Service: Intrusion detection identifies magma and reactive-fluid pathways
DISCRETE geological bodies such as stocks, alteration haloes, and breccia pipes are typically about as wide as they are long in plan view. We have a method of detecting where magmas or migrating fluids seem to have intruded and produced discrete, equant bodies. We show intrusion detection results for two case studies: the Ernest Henry iron oxide-copper-gold deposit in Queensland, and the New South Wales copper and gold porphyry deposits of Cadia and Ridgeway.
Exploration survey datasets typically show numerous features that have pronounced linearity, such as faults, dikes, fold hinge axes, and rock-unit contacts defining where formerly horizontal stratigraphic units have become tilted and exposed. Such features are continuous, and typically are much longer than they are wide.
But equant, or 'blob-like', features of various sizes are typically sprinkled heavily throughout a dataset as well. In plan view, these equant features have an approximately constant width, no matter the compass direction chosen for measurement.
So while a linear geological feature has an obvious strike direction, an equant feature either has a poorly defined strike direction, or none at all.
Equant features tend to occur as the geologically distinct bodies that form when magma intrudes and solidifies, or when a reactive migrating fluid (supercritical or subcritical; magmatic or non-magmatic or mixed) intrudes and modifies a parcel of rock.
Examples of such discrete bodies includes stocks, batholiths, cupolas, alteration haloes, skarns, kimberlites and lamproites, diatremes, steep-plunging mineral lenses, and breccia pipes.
Because of their significant direct and indirect importance in many mineralization styles, we have a method on hand for detecting these magma-intrusion-related and fluid-intrusion-related bodies.
If an intrusion-related body has a magnetic, gravity, geochemical, conductivity or other physico-chemical signature that contrasts strongly enough against the backdrop of surrounding rocks, then its location can be discerned using intrusion detection.
The general procedure involved in this kind of analysis is to seek locations around which data values either decrease in all directions or increase in all directions.
Ernest Henry case study
Ernest Henry, in the Mount Isa Inlier's Eastern Succession near Cloncurry, was discovered in the early 1990s and involves a plunging body of breccia-hosted iron oxide-copper-gold ore. 
The region also hosts the Osborne IOCG deposit, along with many other smaller examples and other deposit types.
The region is thought to be an example of intracontinental orogeny,  with most of the area's copper deposits thought to have formed during west-northwest-directed shortening. 
Plenty of research papers mention that IOCG deposits are structurally controlled .
And in the case of the Ernest Henry region, a good deal of discussion revolves around which specific geological structures are likely to be most prospective for IOCG-style mineralization. For instance, in 2009 researchers James Austin and Tom Blenkinsop reviewed some findings published in the literature about structures. 
"Mineralization within the region has strong structural controls and the major faults in the area may have acted as fluid conduits responsible for mineralization," Austin and Blenkinsop said.
They elaborated: "Carter et al. (1961) and Laing (1998) propose that north and northwest faults host copper-gold mineral occurrences, while northeast faults are generally barren. Ore bodies are localized at fault intersections, jogs and flexures/fault bends. Prospectivity analysis by Mustard et al. (2004) indicated that the main fault orientations favorable for mineralization are north, north-northwest or east-northeast trending structures, particularly jogs or bends. Fault intersections, particularly those of east to northeast and south to southeast-trending faults with all other fault orientations, also appear to be important."
But it seems not much is generally being said about how to arrive at a list of candidates for IOCG-prospective bodies occurring along or near IOCG-prospective structures.
Intrusion detection analysis of geophysical data may permit explorers to fill this information gap in the Cloncurry region (see Figures 1 to 7).
Interestingly, breccias in the Yukon's Wernecke Basin are considered correlatives of those in the Cloncurry IOCG region.  Similarities between Wernecke breccias and those present at Ernest Henry are also said to have been found.  It might be a worthwhile exercise to apply intrusion detection to the area hosting the Wernecke breccias.
FIGURE 1: Reduced-to-the-pole (RTP) magnetic data for the Cloncurry region in Queensland, with the Ernest Henry case study area defined by a yellow boundary.
FIGURE 2: Reduced-to-the-pole (RTP) magnetic data for the Ernest Henry case study area. Locations shown as yellow spots include the copper-containing deposits of Ernest Henry, E1, Monakoff, Australian Margaret, Fairfield, Rocklands, Double Oxide, Chum, Jasper Block, Mt Norna, Jessie, Great Australia, Magpie, Dolomite, Lorena, and King Edward. Ernest Henry is circled in magenta.
FIGURE 3a: Structural network extracted from the Ernest Henry case study area's RTP magnetic data using Fathom Geophysics' computer-vision routines. Structures shown are those with wavelengths of between about 500m and about 6km.
FIGURE 3b: The area's structural network in vectorized form, superimposed on the RTP magnetic data.
FIGURE 4: Results showing features that are magma-intrusion and fluid-intrusion candidates. Features sought in this run are about 2km to about 4km across. Features were extracted from the RTP magnetic data for the Ernest Henry case study area using Fathom Geophysics' computer-vision routines. Reds and oranges indicate relatively large likelihoods, while greens and blues indicate relatively small likelihoods. White areas are where likelihoods were assessed to be negligible.
FIGURE 5: Results showing features that are magma-intrusion and fluid-intrusion candidates (network of colored 'blobs'), with the area's structural network superimposed over the top (black lines).
FIGURE 6: Results showing features that are magma-intrusion and fluid-intrusion candidates (network of colored 'blobs'), and the results of a thresholding procedure, which reveals areas most likely to involve IOCG-style alteration haloes (areas are inside magenta outlines).
FIGURE 7: Thresholding procedure results (in which areas most likely to involve IOCG-style alteration haloes appear inside the magenta outlines) and structural network superimposed on the area's RTP magnetic data.
Cadia-Ridgeway case study
Intrusion detection may also be useful in regions where a single mineralization style displays an array of magnetic-contrast signatures.
This situation is said to occur for the porphyry copper and gold deposits of the Cadia district, which are situated in New South Wales' Lachlan Fold Belt.
The Cadia district displays the gamut of magnetic-contrast features known to occur in porphyry systems, according to geoscience researchers John Holliday and David Cooke. 
"At the district and prospect scale, magnetic physical property contrasts predicted by the porphyry [ore] model include those related to intrusive activity, [because] many of the intrusive complexes driving porphyry mineralization will be to some extent magnetic, and will contrast either positively or negatively with highly magnetic volcanic host-rocks," Holliday and Cooke said in their 2007 paper.
Magnetic contrast is also generated by the various of physico-chemical changes induced in host rocks during the hydrothermal alteration accompanying the intrusion of porphyries, they said.
A further noteworthy complication in the Cadia district is that porphyry-related mineralization seems to straddle a large hiatus. Mineralization is thought to be associated with two different magmatic events occurring more than 10 million years apart. 
Here, with the Cadia-Ridgeway case study, we show how integrating intrusion detection results into an exploration campaign can help explorers develop a clearer picture of the more-subtle, variable signals existing in their magnetic data.
Note that the case study images appearing here (Figures 8 to 15) are small and relatively low-resolution, to fit the purpose of presenting them online.
FIGURE 8: Reduced-to-the-pole (RTP) magnetic data for part of the Lachlan Fold Belt region in New South Wales, with the Cadia-Ridgeway case study area defined by a yellow boundary.
FIGURE 9: Reduced-to-the-pole (RTP) magnetic data for the Cadia-Ridgeway case study area. Locations shown as yellow spots are the deposits, from left to right: Cargo, Ridgeway, Cadia, Cadia East, and Ferndale.
FIGURE 10a: Structural network extracted from the Cadia-Ridgeway case study area's RTP magnetic data using Fathom Geophysics' computer-vision routines. Structures shown are those with wavelengths of between about 800m and about 3km.
FIGURE 10b: The area's structural network in vectorized form, superimposed on the RTP magnetic data.
FIGURE 11: Results showing features that are magma-intrusion and fluid-intrusion candidates. Features sought in this run are about 1.2km to about 2.4km across. Features were extracted from the RTP magnetic data for the Cadia-Ridgeway case study area using Fathom Geophysics' computer-vision routines. Reds and oranges indicate relatively large likelihoods, while greens and blues indicate relatively small likelihoods. White areas are where likelihoods were assessed to be negligible.
FIGURE 12: Image showing the good agreement between the location of mapped intrusions in the Cadia-Ridgeway case study area (shown as black outlines) and the location of features that are magma-intrusion and fluid-intrusion candidates, as extracted by Fathom Geophysics' computer-vision routines (network of colored 'blobs'). Locations of mapped intrusions were obtained from the Geological Survey of New South Wales' 1:250,000 Bathurst geological map.
FIGURE 13: Results showing features that are magma-intrusion and fluid-intrusion candidates (network of colored 'blobs'), with the area's structural network superimposed over the top (black lines).
FIGURE 14: Results showing features that are magma-intrusion and fluid-intrusion candidates (network of colored 'blobs'), and the results of a thresholding procedure, which reveals areas most likely to represent intrusions (areas are inside yellow outlines). A thresholding run done using a more-aggressive cutoff would yield fewer outlined areas. Hover your mouse pointer over the image to show thresholding procedure results superimposed on the area's RTP magnetic data.
FIGURE 15: Thresholding procedure results (in which areas most likely to represent intrusions appear inside the yellow outlines) and structural network superimposed on the area's RTP magnetic data.
References and notes
 M. Webb and P. Rowston (1995) "The geophysics of the Ernest Henry Cu-Au deposit (N.W.) Qld", Exploration Geophysics, 26, 51-59.
 See discussion and citations in: T.G. Blenkinsop, C.R. Huddlestone-Holmes, D.R.W. Foster, M.A. Edmiston, P. Lepong, G. Mark, J.R. Austin, F.C. Murphy, A. Ford and M.J. Rubenach (2008) "The crustal scale architecture of the Eastern Succession, Mount Isa: The influence of inversion", Precambrian Research, 163, 31-49.
 See discussion and citations in: J.R. Austin and T.G. Blenkinsop (2009) "Local to regional structural controls on mineralization and the importance of a major lineament in the eastern Mount Isa Inlier, Australia: Review and analysis with autocorrelation and weights of evidence", Ore Geology Reviews, 35, 298-316.
 See, for example: (1) D.I. Groves, F.P. Bierlein, L.D. Meinert and M.W. Hitzman (2010) "Iron oxide copper-gold (IOCG) deposits through earth history: Implications for origin, lithospheric setting, and distinction from other epigenetic iron oxide deposits", Economic Geology, 105, 641-654, and (2) P.J. Williams, M.D. Barton, D.A. Johnson, L. Fontbote, A. De Haller, G. Mark, N.H.S. Oliver and R. Marschik (2005) "Iron oxide copper-gold deposits: Geology, space-time distribution, and possible modes of origin", Economic Geology, 100th Anniversary Volume, 371-405. In this paper, the authors said: "Structural and/or stratigraphic controls are pronounced, with deposits characteristically localized on fault bends and intersections, shear zones, rock contacts, or breccia bodies, or as lithology-controlled replacements."
 B.L. Crawford, P.G. Betts and L. Ailleres (2010) "An aeromagnetic approach to revealing buried basement structures and their role in the Proterozoic evolution of the Wernecke Inlier, Yukon Territory, Canada", Tectonophysics, 490, 28-46. In this paper, the authors said: "The Paleoproterozoic-Mesoproterozoic transition of Australia and northern Laurentia records one of the largest iron oxide-copper-gold mineralizing events in geological history. Mineralization is recorded in a large belt that extends from the Gawler Craton in southern Australia, through the eastern Mount Isa Inlier and possibly into the Yukon Territory... . The breccias in the Wernecke Basin share many of the characteristics of breccias in the Gawler Craton and Mount Isa."
 M.A. Kendrick, M. Honda, D. Gillen, T. Baker and D. Phillips (2008) "New constraints on regional brecciation in the Wernecke Mountains, Canada, from He, Ne, Ar, Kr, Xe, Cl, Br and I in fluid inclusions", Chemical Geology, 255, 33-46. In this paper, the authors said: "Although regional variations are possible, these [noble gas] data are interpreted to indicate broadly similar processes operated [during brecciation] throughout much of the Werneckes and at Ernest Henry."
 J.R. Holliday and D.R. Cooke (2007) "Advances in geological models and exploration methods for copper +/- gold porphyry deposits", Paper 53, In: Proceedings of Exploration '07: Fifth decennial international conference on mineral exploration, 791-809.
 A.J. Wilson, D.R. Cooke, H.J. Stein, C.M. Fanning, J.R. Holliday and I.J. Tedder (2007) "U-Pb and Re-Os geochronologic evidence for two alkalic porphyry ore-forming events in the Cadia District, New South Wales, Australia", Economic Geology, 102, 3-26.