GEOLOGY 101 — Paleoplacer gold deposits, Pt. 1

Paleoplacer deposits consist of placer concentrations of minerals in which the host material is a consolidated rock (sediment comprising weathered detritus that was subsequently cemented together). The prefix “paleo” simply means “ancient.”

Gold and uranium are the only commodities mined from paleoplacer deposits.

Both commodities can be mined from a single deposit, but often only one or the other is present.

Paleoplacer gold deposits have been mined in the Witwatersrand district of South Africa, the Tarkwaian system of Ghana and in the Jacobina mine of east-central Brazil.

Paleoplacer uranium has been mined at Elliot Lake, Ont., and extracted from the gold deposits of the Witwatersrand district. There is a strong temporal control on paleoplacer uranium occurrences, as these occur only in rocks more than 2.5 billion years old. Gold-bearing paleoplacers are predominantly Archean-aged, but have been mined in rocks as young as 2.1 billion years.

Gold in paleoplacer deposits is present as discrete grains. Uranium occurs as uraninite (UO2-U3O8). Like gold, uraninite is a dense mineral with a high specific gravity (6.5-10 grams per cubic centimeter) compared with common detrital minerals. Uraninite is unstable in oxygen-bearing surface waters, and its presence as detrital grains suggests that the earth’s early atmosphere was oxygen poor. Some researchers, however, suggest that gold and uranium may be at least partly composed of hydrothermal fluid introduced along faults that bound depositional basins.

The host rock in paleoplacer deposits is quartz pebble conglomorate, a rock containing rounded grains of pure quartz up to 32 mm in diameter. The well-rounded nature and relatively equivalent size of the pebbles defines the host sediment as mature. As such, the particles have been subjected to prolonged agitation in an erosional environment.

This type of sediment forms in a regime of intense weathering and corrosion, wherein quartz is the only common rock fragment to survive, owing to its hardness and resistivity to chemical weathering.

Other minerals are locally concentrated with gold and uraninite. As is the case with a placer deposit, these minerals are dense, hard and/or resistant to chemical alteration. Such minerals include pyrite (in paleoplacer deposits fewer than 2.5 billion years old), platinum group metals, chromite, zircon and arsenopyrite. These minerals are intergranular to the quartz pebbles.

The host rock of a paleoplacer deposit can be composed of up to 3% pyrite.

Such rocks are often referred to as pyritic quartz pebble conglomorates.

Owing to the differences in their ages, the host rocks of

Witwatersrand-Brazil and Ghana gold ores have subtle compositional differences. The oldest rocks, those found in Witwatersrand and Brazil, are pyritic. The younger rocks of Ghana are hematitic, further reflecting the presence of oxygen in the atmosphere. Uranium does not occur in these younger rocks.

Coal-like layers of organic matter (kerogen) are closely associated with some ore-bearing conglomorate horizons. Gold and uranium are locally concentrated in these organic layers, which are either the remnants of algal mats or the products of later hydrocarbon migration. According to some authors, this organic matter could represent paleo-angal mats. Should that analyses prove accurate, then the mats trapped gold and uranium in either of two ways: physically (from gold and uranium detritus) or chemically (from gold and uranium dissolved in stream waters).

Gold (and uranium) is concentrated in paleoplacer deposits much as it is in placer deposits, in paystreak-like concentrations. The paystreaks are thin sheets of quartz pebble conglomerate interlayered with thicker beds of sedimentary rocks.

In the Witwatersrand deposits, paystreaks can extend for up to 10 km, but are usually less than 3 metres thick. As such, these paystreaks resemble those found in river (fluvial) sediments of modern-day placer deposits.

Overall, the host sedimentary rocks were deposited in high-energy fluvial conditions, such as in modern-day braided streams that flow from mountainous regions into alluvial plains. The Witwatersrand rocks are fan delta-like sedimentary horizons deposited at the base of hills from which erosion took place.

— The author is a professor of geology at Memorial University in St.

John’s, Nfld.