Quantitative modelling of the segregation of magmatic sulphides: an exploration guide

CIM Bulletin, Vol. 77, No. 864, 1984

A.J. NALDRETT, J.M. DUKE, P.C. LIGHTFOOT and J.F.H. THOMPSON, Department of Geology, University of Toronto, Toronto, Ontario

The segregation of magmatic sulphides from mafic and ultramafic silicate magmas, in many circumstances, leaves the magma with a lower-than-normal Ni/MgO ratio, which is characteristic of both lavas erupted from the magma chamber and olivine crystallizing within it. Two end members of the segregation process are discussed; i) batch segregation and ii) fractional segregation. In the former, the critical factor is the magma/sulphide ratio. Computer modelling suggests that where this ratio has been less than 1000, the magma will be recognizable as anomalously low in Ni, due to sulphide segregation, and thus a good target for exploration. In fractional segregation, the critical factors are the proportion of sulphide to silicate being segregated and the degree of fractionation. Samples believed to represent the liquid portion of komatiitic lava flows from a series of unmineralized sequences in the Abitibi greenstone belt, Canada, Belingwe belt, Zimbabwe, and Barberton Mountain/and, South Africa coincide with the sulphide-free fractionation model. They are in marked contrast to samples from the ore-bearing districts of Kam-balda and Scotia, Western A ustralia in which distinct Ni depletion, corresponding to fractionation of olivine plus sulphide in a ratio of between 100 and 200:1 is recognizable. Ni-depleted olivines a/so mark the sulphide-bearing Dumont intrusion of the Abitibi belt. The Moxie intrusion of northern Maine has distinctly Ni-depleted olivines, as does the nearby and petrologically related Katahdin body. In both of these intrusions, the composition of the olivine allows reasonable predictions to be made of the Ni tenors of sulphide showings within them. At Katahdin, the Ni depletion is so widespread that little hope is help out for the presence of economic concentrations of sulphide. At Moxie, the olivine compositions suggest that the best chance of economic sulphides lies in the southern lobe of the intrusion. Ni-depleted olivines also characterize certain rock units within intrusions of the Insizwa complex, Transkei, Southern Africa. Many millions of tonnes of Ni have apparently been removed by sulphides from these rock units, many more than are contained within the single mineralized occurrence so far identified at Waterfall Gorge.

Keywords: Mineral exploration, Quantitative modelling, Magmatic sulphides, Chalcophile elements, Batch equilibration, Fractional segregation