The Usushwana complex in the Southeastern Transvaal, with special reference to its economic mineral potential

Abstract

The Usushwana Complex consists of a suite of hypabyssal, volcanic and plutonic rocks emplaced after deposition of the Pongola Group and in turn intruded by Pongola granite. The geological history against which the petrology of the complex is discussed sta~ts with the deposition of the Pongola Group sediments including widespread extrusion of basaltic lavas of the Ngwempisi Formation. Deposition of these rocks was followed by deformation, viz. the formation of the Amsterdam syncline and similar synforms east and southeast of Piet Retief and the subsequent disruption of these structures by normal and strike-faulting. During this period of structural deformation ultramafic sills of the Thole Formation, the magma of which bears resemblance to the Geluk type of basaltic komatiite of the Barberton area, were emplaced into the pile of rocks of the Pongola Group. After an erosional hiatus the volcanic phase of the Usushwana Complex ensued with the extrusion of rocks of the Amsterdam and Vaalkop Formations. Except for the rhyolitic Athole Member in the north-central part of the Amsterdam syncline, the Amsterdam Formation consists of pyroclastic rocks of predominantly dacitic composition. They exhibit characteristic macroscopic and microscopic features of ash flow tuffs, e.g. lack bedding and sortins~ variation in the abundance of clasts of foreign material to such an extent that the rock may grade into bodies of pyroclastic breccia in places, broken and corroded crystal fragments, microlites of tridymite which are typical ih the vapour phase of ash £lows as well as crystallites and lithophysae. Many of the diagnostic microscopic textures usually exhibited by a~h flow tuffs have, however, subsequently been destroyed by devitrification and recrystailisation. The Vaalkop Formation, which is developed in the Piet Retief area and farther south, consists of rhyolite ranging from a porphyritic, granophyric rock containing euhedral, totally altered plagioclase laths, to a felsitic type. The volcanic rocks in places constitute the roof of the plutonic phase of the Usushwana Complex. The intrusive rocks are subdivided in the gabbroic Piet Retie£ Suite and the acidic Hlelo Suite. Three types of gabbroic rocks are recognised, i.e. Aormal gabbro, ferrogabbro and quartz ga:Obro. Whereas the ferrogabbro is considered to be an iron-enriched differentiate of the quartz gabbro, no genetic link between these two varieties and the gabbro could be established. Except for sporadic segregations of magnetitite in the ferrogabbro these rocks do not display direct evidence of differentiation. Although the petrological relationships of a fourth type, hyperite, are not quite co~clusive it is included in the Piet Retief Suite because it is usually associated with these rocks, both in the area mapped and in Swaziland. The Hlelo Suite comprises granodiorite and microgranite. The granodiorite is considered to be partly a product of differentiation of the gabbroic suite and partly of assimilation of roof rocks, viz. Vaalkop rhyolite and quartzite of the Pongola Group. In places it grades into microgranite, which in turn cannot always be readily distinguished from the Vaalkop rhyolite, neither in the field nor petrochemically. The microgranite is considered to be essentially a product of remelting of the Vaalkop rhyolite. The geological setting of the Usushwana Complex appears not to have afforded favourable conditions for the formation of ore deposits. The magnetite layers are invariably very restricted and, moreover, the v 2o 5 content is only in the order of 0,25 per cent. Sulphides occur sparingly in the gabbroic rocks and in the basaltic lava of the Ngwempisi Formation. Even where they are somewhat more conspicuous the concentrations of Cu, Ni and Zn remain in the O,OOX to O,OX per cent range and only one exposure of hyperite containing over 0,1 per cent Cu was observed. A geochemical stream sediment survey covering about 750 km 2 was conducted over the northern part of the area mapped. The apparent absence of mineralisation is discussed in the light of petrological evidence.