Sulphur solubility and sulphide immiscibility in silicate melts as a function of the concentration of manganese, nickel, tungsten and copper at 1 atm and 1400 °C

Abstract

Sulphur solubility and immiscibility relationships in silicate melts as a function of the concentrations of Cu, Mn, Ni and W were measured for melts synthesised at 1400 °C and 1 bar. Relationships between fS 2, fO 2 and the S solubility were also investigated. The results were used to extend and calibrate an existing model for the solubility of S in silicate melts to Mn- and W-bearing melt compositions. Mn was found to enhance S solubility. W was found to have little effect on S concentration. Ni stabilised an immiscible sulphide phases at metal contents higher than 0.01 to 0.05 wt.%. Cu was lost from the samples, this was attributed to the formation of an immiscible sulphide phase or devolatilisation of Cu as a Cu–S vapour phase. The data was consistent with an expression for S solubility of the form ln [ S ] = A 0 + ∑ M X M A M + 1 / 2 ln f S 2 f O 2 , where X M is the mole fraction of cation M and the A terms are calibration constants. A M values for Mn and W are 29.5 ± 1.7 and 8.54 ± 6.91 respectively. The expression was tested against other models, and on experimental data that was not included in the calibration. Observed and calculated values were in good agreement. The data for Cu and Ni are consistent with extant thermodynamic models for sulphide saturated silicate melts. These metals effectively stabilise sulphide melt phases, with implications for our understanding of the formation of magmatic sulphide ore deposits and the fractionation of chalcophile elements.