Page 113 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

112 Chapter 4

Fig. 4-20. Concentration-area models for the integrated As-Ni-Cu scores the stream sediment
geochemical data in the Aroroy district (Philippines), based on (A) transformation of the
integrated As-Ni-Cu scores of the point data as a continuous surface and (B) representation of the
integrated As-Ni-Cu scores of the point data as discrete geochemical surfaces. Solid lines are
obtained by least squares fitting through linear parts of the plots. Dots at breaks in slopes of the
lines represent threshold integrated scores (v t ). Lines to the left of any threshold follow a power-
law relation represented by equation (4.6), whereas lines to the right of rightmost thresholds
follow a power-law relation represented by equation (4.7).

contact of the Aroroy Diorite with the Mandaon Formation (see Fig. 3-9). The latter
could be related to feeble hydrothermal alteration in the Mandaon Formation due to the
intrusion of the Aroroy Diorite. The continuous surface of the integrated As-Ni-Cu
scores suggests that the anomalies related to the epithermal Au deposit occurrences and
the anomalies associated to possible hydrothermally altered rocks along the contact
between Mandaon Formation and the Aroroy Diorite seem to overlap rather than run
parallel. This could be a result, however, of the smoothing effect during interpolation of
the point data of integrated As-Ni-Cu scores.
Combining the PC3 (Cu-As) and PC4 (As-Ni) scores as integrated As-Ni-Cu scores
results in downgrading of importance of anomalies (e.g., from anomalous PC3 or PC4
scores to high background integrated scores) in four localities indicated by Roman
numerals in Fig. 4-21. In localities I, II and III, which are underlain by the Aroroy
Diorite, anomalies of PC3 scores (Fig. 4-18), which are mostly due to Cu rather than As
(see Figs. 4-14 and 4-15), and anomalies of PC4 scores (Fig. 4-19), which are mostly
due to Ni rather than As (see Fig. 4-15) are downgraded in importance (i.e., they now
map as high background) after deriving and analysing the integrated As-Ni-Cu scores
(Fig. 4-19). These observations imply that the anomalies of PC3 and PC4 scores in
localities I, II and III are probably non-significant but due to variations in mineralogical
composition of the Aroroy Diorite (see Fig. 3-9). Thus, combining the PC3 (Cu-As) and
PC4 (As-Ni) scores into the integrated As-Ni-Cu scores has positive effects in these
cases. In locality I, which is underlain by the Sambulawan Formation and some alluvial
deposits (see Fig. 3-9), anomalies of PC4 scores (mainly due to As rather than Ni; see
Fig. 4-15) are downgraded in importance (i.e., they now map as high background) after