Combining fractal analysis of mineral deposit clustering with weights of evidence to evaluate patterns of mineralization: Application to copper deposits of the Mount Isa Inlier, NW Queensland, Australia

Abstract

The clustering of mineral occurrences and their spatial associations with particular geological features are critical aspects of mineral distributions for exploration and understanding ore genesis. Variations in the degree of clustering of mineral occurrences or geological features can be measured by fractal dimensions, obtained from a shifting box counting method. Spatial associations between mineral occurrences and geological features can be quantified by the weights of evidence (WofE) method using the contrast value, which increases with the strength of the spatial relationship. A new method is proposed to evaluate mineral occurrence distributions by combining the power of fractal analysis of clustering with the WofE approach. The method compares the correlation between the variation in degree of clustering of mineral occurrences and a geological feature in a study area, with the contrast value of the same feature. The possible outcomes can be simplified into four scenarios, depending on whether the correlation in variation of clustering and the contrast are high or low, respectively. Each outcome has specific exploration implications. If either a high correlation in variation of clustering or a high contrast value is obtained, the geological feature can be used for exploration targeting. The integrated fractal and WofE approach is applied to copper occurrences in the Proterozoic Mount Isa Inlier, NW Queensland, Australia, which hosts large numbers of copper deposits (1,869 occurrences), including the world class Mount Isa copper deposit. Variation in clustering of copper occurrences has a positive correlation with variation in clustering of fault bends ( R = 0.823), fault intersections ( R = 0.862) and mafic rocks ( R = 0.885). WofE results indicate that the copper occurrences are spatially associated with fault intersections and bends and with mafic rocks. Analyses were carried out separately for the two major lithostratigraphic sequences in the Inlier, the Eastern and Western Successions. The Western Succession copper occurrences are apparently more clustered than those of the Eastern Succession, which may reflect a lower degree of exploration and/or geological factors. The association of copper occurrences with mafic rocks compared with fault bends and intersections is greater in the Eastern Succession, which may reflect genetic factors. Correlations in the variation of clustering of mineral occurrences and geological features have a linear relationship with the contrast values, and the spatial association between all geological features and copper occurrences constitute high correlation/high contrast cases. The linear relationship suggests that the geological features that control the clustering of the copper occurrences could be the same features that control their localization.