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The nature of chrysotile asbestos occurrences in southern Africa; a review

1976; Volume: 71; Issue: 1 Linguagem: Inglês

10.2113/gsecongeo.71.1.96

1554-0774

C.R. Anhaeusser,

earthquake and tectonic studies

The principal chrysotile asbestos occurrences in Rhodesia, South Africa, and Swaziland are described, emphasis being placed on the regional geological settings and host rock stratigraphy of the mineralized areas. All the more important asbestos ore deposits in southern Africa are Archean in age ( approximately 2.5-3.5 b.y. old), being associated with ultramafic complexes occurring either as sill-like bodies in greenstone belts or as later cross-cutting intrusions.The ultramafic bodies can be grouped into three varieties. These, in order of decreasing age, are: (1) the layered complexes associated with basaltic and peridotitic komatiite extrusives and forming part of the Lower Ultramafic Unit of southern African greenstone belts, (2) layered ultramafic bodies associated with the intermediate to acid volcanic rocks that constitute part of the Mafic-to-Felsic Unit of greenstone belts, and (3) ultramafic intrusive bodies that postdate the greenstone belts but which are still affected by Archean tectonic disturbances that arise from the emplacement of granites.All the principal asbestos-bearing complexes show magmatic segregation into layered, often cyclically repetitive, differentiation sequences. These single or multicyclic sequences may consist of two or more of the following rock types: dunite, peridotite, harzburgite, Iherzolite, wehrlite, bronzitite-enstatolite, websterite, gabbro, norite, and gabbroic anorthosite. Where fractional crystallization of the ultramafic magma has been most efficient, many layers at, or near, the base of the complexes comprise monomineralic cumulate phases. These commonly consist of dunites and Mg-rich orthopyroxenites. With increasing distance from the base, progressive Mg depletion and Fe enrichment of the successive layers take place. Although chrysotile asbestos may commonly be encountered in all serpentinized ultramafic rock types, optimum development of economically exploitable fiber generally occurs in dunites, peridotites, or harzburgites.The Archean layered complexes were derived from magma of ultramafic composition in contrast to magmas of tholeiitic parentage that gave rise to the great stratiform intrusions like the Bushveld, Stillwater, and many others, including the Great Dyke in Rhodesia. The Great Dyke, unlike the others, however, acts as host to small chrysotile deposits developed in serpentinized dunites or harzburgites.In addition to the asbestos mineralization found in the layered complexes, subordinate deposits, occurring in serpentinized dolomitic rocks associated with the approximately 2.0-b.y.-old Transvaal Supergroup, are briefly described.Whereas faulting and fracturing is generally acknowledged as being largely responsible for the local development of asbestos fiber, examples from the southern African green-stone belts demonstrate that folding is often a dominant regional controlling factor in the localization of asbestos mineralization in ultramafic rocks.