Abstract
The giant Jinchuan nickel-copper-platinum-group element (PGE) deposit is hosted by two individual sub-vertical intrusions, referred to as the western and eastern intrusions (including segment II-W and segment II-E). Exactly how the Jinchuan deposit was formed by a system of sub-vertical magma conduits is still not well understood. This paper reports new major elements, trace elements and PGEs data from the Jinchuan deposit to study the formation mechanism of sulfide ores with different textures and their relationship with the magma conduit system. Our study shows that the PGE tenors of disseminated and net-textured sulfide in segment II-E are significantly lower than segment II-W and the western intrusion, but the Cu/Pd ratios are opposite. In addition, net-textured sulfides in segment II-W show a negative correlation between IPGE (Ir, Ru and Rh) and PPGE (Pt and Pd) in contrast to the positive correlation in segment II-E and the western intrusion. These features indicate the parental magma sources of the western intrusion, segment II-W and segment II-E were originally three different surges of PGE-depleted magma. Modeling of parental magma in the western intrusion, segment II-W and segment II-E suggests that they were formed by the same initial picritic basalt (100 ppm Cu, 1 ppb Ir and 10 ppb Pd) with different prior sulfide segregations (0.0075%, 0.0085% and 0.011%). The three parts of Jinchuan sulfides show that the Pt/Pd and (Pt + Pd)/(Ir + Ru + Rh) ratios decrease from section III-5 toward both sides in the western intrusion and decrease from section II-14 toward all sides, whereas no regular spatial variations occur in segment II-E, showing the different fractionation processes of sulfide melt. The massive sulfides in the western intrusion and segment II-E experienced a ~20% to 40% and ~40% to 60% fractionation of sulfide melt, respectively. We propose that the Jinchuan deposit was generated in a metallogenic system of multiple magma conduits, where sulfides entrained in parental magma experienced different amounts of prior removal.
Highlights
Magmatic nickel-copper sulfide deposits are generated by the accumulation of an immiscible sulfide liquid which is segregated from a host silicate melt [1,2,3]
Because of the preferential partitioning of the chalcophile elements (e.g., platinum-group element (PGE), Ni and Cu) into segregated sulfide liquid, the metal tenors of the sulfides are upgraded by the equilibration of successive batches of silicate magma passing through systems of dynamic magma conduits [12,13,14]
This study demonstrates a new metallogenic model related to three individual magma conduits for the Jinchuan deposit, which provides new insights into the relationship between dyke-like mafic-ultramafic intrusions and metallogenic magma conduit systems
Summary
Magmatic nickel-copper sulfide deposits are generated by the accumulation of an immiscible sulfide liquid which is segregated from a host silicate melt [1,2,3]. Chalcophile elements are commonly used to study sulfide segregation and mineral fractionation [20,21,22], which are important to understand sulfide concentration processes and the evolution of mafic-ultramafic rocks in magma conduit systems [15,23,24]. For the two largest orebodies in the eastern intrusion, sulfides are enriched in different directions [37] and the average contents of PGEs show an order of magnitude difference [29]. Whether they were formed by the same magma conduit or not is still not clear. This study demonstrates a new metallogenic model related to three individual magma conduits for the Jinchuan deposit, which provides new insights into the relationship between dyke-like mafic-ultramafic intrusions and metallogenic magma conduit systems
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