Abstract

Ancient quartz-pebble conglomerates have been the source of most of the gold ever mined and are a major source of uranium and a potential major source of thorium. Conglomeratic ore reefs in the Witwatersrand of South Africa were deposited by rivers that flowed into a great epicontinental lake or sea late in Archean time. The ore reefs are classified as (1) bankets, which consist of pebble-framework conglomerates deposited in the proximal zones of fluvial fans (fan-deltas), and as (2) carbon-seam reefs, which consist of pebbly sandstone and organically derived hydrocarbon that were deposited in the distal zones of fluvial fans. Uraniferous ore reefs in the Blind River-Elliot Lake district are similar to the banket reefs of the Witwatersrand, and are thought to have been deposited in the proximal zones of fluvial deltas formed by rivers that flowed into the sea during early Proterozoic time.

Gold and uranium in these ancient conglomerate deposits appear to be detrital and the ore deposits are inferred to be placers. Because uraninite and pyrite, which both appear to be detrital grains in these conglomerates, are rapidly destroyed under an oxygen-rich atmosphere, the formation of uraniferous quartz-pebble placer deposits is thought to be confined to a time in the Earth’s history before the atmosphere became oxidizing. Therefore, deposits of this type are thought to be confined to fluvial sedimentary rocks that are more than 2100 m.y. old.