The geology of the Dwars river fragment and the ore-minerals of the magnetite-deposit of Kennedy's vale 361 kt. Eastern Transvaal

Abstract

During the field-seasons of 1960 and 1961 an area of 60 square miles was mapped in the vicinity of Kennedy's Vale~ District Lydenburge so acquired is presented on the map. The information The Dwars River Fragment, a mass of metamorphosed rocks of the Pretoria Series, is present in the western part of the area. The Fragment consists of quartzite 9 hornfels, metamorphosed dolomitic limestone and mafic rocks forming sills, belonging to both the Lydenburg and naruleng types as defined by Willemse (1959, p. xliii). These rocks have been folded into a doubly plunging anticline and a syncline plunging south. The Fragment is to be explained in one of three ways. It either represents a "bulge" in the floor of the Bushveld Igneous Complex, a xenolith or a horst between two downfaulted blocks of plutonic rocks" Mafic and Ultramafic plutonic rocks of the 13ushveld Complex were also investigated. A mineragraphic description of the ore minerals of the Merensky Reef is included and an attempt is made to determine the t0nperature range of formation of the sulphides$ Microscopical evidence and X-ray diffraction data point to a range from above 70-6°C. The lower limit is ~ot known. Maghemite and martite are only present at or near the surface of the magnetite deposit on Kennedy's Vale. The existence of intermediate stages in the magnetite-maghemite series is indicated by the experimental data, and the oxidation of magnetite , Ornithogalum Thyrsoides resulted in the formation of predominantly "maghemite" at temperatures below about 500°c, whereas martite formed in minute quantities below 700°C and more extensively above this temperature. A study of the sulphide minerals associated with the ultramafic pegmatoid showed that they should be classified with the pegrnatitic ore-deposits, as defined by Niggli (1954, p.513) as far as their association with rocks of definite pegmatoidal origin is concerned. Regarding their temperature of formation, they do, however, resemble ortho-magmatic deposits. It is proposed that such an assemblage be referred to as pegmatoidal. A statistical investigation of the accuracy and reproducibility of values obtained with a CdS - photocell reflectometer indicates that this instrument is to aid in ore-mineral identification. By means of quantitative reflectivity and micro-indentation hardness measurements, the presence of intermediate members of the magnetite-maghemite series was established.