EQUILIBRIUM CRYSTALLIZATION OF MAGMATIC SULFIDE ORES

Magmatic sulfide ores form by the liquation of superheated sulfide liquid from silicate magmas. If sulfide liquid collects to form a discrete reservoir then it will eventually reach its liquidus temperature and begin to crystallize monosulfide solid solution (mss), magnetite, or both. Currently accepted models of the crystallization of mss from sulfide magmas envision a process of fractional crystallization to yield cumulates rich in compatible elements (usually Os, Ir, Ru, Rh and Ni) and residual sulfide liquids rich in Cu, Pt, Pd and Au. The compositions of magmatic sulfides present serious difficulties for this model. Several experimental studies have shown that at typical natural oxygen and sulfur fugacities Ni is weakly incompatible with mss at over almost the entire crystallization range of natural sulfide magmas. At temperatures close to the solidus of typical magmatic sulfide liquids (i.e., below 950 ºC) the partition coefficient for Ni between mss and sulfide liquid rises slightly above 1, allowing for compatibility of Ni only in the last stages of magmatic evolution. Magmatic sulfide ores apparently of cumulate origin contain too much Ni, Cu, Pt, Pd and Au, and show too small a range of Ir, Ru and Rh contents to have formed by fractional crystallization. A way to resolve this apparent paradox is to abandon the fractional crystallization model and propose instead that sulfide bodies evolve by equilibrium crystallization to near-solidus conditions. Model cumulate compositions in an equilibrium crystallization model with a remaining liquid fraction of 10 to 20% and moderate amounts of trapped liquid provide a close match to ores inferred to represent cumulates in several deposits including Mesamax (Cape Smith Fold Belt) and the Victor Deep (Sudbury). The last few percent of the sulfide liquid may then be tapped to form veins or disseminations rich in Cu, Pt, Pd and Au. Continued crystallization of this evolved liquid can lead to saturation in intermediate solid solution and subsequent Ni enrichment. The reason for a sudden migration of residual sulfide liquid after a protracted episode of equilibrium crystallization remains obscure, but it may be a consequence of late-stage volatile exsolution analogous to second boiling in silicic magmas.