Stream sediment geochemistry

Abstract

Stream sediments are characterized more by their variability than by their uniformity in composition, grain size, sorting and colour. This variability is a function of the geology, terrain, and climate of the catchment areas sampled by the stream, and it provides the essential ingredient for viable stream sediment surveys. Prospectors have always made use of this fact, initially through observation of mineralized boulders and panning of heavy minerals from the stream bed, and more recently by chemical analysis of the finer fractions of stream sediment. Over the past 25 years, routine application of strea m sediment geoche mistry has become accepted by government and intergovernmental agencies and the mining industry as the principal method of low-cost reconnaissance exploration in those areas, favoured by the combination of adequate relief and precipitation, where a suitable, integrated drainage system has developed.At present, surveys that make use of trace analysis of the fine fraction (normally minus-80mesh) of active sediments, although still the most common, are increasingly being questioned. In areas of extreme climate or where information from the fine sediment is no longer adequate, enhancement of anomalies may require sampling of organic material, of specific coarse fractions of inorganic sediment, of selected mineral groups within the sediment that may be isolated mechanically or chemically, or of a combination of these materials. As a result, variations in sampling and analytical methods have been developed to accommodate regional and climatic differences and differing survey aims (Le., mUlti-purpose or strictly prospecting, single or multiple target).This review outlines the present art with selected examples of surveys by both government and industry. The largest single expense is the sample collection cost, particularly the transportation cost to or between sa mple sites, and to optimize this investment the tendency is to collect multiple samples at each site and for multi-element analyses. Mathematical and graphical interrelation of all of these components with the geology, geophysics, and physiography of the area under stUdy, usually requiring computer technology, provides a far greater insight into the exploration targets sought today than the single-sample, single-element approach that was so successful in the past. ResumeLes sediment {luviatiles manifestent frequemment un certain degre de variabilite de composition, granulometrie, triage et couleur. Cette variabilite depend de la geologie, de la nature du terrain et du climat du bassin-versant qui ali mente Ie cours d'eau, et constitue la base d'une prospection serieuse des sediments fluviatiles. Les prospecteurs ont toujours tenu compte de ce fait: autrefois, ils examinaient les blocs mineralises et recueillaient par lavage les mineraux lourds se trouvant dans Ie lit du cours d'eau; de nos jours, ils etudient la composition chimique des plus fines fractions granulometriques des sediments {luviatiles. Depuis 25 ans, les organismes gouvernementa...

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