Page 122 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Catchment Basin Analysis of Stream Sediment Anomalies 121

be interpreted as depletion of uni-element concentrations in stream sediments due to
certain intrinsic or anthropogenic factors or processes. Negative residuals could also
arise, however, if some values in the uni-element data used in the analysis are
‘anomalously’ high values or outliers, which cause upward bias in multiple regression
modeling or in calculation of weighted means and thus result in estimates of local
background uni-element concentrations that may be, for some catchment basins, rather
artificially high. Recognition and removal of such outliers prior to the regression
analysis or the analysis of weighted means may yield unbiased estimates of local
background uni-element concentrations, but only for samples retained in the analysis.
Nevertheless, as mineralisation is partially or completely unknown, absolute values of
estimates of local background uni-element concentrations are trivial but the magnitude
of uni-element residuals is useful in ranking of anomalies.
The magnitude of uni-element residuals (i.e., Y − Y ′ ) is controlled by downstream
i
i
dilution due to mixing of stream sediments from various and mostly non-anomalous
sources in a sample catchment basin, thereby obscuring contributions of anomalous
sources. The relation proposed by Hawkes (1976) re-arranged in equation (5.2) indicates
that uni-element residuals can be corrected for downstream dilution by considering the
sample catchment basin area to enhance uni-element anomalies based on positive uni-
element residuals, which might indicate presence of mineralisation.
To correct uni-element residuals for downstream dilution, Bonham-Carter and
2
2
Goodfellow (1984, 1986), assumed a unit area of 1 km (i.e., A a = 1 km ) for exposed
mineral deposits of interest contributing to stream sediments and defined, by slightly
modifying equation (5.2), a dilution-corrected ‘mineralisation rating’ variable R i as:

′
R = A ( Y − Y + Y′ . (5.8)
)
i
i
i
i
i

However, by using equation (5.8) to correct residuals for downstream dilution, it can be
argued that estimates of local background uni-element concentrations (i.e., Y ′ ) are
i
added back. Rose et al. (1979, pp. 399) point out, nonetheless, that the term Y′ in
A
i
a
equation (5.2) can be neglected if A i is much larger than A a. Carranza and Hale (1997)
2
assumed a small unit area of 1 ha (i.e., A a = 0.01 km ) of exposed anomalous sources,
which is 10× to 200× smaller than the sample catchment basins in their study area. They
then neglected the term Y ′ i A in equation (5.2) and considered Y a to represent dilution-
a
corrected residuals of uni-element concentrations, thus:

Y = 100 A i (Y − Y i ) ′ . (5.9)
×
i
a

Equation (5.9) is adopted here to derive dilution-corrected residuals of uni-element
concentrations in stream sediments.
For our example stream sediment Cu and Zn data, Fig. 5-3 displays the similarity
between the spatial distributions of dilution-corrected uni-element residuals based on
either local background uni-element concentrations estimated via regression analysis