The geology of the manganese and iron deposits north of Postmasburg, C.P.

Abstract

This thesis is based on nearly three years work on the manganese fields of Postmasburg. During this period the writer was able to study in detail every corner of the fields, and make detailed maps of every manganiferous portion, barring a small area at Aucampsrust, which was later visited, and formed the subject of a paper read before the Geological Society of South .Africa. Small portions bearing little manganese in the immediate vicinity of Postmasburg itself were mapped on a smaller scale by two of my colleagues. Because of the lengthy and detailed field work, the subject matter of this thesis is largely based on field observations; supplemented by the study of some mineralogical details in the laboratory. The field study therefore embodies the results of the detailed mapping and observations. The complex geology, dominated by the two major factors in the shape of the unconformity between the Matsap and Transvaal systems, and the post-Mat sap thrusting, is described. From the nature and distribution of the basal member of the Gamagara sediments, conclusions are drawn with regard to the distribution of the members of the Transvaal system in the area in pre-Mat sap times. In the light of such a distribution, the evolution and effect of the post-Matsap tectonics is discussed. Particular attention is paid to the nature of the manganese and iron deposits; and the relation of the one type of deposit to the other; as well as their relations to the enclosing geologioal formations. The probable time and mode of the periods of haematitisation and manganisation is discussed, and the possible effect of the active constituents in the accessory minerals on the mineralisation is alluded to. The interesting assemblage of accessory minerals are also described.