Petrogenesis and economic potential of the Erhongwa mafic–ultramafic intrusion in the Central Asian Orogenic Belt, NW China: Constraints from olivine chemistry, U–Pb age and Hf isotopes of zircons, and whole-rock Sr–Nd–Pb isotopes

Abstract

The Erhongwa mafic–ultramafic intrusion is located in the southern margin of the Central Asian Orogenic Belt in northern Xinjiang where many early-Permian mafic–ultramafic intrusions host important Ni–Cu sulfide deposits. In this paper we report zircon U–Pb age, olivine chemistry and integrated whole-rock chemical and isotopic compositions for the Erhongwa mafic–ultramafic intrusion. This intrusion is composed of lherzolites and gabbroic rocks. The U–Pb age of zircon from a large olivine gabbro sample from the intrusion is 283.1±1.5Ma, which indicates that the Erhongwa intrusion is contemporaneous with the early-Permian sulfide ore-bearing mafic–ultramafic intrusions in the central Tianshan region. Olivine from the Erhongwa intrusion contains up to 89.5mol% Fo and 3000ppm Ni, which are the highest among all known early-Permian mafic–ultramafic intrusions in the region. The occurrence of small sulfide inclusions in the most primitive olivine and significant Ni depletion in more fractionated olivine in the Erhongwa intrusion indicate that sulfide segregation took place during olivine fractional crystallization. The Erhongwa intrusive rocks are characterized by light REE enrichment relative to heavy REE, negative Nb anomalies, positive εNd (t=283Ma) values from +6.3 to +7.7, low initial 87Sr/86Sr ratios from 0.7034 to 0.7036, initial 206Pb/204Pb ratios from 17.8 to 17.9 and zircon εHf values from 8.0 to 15.5. The Erhongwa mafic–ultramafic rocks and coeval A-type granites in the region have similar isotopic compositions but the former have lower Th/Nb ratios than the latter. These similarities and differences are consistent with the interpretation that the Erhongwa magma formed by the mixing of a mafic magma derived from a depleted mantle with a granitic melt derived from a juvenile arc crust. It is deduced that sulfide saturation in the Erhongwa magmatic system was related to the magma mixing event at depth. More significant sulfide mineralization may exist in the lower parts of the Erhongwa intrusion, particularly in the magma conduit of the sulfide-bearing lherzolites of the intrusion. At the regional scale (i.e., the central Tianshan region), early-Permian ultramafic rocks containing olivine as primitive as that in the Erhongwa sulfide-bearing lherzolites are favorable exploration targets for Ni–Cu sulfide deposits.