Page 168 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Analysis of Geologic Controls on Mineral Occurrence 169

The centroid of each of the four mapped units of Nabongsoran Andesite porphyry
intrusions (Fig. 3-9) can be digitised as a point and the four digitised points can be used
in a spatial association analysis via the distance distribution method. The results of this
analysis (Figs. 6-10C and 6-10D) suggest that there is statistically significant (at α=0.05)
positive spatial association between centroids of Nabongsoran Andesite porphyry
intrusions and intersections of NNW- and NW-trending faults/fractures and the positive
spatial association is optimal within 0.8 km of intersections of NNW- and NW-trending
faults/fractures. Within this distance from intersections of NNW- and NW-trending
faults/fractures, all of the centroids of Nabongsoran Andesite porphyry intrusions in the
study area are present and, according to the curve for D, there is about 70% higher
occurrence of Nabongsoran Andesite porphyry intrusions than would be expected due to
chance (Fig. 6-10C). These results suggest that (a) emplacement of Nabongsoran
Andesite porphyry intrusions in the case study area are controlled by intersections of
NNW- and NW-trending faults/fractures and (b) Nabongsoran Andesite porphyry
intrusions contributed to circulation of hydrothermal fluids toward intersections of
NNW-trending and NW-trending faults/fractures. These inferences provide a link, albeit
indirect, between Nabongsoran Andesite porphyry intrusions and epithermal
mineralisations in the case study area as discussed below.
Porphyry intrusions in geological environments characterised by island-arc strike-slip
fault systems are usually accompanied by porphyry Cu mineralisations (Titley and
Beane, 1981). In these geological environments, such as in the Philippine archipelago,
porphyry intrusions and/or porphyry Cu deposits have strong positive spatial
associations with strike-slip fault discontinuities (Carranza and Hale, 2002c), which are
often sites of intersections of strike-slip faults. In addition, there are strong spatial and
temporal relationships between porphyry Cu and epithermal Au mineralisations not only
in island-arc strike-slip fault systems (e.g., Cooke and Bloom, 1990; Arribas et al., 1995;
Hedenquist et al., 1998) but also in continental-arc strike-slip fault systems (e.g.,
Muntean and Einaudi, 2001; Berger and Drew, 2002; Billa et al., 2004). In these
geological environments, epithermal Au mineralisations are found superjacent to (i.e.,
stratigraphically although not necessarily vertically above) porphyry Cu mineralisations,
because the latter are formed at relatively higher temperatures than the former. Based on
these collections of pieces of knowledge, the following hypotheses can be formulated. In
the Aroroy district, it is plausible that there exist porphyry Cu and epithermal Au
mineralisations associated with the Nabongsoran Andesite porphyry intrusions. It is
plausible that porphyry Cu mineralisations associated with the mapped units of
Nabongsoran Andesite porphyry intrusions (Fig. 3-9) have already eroded, meaning that
associated superjacent epithermal Au mineralisations have also already been eroded. It is
plausible, however, that there exist blind Nabongsoran Andesite porphyry intrusions and
blind porphyry Cu mineralisation at sites below the surface where NNW- and NW-
trending faults/fractures intersect, such that only associated epithermal Au
mineralisations are exposed. Therefore, it is plausible that proximity to intersections of
NNW- and NW-trending faults fractures constitutes a proxy spatial evidence for heat