Page 142 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Catchment Basin Analysis of Stream Sediment Anomalies 141

of both PC2 and negated PC3 scores in the northwestern quadrant of the area (Fig. 5-
11B) have been upgraded in importance (i.e., they now map as high anomalies as shown
in Fig. 5-12). In addition, many of the low anomalies of negated PC3 scores in the
eastern parts of the area (Fig. 5-11B) have been enhanced. Combining the PC2 (Ni-Cu-
As) and negated PC3 (As) scores into integrated As-Ni-Cu scores has an overall positive
effect in this case study and is therefore defensible.

Screening of multi-element anomalies with fault/fracture density
The presence of stream sediment uni-element or multi-element anomalies does not
always mean presence of mineral deposits, so it is necessary to apply certain criteria for
ranking or prioritization of anomalies prior to any follow-up work. Criteria for ranking
or prioritization can be related to indicative geological features of the mineral deposit
type of interest or to factors that could influence localisation of stream sediment
anomalies.
In the study area, faults/fractures can influence localisation of stream sediment
anomalies because (a) such geological features are common loci of epithermal Au
deposits, whose element contents find their way into streams due to weathering and
erosion and (b) the presence of such geological features indicates enhanced structural
permeability of rocks in the subsurface, which facilitates upward migration of
groundwaters that have come in contact with and have leached substances from buried
deposits. These arguments suggest that the significance of multi-element stream
sediment anomalies in sample catchment basins can be screened or examined further by
using fault/fracture density as a factor (cf. Carranza and Hale, 1997).
Fig. 5-13A shows a map of faults/fractures in the study area, indicating that the
epithermal Au deposits are localised mostly along certain north-northwest-trending
faults/fractures. A fault/fracture density map can be created by calculating, per sample
catchment basin, the ratio of number of pixels representing faults/fractures in a sample
catchment basin to number of pixels in that sample catchment basin. Most of the
epithermal Au deposit occurrences in the study area are situated in sample catchment
basins with moderate to high fault/fracture density (Fig. 5-13B). In order to further
screen the multi-element stream sediment anomalies (e.g., as shown in Fig. 5-12), the
product of integrated As-Ni-Cu scores and fault/fracture density can be obtained and
then subjected to classification via the concentration-area fractal method.
The results shown in Fig. 5-14 clearly indicate that, on the one hand, anomalies of
integrated As-Ni-Cu scores in the western half of the study area (see Fig. 5-12) are
mostly significant in terms of indicating localities that contain or are proximal to
epithermal Au deposit occurrences. The anomalous sample catchment basins in the
western half of the study area (Fig. 5-14) are aligned along the north-northwest trend of
the epithermal Au deposit occurrences. On the other hand, anomalies of integrated As-
Ni-Cu scores related to the Aroroy Diorite in the eastern half of the study area (see Fig.
5-12) are downgraded in importance (i.e., they now map mostly as background as shown
in Fig. 5-14) after using fault/fracture density in the analysis. This latter result suggests
that the Aroroy Diorite is possibly non-mineralised.