The petrography of rocks of the Bushveld Complex type intersected by boreholes in the Bethal area

Abstract

On geophysical evidence, several Mining Companies undertook drilling programs in the country north-west and north-east of Bethal, Transvaal, in order to test for a possible extension of the Bushveld Igneous Complex. Seven of these boreholes were examined and six intersected an igneous mass consisting of a layered sequence of mafic rocks dipping towards the north and resting on earlier diabase. The seventh borehole showed that this diabase had intruded into the sediments of the Pretoria Series. On the evidence that one of the boreholes intersected the uppermost zone of the Layered Sequence, a complete section could be constructed which shows a thickness for the Sequence of about 1900 metres, of which only 170 metres were not intersected. The Layered Sequence is divided into five zones. These consist of a Basal Zone of peridotite and pyroxenite which pass upwards into Zone I. This zone contains abundant anorthosite and mineralized pegmatoid near the base which give way to norites and gabbros higher up. Zone II is a transition between Zone I, which is poor in magnetite and Zone III, which contains up to six magnetite layers. Zone IV is characteristically dioritic in composition and eleven magnetite seams were intersected in this zone. Trends established from modal analyses and determinations of the composition of the plagiocJase, orthopyroxene and olivine show that cyclical crystallization occurred. There is, however, an overall trend from An55 and Fs32 in the middle of Zone I to An3g 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 to Fa 17 and Fs15 to Fs22 respectively. Partial chemical analyses of samples of the magnetite layers reveal them as being titanomagnetites poor in vanadium. A break in the mineralogical trend of the plagioclase in Zone I, from An45 to An56 over only 35 metres, has given rise to a very persistent mineralogical marker horizon. The mineralized pegmatoid of Zone I gives values of up to 0,65 per cent combined copper and nickel and is considered to have been formed by filter pressing of the intercumulus volatile-rich phase of the Basal Zone. This phase may be responsible for the appearance of oscillatory zoning in the plagioclase of Zone I. Pyrrhotite is the most abundant sulphide mineral in the Layered Sequence except just below the Marker Horizon and at the top of Zone IV, where a high proportion of pyrite is present. The proportion of pentlandite and chalcopyrite decreases upwards towards Zone IV. Pentlandite occurs predominantly as an exsolution product of pyrrhotite. In addition to the narrow exsolution lamellae of augite parallel to the ( l 00) plane in the orthopyroxene, additional broader lamellae, indicative of inverted pigeonite, occur near the top of Zone IV and in the diorites of Zone I just above the Marker Horizon. Fluor-apatite enters as a prominent cumulus phase near the top of Zone IV. On mineralogical grounds the Layered Sequence of the Bethal area can be correlated with that of the Upper Zone of the Bushveld Complex. The predominantly dioritic character of this Sequence implies that the magma migrated from the main chamber at a relatively late stage of its fractionation. The magma continued to crystallize under conditions of higher oxygen fugacity than that in the Eastern Bushveld Complex. The diabases intersected below the Layered Sequence are considered to form part-of the early intruded sill phase of the Bushveld Complex and both the Maruleng and Lydenburg types are present. The Lydenburg type diabase has assimilated sedimentary material, which resulted in enhanced differentiation trends. Associated with the diabase are thermally metamorphosed sediments which grade from pyroxene-hornfels facies to albite-epidote-hornfels facies. Ten new chemical analyses are presented and they have been used to confirm determinations undertaken by other methods.