatic activity control mineralization,for example proper amount of crustal contamination could benefit mineralization.The degree of crustal contamination of the Poyi ultramafic rock with the largest ore-forming scale is less than that of the Pobei complex and larger than that of the Luodong olive gabbro.(5)Magmatic activity controls the migration and distribution of ore-forming materials,which mainlyshow thatseparation and crystallization of olivine and pyroxene in magma lead to sulfur saturation.Meanwhile,the sulphide melting extracts Ni from the residual magma and early crystalline olivine.All these lead to the rock conditions in the prospecting geological conditions in the Pobei area: the output of ultramafic rocks with mineralization potential.That is,the metallogenic ultramafic rock should be found before the prospecting for Ni-Cu deposits.The structural ore-controlling is mainly characterized by the formation and distribution of regional tectonic-controlled metallogenic belts.The Hongliuhe-Yigezitag deep fault is the upwelling channel of parent magma,while the Baidiwa-Yuni River secondary fault is not only the intrusive channel of derived magma,but also ore-conducting and ore-hosting channel.Simultaneously,the magma chamber is located at the intersection of primary and secondary faults.Thus,the structural features for prospecting in the Pobei area are thatthesecondary fractures occure,and to be more specific the distance between the deposits,mineral occurrences and the secondary fracture is <10 km.The inversion of the deep crust structure shows that the Beishan rift in Xinjiang is connected to the deep of the Tarim Basin in the lithospheric mantle.The magma source of mafic-ultramafic rocks and Dark Dikes in the Pobei area tends to the Tarim mantle source area.The combination of the above features indicates that the Tarim mantle plume provides the material and heat for the source area of parent magma of Ni-Cu deposits in Pobei area.This demonstrats that the geotectonic location of pobei area is superior in the prospecting geological conditions.By understanding of the ore-controlling factors and mineralization information of the Pobei Ni-Cu deposit and following the gradual progress principle,,it is proposed that the Ni-Cu ore body is the main objective at the mineral census stage,and the reconnaissance stage is to find the ore-forming ultramafic rocks besides.Correspondingly,the direct indicators of geological prospecting indicators are constructed as follows:(1)surface indicators: peacock petrochemical,Annabergite or gossan;(2)ore type: disseminated ore,penetrating sulfide ore body;(3)mineralogical indicators: nickel pyrite.The indirect indicators of geological prospecting indicators are constructed as follows:(1)Pyroxene peridotite,peridotite;(2)special topography: negative topographic basin in complex;(3)surface indicators: iddingsite petrochemical;(4)alteration indicators: serpentinization.At the same time,the distinguishing criteria of ore-forming rock mass in the Pobei area are summarized as follows:(1)diagenetic period: Late Carboniferous-Permian;(2)distribution of rock mass: distance from secondary faults less than 10 km;(3)Late intrusion sequence: less vein interpenetration,edge of ring structure;(4)composition of parent magma: high temperature and high MgO tholeiite;(5)magmatic contamination degree less than 5%;(6)magmatic sulfur is saturated: Ni in olivine is depleted,and the sulfide content in rock is high;(7)Mineralogical indicators: pyrrhotite,water-bearing minerals;(8)Mineral chemical markers: olivine Fo> 80.Geophysical ore-prospecting criteria are indirect indicators.Ultra-mafic rocks are the main prospecting targets in metallogenic prospecting area prediction,and deep structure of ore-bearing ultramafic rocks is the main exploration targets in ore district.In addition,high magnetic anomaly,low gravity anomaly and low resistivity anomaly of geophysical prospecting correspond to ultramafic rocks.Geochemical ore-prospecting criteria: The ore-forming element Ni is mainly associated with Co,Cu,S,Se,Te,Bi,and Fe.The primary halo is characterized by Te?Fe,Co,Cu,Ni,S,Bi?Se,among which the most important frontal halo element is Se,and Se has secondary enrichment on the surface.In addtion,Ni is positively correlated with Mg,and inverse correlation with Ti and Ca controlled by the combination of rock-forming minerals.In addition,Ni is positively correlated with Ni/Co.Thus,the high value area of Ni /Co indicates the high nickel ore body.The 1:200000 scale water system sediment geochemical anomaly reflects the secondary halo,which also can be used to define the area where the ore-forming ultramafic rocks develop.Remote sensing ore-prospecting criteria: in the extraction of remote sensing mineral content information,the mafic-ultramafic complex corresponds to the high Si-index,low Kaolin mineral content and low carbonate mineral content as a whole,and the ultramafic rock corresponds to the high value area of iron-stained mineral content in the complex in the extraction of remote sensing iron-stained alteration information.According to the indicating targets of each prospecting information,a geological-geophysical-geochemical-remote sensing comprehensive prospecting model of the Ni-Cu deposits in the Pobei area has been built with the metallogenic model as the core.Subsequently,combined with a data-driven valuable variable screening prediction model Elastic Net-Fuzzy WofE,a quantitative prediction of metallogenic prospects was completed in the Pobei area on the scale of 1:50,000 using multi-source data.Afterwards,two A-level,one B-level,two C-level prospecting targets are been proposed.In addition,The Fo-Ni evolution diagrams of olivine show that the parent magma of Poyi-dunite is mixed with high Ni melt produced by pyroxene mantle melting,which is the reason for the large scale of Poyi metallogenesis. |
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