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

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 iron-rich 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.