b'Don Emersons best of Exploration GeophysicsFeatureFigure 4.Range of titanomagnetite compositions found in the major types of igneous rock (after Buddington and Lindlsey 1964).derived by reconstituting magnetite-ulvospinel intergrowthsare in fact much too broad to be useful for classification of within single grains, is very Ti-rich and is close to a paramagneticmagnetic properties. This is because the susceptibility of composition. However exsolution of primary titanomagnetitemost rocks reflects the abundance of accessory minerals, into relatively Ti-poor magnetite, which is ferromagnetic, andparticularly magnetite (sensu lato), which are generally ignored paramagnetic ulvospinel (or ilmenite, if oxidation-exsolutionin petrological classification.occurs) during slow cooling produces grains that are ferromagnetic overall. Although the saturation magnetisation of titanomagnetitesKoenigsberger ratios (Q) can also vary quite widely depends strongly on composition, decreasing almost linearly from(see Figure6), but useful rules-of-thumb can be stated. 480 kAm-1 for pure magnetite to zero for Usp80, the susceptibilityFerromagnetic intermediate to felsic granitoid rocks is only weakly dependent on Ti content for ulvospinel contents ofcontain multidomain magnetite, which is associated with less than ~70% (Clark 1997). Thus the titanomagnetites carried byKoenigsberger ratios less than unity (usually Q 0.5, typically igneous rocks, ranging from gabbroic to granitic compositions, areQ ~ 0.2). Furthermore, the remanence carried by such grains almost invariably ferromagnetic and the susceptibility of the rockis generally unstable and is dominated by viscous remanence is essentially proportional to the modal titanomagnetite (allowingacquired in the recent field. However some, but not all, gabbros, for intergrown paramagnetic phases in composite grains) andnorites and mafic diorites contain ultrafine pseudosingle only weakly dependent on titanomagnetite composition. Thisdomain to single domain magnetite hosted within silicate conclusion differs from that of Grant (1985), who assumed thatminerals, such as pyroxenes, olivine or plagioclase, as well as titaniferous magnetites have much lower susceptibilities thandiscrete multidomain grains. The ultrafine (10 m) grains are Ti-poor magnetite. capable of carrying intense remanence and these rocks may accordingly exhibit Q values substantially greater than unity. Relationship between lithology and magnetic properties Thus, magnetisation by induction can be assumed as a first The data of Figures 5 and 6 are based on magnetic propertyapproximation for the more felsic granitoids, whereas remanent measurements at the CSIRO Division of Exploration and Miningmagnetisation, possibly oblique to the present field, may be over the last 18 years and published studies and compilations.significant for mafic plutonic rocks.The systematic collection of petrophysical data by the geological surveys of Scandinavian countries, in particular, hasBimodal susceptibility distributions reflect ferromagnetic and greatly expanded the quantity and scope of the informationparamagnetic populationsavailable. It is evident from Figure 5 that each rock type exhibits a wide range of susceptibilities and that susceptibility valuesA notable feature of Figure 5 is that the magnetic susceptibilities are not generally diagnostic of lithology. Classical rock namesof a number of rock types have distinctly bimodal distributions. APRIL 2020 PREVIEW 48'