THE COMPOSITION OF SPHALERITE AND ASSOCIATED SULFIDES IN REACTIONS OF THE CU-FE-ZN-S, FE-ZN-S AND CU-FE-S SYSTEMS AT 1 BAR AND TEMPERATURES BETWEEN 250 AND 535 °C

Abstract

Sphalerites and sulfides in selected reaction assemblages of the Cu–Fe–Zn–S, Cu–Fe–S, and Fe–Zn–S systems were crystallized in molten salt fluxes contained in evacuated capsules at temperatures between ∼250 and ∼535 °C. The combined compositional data for sphalerite indicate large increases in the mol% FeS content and the Fe/Cu ratio with decreasing sulfur fugacity at given temperatures. Pyrite and the associated copper-iron sulfides show parallel increases for Cu/Fe ratios in the Cu–Fe–S system, and increases for Fe/Cu and Fe/Zn ratios in the Cu–Fe–Zn–S system. These compositional changes occur over a range of sulfur fugacity–temperature conditions that include many sulfide deposits.A log fS2–temperature (1000/K) composition map has been constructed for sphalerite using Lusk and Bray's (2002) data for a sequence of buffer reactions in the Cu–Fe–S and Fe–S systems. Accordingly, sphalerites with mean mol% FeS compositions of ∼1.7 (i.e. for the reaction: pyrite+idaite=bornite+S (vapor)), ∼4.4 (i.e. pyrite+bornite=chalcopyrite+S), ∼14.4 (i.e. pyrite+chalcopyrite=cubanite (iss)+S), 22.9 (i.e. pyrite+cubanite (iss)=pyrrhotite+cubanite (iss)+S), and ∼21.5 (i.e. pyrite=pyrrhotite+S) were obtained. A slight increase in the mol% FeS content of sphalerite with decreasing temperature was obtained for the pyrite=pyrrhotite+S buffer reaction at temperatures below ∼550 °C. The copper contents of sphalerite are generally low, ranging between ∼3 and ∼0.5 mol% CuS, and are highest in sphalerites associated with bornites.The equation: log10fS2=11.01−9.49 (1000/K)+[0.187−0.252 (1000/K)] (mol% FeS)+[0.35−0.2 (1000/K)] (mol% CuS in sphalerite) can be applied to sphalerites that have equilibrated with buffer assemblages containing pyrite, or pyrite and pyrrhotite, in both the Fe–Zn–S and Cu–Fe–Zn–S systems at temperatures (K is in Kelvin) between 250 and 550 °C at 1 bar. This equation incorporates present data and a selection of published experimental data that is fitted to within 10%.