Mg–Sr–Nd isotopic constraints on the genesis of the giant Jinchuan Ni–Cu–(PGE) sulfide deposit, NW China

Abstract

Magmatic sulfide deposits are principal resources of nickel and platinum-group elements (PGE) in the world. These deposits form as the result of the concentration of immiscible sulfide-liquid droplets segregated from mafic or ultramafic magma. Sulfide saturation in the magmas is commonly linked to contamination by the continental crust. The Jinchuan Ni–Cu–(PGE) deposit is the largest single magmatic sulfide deposit in the world. It is hosted by a small mafic–ultramafic intrusion in the North China Craton. The Jinchuan mafic–ultramafic intrusion intruded Paleoproterozoic marbles, gneisses, and granitic migmatites. This paper reports the magnesium, strontium and neodymium isotopic compositions of the intrusion and the country rocks. Olivine, pyroxene and whole rocks of the lherzolites from the Jinchuan intrusion have δ26Mg values ranging from −0.40‰ to −0.26‰, with a weighted mean value of −0.31 ± 0.06‰ (2SD), which is slightly lighter than expected in a mantle-derived magma. The marble country rocks have relatively light δ26Mg values varying from −2.00‰ to −1.17‰, with a weighted mean value of −1.53 ± 0.02‰ (2SD). The variations are attributed to the interaction between basaltic magma and marble country rocks as the principal mechanism which generated the low Mg isotopic signatures in the Jinchuan deposit. The (87Sr/86Sr)i and εNd values of the lherzolites vary from 0.706615 to 0.714612 and from −8.29 to −6.13, respectively. The marble country rocks have relatively high (87Sr/86Sr)i ratios (0.713662 to 0.740069) and low εNd values (−25.43 to −12.41). The negative Nb anomalies in all samples from the intrusion indicate that the parental magma underwent crustal contamination. Mg–Sr–Nd isotopic mixing calculations indicate that the Jinchuan intrusion underwent 10–20% bulk contamination involving the lower continental crust, followed by <10% bulk contamination by marbles. Our petrological and geochemical investigation of the Jinchuan intrusion provides evidence that a combination of the two processes led to the formation of the ore deposit. The assimilation of lower crustal rocks reduced the sulfur solubility; the assimilation of marble country rocks made the magma more oxidized and hence decreased the sulfur solubility further. The results from this study show that Mg isotopes are useful in the study of carbonate assimilation by mafic magma, and marble assimilation by basaltic magma is an important mechanism to produce sulfide saturation in the genesis of Ni–Cu–PGE sulfide deposits.