Page 157 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

158 Chapter 6

then be made as the first record in the data file to be used in creating a Fry plot (although
this might be software dependent).
The Fry plots for the 13 occurrences of epithermal Au deposits in the Aroroy district
show prominent 150-180º (or 330-360º) trends (Figs. 6-6A and 6-6B), which indicates
structural controls by north-northwest-trending faults/fractures (see Fig. 5-13). The
overall north-northwest trend of the 13 occurrences of the epithermal Au deposits in
Aroroy district is, independent of the Fry plots, perceivable from the map shown in Fig.
3-9. So, one could say that Fry plots of mineral deposit occurrences can be biased by
pre-existing data. Nevertheless, the Fry plots of the 13 occurrences of epithermal Au
deposits in the study area show at least two patterns that are not obvious in the map of
the original data points. Firstly, the Fry plots suggest the presence of north-northwest
trending corridors of epithermal Au deposits (Fig. 6-6A), which seem to be regularly-
spaced at about 2 km intervals. These regularly-spaced corridors possibly represent
parallel district-scale hydrothermal systems controlled by the general spacing of north-
northwest trending faults/fractures (see Fig. 5-13). Secondly, the rose diagram of
orientations of only pairs of Fry points within 3.6 km of each other (Fig. 6-6C) shows a
subsidiary 120-150º (or 300-330º) trend, which suggests that northwest trending
faults/fractures are important local-scale controls on epithermal Au mineralisations in the
study area. Thus, the Fry plots of the 13 occurrences of epithermal Au deposits in the
Aroroy district complement as well as supplement the results of the point pattern
analysis (in terms of regularity of spatial distribution) and the fractal analysis (in terms
controls at local- and district-scales).

Knowledge synthesis and results of spatial analysis
In order to develop a model of plausible geologic controls on localisation of the type
of mineral deposits sought in a study area, it is instructive to synthesise the results of
analyses of spatial distribution of mineral deposits of the type sought with published
knowledge of geological processes relevant to the formation of mineral deposits of the
type sought. There is no particular way of doing the synthesis. The following discussions
provide an example of doing so for the case study area.
Based on a review of models of epithermal precious metal deposits worldwide, there
is a variety of geologic settings in which such type of mineral deposits can occur
(Mitchell and Garson, 1981; Sawkins, 1989; Robb, 2004). Igneous activity plays an
essential role in the formation of most epithermal Au deposits in terms of providing the
heat necessary to generate hydrothermal convection cells (White and Hedenquist, 1990).
In most cases, epithermal Au deposits are spatially and temporally associated with
subaerial volcanic rocks and/or their related subvolcanic intrusions (Sillitoe and
Bonham, 1984). Subaerial volcanism may occur in a variety of tectonic settings,
although it occurs mostly along volcanic arcs in subduction tectonic settings
characteristic of oceanic-continental or oceanic-oceanic plate collision environments (Le
Pichon et al., 1973). Strong structural control is almost universally recognised for
various types of gold deposits, including epithermal gold deposits (Henley, 1990; Henley
and Adams, 1992). Faults/fractures in the near-surface enhance the permeability of