Page 184 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Analysis of Geologic Controls on Mineral Occurrence 185

Fig. 6-16. Conceptual model of structural control on epithermal mineralisation in the Aroroy
district (Philippines). Dilational jogs or zones of extension arise from en echelon strike-slip
fault/fracture segments or from intersections of strike-slip faults/fractures (see Fig. 6-7). In case of
the former in the study area, a set of conjugate shears (in this case the dashed NW-trending faults)
is possibly relatively passive. Within a dilational jog, a mesh of interlinked shear faults/fractures
and extension faults/fractures is developed and the extension faults/fractures are favourable sites
for deposition of epithermal veins. See text for further explanation.

control on epithermal mineralisation in the case study area can be drawn (Fig. 6-16) and
postulated as follows.
The stronger positive spatial association of the epithermal Au deposit occurrences
with the NNW-trending faults/fractures than with the NW-trending faults/fractures
suggests that the former were the more active structural controls on the formation of the
epithermal Au deposits. Dilational jogs or zones of extension faults/fractures, arising
from en echelon NNW-trending strike-slip faults/fractures or from intersections of the
NNW- and NW-trending strike-slip faults/fractures (see Fig. 6-7), provided conditions
(i.e., structural permeability and hydrothermal fluid flow) that favoured epithermal
mineralisation (Fig. 6-16). On regional- to district-scales, hydrothermal fluids were
circulated or focused towards dilational jogs or intersections of strike-slip faults/fractures
(e.g., Laing, 2004) during seismic activity (cf. Sibson, 1987, 1996, 2000, 2001; Bellot,
2008). Then, on a local-scale, hydrothermal fluids were focused towards extension
faults/fractures in a mesh of interlinked shear faults/fractures and extension
faults/fractures within dilational jogs (cf. Blenkinsop and Kadzviti, 2006; Nelson, 2006).
These interpretations of presence of regional- to local-scale dilational jogs and regional-