Page 105 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

104 Chapter 4

Fig. 4-14. Spatial distributions of different background levels of Co in stream sediments, Aroroy
district (Philippines), based on thresholds defined from the (A) continuous surface of the Co data
and (Fig. 4-12D and Table 4-I) (B) discrete catchment basin surface of the Co data (Fig. 4-13D
and Table 4-I). VL = very low; L = low; H = high; VH = very high. Triangles represent locations
of epithermal Au deposit occurrences, whilst thin black lines represent lithologic contacts (see Fig.
3-9).

Mandaon Formation. In the southwestern portion of the area, the high (to very high)
background Co values are spatially associated with the Nabongsoran Andesite, large
outcrops of which have been observed to be intrusive into the Lanang Formation
although much smaller outcrops (not mappable at the scale of the map in Fig. 3-9) have
been observed to be intrusive into the Mandaon Formation. The high (to very high)
background Co values in the southwestern portion of the area are plausibly due to the
more mafic facies in the Nabongsoran Andesite. Similarly, the high background Co
values in the northwestern portion of the area are plausibly due to the more mafic facies
in smaller intrusive units of the Nabongsoran Andesite, which are not mappable at the
scale of the map in Fig. 3-9. However, the strong correlation of Co with Mn (see Table
3-V) suggests that the high (to very high) background Co values in the southwestern
portion of the area are also plausibly due to scavenging by co-precipitation with Mn-
oxides in the surficial environment. Note in Fig. 4-11 that the southwestern portion of
the area is characterised by topographic lows (i.e., low-energy environment), into which
westerly and southwesterly flowing streams carry their load. It is plausible that
hydromorphically dispersed elements (e.g., Co, Mn) derived from weathered rocks