Page 158 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Analysis of Geologic Controls on Mineral Occurrence 159

potential host rocks. Regional faults are also important (Mitchell and Balce, 1990),
perhaps in guiding the emplacement of magmatic heat sources and influencing structural
permeability and subsequent hydrothermal activity (Hedenquist, 1986; Sillitoe, 1993).
Based on a review of the general characteristics of epithermal systems in the
Philippines, most epithermal Au deposits in the archipelago, including those in the
Aroroy district, are deposited along volcanic arcs during the mid-Miocene to mid-
Pliocene (Mitchell and Balce, 1990; Mitchell and Leach, 1991; Yumul et al., 2003). The
geochemical nature of epithermal mineralisations in the Philippines and geochemical
anomalies (which include As) associated with such mineralisations are described by
UNDP (1987). The epithermal Au deposits are largely in the form of veins or vein
breccias and stockworks, indicating strong structural controls. There is no direct
evidence of genetic association between magmatism and the epithermal systems,
although a causal relationship is implied by analogy with epithermal Au deposits
elsewhere (e.g., Hedenquist and Henley, 1985; Singer, 2000). In most of the epithermal
gold districts in the Philippines, including those in the Aroroy district, the
lithostratigraphic succession comprises predominantly clastic andesitic or dacitic rocks
lying unconformably on folded basement rocks. Almost all epithermal gold deposits in
the Philippines are hosted by andesitic rocks and are commonly associated with minor
intrusions of andesitic porphyry plutons or less commonly with dacitic porphyry plutons.
Based on further review of the structural controls of epithermal gold deposits and the
geotectonic settings in the Philippines, epithermal mineralisations in the archipelago,
including those in the Aroroy district, have no apparent genetic association and display
lack of spatial relationship with regional fault systems, such as the sinistral strike-slip
Philippine Fault system (Fig. 6-7A), which actually cuts through most of the Philippine
archipelago (Aurelio et al., 1991). However, the epithermal mineralisations are
commonly situated along subsidiary faults or splays of regional fault systems (Mitchell
and Balce, 1990; Mitchell and Leach, 1991). This is also apparently the case for the
epithermal Au deposits in the Aroroy district. Many of the faults/fractures in the Aroroy
district (Fig. 5-13) are plausibly subsidiary structures of the regional sinistral strike-slip
Sibuyan Sea Fault, which is a branch of the Philippine Fault system (Fig. 6-7A). The far-
field stress (i.e., principal regional stress axis) that generated the Philippine Fault is
generally oriented east-west (Aurelio et al., 1997). By inference from the directions of
the strike-slip motions of the Sibuyan Sea Fault and the Philippine Fault, the near-field
stress (i.e., principal district-scale stress axis, σ 1) in the vicinity of the Aroroy district is
probably oriented towards about 150º (or 330º) (Fig. 6-7A). If this is the case, and in
accordance with theoretical wrench tectonics or fault mechanics (Jaeger and Cook, 1976;
Mandl, 1988), then the north-northwest and northwest trending faults/fractures in the
Aroroy district (Fig. 5-13) are subsidiary conjugate faults/fractures likely associated
directly with the Sibuyan Sea Fault and to a lesser extent with the Philippine Fault. In
addition, according to Aurelio et al. (1991), the sinistral strike-slip motion along the
Philippine Fault initiated in about late Early Pliocene. This, in turn, suggests that the
Philippine Fault can be implicated in the emplacement of the Pliocene Nabongsoran
Andesite porphyry intrusions (Baybayan and Matos, 1986; JICA-MMAJ, 1986), which