Geologists identify rocks mainly through identifying the minerals they contain. These might include the minerals which make up the majority of rocks we see at the Earth’s surface, such as quartz or feldspars. They could also be minerals which are more commonly associated with ore deposits, such as pyrite (fool’s gold), pyrrhotite or magnetite. Geologists identify these minerals by their unique properties such as hardness, colour, crystal form and cleavage, streak, how heavy the mineral is, or how magnetic it is. The latter two properties are termed ‘physical properties’, namely density and magnetic susceptibility.
The physical properties of geological materials are the link between geology and geophysics. A high density area of the Earth will produce a gravity high; a low density area will produce a gravity low. Likewise, an area with high magnetic susceptibility will produce a magnetic high. These geophysical responses are linked to the minerals contained within the rocks in those areas; an area of rock which contains more dense minerals has a higher density and will thus produce a gravity high. This link between mineralogy, physical properties and geophysical responses is the key to mapping the signatures of ore deposits using geophysics. Often the processes which form a mineral deposit will produce minerals which have vastly different physical properties to the minerals already formed in the host rocks. These differing physical properties resulting from the processes of mineralisation can produce a geophysical response.
Recent developments in technology allow for the mapping of the distribution of physical properties derived from geophysical data in 3D. These developments, which utilise geophysical inversions of gravity and magnetic data (Williams et al 2009), have produced 3D models of density and magnetic susceptibility. In a project conducted between 2006 and 2008 for the Predictive Mineral Discovery Co-operative Research Centre (pmd*CRC), the authors examined the 3D signatures of ore deposits in the Cobar region of New South Wales.
