The Major Nickel-Copper Sulfide Deposits And Their Genesis Of China

The major nickel-copper sulfide deposits or Ni-and Cu-bearing in-trusions of China are represented by Jinchuan, Hongqilin, and Limahemasses. They occur in passive continental margins, back-are exten-tienal basins, and continental rifts respectimely, and may be dividedinto three types. Although there are differences in their metallogenictectonic settings, the connections with regional extending are theircommon characteristics.In those deposits, the stable sulfide mineral assemblage is allpyrrhotite-pentlandite-chalopyrite. The ores have all sider-onitictexture, unmixing-ball texture, impregnation structnne, and, secon-darly, compact-massive structure. Usefull components are Ni, Cu, Coand S. Those features are in accord with that at abroad.However, the differences are certain compared with the same nickel-bearing intrusions in the other regions of the world. The distinguienshed aspects are as follow: 1) the masses of China are all small-selzedcomposite lopolith, dyke and vein or monolith lenticules, which, ob-viously, differ in some great layered complex such as Bushveld and SSudbury complex and so on; 2) the basicity of the masses at home ishigher than that at abroad. The deposits are all confined to the latesupplement injected ultramafic or monolith ultramafic mass in thecomppsite mass without any exception, to this day no deposit associ-ated with the mafic mass has been found, and the known host rocks arealmost dunite, lherzolite, peridotite, olivdne pyroxenolite and pyr-oxenolite, etc, whereas the host rocks at abroad are mainly norite;3) the nickel-copper metallogenic period in geological history inChina is Caledonian-Hercynian period, whereas that at abroad isprecambrian and predomonately Archean and Suberathem. Those differe-nces should be attributed to that the Sino-Continental was locatedin given position in global tectonics.Detailed inwstigations on mineralogy, petrology and geochemistryhave clarified the scale and enrichment degree of the orebodies norelation to rock type and scale of the masses, a large-seized depo-sit may occurs in a small mass. For example, the nickel-copper sul-fides from Limahe deposit make up 20％of the nock volume, from Hong- qilin No.7 deposit 40-75％, and from Jinchuan deposit 30-45％. The con-tent of nickel-copper sulfides is much more than their solubility inthe mafic-ultramafic magmas, which conld'nt result from the magmaticon-site-liquation but were bound to result from the magmatic typhonicdifferetiation. It appears that the conclusion on no large deposit ina small mass drawn by T. L. Vogt is confined. This opens wast vistasto the search for new nickel-copper sulfide deposits.The data from sulphur isotope, rare earth and incompatible elementsshaw that the nickel-copper-bearing magmas derived from high-degreepartial melting of mantle lherzolite with exceeding amount of radio-active elements as U and Th. The calculation of pressure and tempera-ture in magma-source and the quantitative modeling of immiscibilitybetween silicate and sulfide indicate that the partial melting degreeof mantle-source-rock from Limahe, Hongqilin and Jinchuan masses maybe reach to 20-30％, 30-45％and 35-52％respectively, which are all higherthan that of any basaltic even tholeiitic magmas. The study also shawsthat the metallogenic speciality of ore-bearing magmas is related tothe partial melting degree of mantle rocks.The processes or evoloution of nickel-copper-bearing magma underwentthe following 3 stages:1) the primitive enrichment under mantle's condition, related tothe melting and escaping of the basaltic components of upper mantle,this stage would make mantle sulfide increasing of 1.5～4.0％.2) immiscibility of silicate-sulfide in mantle-magma-re-servoir, the fact wich lead to liquid unmixing between silicates andsulfides is sulphur and Ni, Cu metals solubility in primitive magmas.3) the liquid-gravity-differentiation in intermediate chamber aboveMoho-discontinuty. In this stage, owing to full liquid-gravity differ-entiation in a stable structural envirement, the upper parent magmabecame a basic rich in Si, Al, alkali, low in Ca.and poor in Mg, orthe top—a intermediaye magma, the lower—a ultramafic magma low inAl, poor in Ca, alkali and rich in Mg, sulfide, the bottom—sulfide oremagma.During multiple reactivation of fracture structures the magma in-truded into the chambers on differet levels in upper crust, crystall-izing and forming different types of nickel-copper sulfide deposits.The rock-and oreOforming mechanism mentined above may be called "ma-gmatic multiple-stage differentiation model of the genesis of nickel-copper sulfide deposits".