Diamonds in the Apple Formation?

Claude Dion and Jean Goutier
Direction de Géologie Québec

The paleoplacer model

Paleoplacers contain significant amounts of gold or uranium. The best example is the colossal Witwatersrand Au-U district in South Africa, from which more than 48,670 tonnes of gold and 165,000 tonnes of U₃O₈ have been extracted between 1886 and 2000, i.e. roughly 40% of all the gold extracted since the early days of mankind (Frimmel and Minter, 2002). What is less well known however is that some paleoplacers also contain important concentrations of detrital diamond, similar to recent placer deposits in Namibia, Brazil or India. Although the productivity of these recent or older deposits is generally lower than that of primary deposits (kimberlites or lamproites), their economic impact is fairly significant given the superior quality of recovered diamonds.

In his review of diamond-bearing paleoplacers, Konstantinovskii (2003) noticed a discontinuous and irregular increase in the number of these paleoplacers through geological time, a growing trend that may be related to an increase in kimberlitic volcanism through time. This author reports the existence of a few Archean diamond-bearing paleoplacers, namely those in the Witwatersrand Basin in South Africa and Nullagine in Western Australia. Prior to the development of modern crushing methods, diamonds were reportedly recovered from the processing of Witwatersrand gold ore (Roscoe and Minter, 1993). Several stones of interesting size and quality were also recovered from uraniferous quartz pebble conglomerates in the Nullagine area, in the Pilbara craton of Australia. In both cases, the primary source of diamonds (kimberlites) is unknown and has probably been completely eroded.