During the last Ice Age, the ice sheets that covered Canada ground down and eroded the bedrock beneath them. Ore deposits were also eroded and sometimes picked up and dispersed by the ice in its relentless flow.

Studies by GSC scientists and others indicate that debris eroded from orebodies (figure 1) was dropped mostly within a few kilometres of its bedrock source, in till, the sediment deposited directly from glacial ice.

Drift prospecting is the study of glacial sediments (drift) to identify economically significant minerals in the sediments and to trace them up-ice to their source, which may be a bare outcrop or heavily drift-covered bedrock.

Drift prospecting research in Canada started in the 1950s and has been expanding since the early 1970s, concentrating on the properties of till and on the anatomy of dispersal trains. Staff of the GSC's Terrain Sciences Division have been responsible for developing drift prospecting methods in Canada. Methods were originally developed in Finland and other glaciated countries. We have refined and expanded these methods and Canadian scientists are now recognized as experts in the field.

Geologists search for evidence of ore (ore fragments, geochemical anomalies) in till. Ore debris is usually spread across the landscape in a dispersal train (figure 2), an elongate patch of ore-enriched till, oriented parallel to ice flow. Near the source, at the 'head' of the train, concentration is greatest; farther down-ice, ore content declines gradually to a slightly-above-normal level, forming the 'tail'.

Features of Dispersal Trains

Small trains, studied during mineral exploration, have at least five features in common. First, they are thin in comparison with their length and width: most are 500 to 10 000 m long, 100 to 1000 m wide and only 1 to 5 m thick. Second, they are hundreds to thousands of times larger in area than their bedrock sources. Third, they have sharp lateral and vertical contacts with the enclosing till. Fourth, they climb gently within the enclosing till, down-ice from their sources, meaning the head of the train may be buried but the tail may reach the surface. Fifth, many but not all, trains show a down-ice decrease in the concentration of ore components (figure 3).

A dispersal train of distinctive boulders, minerals, trace or major elements, or radioactive components forms a much larger mineral exploration target (figure 4) than does an ore deposit itself. The broad tail of a train is usually detected first.

Discovery

Dispersal trains are usually found by prospecting and geochemical or mineralogical studies of till. Prospectors look for ore boulders protruding from the till. Till samples, collected at the surface or from drillholes on a regular grid covering the favorable bedrock, are analyzed chemically or mineralogically to measure metal or mineral content. Results are plotted and contoured on a map of sample locations - a flame-shaped area of enrichment on that map identifies a dispersal train.

Glacial dispersal trains can be hundreds of kilometres long. Trains of this size are detected only when the area sampled is very large and when the train's characteristic lithological component is present in adequate amounts to make it distinctive against background rock types in the dispersal area. For drift prospectors, large trains are problematic because the till's exotic lithology can overwhelm the 'signal' from mineralized debris eroded from local sources (i.e., small dispersal trains). Large trains can best be detected by 'reconnaissance' scale till sampling at a density of one sample per 100 km2.

Smaller dispersal trains derived from individual rock units and distinctive belts of rock are more likely to be detected early in mineral exploration programs. Local-scale sampling, at one sample per 1 km2, will suffice to define which parts of a favorable bedrock unit are most metalliferous, and even to detect dispersal train tails derived from small mineralized sources.

Detailed sampling, in which sample density is about one sample per hectare, is designed to locate the heads of dispersal trains. This sampling density would normally be used to trace trains up-ice or test geophysical anomalies or favorable geological contacts.

A large number of Canadian ore deposits have been found using drift prospecting, including deposits of gold, copper, zinc, uranium, and rare earth elements. The discovery of diamonds at Lac de Gras in the Northwest Territories was a direct result of drift prospecting.