Page 175 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

176 Chapter 6

5. For each point P jx (i.e., points of interest), determine d jx from an AP. Likewise, for
each point P jy (i.e., points at regular intervals along ⊥L i), determine d jy from an AP.
These can be achieved via an overlay operation between point map P jx (or P jy) and a
map of distances from an AP.
6. Calculate r d jx d jy for every pair of d jx and d jy. Create plots of r d jx d jy versus Y j.
7. To test if values of r d jx d jy are significantly different from zero, a t-value can be
calculated as (Davis, 2002):

r d d n− 2
t= jx jy , (6-4)
1− (r d jx d jy ) 2

which has n-2 degrees of freedom, where n is equal to the number of P jx and thus
equal to the number of P jy. A calculated t-value is then compared with a critical t-
value at a certain significance level from published statistical tables.
8. Repeat steps 5-8 each time for a different AP.
9. Calculate the mean proximal- r d jx d jy and the mean distal- r d jx d jy . This procedure
aims to determine whether the spatial association between P jx and L i is positive (i.e.,
mean proximal- r d jx d jy > mean distal- r d jx d jy ) or negative (i.e., mean proximal-
r d jx d jy < mean distal- r d jx d jy ).

The application of the distance correlation method to quantify the spatial associations of
the 13 epithermal Au deposit occurrences with the different sets of linear and point
structural features in the case study area yielded satisfactory results (discussed below)
from all of the nine APs (Tables 6-III to 6-VI and Fig. 6-14).
Table 6-III and Fig. 6-14A indicate that there is positive spatial association between
the epithermal Au deposit occurrences and NNW-trending faults/fractures in the study
area and the positive spatial association is optimal within 0.2 km of NNW-trending
faults/fractures. Inspection of the individual distances between the epithermal Au
deposits and NNW-trending faults/fractures shows that eight (or 62%) and nine (or 69%)
of the 13 epithermal Au deposits are within 0.1 km and 0.2 km of NNW-trending
faults/fractures, respectively. These results are similar to the results of the distance
distribution analysis (see Figs. 6-9A and 6-9B).
Table 6-IV and Fig. 6-14B indicate that there is positive spatial association between
the epithermal Au deposit occurrences and NW-trending faults/fractures in the study
area and the positive spatial association is optimal within 0.8 km of NW-trending
faults/fractures. Inspection of the individual distances between the epithermal Au
deposits and NW-trending faults/fractures shows that seven (or 54%) and nine (or 69%)
of the 13 epithermal Au deposits are within 0.6 km and 0.8 km of NW-trending
faults/fractures, respectively. These results are similar to the results of the distance
distribution analysis (see Figs. 6-9C and 6-9D).