Geochronological, geochemical, and mineralogical characteristics of the Akechukesai-I mafic–ultramafic complex in the eastern Kunlun area of the northern Tibet Plateau, west China: Insights into ore potential

Abstract

The recently discovered Akechukesai mafic–ultramafic complex is located in the East Kunlun Orogenic Belt, northern Tibet Plateau, China. Two mafic–ultramafic complexes (Akechukesai I and Ⅱ) intrude marble of the Cambrian–Ordovician Tanjianshan Group. Cu–Ni sulfide mineralization occurs in these complexes. Pyroxenite in the Akechukesai-I complex has an age of 422 ± 10 Ma(1σ), similar to that of the Xiarihamu and Shitoukengde Cu–Ni deposits of the East Kunlun area. The positive εHf(t) values (1.7–4.9) and SiO2, TiO2, and Al2O3 contents of clinopyroxene from pyroxenite suggest that the parental magma of the Akechukesai-I complex was derived from depleted mantle composed of garnet lherzolite and spinel lherzolite in varying proportions. Pyroxenite of the Akechukesai-I complex has (87Sr/86Sr)i ratios of 0.70993–0.71405, εNd(t) values of −1.97 to −6.64, (206Pb/204Pb)i ratios of 17.113–18.994, (207Pb/204Pb)i ratios of 15.548–15.673, and (208Pb/204Pb)i ratios of 37.066–38.650, which together indicate a depleted mantle source with 18–35 wt% upper-crustal contamination. δ34S values of sulfide minerals are 5‰–11.5‰ with S/Se ratios of ore-bearing pyroxenite being in the range 3738–15,501, both being higher than mantle values (−2‰ to +2‰ and 2850–4350, respectively) and indicating the addition of crustal S. The incorporation of primitive magma into the marble host during ascent caused a reduction in sulfide solubility owing to increased oxygen fugacity of the magma, leading to S saturation. These observations and the geodynamic setting indicate that the complex is a favorable environment for mineralization.