The Character and Economic Significance of Precambrian Paleoweathering and Erosion Surfaces in Southern Africa
Andrew Button, Noel Tyler, 1981. "The Character and Economic Significance of Precambrian Paleoweathering and Erosion Surfaces in Southern Africa", Seventy-Fifth Anniversary Volume, Brian J. Skinner
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A disproportionately large number of mineral deposits is associated with Precambrian weathering and erosion surfaces. In southern Africa such surfaces range in age from 3,200 to 600 m.y. and are developed on rocks as diverse as granites, volcanics (both basic and acid), arenites, shales, carbonates, and iron-formations. The sericitic arenites that host the auriferous-uraniferous conglomerates of the Witwatersrand and other basins of comparable age were almost certainly derived from granitic paleosols. There is some evidence to suggest that uranium was leached by paleoground water during the formation of such paleosols. We suggest that some of this uranium may have become fixed in diagenetic titania skeletons in conglomerate paleo-aquifers. In addition to chemical concentration factors, it has long been recognized that erosive truncation and reworking of older conglomerates were important ways of creating mechanical concentrations of gold and uranium along early Proterozoic and late Archean unconformities.
In several areas in southern Africa basic lavas were weathered during the early Proterozoic or earlier times to yield an alumina-rich (≥ 30% Al2O3) paleosol. Such paleosols represent a vast resource of nonbauxitic alumina.
Unconformities that cut across Precambrian carbonate rocks are marked by paleokarst surfaces, which have acted as emplacement loci for deposits as diverse as the fluorite (lead, zinc) deposits of the western Transvaal to the lead-zinc-copper deposits in the Tsumeb mine of South West Africa-Namibia. Manganese deposits are characteristically associated with erosion surfaces cut across early Proterozoic carbonate formations both in South Africa and in Western Australia. Apparently the paleoatmosphere some 2,200 to 2,300 m.y. ago contained enough oxygen to oxidize (and fix on the unconformity) both Mn and Fe that had previously been held in divalent form in carbonate minerals.
Erosion surfaces that cut across iron-formations are commonly mantled by iron-formation clast conglomerates. These conglomerates are sometimes enriched, presumably by paleoground water, to high-grade, conglomerate-textured hematite ore.
The vast period of geologic time represented by unconformities was probably the most potent factor in mineral concentration. Given time, normal geologic processes can achieve a high degree of concentration of minerals. Other important factors identified include paleotopography (due to differential erosion), solution by (and precipitation from) paleoground water, biological action, and porosity contrasts adjacent to unconformities. Continued study of Precambrian paleoweathering and erosion surfaces promises to be an exceptionally fertile field for research, both in mineral deposits science, and in advancing our understanding of the fundamental questions of terrestrial atmospheric-hydrospheric evolution.
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