Developing the tools for geological shape analysis, with regional- to local-scale examples from the Kalgoorlie Terrane of Western Australia

Geological map data are often underused in mineral-exploration programs, wh ich rely increasingly on regolith geochemistry and geophysical and other re motely sensed data to generate exploration targets. However, solid geology maps, which are progressively being upgraded due to improved interpretation s of superior, remotely sensed images and airborne geophysical data, can be useful in targeting specific types of mineral deposits, which formed late in the evolutionary history of the host terrane. In such terranes, the pres ent map geometry is essentially the same as that at the time of deposit for mation. This is the case for orogenic lode-gold deposits, which commonly sh ow predictable structural controls and/or structural geometry. Thus, the sh ape of a rock body, or combinations of structures and rock bodies, may prov ide an important guide to the exploration potential for orogenic lode-gold deposits. However, until recently, there has been a dearth of techniques to quantify the various properties of shape, and hence test the potential of the two-dimensional shape of geological bodies in map view as an exploratio n tool. Integrating techniques from the field of pattern recognition with a modern Geographical Information System (GIS) can provide the shape-analysi s tools required to investigate the geometries of geological shapes. Two-di mensional shape analysis is now possible through the calculation of several shape metrics including, but not restricted to, aspect ratio, blockiness, elongation, compactness, complexity, roundness, spreadness and squareness. Methods are developed for describing the geometries of rock units about min eral deposits, or any geological features, at any scale, which for the firs t time makes it possible to compare shapes. These shape-analysis techniques are tested using orogenic lode-gold deposits, particularly those in the Ka lgoorlie Terrane of the highly auriferous Late Archaean Norseman-Wiluna Bel t of Western Australia. On a global scale, shape analysis indicates that th ose greenstone belts whose volcanic rock sequences have high elongation and relative low roundness, complexity and aspect ratio (e.g. Kalgoorlie Terra ne) are likely to be the most richly endowed in gold. On a more local scale , characteristics of the shape of geological features around the Golden Mil e deposit are calculated and used to test the likelihood of occurrence of g old deposits with similar geometry elsewhere in the Kalgoorlie Terrane. The area with the most closely matching shape, on the basis of a 2 km clipping -circle radius, chosen on the basis of available proximity-analysis data, c orresponds to the recently discovered Ghost Crab deposit, illustrating the potential of the shape analysis methodology in mineral exploration. Shape a nalysis is, at least in part, scale dependent, due to the inherent problem of being able to define rock boundaries more precisely in units that have s trong geophysical signatures than those with weak signatures in poorly expo sed terranes. Overcoming this problem is a challenge to the application of this methodology.