Xiangshanxi composite Cu-Ni-Ti-Fe deposit belongs to comagmatic evolution product: Evidences from ore microscopy, zircon U-Pb chronology and petrological geochemistry, Hami, Xinjiang, NW China
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摘要: 香山西岩体是东天山地区发现的唯一的一个铜镍-钛铁复合型含矿岩体,前人对香山西铜镍-钛铁矿床成因认识一直存在争议。本次通过矿相学研究结合电子探针分析,在香山西铜镍矿石中发现了铜镍硫化物与钛铁氧化物共生的现象,并首次测定了香山西钛铁辉长岩的SIMS锆石U-Pb年龄为278.6±1.8Ma(MSWD=1.2),与前人测得香山中岩体角闪辉长岩的锆石U-Pb年龄基本一致,证实了香山西铜镍矿与钛铁矿为共生关系,两者空间上渐变过渡。岩石地球化学研究表明,香山岩体(包括钛铁辉长岩)为同源岩浆经结晶分异作用形成的一套镁铁-超镁铁质岩体,具有拉斑玄武质岩浆分异演化趋势,并且经历了与地壳物质的混染。与中段和东段岩石相比,香山西辉长岩类具有相对高的REE、SiO2(平均48.99%)、K2O+Na2O(平均3.43%)、TiO2(平均1.26%)和明显低的Mg#值(平均62.4),表现出分异演化程度高于中段和东段。经反演得出香山岩体原始岩浆含MgO约10%,FeO约9%,TiO2约1.14%,与同一成矿带内其它含铜镍矿岩体原始岩浆成份相近,应为 普通的拉斑玄武质岩浆。因此,香山西钛铁矿床的形成机制可能是由普通的拉斑玄武质岩浆,经历了较高程度的分异演化导致钛铁氧化物在残余岩浆中逐步富集,岩浆在上升侵位过程中与地壳物质的混染,不仅促使了岩浆中硫化物的饱和,同时提高了岩浆体系的氧逸度,进而促使了钛铁氧化物结晶沉淀。对香山西钛铁矿床成因研究的启示意义在于,除我国攀西地区高钛的玄武质岩浆外,普通的拉斑玄武质岩浆,在有利的分异演化条件下(如初始低氧逸度、相对高度的分异演化、后期与地壳物质混染)也有可能形成大型钛铁矿床。Abstract: So far,Xiangshanxi intrusion is the unique complex that was discoveried hosting both Ti-Fe oxide and Cu-Ni sulfide mineralizations in eastern Tianshan area. The genetic relationship between Ti-Fe oxide and Cu-Ni sulfide has been debated because of the lack of precise age data or trace element and microscopy evidences. This article studied the geology, petrology, geochemistry and geochronology of the Xiangshan intrusion. By detailed observation in the field and under the microscope and electron microprobe analysis, we found the complicated paragenetic relationships between Cu-Ni sulfides and Ti-Fe oxides in disseminated Cu-Ni ore from Xiangshanxi. By high-precision SIMS zircon U-Pb dating,we obtained the concordant age of 278.6±1.8Ma(MSWD=1.2)for Ti-Fe-bearing gabbros in Xiangshanxi, the age is almost identical to that of the Cu-Ni sulfide-bearing hornblend gabbro in the middle part of Xiangshang intrusion. These evidences provide the support for paragenetic relationships between Cu-Ni sulfide and Ti-Fe oxide mineralization in Xiangshanxi. The petrochemical characteristics of Xiangshan intrusion indicate that Xiangshan mafic-ultramafic complex (including Ti-Fe-bearing gabbro in Xiangshanxi) are products of comagmatic evolution and show a trend of tholeiitic magmatic differentiation. Compared with middle part and eastern part of Xiangshan intrusion, Xiangshanxi gabbros are relatively richer in REE, SiO2 (average value 48.99%)and K2O+Na2O(average vaule 3.43%)and TiO2(average value 1.26%),but poorer in MgO ( average Mg# value 62.4 ),which show the features of higher degree fractionation and relatively late-stage of magma evolution. By inversion of primiary magma composition, the result indicate that the parent magma of Xiangshan intrusion belongs to tholeiitic basalt magma with about 10% MgO, 9% FeO and 1.2% TiO2. So, a likely ore-forming mechanism for Xiangshanxi Ti-Fe dposit may be that the common parent tholeiitic magma became Ti-Fe enrichment in residual liquids through relatively high degrees of fraction crystallization. During the magma rising into crust, crustal contamination may not only induce sulfide saturation in magma, but also increase in fO2 of magmatic system, and therefore change the trend of tholeiitic magma differentiation, leading to Ti- Fe oxide crystallization. From a genetic viewpoint, ore-formation of Xiangshanxi Ti-Fe deposit in eastern Tianshan area suggests that except for high-Ti bastal magma in Panxi area of Sichan Province, the common tholeiitic magma (not so Ti-Fe enrichment ), in favorable conditions, such as lower fO2, relatively high degree of fractionation and crustal contamination in relatively late stage of magma evlution, may also form large scale Ti-Fe oxide deposit.
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