b'Don Emersons best of Exploration GeophysicsFeatureMagnetic petrology of igneous intrusions: implications for exploration and magnetic interpretationDavid A. Clark CSIRO Exploration and MiningPO Box 136,North Ryde, NSW, 2113.AbstractMagnetic petrology integrates rock magnetism andThe mineral exploration industry has now reached a stage conventional petrology in order to define the processes thatwhere the ability to acquire, process and present magnetic create, alter and destroy magnetic minerals in rocks. By relatingsurvey data far outstrips capacity to interpret the surveys. There magnetic mineralogy, bulk magnetic properties, petrologyis often far more geological information in these very large and geochemistry to observed magnetic anomalies andata sets than can be presently extracted in the time available understanding of the geological factors that control magneticfor interpretation. Better understanding of the relationships signatures is obtained, which can be used to improve geologicalbetween magnetic signatures and geology can facilitate the interpretation of magnetic surveys. interpretation process and produce more reliable geological The magnetic properties of igneous intrusions, and henceinterpretations.the magnetic anomalies associated with them, reflect bulkA crucial limitation of interpretation of magnetic surveys arises rock composition, redox state, hydrothermal alteration andfrom the fundamental non-uniqueness of potential field source metamorphism. These geological variables are in turn controlleddistributions. This ambiguity in source geometry can only be by tectonic setting, composition and history of the source region,addressed by constraining models. The most important control depth of emplacement and nature of wall rocks. The fundamentalon the reliability of magnetic models is information on magnetic control on magnetic mineralogy and bulk magnetic propertiesproperties. Understanding of the factors that determine is partitioning of iron between silicate and oxide phases, whichmagnetisation intensities and directions for the geological is strongly influenced by oxidation ratio. This paper reviews andunits within the survey area is essential for resolving geological synthesises information on relationships between the chemistry,ambiguity in order to produce a reliable interpretation of mineralogy and metallogenic associations of igneous intrusionssubsurface geology.and their magnetic properties. Although links between magneticIgneous intrusions comprise a substantial portion of many properties and broad rock names are tenuous, refined rockgeological provinces and intrusive-related mineralisation classification enables magnetic properties to be predicted withis a major exploration target. Information on the magnetic reasonable confidence. petrology of igneous intrusions should therefore assist Oxidised, magnetite-series, and reduced, ilmenite-seriesgeological mapping and an understanding of the relationships granitoids have quite distinct metallogeny. Cu, Mo and Aubetween magnetic properties and metallogenic associations are associated with oxidised granitoids and Sn with reducedof intrusions is important in exploration for intrusive-related granitoids. Fractional crystallisation, which has a distinctiveore deposits. Extensive background material that cannot be magnetic expression, plays an important role in generatingincluded in this summary paper can be found in Clark et al. magmatic-hydrothermal ore deposits. Hydrothermal alteration(1992a).profoundly affects magnetic properties, in a generally predictable fashion. Implications for interpretation of magnetic anomaliesPrinciples of magnetic petrologyassociated with igneous intrusions and recognition of magnetic signatures of potential intrusive-related ore deposits are adduced. What is magnetic petrology?Keywords:magnetic petrophysics, magnetic petrology, magneticMagnetic petrology integrates rock magnetism and anomalies, rock magnetism, interpretation of magneticconventional petrology to characterise the composition, surveys, igneous intrusions, granitoids, granitoid- abundance, microstructure and paragenesis of magnetic related mineralisation minerals in order to define the processes that create, alter and destroy magnetic minerals in rocks. By relating magnetic Introduction mineralogy, bulk magnetic properties and petrology to observed magnetic anomalies an understanding of the The magnetic method has been widely used in mineralgeological factors that control magnetic signatures is obtained, exploration for decades. Recent improvements in magneticwhich can be used to improve geological interpretation of data acquisition, processing and presentation and reducedmagnetic surveys. Dunlop and Ozdemir (1997) have provided airborne acquisition costs have increased the utility anda comprehensive and up-to-date overview of rock magnetism. importance of magnetic surveys, particularly high resolutionThere is no corresponding textbook on magnetic petrology. aeromagnetic surveys. Increasingly, high quality surveys ofUseful reviews of magnetic petrological principles have been large areas are becoming available at reasonable cost. Thisgiven by McIntyre (1980), Grant (1985) and Frost (1991a). has led to increasing emphasis on magnetic methods in areaClark et al. (1992b) presented several magnetic petrological selection and regional mapping, as well as prospect-scalecase studies. Clark and Emerson (1991) summarised magnetic mapping and drill targeting. properties of rocks and some principles of rock magnetism and APRIL 2020 PREVIEW 44'