GEOLOGY 101 — Paleoplacer gold deposits, Pt. 2

.BDerek Wilton

Paleoplacer gold and uranium deposits are generally mined as underground operations, as the hard-rock host material is usually deep beneath the earth’s surface.

The gold mines in South Africa’s Witwatersrand region have reached depths of 4 km, and are the deepest operations in the world. The deepening of these mines increases production costs, however, and is part of the reason that gold production in South Africa dropped to 500 tonnes in 1997 from 1,000 tonnes per year in the 1970s.

Ore from paleoplacers is crushed, and sometimes leached, in order to extract gold. These techniques differ from those used at placer operations, where the gold, which is contained in unconsolidated host rock, is won through gravity processing techniques.

Average grades at the Witwatersrand deposits are about 9.2 grams gold per tonne, but have been as high as 19.4 grams. Tonnages are in the order of 4 billion tonnes.

Paleoplacers have been extremely important in terms of the world’s gold and uranium resources. Before the explosion of interest in gold deposits in the 1980s, paleoplacer deposits accounted for 75% of the world’s gold resources and up to 50% of its uranium resources. More than 42,500 tonnes of gold have been extracted from the Witwatersrand district since mining began there in 1886. The paleoplacers near Elliot Lake, Ont., produced in excess of 140,000 tonnes of uranium, whereas South African paleoplacers have produced more than 130,000 tonnes of uranium.

These deposits form through a subtle interplay between tectonic forces and paleoenvironmental conditions. The sedimentary host rocks form on erosional surfaces that have developed on old rocks. Paleoplacers form in a high-energy fluvial (river) system. Dense detrital gold and uranium grains are deposited when the river flow is no longer fast enough to keep them in motion.

Exploration efforts for paleoplacer deposits are usually concentrated on areas that exhibit these geographical properties.

Unlike epithermal or mesothermal lode gold occurrences, paleoplacer deposits are not associated with broad alteration halos (a chemical and mineralogical change in rocks surrounding certain types of gold deposits), which can be used to map potential deposits.

Although geophysical surveys are of little use in the exploration for paleoplacer gold deposits, radiometric surveys can be useful in the search for paleoplacer uranium deposits. These surveys, which employ radioactivity, map the distribution of uraninite, the mineral from which uranium is extracted.

— The author is a professor of geology at Memorial University in St. John’s, Nfld.