Partition of Ni between olivine and sulfide and its application to Ni-Cu sulfide deposits

Equilibration of natural olivine with (Fe, Ni)S in sealed silica glass tubes yields a value for the distribution constant for Ni/Fe exchange (K_D3) of 27.7±3.5, for 1,200° C, product olivine and sulfide compositions in the ranges 96 to 97 mol% Fo and 15 to 70 mol% NiS, respectively, and run durations of 28 days. Electron microprobe analysis of product olivine was made on ～50 μm diameter grains separated from the sulfide matrix to eliminate the severe matrix interference experienced with in situ determination of trace and minor amounts of Ni. The present K_D3 value is in general agreement with previous experimental results indicating K_D 3～30 in the temperature range applicable to hypothesized magmatic models for Ni-Cu sulfide ore formation and with inferred K_D3 values for the temperature range 400 to 600° C. The olivine and sulfide compositions of a selection of Ni-Cu sulfide deposits yield K_D3 values appreciably lower than the corresponding equilibrium values, which argues against concentration of the sulfides to ore grade in their present locations as early magmatic sulfides. However, K_D3 is significantly higher for Ni-Cu sulfide deposits metamorphosed to medium and high grade conditions. The good correlation of increasing approach to chemical equilibrium with increasing grade and pervasiveness of metamorphism is related to the marked temperature dependence of the mobility of Ni in olivine.