Page 137 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

136 Chapter 5

B. In contrast to the spatial distributions of the measured As values (Fig. 5-9A), the
spatial distributions of the standardised As residuals show not only enrichment of As
along the north-northwest trend of the epithermal Au deposit occurrences but also
enrichment of As in the eastern parts of the area underlain by the Aroroy Diorite (Fig. 5-
9B). These results demonstrate the value of estimating and then removing local
background uni-element contents attributable to lithology from measured uni-element
concentrations in stream sediments. The standardised uni-element residuals are then
further subjected to downstream dilution correction to identify significant anomalies.

Correction for downstream dilution and classification of anomalies

Equation (5.9) is applied to the standardised uni-element residuals in order to correct
for downstream dilution. Note that the standardised geochemical residuals are the values
used for the term Y − Y ′ in equation (5.9). The dilution-corrected uni-element residuals
i
i
are then subjected to fractal analysis via the concentration-area method (Cheng et al.,
1994) demonstrated in Chapter 4. In this analysis, negative dilution-corrected uni-
element residuals cannot be considered in a log-log graph of the concentration-area
relation. This is not a concern, however, because negative dilution-corrected uni-element
residuals can already be considered to represent background populations and thus
excluded in the analysis, leaving high background to anomalous populations to be
recognised from the positive dilution-corrected uni-element residuals.
Fig. 5-10A shows that the log-log graph of the concentration-area model for positive
dilution-corrected As residuals follows power-law relations indicating multifractal
patterns in the data. Three straight lines satisfactorily fit the concentration-area plots,
indicating the presence of three populations that can be separated by threshold values at
the breaks in slopes of the straight lines. These three populations, from lowest to highest
values, are considered to represent high background, low anomaly and high anomaly of
As. The three recognised thresholds, plus a threshold for the highest negative or zero
dilution-corrected As residual to represent upper limit of background, are used to display
the spatial distributions of the background and anomalous populations of dilution-
corrected As residuals (Fig. 5-10B).
The spatial distributions of the classified dilution-corrected As residuals (Fig. 5-10B)
show a series of anomalies that follow the north-northwest trend of the epithermal Au
deposit occurrences and indicate some low anomalies in the eastern sections of the area
underlain by the Aroroy Diorite. These latter anomalies cannot be readily considered as
significant or not without considering other uni-element anomalies. Fractal analysis via
the concentration-area method was also applied to separate background and anomalies in
the dilution-corrected residuals of Cu, Zn, Ni, Co and Mn, so that analysis of multi-
element anomalies can be performed.

Modeling of multi-element anomalies via principal components analysis

Principal components analysis is performed on a subset of samples (n=93 out of the
total 135) with anomalous dilution-corrected residuals for at least one of the elements