Page 116 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Geochemical Anomaly and Mineral Prospectivity Mapping in GIS
by E.J.M. Carranza
Handbook of Exploration and Environmental Geochemistry, Vol. 11 (M. Hale, Editor)
© 2009 Elsevier B.V. All rights reserved. 115

Chapter 5

CATCHMENT BASIN ANALYSIS OF STREAM SEDIMENT ANOMALIES

INTRODUCTION
Stream sediments are commonly used as the chief sampling medium in exploration
geochemical surveys. They represent, at any point along drainage systems, composite
materials derived from the weathering and erosion of one or more sources upstream. It
follows that uni-element concentrations in stream sediments are derived from multiple
(mostly background and rarely anomalous) sources. In most cases, a major proportion of
variation in stream sediment uni-element concentrations is due to lithologic units
underlying the areas upstream of sample points. Rose et al. (1970) have demonstrated
that certain chemical contents of stream sediments have (a) positive relationships with
areas of the lithologic units in a catchment basin and (b) negative relationships with the
total area of a catchment basin. Hawkes (1976) considered such relationships as due to
downstream of dilution of chemical contents of stream sediments and suggested an
idealised formula that relates the measured uni-element concentrations (Y i) in stream
sediment sample i and the area of its sample catchment basin (A i) to the uni-element
concentrations in and surface areas of source materials in the sample catchment basin:

Y i A = Y a A + Y′ i (A − A a ) (5.1)
i
a
i

where Y a represents the uni-element concentrations due to anomalous sources occupying
a cumulative area A a, Y′ represents the uni-element concentrations due to background
i
sources occupying a cumulative area equal to A i–A a. Hence, if a catchment basin
contains only background sources, then Y i is equal to Y′ ; whereas if a catchment basin
i
contains anomalous sources (e.g., mineral deposits), then Y i is greater than Y′ . As stream
i
sediment geochemical exploration aims to identify catchment basins containing
anomalous sources, equation (5.1) can be re-arranged as:

′
)
Y a A = A ( Y − Y + Y i A ′ a (5.2)
i
i
a
i

and the term Y aA a can be considered an ‘anomaly rating’. The term A i (Y − Y i ) ′ in
i
equation (5.2) is equivalent to the ‘productivity’ of a catchment basin (Polikarpochkin,
1971), which was demonstrated by Moon (1999) to be useful in identifying anomalous
catchment basins.