The geology and geochemistry of the Waterfall Gorge section of the Insizwa Complex with particular reference to the origin of the nickel sulfide deposits

Abstract

The Jurassic-aged mafic rocks from the Waterfall Gorge section of the Insizwa layered complex, Transkei, range in composition from olivine gabbro and picrite (Basal zone), through a series of gabbros (Central zone), to monzonites (Roof zone). The compositional data for the mineral phases (olivine, plagioclase, bronzite, and augite) show significant features: in the basal gabbro the trend in compositional variation is the reverse of that seen in the Central zone of the intrusion; and olivines from the picrite unit are the most forsteritic in the intrusion, but they are depleted in Ni compared to olivines from the basal gabbro and Central zone. The data for olivines from the picrite unit at Tabankulu exhibit a similar but more significant degree of depletion.Mineral compositional and whole-rock geochemical data for the basal olivine gabbro suggest that olivine control is primarily responsible for the compositional variation in this unit at Insizwa, however, to account for the upward increase in the forsterite content of the cumulus olivine at the base, the intrusion of successively less-fractionated batches of a low MgO magma is suggested.We suggest that the depletion of the olivines in Ni can be attributed to the equilibration of the parent magma with sulfide and we show that these data are consistent with experimental distribution coefficient data for Ni between olivine and sulfide, permitting broad constraints to be placed on the grade, as well as the tonnage and location of the ore. Extrapolation of these conclusions to the Tabankulu intrusion suggest that the highly Ni-depleted olivines of the picrite have equilibrated with a magma from which a particularly high proportion of sulfide has segregated. Geophysical data fail to detect a massive orebody at the base of the intrusion, though several small bodies may exist. It is suggested that saturation may have occurred at depth, and only a small amount of sulfide has been transported to the surface in the picrite and concentrated in depressions at the base of the intrusion.Massive and disseminated magmatic Ni sulfide mineralization is confined to the lower Basal zone and shows a consistent fractionation into pyrrhotite-pentlandite-rich ore at the base and chalcopyrite-rich ore at the top (in both massive sheets and globular ore).The cause of sulfur saturation cannot be attributed to the reduction of the solubility of sulfur in the magma by increasing the SiO 2 concentration (contamination), despite the close association of the ore with granophyric segregations in the basal olivine gabbro. Indeed, the major and trace element data argue strongly against wholesale assimilation of crust at any stage in the production of the basal gabbro.