Petrogenesis and sulfide saturation in the Yueyawan Cu-Ni sulfide deposit in Eastern Tianshan, NW China

Abstract The Yueyawan mafic intrusion hosted Cu-Ni mineralization is located at the Dananhu island arc in Eastern Tianshan Orogenic belt, NW China, and is unique in its paucity of ultramafic rocks. It formed by two intrusive phases: the phase I is composed of gabbro, troctolite and olivine gabbro, and the phase II is composed of fine-grained olivine gabbro and fine-grained troctolite. The phase I troctolite yielded SMIS U-Pb age of 283.4 ± 2.0 Ma, broadly coeval with the regional Cu-Ni sulfide mineralization. The mineral components and geochemistry characteristics suggest that the phase I rocks mainly occurred plagioclase fractionation whereas the phase II rocks primarily occurred olivine fractionation. Some plagioclase grains show reverse zoning with 63.4 to 52.1 from rim to core indicating the intrusion underwent magma mixing. The olivine is FeO-rich (16.61–32.58%), and its forsterite (61.8–80.7 mol%) and Ni (358–1218 ppm) contents are relatively low. The olivine and clinopyroxene thermobarometry show that the later pulse magma has higher temperature and higher pressure than the early pulse magma. The estimated equilibrated magma was likely much highly-evolved with 8.49 wt.% MgO and 11.02 wt.% FeO. The sulfide δ 34S values (–0.61 to 2.87‰) are consistent with those of the Cu polymetallic deposits in Kalatag and the Se/S ration of most samples range from 125 to 180×10-6 suggest that crustal assimilation caused prominent addition of external sulfur into the magma. The olivine crystallization, crustal sulfur addition and pressure rising were likely the triggers of sulfide saturation at Yueyawan deposit.