Abstract

The Palabora Igneous Complex, located in the Archean Shield of northeastern Transvaal, South Africa, is unique amongst many described African alkaline complexes in that its carbonatite member is the site of an economic deposit of copper ore. Magnetite, uranothorianite, and baddeleyite are subsidiary products of the copper mining venture, while the ultramafic rocks of the Complex are also host to economic deposits of apatite and vermiculite.The Palabora Igneous Complex resulted from an alkaline intrusive cycle which emplaced, in successive stages, pyroxenite, syenite, and ultrabasic pegmatoids. Some products of metasomatism exist. The pyroxenite intruded first in a north-south elongated, kidney-shaped stock which covers an area of approximately 1,655 hectares. A corona of feldspathic pyroxenite, formed by interaction with the Archean gneiss country rock, is peripherally distributed. Syenite plugs were forcibly injected into the gneisses surrounding the main pyroxenite mass, followed by an extended period of nonviolent and of partly metasomatic activity which formed irregular, vertically disposed ultrabasic pegmatoids at three centers in the pyroxenite pipe and caused some fenitization of the Archean gneisses in contact with the pyroxenite. During this latter stage the foskorite and banded carbonatite in the centrally located pegmatoid body were emplaced.Subsequent fracturing of the consolidated infilling of this latter subsidiary pipe and renewed igneous activity led to the intrusion of a dikelike body of transgressive carbonatite at the intersection of two prominent fracture zones and a stockwork of transgressive carbonatite veinlets cross-cutting the older rocks along preferred trends. Intensive postcarbonatite fracturing along pre-existing zones of rupture provided channels for copper-bearing mineralizing solutions to permeate the carbonatite-foskorite pipe infilling with veinlets of copper sulfide and other subsidiary sulfides.A late-stage copper mineral, valleriite, occurs as replacement intergrowths and coatings along grain boundaries, cracks, and cleavage planes in the other copper sulfides and in the gangue minerals. Valleriite has poor flotation characteristics, thereby causing losses in the flotation of the copper sulfide minerals which are thus coated.The ultrabasic pegmatoid bodies consist of two main rock types with constituent minerals in varying proportions, i.e., an olivine-phlogopite-diopside rock and a diopside-phlogopite-apatite rock. In places the constituent minerals of these rocks attain pegmatoid dimensions. Hydration of the phlogopite in the weathered zone of the pegmatoid bodies has formed commercial deposits of vermiculite.Apatite is an important mineral constituent of pyroxenite, foskorite, and some of the pegmatoid rocks, attaining economic concentrations over large areas.The complex nature of the copper-magnetite and vermiculite orebodies and the large-scale mining methods employed for their exploitation necessitate meticulous evaluation and grade control techniques to ensure accurate short-term mine planning. The evaluation and grade control procedures involve an unusual combination of visual and laboratory methods, which have greatly contributed to the efficiency and low production costs of the operations.

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