Hydrothermal alteration associated with porphyry-style mineralisation typically comprises concentric near-circular alteration zones surrounding a roughly circular central intrusion. The intrusion itself and the proximal alteration zone are usually associated with positive magnetic anomalies whilst the outer alteration zones are much less magnetic. Because the country rocks are usually magnetic, this pattern of alteration produces a central magnetic 'high' surrounded by an annular magnetic 'low'. In collaboration with Barrick Gold, we have developed an automatic image analysis system for gridded data that provides an efficient, accurate and non-subjective way to identify the magnetic response of an idealised porphyry mineralising system within magnetic datasets. The method finds circular anomalies that are associated with the central intrusion and inner alteration zone of the porphyry system using a circular feature detection method called the circular feature transform. Then their boundaries are approximated by deformable splines that are drawn to the locations of maximum contrast between the amplitudes of the central 'high' and surrounding area of lower magnetisation.
CET Porphyry Detection Extension for Geosoft Oasis Montaj
– commercialised software product marketed by Geosoft
Holden, E. J., Fu, S.C., Kovesi, P., Dentith, M.C., Bourne, B., Hope, M., 2011, Automatic Identification of Responses from Porphyry Intrusive Systems within Magnetic Data using Image Analysis, Journal of Applied Geophysics, 74, pp.255-262