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Meaningful techniques for presenting geochemical data from regional stream sediment surveys have long been a difficulty for explorationists. Contouring and image analysis methods can create artifacts which misrepresent the data whereas its display as point values may not portray the spatial variation inherent to the data set.
Further, the geochemistry of a stream sediment sample is often most influenced by the geology of the sediment source area. Coding the sample site by its underlying geology may not accurately represent the site and may result in the misidentification of anomalies.
An effective solution to this problem is to utilize the catchment basin of each sample site to define its zone of influence. This method can be used to:
- Define the actual areal coverage of a survey
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- Reclassify the geological influence on each sample based on its source area
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- Redefine the thresholds which separate anomalous from background populations
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RGS catchment,
North Gataga area
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Consequently, the BC Geological Survey has instituted a program of capturing catchment basin data for new and previous RGS areas. Each catchment basin map is produced from 1:50 000 scale topographic maps. Catchment basins are defined by the topographic height of land which divided one drainage from another.
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The resulting polygons are digitized with each polygon labeled to correspond to its RGS sample number. On occasion, nested polygons are produced where two samples were taken from successive sites on the same stream; in these cases the downstream polygon is defined to end at the upstream sample site. Areas of each polygon are calculated during the digitizing procedure. The corresponding RGS data can then be joined to each digital polygon record for interpretation.
Examples of catchment basin studies, interpreting RGS data:
Sibbick, S.J. (1994): Preliminary Report on the Application of Catchment Basin Analysis to Regional Geochemical Survey Data, Northern Vancouver Island (NTS 92L/03,04,05 and 06); in Geological Fieldwork 1993, Grant, B. and Newell, J.M. , Editors, B.C. Ministry of Energy Mines and Petroleum Resources, Geological Fieldwork 1993, Paper 1994-1, pages 111-117.
S.J. Sibbick and K.A. Laurus (1995): Integrated Geological and Geochemical Map for the Prediction of Intrusion-related Mineralization, Northern Vancouver Island, B.C. Ministry of Energy, Mines and Petroleum Resources, Open File 1995-12.
P. Matysek and W. Jackaman: B.C. Regional Geochemical Survey Anomaly Recognition, an Example Using Catchment Basin Analysis (PDF 737Kb) (103I, 103J), B.C. Ministry of Energy Mines and Petroleum Resources, Geological Fieldwork 1995, Paper 1996-1, pages 185-190.
For more information contact Ray Lett, Regional Geochem Program Unit.
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