Active stream sediments are composite samples of weathering products. The composition of sediment at any site is a product of weathering, erosion, transport and deposition. The ultimate source of sediments is rocks exposed in the drainage basin and the cover material.

The geochemical response of sediments at any site is the result of a combination of physical, chemical and biological processes. Physical processes that break down rocks into smaller particles include glacial plucking, thermal expansion and contraction of rocks, and expansion due to the crystallization of salts or the freezing of water in fissures. The movement of rock fragments by wind, ice and water also causes abrasion, producing increasingly fine particles. These smaller particles result in the exposure of larger surface areas to chemical attack, as well as biological processes that affect acidity and complexation reactions. Roots also contribute to the breakdown of rocks into smaller particles.

Chemical weathering includes a range of reactions occurring when minerals react with water containing dissolved materials that increase mineral solubility. Generally, chemical weathering differs from other forms of weathering by its ability to separate different elements in the surficial environment because of the differing reactivities of these elements. Climate, biological activity, parent materials, topography and time influence chemical weathering.

Regional lake sediment and water data are used to evaluate the economic mineral potential of large areas of the Canadian Shield, and can also provide environmental baseline data. A systematic lake sediment and water sampling program started by the GSC in 1973 continues to the present day. Interpretation of lake sediment data is based on the premise that element values of centre-lake sediments will indicate to some degree the chemistry of underlying or adjacent bedrock, or outline anomalous dispersal patterns of up-ice mineralization.

The geochemical signature of any individual lake sediment sample is influenced by a number of factors. The main factors are bedrock and glacial geology; others are mineralization, climate, physiography, and vegetation, as well as the geochemical characteristics (mobilization, transport, deposition) of individual elements as influenced by different conditions of pH-Eh and/or the presence/absence of other elements.

Typical modern lake sediment from a Shield lake will consist of varying amounts of organic gels, organic sediments, and inorganic sediments (Timperley et al., 1973). Burning off the organic fraction and weighing the remaining material determine proportions of organic versus inorganic materials. Typical values range from an average of 87 per cent inorganic material from lake sediments near Contwoyto Lake, NWT, to 58 per cent in northern Saskatchewan lakes (Friske, 1991). In general, the inorganic component of lake sediment, made up of various combinations of sand, silt, and clay, with silt and clay predominating forms more than half of a lake sediment sample by weight (Timperley et al., 1973). A considerable proportion of the inorganic fraction can consist of material washed into the lake by streams flowing over the surrounding glacial deposits, which are in turn a product of complex glacial processes.

The advantages of waters as a sample media in mineral exploration result from their reactivity with rocks and soils and their physical mobility. Element abundances vary in natural waters according to the solubility of their source minerals and the availability of surface-active particulates, which remove solutes from solution by adsorption or chemical complexation. The solubility of source minerals is controlled by pH, redox conditions and salinity of the natural water.