Page 117 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

116 Chapter 5

In another study, Pan and Harris (1990) proposed a generalised equation to predict
uni-element concentration (Y s) at a source as a function of uni-element concentration (Y i)
in a stream sediment sample i and the distance (D i) of that sample from the source:

Y s = D i θ sin α (5.3)
Y
i

where θ is a uni-element coefficient and α is the angle of topographic slope. The
generalised equation proposed by Pan and Harris (1990) worked well in their case study
with discrete anomalous sources of Au, Ag and Cu in soil. It can be noted, however, that
equation (5.3) does not take into account the area and uni-element background in a
sample catchment basin. Although background concentrations of Au can be relatively
insignificant compared to anomalous concentrations of Au, it is important to consider
background of other elements in order to recognise anomalies.
Analysis of stream sediment geochemical data alone can therefore be insufficient for
recognition of significant anomalies. Effective interpretation of stream sediment
geochemical data requires integration of every available piece of spatial information
pertinent to the zone of influence of every stream sediment sample location – its
catchment basin. The relation in equation (5.2) indicates that, in order to recognise
stream sediment anomalies, (a) background uni-element concentrations Y ′ must first be
i
estimated for each sample catchment basin, (b) background uni-element concentrations
Y′ must then be removed from measured uni-element concentrations Y i (i.e., Y i–Y′ )
i
i
leaving geochemical residuals, which may include significant anomalies (i.e., derived
from mineral deposits) and (c) geochemical residuals must be corrected for downstream
dilution by taking into account area of sample catchment basin (i.e., A i (Y − Y i ) ′ ) to
i
enhance anomalies. By considering areal proportions of lithologic units in every sample
catchment basin, it is possible to estimate local background uni-element concentrations
due to lithology in every sample catchment basin (Bonham-Carter and Goodfellow,
1984, 1986; Bonham-Carter et al., 1987; Carranza and Hale, 1997). Instead of areal
proportions of lithologic units in every sample catchment basin, Peh et al. (2006)
demonstrated that linear proportions of lithologic units along perennial streams in every
sample catchment are also useful in estimating local background uni-element
concentrations.
The objective of this chapter is to explain techniques, which can be implemented in a
GIS, for catchment basin analysis of uni-element anomalies in stream sediments in order
to (a) estimate local uni-element background concentrations due to lithology and (b)
derive and correct uni-element residuals for downstream dilution. Dilution-corrected uni-
element residuals are then used in the analysis of uni-element and multi-element
anomalies. These techniques are then demonstrated in a case study of the same stream
sediment geochemical data used to demonstrate the EDA and fractal analysis explained,
respectively, in Chapters 3 and 4.