Geochemistry, age and origin of some porphyry intrusions in the Onverwacht group, Barberton greenstone belt

Field investigations in the vicinity of the Msauli Mine have shown that a number of porphyry stocks occurring in this area are intrusive into the pre-folded metavolcanic formations of the upper Onverwacht Group. The age of one of these porphyry stocks has been determined by means of Rb-Sr whole rock isotopic analyses and yielded an age of 2 766 Ma with an initial 87Sr/86Sr ratio of 0,0705 9. Geochemically, the Msauli porphyries are classified as granitic to granodioritic in composition and thus differ from many other porphyries in the Barberton greenstone belt. Trace element modelling, using K, Rb, Sr and Ba, was undertaken in an attempt to distinguish between two potential source rocks, namely, tonalite/trondhjemite or amphibolite (tholeiite basalt). None of the calculations, however, resulted in a model magma similar to the porphyry rock. Instead indications are that mechanical mixing of the two potential source rocks resulted in a magma whose composition approaches that of the porphyries. The latter process, therefore, is favoured by the authors for the petrogenesis of the Msauli porphyries. The trace element modelling further suggests that the porphyries were formed by between 0 and 30 per cent fractional crystallization. This process is reflected in the existence of isolated phenocrysts within a fine-grained matrix