The geology of the bushveld ingneous complex east of the kruis river cobalt occurrence, North of Middelburg, Transvaal

Abstract

The area investigated covers the south-eastern flank of a dome-like structure in the Bushveld gabbro. This gabbro is intersected by two sets of faults which are older than the Bushveld granite. The investigation bears out the contention that the sedimentary rocks in the Moos River area do not represent an inclusion in the gabbro but that they represent part of an anticlinal fold caused by doming during the emplacement of the Bushveld granite. Xenoliths in the gabbro belong to the Smelterskop Stage. A large xenolith of quartzite occupies an extensive area in the north-west, and numerous smaller xenoliths are composed of highly altered Dullstroom lava. The Rooiberg felsite has a considerable development (probably a maximum) in this area. It attains a thickness 8f 10,180 ft and is subdivided into three zones: (i) a Lower felsite which consists of micrograpbic felsite and leptite, (ii) a Variable felsite, mainly black, amygdaloidal and pseudospherulitic, and (iii) an Upper felsite which is red, glassy and porphyritic. The total thickess of the roof-rocks of the Main Plutonic Phase is 14,680 ft. They consist of felsite and granophyre. Three different types of sranophyre seem to be present the Bushveld Complex. One type, the so-called "Rooiberg granophyre" is found as large sheet-like masses at the base the Rooiberg felsite in the area investigated, and is considered to have originated by melting of the felsite during, emplacement of the gabbroic rocks. Another type represents a chill-phase of the Bushveld granite and a third type, termed "paragranophyre" is not present in the area investigated but has been described by other workers as having originated by metasomatism of quartzo – feldspathic sediments. Four new chemical analyses of felsite and related rocks are given. These, together with chemical data on Rooiberg felsite and Bushveld granite published previously are plotted on various variation diagrams. According to these diagrams the Rooiberg granophyre and the leptite are related to the Rooiberg felsite. The composition of the granophyre, related to the Bushveld granite, varies considerably and could not be separated from the Rooiberg granophyre on the grounds of chemical composition. Although neither the Merensky Reef nor the Main Magnetitite Seam is present, rocks of the Main and Upper Zones of the Main Plutonic Phase of the Bushveld Complex are developed. A valuable marker in the Main Zone in this area is a gabbro which contains spherical inclusions of pyroxenite. This marker is designated the "Tennis-ball marker" and is situated about 2200 ft above the Needle-norite which in turn was found to be 1000 ft above the Merensky Reef in other localities. The gabbroic rocks are described and much attention is given to a so-called nesophitic texture of the orthopyroxene, a common texture in these rocks. The nesophitic orthopyroxene is orientated so that its crystallographic c-axis is parallel to the plane of igneous lamination. This, as well as the presence of augite lamellae orientated at random in this orthopyroxene is explained as being due to directed pressure of the superincumbent crystal mass during the inversion of the pigeonite. The Upper Zone is developed only in the eastern part of the area and thins out gradually towards the west. This is explained as the result of a discordant relationship of the gabbroic rocks towards the roof. Outcrops are generally poor and rock types present are ferrodiorite, diorite, granodiorite and magnetitite. The granodiorite is not considered to be a product of magmatic differentiation, but rather to have formed by assimilation of felsitic rocks. Magnetitite is present as seams and as a plug on Diepkloof 186-J .S. For comparative purposes magnetitite from plugs on Haakdoorndraai 169-J.S. and Maleeuwskop (both to the north of the area in question) was investigated. According to the low V20 5 content the magnetitite present is considered to be high up in the succession of the Upper Zone. The magnetitite reveals several interesting exsolution phenomena, most of which may be explained as the result of the oxidation of ulvite to ilmenite. In this process of oxidation and subsequent migration of the constituents of the ilmenite, diffuse proto-ilmenite, lamellar and worm-like concentrations of, ilmenite and intragranular graphic ilmenite in magnetite originated. Leucoxene is a common alteration product of the ilmenite.