北阿尔金地区超基性岩地球化学特征及其成矿潜力分析

Geochemical characteristics and metallogenic potential of ultrabasic rocks in North Altun area, Xinjiang

北阿尔金地区经历了新元古代中期—早寒武世北阿尔金洋裂解形成及扩张、中寒武世—晚奥陶世北阿尔金洋双向俯冲-增生和早志留世—早泥盆世北阿尔金洋碰撞造山作用,在漫长的构造演化过程中保留了较多的基性-超基性岩体,地球化学研究显示,北阿尔金地区至少存在两种构造环境下的超基性岩。第一种超基性岩的微量元素特征显示其几乎没有受到地壳物质的混染,铂族元素含量低于原始地幔,推断其可能来源于原始地幔20%左右的部分熔融,形成于大陆裂谷的构造环境,为洋盆快速拉张时期幔源岩浆的快速上升侵位,代表了北阿尔金洋的初始裂解期间的岩浆活动。第二种超基性岩源区较为复杂,经历了较为强烈的地壳物质的混染,该组样品所在的蛇绿岩带具有俯冲带环境下蛇绿岩的特征,可能形成于汇聚背景下的弧后盆地环境,洋壳的俯冲携带大量的水及大陆边缘陆壳物质进入地幔岩浆源区,代表了北阿尔金地区在早古生代的古洋盆的俯冲消减过程。第一组样品的m/f值平均为1.64,为无矿的富铁质超基性岩,不具有Cu-Ni成矿潜力,铂族元素分析显示,其母岩浆可能为玄武质岩浆和超镁铁质岩浆混合的结果,为硫不饱和岩浆,经历过有限的硫化物熔离作用;第二组样品的铂族元素和Ni的含量较高,主要来源于地幔,未经历硫化物的熔离作用,m/f值平均17.95,为与铬铁矿有关的镁质超基性岩,具有形成铬铁矿的潜力,但形成铜镍硫化物矿床的潜力不大。

North Alun area has experienced complicated tectonic evolution process and, as a result, quite many ultrabasic rocks were formed in a post-collision extension background throughout the area, and geochemical characteristics of these rocks show that ultrabasic rocks in the study area have two kinds of different magmatic sources, evolution processes and tectonic backgrounds. The trace element characteristics show that the first group samples are almost free from the contamination of crustal substances and that the PGE content is lower than that of the primitive mantle. It can be inferred that the first group samples might have come from about 20% partial melting of the primitive mantle and the samples might have been formed in the continental rift tectonic environment because of the rapid rise and emplacement of mantle-derived magma during the quick extension of the oceanic basin,representing the magmatic activity of North Altun Ocean during the initial cleavage period. The geochemical characteristics of the second group samples show that their source region was more complicated and experienced a relatively strong contamination of crustal substances. The research shows that ophiolite belt contained in the samples has the characteristics of ophiolite in a subduction zone environment;therefore, it can be inferred that it was probably formed in a back-arc basin environment under the convergence background, the subduction of oceanic crust that carried a large amount of water and continental crustal substances in the continental margin entered the mantle magma region, suggesting that the subduction process of the ancient oceanic basin of North Altun area occurred in the Early Paleozoic. The average m/f value of the first group samples is 1.64, indicating that the samples are barren ironrich ultrabasic rocks having no metallogenic potential of Cu-Ni deposits. The analysis of platinum group elements shows that the parental magma might have resulted from the mixing of basaltic magma with ultramafic magma, which was sulfur-unsaturated magma, and the samples experienced limited sulfide liquation. The average m/f value of the second group samples is 17.95,indicating that the samples consist of magnesian ultrabasic rocks associated with chromite, and they have no metallogenic potential of chromite;the values of platinum group elements and Ni are comparatively high, and these elements were mainly from the mantle,and did not experience sulfide liquation. Therefore, the metallogenic potential of Cu-Ni sulfide deposits of the two groups of samples is quite low.