Fractional crystallisation of sulfide melts as illustrated at Noril'sk and Sudbury
Naldrett, Anthony J.; Ebel, Denton S.; Asif, Mohammed; Morrison, Gordon; Moore, Chester M.
European Journal of Mineralogy Volume 9 Number 2 (1997), p. 365 - 378
published: Jun 26, 1997
manuscript accepted: Nov 12, 1996
manuscript received: May 6, 1996
ArtNo. ESP147050902013, Price: 29.00 €
Abstract Study of the compositional variation in ores at Noril'sk and Sudbury indicates that Ni increases initially and then decreases with increasing Cu. Pt, Pd and Au increase with increasing Cu, while Rh, 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 relationships between mss and liquid in the Fe-Ni-S, Cu-Fe-S and Cu-Fe-Ni-S systems indicate that mss-liquidDCu can be approximated by an extrapolation from ˜0.28 where the liquid contains 2 wt.% Cu to 0.1 where it contains 30 wt%, and mass-liquidDNi increases with decreasing temperature, exceeding 1 when the Cu content exceeds about 20 wt.%. An additional observation is that the "wetting" ability of sulfide liquids with regard to solid silicates increases greatly with increasing Cu content of the liquid. When these experimental data, in conjunction 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 analytical data, together with observations on the occurrence of pentlandite in very Cu-rich ores, suggests that pentlandite may crystallise as a liquidus phase from these highly fractionated liquids