Sulphides in the Skaergaard intrusion, East Greenland

Abstract

In the chilled marginal gabbro of the Skaergaard intrusion are small specks of chalcopyrite and bornite; the amount of sulphur is .005 percent. This is taken to be representative of the amount in the 300 km 3 of original Skaergaard magma.In the layered rocks, up to the latest ferrogabbros, sulphides occur in small composite units mostly about 0.4 mm across and forming about .02 percent by weight of the rock. The sulphides are bornite, digenite and chalcopyrite, with some covellite replacing the digenite. In melanocratic bands consisting largely of the heavier crystal fractions, copper sulphides are ten times as abundant as in average rock. It is concluded that immiscible droplets of dominantly copper sulphide liquid formed in the silicate liquid at this stage and could accumulate, as a result of gravity, along with the heavy, primary precipitate silicate and oxide minerals.In the fayalite ferrogabbros at the top of the layered series droplet-shaped patches of pyrrhotite (mostly altered to marcasite) with a little chalcopyrite but no bornite make up about 2 percent of the rock. At this stage in the fractionation an immiscible sulphide liquid, rich in ferrous sulphide, had formed. In the latest rock fraction, the acid granophyre, pyrrhotite or pyrite with a little chalcopyrite is patchily present up to 4 percent by weight of the rock.The composition of the latest pyrrhotite has been determined and the composition of earlier immiscible copper sulphide and iron sulphide liquids has been indirectly estimated. The copper-rich sulphide liquid apparently began separating from the silicate magma when the amounts of copper and sulphur in the magma were about .02 and .01 percent respectively. This is considered to represent the solubility of the copper sulphide in the ordinary basic magma at the time of formation of the hypersthene olivine gabbros. With the changing composition resulting from further fractionation the solubility of the copper sulphide in silicate magma increased until the amounts at saturation were .05% Cu and .03% S. The iron sulphide matte separated later when the sulphur content of the silicate magma had risen to about .06 percent and the ferrous iron was about 15 percent. The partition of Cu, Ni and Co between silicate and sulphide liquids is roughly determined at various stages of the fractionation.The Skaergaard sulphides are low in nickel because, by the time an immiscible iron sulphide liquid separated, nickel had been reduced to very low amounts in the magma as a result of its abundant entry into early olivine and pyroxene. Nickel does not enter the copper-rich sulphide liquid so readily as the later iron-rich sulphide liquid. Cobalt is strongly concentrated in the copper-rich liquid and moderately in the iron-rich. The amounts of palladium, indium and gold have also been determined in selected rocks, from which it appears that at low concentrations the ease of entry of these elements with the partial exception of gold into the sulphide liquid is not markedly greater than into the silicate.It is suggested that nickeliferous iron-rich sulphides such as the disseminated ores of the Bushveld and Insizwa which separated at an early stage in the crystallization of the magma were formed because there was a relatively high initial sulphur content.