Abstract

We present the first in-situ Sr isotopic and mineral geochemical results for plagioclase in mafic to ultramafic rocks of the Waterberg Project of the far northern Bushveld Complex. Initial strontium isotopic compositions of plagioclase are dominantly in the range of 87Sr/86Sri (Sri) 0.7065 to 0.7075, differing from the rest of the Northern Lobe, which has higher values with an average Sri values of 0.708. Based on mineralogy and geochemistry, the intrusion was previously interpreted to consist of two main pulses. However, the Sr isotopic data exhibit relatively little stratigraphic variation over a wide range of rock types. The results of this study, therefore, represent a paradox: Compositionally clearly different magmas have nearly identical initial Sr ratios, whereas the compositionally somewhat similar magmas of the Northern Lobe display statistically significantly different initial Sr ratios. We suggest, therefore, that the observed Sr values represent a proxy for the residence time and concomitant country rock assimilation of magma in the lower crust and in the case of the Waterberg Project, indicate one common staging chamber for all magmas. Furthermore, the data presented here, together with recently published studies support a disconnect between rocks north and south of the Hout River Shear Zone, and indicate two separate, mineralized, magmatic basins.

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