Abstract: The formation of placer deposits has several prerequisites. They include a source for the gold, a mechanism to concentrate it, and time for the concentrating mechanism to work. The source of the gold could be gold-quartz veins, gold-bearing sulphide deposits, or previously formed placer deposits.

Concentration of gold in placer deposits occurs through erosion. Concentrations may occur in residual eluvial and deluvial deposits overlying and close to a lode gold source. Most placer deposits have, however, been further concentrated through the action of moving water in streams or on beaches.

Typical sites for accumulation of placer gold deposits occur where the rate of flow decreases. Such sites include the inside of meander curves, at decreases in stream gradients, at the confluence of creeks, and where physical obstructions such as irregularities in the bedrock surface, or the surface of the sand and gravel occur. In deposits of meandering streams, gold occurs in distinct pay-streaks at the base of the deposits. In deposits of braided streams, however, the gold occurs erratically throughout the deposits.

A variety of sampling methods and other techniques can be used to explore and evaluate placer deposits. Black and white aerial photographs and infra-red photographs are used to identify areas of interest. Geological mapping of placer deposits is done to establish the distribution of the horizons bearing valuable minerals and identify potential impediments to mining.

Bulk samples are taken to determine the valuable mineral content of the deposits. Both small-scale panning and large-scale mechanical excavation are used to obtain bulk samples.

Authors: Debicki, R.L.

Map Scale: 1 : 0

Citation: Debicki, R.L., 1983. Placer deposits: Their formation, evaluation, and exploitation, Yukon. In: Yukon Placer Mining Industry 1978-1982, Debicki, R. L. (comp.), Indian & Northern Affairs Canada/Department of Indian & Northern Development: Exploration & Geological Services Division.