PETROLOGICAL MODEL FOR THE FORMATION OF NORIL'SK NICKEL-COPPER DEPOSITS

Abstract

This paper presents a model for the formation of nickel-copper sulfide ores of the Noril'sk deposits, which originated as a result of the fluid sulfurization of iron-rich differentiates in picritic layers of gabbro-dolerite intrusions. The generated ore melts were immiscible with silicate liquids, which is recorded in the widespread occurrence of droplike ores in the picritic layer. Most of the ore melt left its place of origin, sank to the lower part of intrusions, and formed copper-rich pocket ores in taxitic gabbro-dolerites (taxitic layer) directly below the picritic layer and nickel-rich massive ores at the base of intrusions. Such a distribution of copper and nickel in the sulfide ores is correlated with their different platinum metal contents and agrees with the structure of sulfide drops in the ores of the picrite layer, which are separated into the upper copper and lower nickel zones. These relationships are controlled by the differentiated downward migration of ore magmas from the picritic layer owing to copper-nickel splitting. High-grade copper ores have another genesis and were formed from residual melts during crystallization of massive ores. These copper-rich ores build up aureoles around the massive ores and penetrate sedimentary rocks underlying the intrusions. The model of fluid sulfurization adequately explains the geochemical features of sulfide deposits, which show an upward enrichment in selenium and the light sulfur isotope. The emplacement of the ore-bearing intrusions of the Noril'sk deposits occurred through the magmatic replacement of platform rocks, which provides direct evidence for the fluid character of the magmatism. The petrological model presented in this paper is the only way to reconcile the tremendous scale of sulfide mineralization in the Noril'sk deposits and the limited sizes of their parent intrusions.