Direct targeting of mineral deposits using magnetic data may be facilitated by hydrothermal alteration associated with the mineralising event if the alteration changes the magnetic properties of the host rock. 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'.This paper presents an automatic image analysis system for gridded data that provides an efficient, accurate and non-subjective way to seek 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 radial symmetry transform. Next, their boundaries are traced using deformable splines that are drawn to the locations of maximum contrast between the amplitudes of the central 'high' and surrounding area of lower magnetisation.Experiments were conducted on magnetic data from Reko Diq, Pakistan; a region known to contain numerous occurrences of porphyry-style mineralisation. The predicted locations of porphyry systems closely match the locations of the known deposits in this region.This system is suitable as an initial screening tool for large geophysical datasets, therefore reducing the time and cost imposed by manual data inspection in the exploration process. The same principles can be applied to the search for circular magnetic responses with different amplitude characteristics. (C) 2011 Elsevier B.V. All rights reserved.