oa South African Journal of Geology - The petrography of the Bushveld Complex intersected by boreholes in the Bethal area

Volume 78 Number 3
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590



Seven boreholes drilled through the cover of Karroo rocks in the Bethal district were studied; six of the boreholes intersected a northward-dipping extension of the Bushveld Igneous Complex. Except for 1 70 m of the Layered Sequence, which was not intersected, the complete section of about I 900 m could be constructed.

The Layered Sequence was divided into five zones: these consisted of a Basal Zone of peridotite and pyroxenite which rests on a diabase of the early sill phase of the Bushveld Complex. Associated with the diabase are thermally metamorphosed sediments of the Pretoria Series. The Basal Zone passes upwards into Zone 1 which contains abundant anorthosite and mafic pegmatoid, both of which occasionally contain significant amounts of pyrrhotite and pyrite. Minor amounts of chalcopyrite and pentlandite are usually associated with the sulphides. A possible fresh injection of magma may have been responsible for a break in the mineralogical trend in the plagioclase of Zone I, from An45 to An56 over only 35 m, which has given rise to a very persistent mineralogical marker horizon . .

Zone II consists mainly of norite and gabbro and forms a transition between Zone I, which is poor in magnetite, and Zone III which is characterized by magnetite-rich gabbro, a prominent anorthosite layer and six magnetite layers. Zone IV is predominantly dioritic in composition; two anorthosite layers and eleven magnetite seams were intersected in this zone.

Trends established from modal analyses and determinations of the composition of the plagioclase, orthopyroxene and olivine show that cyclical crystallization had occurred. There is, however, an overall trend from An55 and Fs32 in the middle of Zone I to An38 and Fs62 at the top of Zone IV. The olivine and orthopyroxene of the Basal Zone are poor in iron, with values ranging from Fa6 and Fs17 to Fa15 and Fs22 respectively. Apart from its occurrence in the Basal Zone, olivine reappears only at the top of Zone IV, where it is greatly enriched in iron at Fa75.Fluor-apatite enters as a prominent cumulus phase near the top of Zone IV.

Zones I to IV of the Bethal area can be correlated with the Upper Zone of the Bushveld Complex, and the Basal Zone with the equivalent horizon in other parts of the Bushveld Complex. The absence of both the Main and Critical Zones in the Bethal area may be due to the migration of fractionated magma from the main magma chamber. It is considered more probable, however, that the Bushveld Complex rocks in the Bethal area represent a separate injection centre.

Crystallization occurred under conditions of faster cooling and higher oxygen fugacity than in the case of rocks of the Upper Zone in the eastern Bushveld Complex.

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