Geochemistry of the Sudbury igneous complex: a model for the complex and its ores.

Abstract

Regional gravity and magnetic data suggest that the Sudbury igneous complex is underlain at 5-8 km depth by a large mass of mafic and ultramafic rock which is not part of the complex. The marginal rocks probably crystallized in situ from a magma contaminated by felsic country rocks. REE profiles and major elements are consistent with a model of a 1:2 mixture of basement quartz monzonite and tonalite combined on a 1:1 basis with a fairly primitive flood basalt. Mafic and ultramafic inclusions have REE profiles similar to those of the main mass and sublayer and it is suggested that the inclusions were derived from cumulate layers formed during fractionation of the sublayer magmas. The contamination which led to the SiO2-rich composition of the complex is thought to be the cause of the sulphide segregation. The sulphide and ultramafic and mafic cumulates were injected into fractures beneath the complex and cooled and fractionated, while residual magma rose to form the present sublayer. The ore deposits were formed by deeper sills cutting overlying sills and concentrating the sulphides. The main mass of magma cooled slowly becoming contaminated more rapidly but fractionating less rapidly than the sublayer. It was injected in pulses at the same time as the sublayer.-R.E.S.