Cu-Ni mineralization in Early Permian mafic complexes in the Kalatage area of eastern Tianshan (NW China): Petrogenetic constraints from geochronology, geochemistry, and Hf-Sr-Nd-Os isotopes

Abstract

The Early Permian was a critical period in the long history of accretion and collision of the southern Altaid orogen. Early Permian mafic-(ultramafic) intrusions and mafic–ultramafic-related sulfide Cu-Ni deposits are common in the eastern Tianshan, but the sulfide Cu-Ni deposits related to the mafic intrusions are still poorly studied, and yet provide key information on the ambient tectonic evolution. In order to help understand their petrogenesis, we report new data on the Early Permian mafic intrusions and related sulfide Cu-Ni deposits in the Dananhu arc of the eastern Tianshan in NW China. The ore-bearing Yueyawan complex is a well-differentiated, zoned mafic intrusion consisting of troctolite, olivine gabbro, layered gabbro, gabbro, and leucogabbro from the bottom upwards. The ore bodies are primarily strata-bound within olivine gabbros and troctolites at the bottom of the complex. The ores are more enriched in Cu than Ni than the more enriched, disseminated, veined and massive equivalents. Gabbros from the mafic complex yield zircon crystallization ages that range from 265 Ma to 273 Ma. The gabbros are tholeiitic and characterized by typical subduction-related geochemical signatures with low TiO2 contents, enrichments in Rb, Ba, U, K, Pb, and Sr, depletion in Nb and Ta, positive Eu and Ti anomalies, high εNd(t) values of +7.44 to +8.06, zircon εHf(t) values of +11.68 to +20.58, and low (87Sr/86Sr)i (0.703322–0.703416). The gabbroic rocks have positive correlations between SiO2 and TiO2, Al2O3, CaO, Nb, and Y, and negative correlations between SiO2 and MgO, Ni, and Co. The (187Os/188Os)i and γOs values of sulfides from the Yueyawan complex that contains Cu-Ni deposits range from 0.266 to 0.372 and 112 to 197, respectively. These geological, petrological, and geochemical characters indicate that the gabbros were mantle-derived, and that fractionation and crustal contamination were essential for the genesis of the magmatic Cu-Ni sulfide deposits. After integrating our data with those of previous investigations, we conclude that the protoliths of the sulfide deposits were probably derived from an asthenospheric garnet-free mantle in a subduction setting in the Early Permian. Oblique ocean-ridge subduction and a resultant slab window plausibly induced asthenospheric upwelling that gave rise to the generation of the mantle-derived melts, which likely moved into strike-slip faults through a slab window in the supra-subduction zone. Strong fractionation, crustal contamination, and multi-stage eruptions led to formation of the Cu-Ni sulfide deposits.