The nature and origin of the rocks of the lower Griquatown stage and the associated deposits of amphibole asbestos in the Northern Cape, with special reference to the Koegas-Prieska area

Abstract

Unweathered specimens of rocks or the Lower Griquatown Stage, as well as of the associated amphibole asbestos, have only become available for investigation during the course of the past ten years. Detailed microscopic, chemical and X-ray examination of unaltered specimens obtained from the deepest mines and bore-holes showed that the rock exposed above the water-table has been altered to such a degree that conclusions regarding its origin cannot be based on examination of specimens or this rock. Not one of the existing theories explains satisfactorily the origin of the rock or of the associated amphibole asbestos, and the persistence of economic deposits of asbestos in depth has been open to doubt. From investigations carried out in the field and in the laboratory, the writer has come to the conclusion that the banded ironstone was formed from material precipitated chemically. The nature or the material precipitated was controlled by the chemical environment in the basin of deposition~ The effects of regional metamorphism are negligible and the amphibole asbestos was not formed as a result of stress set up during periods of regional folding, but crystallized directly from a colloidal precipitate of parent-material or amphibole. The fibrous habit of the amphibole asbestos is the result of crystallization of minute needles of amphibole perpendicular to an initiating surface of pre-existing magnetite. The crystallization of the amphibole asbestos took place between the period of mild Pre Loskop and intense Post-Waterberg deformation, and the quantity of asbestos developed is often related to the gentle Pre-Loskop folding. 1he writer feels that these views on the origin of the amphibole asbestos and the rocks of the Lower Griquntown Stage offer a satisfactory explanation for the distribution of the asbestos and its fibrous habit. There is no longer any doubt as to the persistence of amphibole asbestos with increasing depth, both as regards quality and quantity, and new, completely hidden deposits have been discovered by application of the principles outlined in this thesis. This knowledge, combined with the development of suitable modern techniques of exploration, assures the asbestos industry of on adequate supply of raw material for the foreseeable future.