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Geological Society, London, Special Publications; 1987; v. 33; p. 347-354;
DOI: 10.1144/GSL.SP.1987.033.01.23
© 1987 Geological Society of London

Proterozoic Volcanic Suites of Africa

Alteration of Late Middle Proterozoic Volcanics and its Relation to Stratabound Copper-Silver-Gold Mineralization Along the Margin of the Kalahari Craton in Swa/Namibia and Botswana

G. Borg & K. J. Maiden

Department of Geology, University of the Witwatersrand, Johannesburg, South Africa

Metavolcanic and metasedimentary rocks of late Middle Proterozoic age are preserved in basins along the northern margin of the Kalahari Craton. Three broad stratigraphic divisions are recognized: a lower acid extrusive unit, a basic extrusive/red bed unit and a shallow marine unit.

The basalts of the basic extrusive/red bed unit are interpreted to be continental within-plate basalts. Copper mineralization occurs as native copper in amygdaloidal altered basalt, and as sediment-hosted, stratiform sulphide deposits at the base of the shallow marine unit.

The rocks have been strongly altered during regional metamorphism which reached temperatures up to 350°C. Massive domains were metamorphosed to oligoclase-chlorite-epidote-haematite rocks. In permeable zones such as flow top breccias, amygdaloidal zones, and regions of secondary, tectonically induced fracturing, the basalts have been altered to epidote-quartz-chlorite-haematite-muscovite rocks. During this epidote alteration copper, zinc and cobalt were leached from the basalts, thus providing a possible metal source for the sediment-hosted deposits.

A model for multiphase ore genesis is presented in which basalt alteration is the main metal source for epigenetic mineralization.