Canada's National Geochemical Reconnaissance (NGR) Program is an ongoing reconnaissance-scale drainage sediment and water sampling program that began in 1975 with lake sediment surveys in Saskatchewan and Manitoba. Areas of coverage have been incrementally increased across the country so that today geochemical data are available for over 83,000 lakes and 78,000 streams in an area equivalent to one-fifth of the country. Early surveys included atomic absorption spectrophotometry and colorimetric data for 12 or 13 elements in sediments, along with uranium, pH and fluoride values in waters. The most recent open files contain data for 50 or more variables in both sediments and waters from a combination of ICP-MS, ICP-ES, ion chromatography, INAA and specific methods. The reports were originally used by the mineral exploration industry to search for uranium and base metals, but as analytical techniques evolved, data became available for gold and other precious metals, rare earths, and now, with the most recent release, diamonds.
Collection and analysis of drainage sediments and waters is an established method of mineral exploration in Canada. Large areas of the country can be surveyed rapidly and at a relatively low cost. Stream sediments are the most widely used media for exploration at intermediate to regional scales and suitable drainage systems exist in many areas of the country. Lake sediments are suited to glaciated basement terrains such as found over much of the Canadian Shield. The importance of waters has increased as the understanding of the chemistry of natural waters has increased along with the improvement in the sensitivity of analytical techniques, particularly ICP-MS.
About the Sample Media Stream Sediments 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.
Samples are collected in Kraft paper bags (12.5 cm x 28 cm with side gusset) that are two-thirds filled with silt or fine sand collected from the active stream channel. Silt samples are collected after water samples. Commonly, the sampler collects handfuls of silt from various points in the active stream channel while moving gradually upstream. If the stream channel consists of clay or coarse materials from which suitable sample is scarce or absent, a moss mat sample might be collected.
The Kraft paper bags containing the silt samples are shipped to a commercial lab, where they are air-dried at temperatures below 40ºC and sieved through a minus 80-mesh (177 µm) screen. Quality control is critical to the success of the NGR program and includes the insertion of control reference and blind duplicate samples into each block of twenty sediment samples.
Lake Sediments 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.
A bottom-valved, hollow-pipe sampler is used to collect one kilogram or so of wet lake sediment. The sampler is vented at the top, allowing the top few centimetres of sediment to escape so that possible contamination in the upper levels of sediment can be avoided. Typically, one kilogram of the organic gel, the preferred collection material, is about 95 per cent water, and once dried, about 50 grams of material remain for analysis.
Waters (Lakes and Streams) 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.
Stream waters are sampled in mid-channel, from flowing water where possible. Lake waters are collected from below the surface of the lake from the same location the sediment sample is collected. Samples are contained in HDPE bottles. Until recently, water samples were analyzed only for uranium, fluoride and pH, but with the advent of sensitive and relatively inexpensive multi-element analysis by inductively coupled plasma-mass spectrometry (ICP-MS), data for up to 50 or more elements will become available.
Quality Control One of the most important characteristics of NGR surveys is the structure of the sampling routine. Each block of 20 consecutive field numbers consists of 17 routine field samples, a field duplicate sample, a blind (analytical) duplicate sample and a control reference sample. The field duplicate sample is a separate sample collected at one of the 17 routine sites, at the discretion of the sampling team. One number, always the first in a block of 20 (i.e. 001, 021, 041, etc.) is reserved for a blind duplicate. The sample preparation laboratory splits a sample in the block, preferably one of the field duplicate samples, and places one of the splits into the blind duplicate position. A randomly pre-selected number within a block of 20 is reserved for a control reference sample. Control reference samples are lake or stream sediments with well-established analytical values.
Field duplicates, blind duplicates and control reference samples are incorporated in every block of 20 samples, and are used to monitor and control sampling and analytical variance. As a result of stringent quality control and consistency of analytical methods over time, it is possible to generate a regional compilation for an element with minimal boundary effects between different surveys using the same analytical method.
About the Geochemical Database Data are stored in files consisting of one record (or row) for each site, with fields for sample number, geographic coordinates (NAD27), field observations, and analytical data. Occasional missing items in any field may be the result of observations not made at the time, in the case of site observations, or, in the case of analytical data, samples with insufficient material for analysis.
Each analytical data field is populated by results derived from identical or similar analytical methods. In cases where analytical methods are significantly different, analytical results are shown in separate fields. For example, results from two different analytical procedures are listed for elemental concentrations of gold in sediments. Until 1989, samples were analyzed using a combination of fire assay and neutron activation (FA-NA) techniques. In 1990, a new method, non-destructive instrumental neutron activation analysis (INAA) replaced the fire assay-neutron activation method. In time, thousands of samples not originally analyzed for gold were reanalyzed by INAA.
In some cases, for some elements, analytical results obtained by two methods are shown for the same sample. Before attempting to compare these data, some caution should be exercised. Generally, analytical results for elements such as As, Co, Fe, Sb, U and W in sediments are either 'partial' or 'total.' An example of a partial extraction method is an HNO3 - HCl (aqua regia) digestion. Silicates such as zircon and pyroxene remain relatively unaffected by this digestion. However, data for these elements obtained by INAA are 'total' data. Hence, median values for sediments determined with a partial extraction method will usually be somewhat lower than those for INAA data.
Original observations for stream width, stream depth and lake depth were in feet. Although every attempt has been made to convert all data to metres, there may be cases, particularly in earlier data, of measurements in the original imperial form.
Coordinate data are given as geographic coordinates (latitude/longitudes), in decimal degrees, using a NAD27 (Clarke 1866 spheroid) datum.
References and Further Reading
Friske P.W.B. 1991: The application of lake sediment geochemistry in mineral exploration; Geological Survey of Canada Open File 2390, Paper #4, p. 4-1 to 4-20. Friske, P.W.B. and Hornbrook, E.H.W. 1991: Canada's National Geochemical Reconnaissance programme; Transactions of the Institution of Mining and Metallurgy, Section B, Volume 100, p.47-56. Govett, G.J.S. (Editor) 1994: Handbook of Exploration Geochemistry, Vol. 6 (Drainage Geochemistry), edited by M. Hale and J.A. Plant; Elsevier Science B.V., 766 p. Timperley, M.H., Jonasson, I.R., and Allan, R.J. 1973: Sub-aquatic organic gels: A medium for geochemical prospecting in the southern Canadian Shield; Geological Survey of Canada Report of Activities, Paper 73-1, Part A, p. 58-62.
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