Abstract

Suggestions that the Ore-Shale of the Lower Roan Group is part of a series of sedimentary fades produced by the north-easterly transgression of the sea over dominantly arenaceous strata of littoral, deltaic, lacustrine and continental origin have been viewed with scepticism by the author. The lack of proven transition facies from silty carbonaceous argillites, of the type found close to the south-western limits of the Ore-Shale in the Zambian Copperbelt, to dolomites found further to the SW, has tended to accentuate this feeling. The intensity of exploration drilling in these areas must imply that, whatever the cause, the cut-out of the Ore-Shale is rapid. At the same time a 'normal' Lower Roan succession of the Chambishi type suddenly becomes attenuated and dolomites and dolomitic/chloritic schists are found immediately overlying argillaceous gritty sandstones, conglomerates and arkosic quartzites characteristic of Footwall Formations. Locally this contact may be reddened, porous and brecciated. Amphibolite bodies of the type normally found at a much higher stratigraphic level in the Upper Roan Group are present a short distance above this contact. It is thus proposed that this feature represents a hiatus in the succession and that all rocks from the Ore-Shale to the top of the Lower Roan, and possibly into the base of the Upper Roan, are missing due to non-deposition and/or erosion. This interruption of the succession coincides with the western cut-out of the Ore-Shale from the Konkola Dome in the NW, down to the western end of the Roan Basin in the S.

In view of various authors' contention that the Zambian Copperbelt is the site of an ancient aulacogen, the above observations take on an important genetic significance. The present paper thus reviews the location of copper-cobalt mineralization in relation to sedimentary facies changes and cut-outs, structural features and amphibolitic bodies, and proposes a geotectonic model for the Copperbelt which, it is hoped, provides a more satisfactory and comprehensive explanation for the facts. This model also provides a source for the metals which were converted to sulphides by diagenetic processes. It is believed that the complex mineral chemistry of the Ore-Shale associated ores and, in particular, the presence of localized concentrations of Mo, Co, Ag, As, Bi and Te-bearing phases, is best explained by high level seepage of magmatically enriched hydrothermal solutions along Precambrian rift systems and associated fractures. These metals were either dispersed by wave and current action in the sedimentary environment or were concentrated by porosity and physico-chemical and bacteriogenic controls in the diagenetic environment.

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