Mechanisms to explain the loss of heavy minerals from the Upper Palaeozoic tillites of South Africa and Australia and the late Precambrian tillites of Australia



Heavy mineral studies on Pleistocene tills from North America, Upper Palaeozoic tillites of South Africa and Australia, and late Precambrian tillites of South Australia show that the heavy mineral suites of the Pleistocene tills are dominated by amphiboles, the Upper Palaeozoic tillites by garnet, and the late Precambrian tillites by zircon and tourmaline. About half of the garnets in the Upper Palaeozoic tillites show evidence of having been rounded, and retain delicate surface chattermark trails, which indicates that these garnets have not undergone chemical attack since deposition. Although the remainder of the garnets show, by way of etching, that intrastratal solutions were active in the sediments, it is suggested that amphiboles, pyroxenes and epidote, which must have been present in the original Upper Palaeozoic heavy mineral suites, were lost primarily by the action of sorting and mechanical abrasion in beach environments prior to, and during interglacial periods. The absence of garnet and the etching of tourmaline and zircon in the late Precambrian tillites is attributed to the action of alkaline intrastratal solutions over the long time interval during which the tillites were buried in the Adelaide Geosyncline.