Page 76 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Exploratory Analysis of Geochemical Anomalies 75

Fig. 3-17. Anomalies in the log e -transformed As data subsets according to rock type at sample
points and exclusive of As censored values (see Table 3-IV), Aroroy district (Philippines).
Anomalies are based on threshold defined as (A) mean+2SDEV, (B) median+2MAD and (C)
boxplot UW. Triangles represent locations of epithermal Au deposit occurrences. Light-grey lines
represent lithologic contacts (see Fig. 3-9).

defined by each of the three methods are apparently non-problematic (Table 3-IV), so
they are mapped to study the anomalies and compare the performance of the three
methods (Fig. 3-17). It is obvious that the As anomalies based on threshold defined as
median+2MAD show the best spatial associations with the known epithermal Au deposit
occurrences in the case study area (Fig. 3-17B). The northwestern most cluster of As
anomalies are associated with hydrothermally altered volcano-sedimentary rocks. The
As anomaly map in Fig. 3-17B is even better than the As anomaly map in Fig. 3-16B. In
the former, there are 18 anomalous samples (or at least 13%) of the 135 samples, which
is probably why the median+2MAD threshold outperforms the boxplot UW threshold as
well as the mean+2SDEV threshold.

Analysis of inter-element relationships
In the preceding analysis of the uni-element data distributions, it can be perceived
that probably the most dominant inter-element relationships in the study area is due to
lithology (see Table 3-II and Fig. 3-13). That is, parts of the study area underlain by
diorite have relatively lower concentrations of the elements under study compared to
parts of the study area underlain by dacitic/andesitic volcano-sedimentary rocks. It is
important to further unravel other inter-element relationships in the data, which may be
useful in the interpretation of significant geochemical anomalies. For example, in this
case study it is instructive to determine further (a) whether or not anomalies of As are
plausibly due to scavenging by Mn-oxides and (b) whether there are inter-element