Page 206 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

208 Chapter 7

TABLE 7-V

Examples of scores assigned to evidential classes in individual evidential maps portraying the
recognition criteria for epithermal Au prospectivity, Aroroy district (Philippines). Range of values
in bold include the threshold value of spatial data of optimum positive spatial associations with
epithermal Au deposits in the case study area.

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Proximity to NNW 1 Proximity to FI 2 Proximity to NW 3 ANOMALY
Range (km) Score Range (km) Score Range (km) Score Range Score
0.00 – 0.08 8.0 0.00 – 0.39 8.0 0.00 – 0.18 8.0 0.00 – 0.06 1.0
0.08 – 0.15 8.5 0.39 – 0.58 9.0 0.18 – 0.36 8.5 0.06 – 0.10 2.0
0.15 – 0.23 9.0 0.58 – 0.80 9.5 0.36 – 0.54 9.0 0.10 – 0.16 4.0
0.23 – 0.32 9.5 0.80 – 1.09 10.0 0.54 – 0.75 9.5 0.16 – 0.25 6.0
0.32 – 0.41 10.0 1.09 – 1.40 9.0 0.75 – 1.01 10.0 0.25 – 0.29 8.0
0.41 – 0.52 9.0 1.40 – 1.80 7.0 1.01 – 1.29 8.0 0.29 – 0.37 10.0
0.52 – 0.71 7.0 1.80 – 2.32 5.0 1.29 – 1.65 6.0 0.37 – 0.49 9.5
0.71 – 1.06 5.0 2.32 – 2.92 3.0 1.65 – 2.24 4.0 0.49 – 0.78 9.0
1.06 – 1.73 3.0 2.92 – 3.62 2.0 2.24 – 3.02 2.0
1.73 – 3.55 1.0 3.62 – 5.92 1.0 3.02 -5.32 1.0
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1 NNW-trending faults/fractures. Intersections of NNW- and NW-trending faults/fractures. NW-
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trending faults/fractures. Integrated PC2 and PC3 scores obtained from the catchment basin
analysis of stream sediment geochemical data (Chapter 3).
multi-class index overlay modeling applied to mineral prospectivity mapping can be
found in Harris et al. (2001b), Chico-Olmo et al. (2002), De Araújo and Macedo (2002)
and Billa et al. (2004).
Examples of scores assigned to classes in individual evidential maps representing the
recognition criteria for epithermal Au prospectivity in the case study area are listed in
Table 7-V. The evidential maps of proximity to structures each contain 10 classes
obtained by classifying the range of map distances to a set of structures into 10-
percentile intervals. The scores assigned to proximal classes within the range of distance
of optimum positive spatial association with known epithermal Au deposits are
substantially higher than the scores assigned to distal classes. The evidential map of
multi-element stream sediment geochemical anomalies contains only eight classes
because the lowest values represent 30 percentile of the data and the remaining values
are classified into 10-percentile intervals. The scores assigned to the upper 30 percentile
classes, which have optimum positive spatial association with known epithermal Au
deposits, are substantially higher than the scores assigned to the lower 70 percentile
classes. The individual evidential maps are assigned the same weights (W i) as those used
in the binary index overlay modeling (see Table 7-II).
The prospectivity map derived via multi-class index overlay modeling (Fig. 7-9A)
shows the influence of the catchment basin stream sediment multi-element anomalies
and NNW-trending linear patterns indicating the influence of proximity to the NNW-
trending faults. These indicate consistency of the slightly higher pairwise importance
rating given to the stream sediment anomalies compared to the proximity to NNW-