The Bushveld igneous complex in the Stoffberg area, Eastern Transvaal, with special reference to the magnetite seams

Abstract

In the Stoffberg area a portion of the Main Zone, and the whole of the Upper Zone of the Bushveld. Igneous Complex have.been emplaced. The floor of the Bushveld Complex consists of so called Dullstroom andesite belonging to the Smelterskop Stage of the Pretoria Series. The structure of the region is characterised by a gentle westward dip. ~ fault, referred to as the Laersdrif Fault, has displaced the sedimentary and the igneous rocks. The horizontal separation of the Main Magnetitite Seam of Subzone Bis about 20,000 feet, and the vertical displacement about 3,700 feet. North of the fault the structural dip is from Oto 12° W, and south of it the dip increases to about 25° w. Impure quartzite of the Smelterskop Stage has been altered, in some places to such an extent that it may be termed a leptite. The gabbroic rocks of the Bushveld Complex were responsible for the metamorphism. The Dullstroom andesite was also metamorphosed, mainly along the top of the succession, into pyroxene hornfels which has a granulitic texture. Xenoliths of the hornfels settled within the gabbroic magma. Layers of felsitic rocks have formed within the andesite; chemical analyses, Niggli values and norms of the andesite and the felsitic rocks are given. The "roof" of the Bushveld Complex in the Stoffberg area is made up of various types of felsitic rocks (Rooiberg felsite). Based on the degree of metamorphism, textural. Relationship, volumetric mineral composition, analyses and grain-size, the Rooiberg Felsite has been subdivided into the following; 1. Basal felsite and altered quartzite. 2. Red granophyric felsite. 3. Spherulitic felsite. 4. Main felsite with bands of quartzite. Granite and granophyre have.been intruded mainly into the Basal felsite along its whole length of strike. A persistent sill of diabase was located within the felsitic rocks. The gabbroic rocks of the Main Zone of the Bushveld Complex are composed of layers of hypersthene gabbro, hyperite and norite. A lens of magnetite gabbro is interlayered in the gabbroic rocks. A peculiar type of black gabbro was found to owe its colour to the presence of minute grains of magnetite in the plagioclase feldspar. The magnetite gabbro of the Upper Zone follows on the gabbroic rocks of the Main Zone. In the southern part of the area the two zones are separated from each other by a large xenolith of pyroxene hornfels and by microgranite of uncertain derivation. The contact between the Main and the Upper Zone is not clearly defined, the main difference being that magnetite is present in amounts exceeding 0.5 per cent in the latter. Magnetitite seams have formed only in the magnetite gabbro suite which is made up of various rock types including hypersthene gabbro, hyperite, norite, olivinehypersthene gabbro, olivine diorite, diorite and fayalite diorite. In all these rocks magnetite is present in amounts from 0.5 to as much as 15 per cent. Four subzones of magnetitite viz, A, B, C and D have formed in the magnetite gabbro of the Upper Zone. Subzone Bis economically the most important in that it contains about 1.6 per cent v2o5 • The so-called Main Seam of this Subzone is some 6 feet thick in the Stoffberg area. Northwards it becomes progressively thicker. In Sekhukhuneland it is 10 feet thick. The magnetitite seams of Subzone A, Band Care associated with hypersthene gabbro, hyperite and norite whereas the seams of Subzone Dare found in olivine diorite and diorite. The v2o5 content of the seruns of Subzone C is about 0.6 per cent, and that of Subzone D about 0.3 per cent. The Ti02-content of the different sea.ms varies from 8 to 23 per cent. Sever-al magnetitite pipes ~f similar composition to that of the seams have been emplaced. Quantitative spectrographic determinations for vanadium and titanium were done. Unit cell determinations suggest that magnetite reached various stages of alteration to hematite. Ore reserves of the Main Seam of Subzone B have been calculated. Fractional crystallisation under the influence of gravity is considered to be the major cause of the rhythmic layering of the gabbroic rocks of the Bushveld Complex. However, with regard to the magnetitite seams, it is argued that partial separation of the Fe-Ti fraction of the magma took place before crystallisation commenced.