Noble gas isotopic constraints on the origin and evolution of the Jinchuan Ni–Cu–(PGE) sulfide ore-bearing ultramafic intrusion, Western China

Abstract

The Jinchuan Ni–Cu–PGE sulfide deposit in Western China is the largest single magmatic sulfide deposit hosted by a small ultramafic intrusion in the world. The abundances and isotopic compositions of He, Ne and Ar in 23 silicate and sulfide mineral separates from peridotites, pyroxenites and sulfide ores have been determined by stepwise heating to investigate the formation and evolution of the intrusion and associated sulfide ores. Intriguingly, the volatiles released from elevated temperatures (400–1300°C) are all characterized by low 3He/4He (<1Ra) that are close to crustal value, varying 3He/4He, 21Ne/22Ne, 20Ne/22Ne and 40Ar/36Ar ratios suggest a mixture of crust, mantle and air components. Numerical modeling reveals that the apparent “crustal signatures” of the noble gases released from the Jinchuan samples are due to significant radiogenic production from U and Th decay after the crystallization from magma derived from a subcontinental lithospheric mantle at ~830Ma. In contrast, the noble gases released at low temperatures (200–400°C) are characterized by abundant atmosphere-origin components, suggesting that they are mainly derived from secondary fluid inclusions. The noble gases in the 400–1300°C fractions collectively exhibit a negative correlation between 3He/4He and 4He, and a positive correlation between 40Ar/36Ar and 40Ar, which are consistent with significant radiogenic ingrowths. The sulfide separates have slightly higher 3He/4He ratios than coexisting silicates in the intrusion, indicating more abundant radiogenic components in the latter. Mass balance calculations of He and Ar isotopic mixing models indicate that about 20% crustal fluid and 63% air saturated fluid (ASF) were added into ore-forming magma. Our data are consistent with a recent suggestion by some researchers that significant amounts of crustal fluids were added to the parental magma of the Jinchuan intrusion.