Magmatic sulfide ores form as the result of droplets of an immiscible sulfide-oxide liquid developing within silicate magma and then becoming concentrated in a particular location. Certain elements, notably the Group VIII transition metals Fe, Co, Ni, Pd, Pt, Rh, Ru, Ir and Os together with Cu and Au, partition strongly into the sulfide-oxide liquid, and thus become concentrated with it. An understanding of deposits of this type requires an understanding of the chemical and physical processes that are involved. To help in this, we first discuss (section 2.1) factors governing the solubility of sulfide in mafic and ultramafic melts. This is followed by a discussion of the partitioning of elements between sulfide-oxide liquid and silicate magma or olivine, and the influence of mass ratios between sulfide liquid and silicate melts (“R” and “N” factors) on the composition of sulfides. Some relevant phase equilibria are discussed in section 2.3. Section 2.4 is devoted to fractional crystallization of sulfide melts which is particularly relevant to the formation of valuable PGE-rich zones within deposits such as are known at Noril’sk and Sudbury and elsewhere. In a final section, 2.5, the poorly understood problem of how sulfur enters a magma from an external source is briefly discussed.
KeywordsFractional Crystallization Oxygen Fugacity Basaltic Magma Silicate Magma Sulfide Liquid
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