Till is a favoured sample medium for locating mineral deposits in glaciated shield terrains of Canada and Fennoscandia because it best reflects the primary composition of the bedrock source area. In the sampling phase, an important and costly component of till surveys, sample density, sample depth and sample method must be chosen according to the needs of the exploration program. Surface till sampling methods in forested areas differ from those used in permafrost terrain. However, in both areas, concentrations of labile ore minerals and their products of decomposition can be detected in the fine fraction (< 2 mm) of weakly oxidized till. In thin drift-covered areas, till samples are collected by hand excavation or trenching at < 5 m depth. In areas of thicker drift, more expensive methods such as reverse circulation rotary drills, rotasonic drills and portable drills are used to collect till samples at depth and to determine lateral and vertical variations in till geochemistry. Laboratory methods are an essential part of till geochemical surveys. The choices of the size fraction and analytical methods are determined by the nature and composition of the expected bedrock target, and by costs.
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Drift Exploration in Glaciated Terrain
This volume describes the use of till geochemical and indicator mineral methods for mineral exploration in glaciated terrain of Canada. The principles and examples described in this volume wil have direct applications for exploration companies and prospectors exploring for diamonds, precious and base metals and uranium in glaciated parts of North America, northern Europe and Asia and mountainous regions of South America.
The first two papers in this volume provide an introduction to glaciated terrain and the two styles of glaciation that have affected the world, continental glaciers in broad flat lying Shield areas and alpine glaciers in mountainous terrain. Sampling techniques are described next, followed by an introduction to the use of heavy minerals. Heavy mineral methodss have become an important exploration tool in glaciated terrain for gold and base metals and, in the last ten years, for diamonds. Lake sediments and biogeochemical methods are also included in this volume as a complement to geochemical and indicator mineral methods in glaciated terrain. A chapter on GIS has been included because data interpretation and display are important and essential parts of any regional or detailed geochemical survey. The remainder of the volume is case studies for the three main glaciated terrain tyes in Canada: Shield, Appalachia and Cordillera