FRACTIONAL CRYSTALLIZATION OF SULFIDE ORE LIQUIDS AT HIGH-TEMPERATURE

Tie line compositions of (Fe, Ni, Cu)(1-x)S (monosulfide solid solutio n, mss) and Fe-Ni-Cu-S liquid, in the presence of sulfur vapor, have b een quenched from temperatures between 1,050 degrees and 1,180 degrees C. More than 80 bulk compositions on both sulfur-rich and sulfur-poor sides of the mss field were investigated by sealed silica tube techni ques. Qualitative observations of wetting behavior suggest increasing mobility in a silicate host rock, as sulfide liquids become more Cu ri ch. Partition coefficients for copper greater than or equal to 0.25, D -Cu(mss/liq) are obtained from Ni-bearing experiments with similar to 2 wt percent Cu and no quench phases. The equation D-Cu(mss/liq) = 0.0 003 x T(degrees C) + 0.0310 x (wt % S) + 0.0069 x (wt % Cu) - 1.3450 d escribes partitioning observed in Ni-free experiments above 1,000 degr ees C. Above 1,000 degrees C, the Ni distribution coefficient D-Cu(mss /liq) decreases with increasing temperature and/or Cu sulfur content o f the liquid. These results yield improved models describing the fract ional crystallization of natural sulfide liquids. Major element (Fe, N i: Cu, S) compositions of ores from the Sudbury district are shown to be entirely consistent with fractional crystallization at temperatures above 1,000 degrees C, with tile possible exception of rare samples e nriched in both Ni and Cu. Sulfide liquids fractionating Ni-Fe-rich ba nging-wall ores at Sudbury, Ontario, must have been less Cu rich than previously thought. At temperatures above 950 degrees C, reduced sulfu r activity in residual liquids can result in massive late-stage bornit e-rich ores.