The Genesis And Metallogenic Model Of Jinchuan Cu-Ni(PGE) Magmatic Sulfide Deposit

The Jinchuan Cu-Ni(PGE) magmatic sulfide deposit, which contains >500 million metric tons(Mt) of sulfide ores with grades of 1.1 wt percent Ni and 0.7 wt percent Cu, is one of the three largest magmatic sulfide deposits in the world, and the largest magmatic sulfide deposit in China. The metals production of Ni, Cu, Co and PGE for Jinchuan deposit will determine the sustainable development of these metals for China. In addition to its economic significance, the Jinchuan metallogenic model is one of the most important examples for magmatic liquation deposit in mineral deposits. Otherwise, the Jinchuan deposit is intriguing to exploration geologists and researchers due to its unusual geologic characteristics.The Jinchuan Cu-Ni(PGE) magmatic sulfide deposit was discovered in 1958. And a lot of study have been done by the institutes and universities at home and abroad. On the basis of previous data, this paper sampled the typical ultramafic rocks and ores at the lower part of Jinchuan deposit(below the elevation of ~1250 m). We have carried out integrated geochronological, petrology and geochemical study. These zircon U-Pb dating and geochemical data are used to interpret the geotectonic beckground, the characteristics of parental and primary magma, to evaluate the magma evolution process and the mechanism of sulfide saturation and segregation of Jinchuan deposit, to discuss the mineralization and depth process of Jinchuan Ni-Cu(PGE) sulfide deposits. The most important innovative findings from this study are summarized below:1) In Longshouhan terrane, the age of Jinchuan ultramafic intrusion and associated sulfide mineralization is 839.4 ± 2.5 Ma. Our new zircon U-Pb dating indicates that the mafic dykes and Xijing mafic intrusion were emplaced at ~424 Ma, >400 Ma younger than the Jinchuan sulfide ore-bearing ultramafic intrusion. Otherwise, the Xiaokouzi mafic intrusion formed in ~280 Ma. Obviously, it is difficult to accept the mantle plume model, and we believe that the geotectonic background of Jinchuan deposit should be extensional environment which is related to the Rodinia breakup between 830 and 790 Ma.2) The Fo contents of olivine crystals of KJinchuan deposit is 80~85.5. The compositional variations of primary olivine from the sulfide-poor samples can be explained by a small degree of olivine crystallization(<5%) from a basaltic magma followed by local re-equilibration of the olivine with up to 30% trapped silicate liquid. In the sulfide-bearing samples the compositions of primary olivine record the results of olivine-sulfide Fe–Ni exchange that occurred after the trapped silicate liquid crystallized.3) The oxidation states of the different segments of Jinchuan deposit are similar, varying within a small range between QFM+0.3 and QFM+1; the MgO content of parental magma is 1.79 wt%, a typical high Mg basalts; the MgO content of primary magma is 18.1 wt%, which was formed by the partial melting of primitive; the simulated crystallization sequence of Jinchuan deposit is olivine �' orthopyroxene �' clinopyroxene �' plagioclase +clinopyroxene.4) The combination of large negative εNd values and positive γOs values(30-150) for the Jinchuan samples are best explained by contamination of mantle-derived(including SCLM) magma with small amounts of upper crustal materials. Mixing calculations based on zircon Hf isotopes and whole rock Sr–Nd isotopes indicate that the Jinchuan magma experienced up to ~20% contamination with the bulk upper crust.5) The multiple S isotopic compositions of the Jinchuan sulfide ores provide clear evidence that both Archean(elevated ?33S up to 2.67 ‰) and post-Archean(elevated δ34S up to 8 ‰) crustal sulfur was involved in the generation of the Jinchuan deposit.6) If the Jinchuan primary magma was generated at 75 km, the maximum S content in the magma estimated is 1111 ppm. This value is 1763 ppm less than the value that is required to induce sulfide saturation in the Jinchuan parental magma with assumed at 2 kb(6 km). The ~30 wt% olivine fractional crystallization alone will induce sulfide saturation in the Jinchuan magma. However, after such a high degree of olivine fractionation the residual magma would have been too depleted in Ni to form the sulfide ores with high Ni tenors that characterize the Jinchuan deposit. Therefore, it is likely that other processes such as crustal contamination played a more important role in triggering sulfide saturation in the Jinchuan magmatic system.7) Our new model showed that the R factor of different segments in Jinchuan deposit is 150~1000, and reduce from east to west, which indicated that Jinchuan deposit formed in an open system such as a flow-through system. The R-factor could increase further if the sulfide reacted with new, sulfide-unsaturated magma. Accordingly, the initial content of Pd in the parental magma of the Jinchuan deposit is estimated to be ~1 ppb. This value is only about 1/10 of the average value for PGE-undepleted picrites associated with continental flood basalts, indicating that the Jinchuan parental magma indeed experienced previous sulfide segregation.8) The eastward increase of PGE tenors in the deposit is explained by metal upgrading by a new surge of magma moving eastward through a pre-existing sheet-like conduit where immiscible sulfide liquid from the previous magma was deposited.9) This paper called “I6 concealed orebody” as segments I-a and II-W-a. The geological form of segments I-a and II-W-a indicated that the magma was moved from west to east and from down to up. The segments I-a and II-W-a were formed by a different parental magma with lower Pd/Ir than that for the other ore zones of the Jinchuan deposit. This explanation is supported by the fact that:(1) the depletions of IPGE in these zones are not coupled by enrichments in Pd and Cu, which rules out MSS fractional crystallization;(2) The wholerock LOI values are not higher in these two zones than in the other parts of the deposit, which close out the possibility of post-magmatic hydrothermal overprinting.We suggest that the depletion of IPGE in the magma resulted from fractional crystallization of Cr-spinel and olivine, which is consistent with the observation that olivine and Cr-spinel are generally more fractionated in the IPGE-depleted ore zones than elsewhere in the deposit and that IPGE are compatible whereas PPGE are incompatible in these two minerals in basaltic system.