Page 138 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Catchment Basin Analysis of Stream Sediment Anomalies 137

Fig. 5-10. (A) Log-log plot of concentration-area fractal model of positive dilution-corrected As
residuals, Aroroy district (Philippines), showing two thresholds (dots) that separate high (H)
anomaly, low (L) anomaly and high background populations in the data. (B) Spatial distributions
of high background and anomalous populations of dilution-corrected As residuals based on
thresholds recognised in the concentration-area plot. Background class pertains to negative
dilution-corrected As residuals, which are not represented in the log-log plot of the concentration-
area relation. Triangles in the map represent locations of epithermal Au deposit occurrences,
whilst thin black lines represent lithologic contacts (see Fig. 3-9).

under study. Prior to PCA, descending ranks of n to 1 (i.e., considering 1 as the lowest
rank) are assigned to descending n values of dilution-corrected uni-element residuals and
tied ranks are averaged. A Spearman rank correlation matrix is computed for rank-
transformed dilution-corrected uni-element residuals, and is used in the PCA (e.g.,
George and Bonham-Carter, 1989; Carranza and Hale, 1997). Table 5-IX shows the
results of the PCA.
The PC1, accounting for about 34% of the total variance, plausibly represents a Co-
Zn-Mn multi-element association reflecting metal-scavenging by Mn-oxides in the
drainage environments in most parts of the area; Cu, Ni and As have antipathetic, weak
sympathetic and very weak sympathetic relations, respectively, with this multi-element
association. The PC2, accounting for about 22% of the total variance, represents a Ni-
Cu-As association having an antipathetic relation with Mn and thus plausibly reflects an
anomalous multi-element association. The association between Cu and As in PC2
plausibly reflects enrichment of these elements in stream sediments due to weathering
and erosion of anomalous sources containing sulphide (chalcopyritic and arsenopyritic)