FRACTIONAL CRYSTALLIZATION OF SULFIDE MELTS AS ILLUSTRATED AT NORILSKAND SUDBURY

Study of the compositional variation in ores at Noril'sk and Sudbury i ndicates that Ni increases initially and then decreases with increasin g Cu. Pt, Pd and Au increase with increasing Cu, while Ph, Ru, Ir and Os decrease until the Cu content of the liquid reaches 30-35 wt.%, at which stage they start to increase. Experimental data on tie-line rela tionships between mss and liquid in the Fe-Ni-S, Cu-Fe-S and Cu-Fe-Ni- S systems indicate that D-mss-liquid(Cu) can be approximated by an ext rapolation from similar to 0.28 where the liquid contains 2 wt.% Cu to 0.1 where it contains 30 wt%, and D-mass-liquid(Ni) increases with de creasing temperature, exceeding when the Cu content exceeds about 20 w t.%. An additional observation is that the ''wetting'' ability of sulf ide liquids with regard to solid silicates increases greatly with incr easing Cu content of the liquid. When these experimental data, in conj unction with literature data for partition coefficients of PGE between mss and iss on one hand and sulfide liquid on the other, are applied to interpreting the natural observations on element variation in ores, the conclusion is reached that mss is the principal sulfide liquidus phase until the Cu content of the liquid reaches 30-35 wt.%, at which stage iss takes its place. It is when this occurs that Rh, Ru, Ir and Os stop decreasing and start to increase with fractionation. The analy tical data, together with observations on the occurrence of pentlandit e in very Cu-rich ores, suggests that pentlandite may crystallise as a liquidus phase from these highly fractionated liquids.