Abstract

The partitioning behavior of Fe, Co, Ni, and Cu between immiscible sulfide liquid and basaltic melts has been studied using evacuated silica-tube techniques at temperatures above 1,255 degrees C. The observed Nernst partition coefficients (D) are 274 + or - 34, 245 + or - 33, and 80 + or - 15 for Ni, Cu, and Co, respectively, between sufide liquid and a normal basaltic melt (with 8.3 + or - 0.3 wt % MgO) at 1,255 degrees C; the distribution coefficients (K D = (Me/Fe) (sub sulfide liquid) X (Fe/Me) (sub silicate melt) where Me = Ni, Cu, and Co) are 42 + or - 7, 35 + or - 6, and 15 for Ni, Cu, and Co, respectively. The composition of silicate melts was observed to have a significant effect on the partitioning of Ni and Cu. For andesitic silicate melts (with 4.5 wt % MgO), Ni is more sulfophile than Cu, whereas for a more basic melt (with 13.5 wt % MgO) Ni behaves as if it were less sulfophile than Cu. The behavior of Ni is explained by its high octahedral site preference energy and by the decreasing proportions of available octahedral sites for transition metal ions with a decreasing basicity of silicate melts (Burns and Fyfe, 1964).The composition of sulfide liquids, especially the concentration of Ni and Cu and Cu/(Cu + Ni) ratios in these liquids as a function of the composition of silicate magma, was estimated by extrapolating the present experimental results to ultramafic magmas and applying them to a typical natural komatiitic magma (Naldrett and Turner, 1977). The estimated compositions are in reasonable agreement with the reported compositions of natural Ni and Ni-Cu sulfide deposits. It is concluded that the composition of silicate magma determines not only the initial concentration of ore metals but also their partitioning behavior between immiscible sulfide liquid and silicate magma--the two important factors determining the composition of massive, magmatic Ni-Cu sulfide deposits.

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