The formation of a magmatic Cu[sbnd]Ni sulfide deposit in mafic intrusions at the Kalatongke, NW China: Insights from amphibole mineralogy and composition

Abstract

The early Permian Kalatongke CuNi sulfide deposit in the Central Asian Orogenic Belt (CAOB) is the most important CuNi deposit in northern Xinjiang, NW China. It is hosted mainly by mafic intrusions, which is in contrast to deposits in the Eastern Tianshan area of eastern Xinjiang that are hosted mainly by ultramafic intrusions. The controls on sulfide mineralization in the Kalatongke mafic intrusions are poorly constrained relative to the ultramafic intrusions. Here we use amphibole compositions to trace the nature, oxygen fugacity, and water content of the Kalatongke magmas. The results, together with data for other NiCu mineralized magmas in Eastern Tianshan, are used to clarify the formation mechanism of sulfide mineralization in mafic intrusions. Amphiboles in the Kalatongke intrusions are divided into three types based on petrographic features: poikilitic amphiboles in olivine norite and norite (Type 1), which formed by reactions between olivine and pyroxene and interstitial residual melt; zoned amphiboles in norite (Type 2), generated by contamination processes; and needle-shaped amphiboles in diorite (Type 3), which indicate rapid cooling during the early stage of magma evolution. Modeled results show that the melts in equilibrium with amphiboles and clinopyroxenes exhibit enrichment in Th, U, and light rare earth elements (LREE) over heavy REE (HREE). Combined with amphibole and clinopyroxene chemistry, this indicates that the parental magma of the Kalatongke intrusions was strongly contaminated by continental crust during ascent. The oxygen fugacity of the magma during fractionation was evaluated using amphibole chemistry and olivine–sulfide equilibration. The oxygen fugacity of the magmas varied during their evolution, with the Kalatongke deposit magma showing relatively little variation compared with the deposits related to ultramafic intrusions in Eastern Tianshan. The oxygen fugacity of ore-forming magma thus varied significantly during the early stages of magma emplacement. Moreover, the calculated water content of melts in equilibrium with Kalatongke amphibole and clinopyroxenes averages 4.37 wt% in. These lines of evidence indicate that a highly hydrous and relatively oxidized magma favors the formation of sulfide deposits in mafic intrusions. It is inferred that sulfide saturation was delayed to the onset of mafic magma crystallization, rather than trapped in the early crystallized olivine cumulate within the staging magma chamber.