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Regional till geochemical surveys in the Canadian Cordillera: sample media, methods and anomaly evaluation

By
Victor M. Levson
Victor M. Levson
British Columbia Geological Survey, 1810 Blanshard Street, Victoria, British Columbia V8V 1X4, Canada (e-mail: Vic.Levson@gems9.gov.bc.ca)
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Published:
January 01, 2001

Abstract

Basal tills have become a widely used regional geochemical sampling medium in recent years in the Canadian Cordillera. They reflect the primary composition of the source bedrock and contrast with B-horizon soil that can be developed on a variety of glacial and non-glacial surficial sediment types. Detailed sedimentological data are critical to collect and they are used to differentiate basal tills from other visually similar sediments including englacial and supraglacial tills, colluvial debris flow deposits, and very poorly sorted, glaciofluvial or glaciolacustrine sediments (e.g. diamictons or gravelly muds). The variable transport and depositional processes that form these different sediments make interpretation of geochemical data difficult. Deep (usually > 0.75 m) C-horizon sampling of basal till minimizēs the complicating effects of pedogenesis, weathering, surface washing and gravity remobilization of the tills. The latter processes, particularly pronounced in the wet, steep terrain, typical of much of the Canadian Cordillera, lead to depleted concentrations of heavy minerals (notably Au) and hydromorphic dispersion of mobile elements in the near surface sediments. Also, elements that are preferentially concentrated in the fine fraction can be selectively removed by surface waters.

Offset sampling lines, oriented perpendicular to the dominant ice-flow direction, are most effective for detecting regional geochemical anomalies which are typically narrow and elongated parallel to ice-flow. Erratics trains and till anomalies are usually a few to several kilometres long and up to one or more kilometres wide. For some metals such as Au, anomalies are generally larger and more readily detected in till than in B-horizon soil. Surface till anomalies reflect up-ice sources and not the immediately underlying bedrock; down-ice displacements of > 500 m often occur in areas of thick till. Basal till anomalies usually can be traced to source along linear transport paths reflecting topographically controlled valley-glacier flow in mountainous areas and unidireccional ice-sheet flow in many plateau areas, chiefly representative of the last glacial event. Interpretations of till geochemical data are enhanced with a clear understanding of the surficial and bedrock geology, Quaternary stratigraphy, ice-flow history and down-ice dispersal characteristics around known mineral deposits.

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Contents

Geological Society, London, Special Publications

Drift Exploration in Glaciated Terrain

M. B. McClenaghan
M. B. McClenaghan
Geological Survey of Canada, Canada
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;
P. T. Bobrowsky
P. T. Bobrowsky
British Columbia Geological Survey, Canada
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;
G. E. M. Hall
G. E. M. Hall
Geological Survey of Canada, Canada
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;
S. J. Cook
S. J. Cook
Hudson Bay Exploration and Development Co. Ltd., Canada
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Geological Society of London
Volume
185
ISBN electronic:
9781862394339
Publication date:
January 01, 2001

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