The sulphides in the layered sequence of the Bushveld igneous complex

Abstract

The Bushveld Complex, which is the largest mafic and ultramafic complex of its kind in the world does not contain copper and nickeliferous ores, with the exception of the Merensky Reef and the sulphide pegmatoids of the Western Transvaal. Ores belonging to the latter type have almost been worked out. However, minor quantities of disseminated sulphides are found throughout the whole sequence of layered rocks of the Main Plutonic Phase, the Hypabyssal Phase, the Late Plutonic Phase and the Volcanic Phase. The investigation reported here is restricted to the rocks of the Main Plutonic Phase and the Late Hypabyssal Phase. From field observations and bore-hole investigations it is clear that cross-cutting relationships exist between the rocks of the Basal Zone and the sediments of the Pretoria Series in the Western Transvaal. Huge blocks of metasediment were lifted from the floor to form inclusions in the plutonic rocks. The bore-hole results reveal that in the deepest parts of the basin the floor of the Complex has not yet been explored, particularly in the region west of the Pilanesberg. It was found that disseminated sulphide seldom exceeds 2 per cent by volume of the rock. Abnormal concentrations of sulphide are known only from certain parts of the Basal Zone, from sulphide-bearing pegmatoids, from the Merensky Reef and from the mineralized anorthosites below the Main and the Uppermost Magnetitite Seams. Generally the sulphides constitute approximately O. 05 per cent by volume of the layered rocks. The sulphides are the three modifications of pyrrhotite (troilite, hexagonal and monoclinic pyrrhotite, distinguished from one another by X-ray diffraction and etching methods), pentlandite, chalcopyrite, cubanite, pyrite, mackinawite, zinc-blende, bornite, galena(?), safflorite-loellingite, linaeite, millerite, chalcopyrrhotite( ?) and the nickel-arsenic-bearing minerals, gersdorffite, maucherite, niccolite, rammelsbergite( ?) and ullmannite( ?) . The secondary minerals are valleriite, melnikowite-pyrite, bravoite and covellite. Not much attention was paid to the minerals containing the platinum group metals, although a few have been identified; sperrylite, braggite, cooperite and michenerite( ?) . A differentiation trend was established in the mineralogical composition of the sulphide phase. From the base to the top of the Main Plutonic Phase there is a decrease in pentlandite and an increase in chalcopyrite. In the Upper Zone chalcopyrite decreases with an increase of pyrrhotite. The sulphides in any pegmatoid, which is considered to be a differentiation product of the host-rock, are also enriched in copper sulphide and pyrrhotite in comparison with the adjoining country-rock. In addition to this differentiation trend, the sulphide of the mafic rock, which contains little sulphide (0. 1 %), consists mainly of chalcopyrite. Mackinawite in pentlandite appears in these rocks below the Merensky Reef, in association with metal-rich sulphides such as troilite and cubanite. Galena (?) is present only in higher horizons of the layered sequence. All the sulphides involved in the Fe-Ni-S system exsolved from a solid solution of the monosulphide (FeNi)1 –x S, above 863 C.