Concepts of the Genesis of Sulfide Nickel-Copper Deposits

Abstract

Abstract Ideas about the genesis of sulfide nickel-copper deposits largely rely on the geology and type of major deposits or new discoveries of the times in which they were proposed. Nickel-copper sulfides in the Norwegian gabbronorite intrusions and in the Sudbury Complex were the early examples supporting the conclusion that mafic intrusions were the main host rocks for sulfide nickel-copper deposits. Experimental work by the Norwegian geochemist J. H. L. Vogt at the beginning of the twentieth century created a firm foundation for the hypothesis of magmatic segregation. However, some of the geologic observations of the Sudbury deposits were in contradiction to the magmatic segregation theory, and many authors considered hy-drothermal processes as the main accumulation mechanism of Sudbury-type Ni-Cu sulfides. These two main theories, magmatic and hydrothermal, still prevail in genetic considerations today. The discovery of shock metamorphic structures around Sudbury led to the idea that the Sudbury Complex was an old meteorite impact site and that the mafic intrusion with Ni-Cu sulfides followed the structure of a meteorite crater. After lively discussion, this idea has been widely accepted, but the type of magmatism, composition of primary melts, assimilation of silicate magma, and emplacement as well as localization of sulfides are still under study. Discoveries of ultramafic-hosted nickel sulfides in Manitoba, Canada, and, later on, the komatiite-associated massive nickel sulfides in Western Australia awakened the discussion of genetic models of nickel sulfides in ultramafic host rocks, which has continued until today. Some studies published in the 1980s indicated that the komatiitic ultramafic flows of the Kambalda area, Western Australia, formed deep thermal erosion channels in the underlying supracrustal sequence, and the lavas assimilated volcanic and sedimentary material. The massive Ni-Cu sulfides formed from the assimilated barren sedimentary sulfides and accumulated at the basal depressions of the komatiitic flows. Experimental studies widened knowledge of the origin of the Ni-Cu sulfides. In the 1960s the sulfide mineral stabilities and composition of sulfide phases were the main targets of study. In the 1970s the sulfide-silicate melt system was examined through the calculation of distribution coefficients between sulfides and silicates. Also, the importance of the magma/sulfide mass ratio (R factor) was discovered. Sulfur isotope studies of the Norilsk deposit, Siberia, proved that sedimentary sulfur was extracted from underlying sediments and reacted with metals in mafic magma to form Ni-Cu sulfide deposits. Since then, the origin of sulfur in Ni-Cu sulfides has been a topic of continuous interest. Geochemistry of platinum-group elements (PGE) forms an essential part in the recent studies of Ni-Cu sulfides. Not only are these elements important additional commodities of the deposits, but their distribution has proved to be an important indicator of the evolution of mafic magma. A debate on the origin of platinum-group element deposits is revolving around magmatic and hydrothermal models, resembling the discussion of the genetic theories of Ni-Cu sulfides. Recent studies prove that, owing to high distribution coefficients, magmatic sulfides accumulate a high proportion of platinum-group elements, but the elements are also remobilized by hydrothermal fluids and accumulated with them in suitable environments, forming high-grade deposits. Magmatic sulfides have been a target of international research programs, and IGCP project 161 has widened the knowledge of this type of ore deposit. It is now well known that the variation in composition and structure of magmatic sulfides and platinum-group elements is linked with the origin and composition of mafic magmas, fluid dynamics, emplacement of intrusion, magma mixing and assimilation, crystal fractionation, and partitioning of elements between sulfide and silicate melts, as well as hydrothermal and metamorphic remobilization.