The Studies On The Sulfide Saturation Mechanism Of Jinchuan Magmatic Cu-Ni Sulfide Deposit

Jinchuan magmatic Cu-Ni sulfide deposit is the third largest world-class Cu-Ni deposit. It has an intrusion with a small outcropping size of only 1.34km2 but a high sulfide/silicate ratio. Jinchuan intrusion mainly consists of mafic rocks whose major rock-forming minerals include olivine, clinopyroxene, othopyroxene etc. The EMPA analysis for the Fo value of olivines from distinct rock types of Jinchuan intrusion shows that the Mg# value ranges from 80-83. The narrow range indicates that the surroundings where sulfide deposits are part of open magmatic systems facilitating the injection of post multi-stage magmas. Sharp contact boundaries are commonly seen between rocks of distinct grain sizes and ores of distinct types. The boundaries indicate that the magmas had achieved metal preconcentration prior to reaching its present chamber or the magmas reached sulfide saturation during the ascent into magma chamber. The sulfide saturation mechanism mainly involves discriminating magma source, magma composition determination, measuring the partial melting degrees of mantle, the sulfur content of melts, different factors casting effects on the sulfur capacity of magmas, the dominance of the factors, the SCSS (sulfur content at sulfide saturation) of magmas, etc. This study reports that Jinchuan primary originates from enriched mantle as a result of partial melting of～30% degrees. Trace element characteristics suggests that Jinchuan primary magma has undergone crustal contamination of 15%～40% degrees. Sulfur isotope study shows that sulfur of Jinchuan comes from mantle instead of crust, precluding the possibility that crust sulfur addition may be an attribute to saturation. Based on reliable assumption, the sulfur content of Jinchuan primary magma is estimated to be～1000ppm. According to the correlation between the SCSS and depth of basaltic magmas, the SCSS of Jinchuan primary magma is greatly likely to be 950ppm-1100ppm. Therefore, the possible sulfur saturation mechanism is presented as follows. After partial melting of ～30% in the enriched mantle, Jinchuan primary magma comes into being. Her sulfur content is likely 1000ppm near the possible sulfide saturation of 950ppm-1100ppm. During her ascent into magma chamber,～15%-40% degrees of crustal contamination brings her an easy sulfide saturation.