b'Don Emersons best of Exploration GeophysicsFeaturegranitoids (POG). Anorogenic granitoids fall into three categories: Rift Related Granitoids (RRG); Continental Epeirogenic Uplift Granitoids (CEUG); and Oceanic Plagiogranites (OP). Although the occurrence and abundance of magnetite was not noted by these authors, the detailed information on chemistry and mineralogy allows broad conclusions on likely magnetite contents to be inferred by comparison with other studies.Granitoid classification based on Fe-Ti oxide mineralogyIshihara (1977) instigated a descriptive classification of calc-alkaline granitoids into a magnetite-series and an ilmenite-series, based on their characteristic iron-titanium oxide mineralogy. This classification can be directly related to magnetic properties and has important exploration implications, because of the association between metallogeny and the magnetite-series/ilmenite-series classification (Ishihara 1981). The characteristic accessory mineralogies of the two categories of granitoid are: magnetite-series0.1-2 vol % magnetiteilmenite; plus haematite, pyrite, sphene, oxidised Mg-rich biotite; and ilmenite seriesmagnetite absent, ilmenite ( 0.1 vol %) pyrrhotite, graphite, muscovite, reduced Fe-rich biotite.Thus magnetite-series granitoids are ferromagnetic (MFM to SFM), with susceptibilities in the approximate range 3800-75,000106SI (300-6000 G/Oe), whereas ilmenite series granitoids are paramagnetic. Pyrrhotite is present in ilmenite-series granitoids in very minor amounts and cannot contribute significantly to the susceptibility, particularly since much of the pyrrhotite present in ilmenite-series granitoids is the hexagonal Figure 11.(a) Classification of comagmatic igneous rock series as alkalic,variety (Whalen and Chappell 1988), which is weakly magnetic. alkali-calcic, calc-alkalic and calcic on the basis of the alkali lime index (PeacockMagnetite-series granitoids are significantly more oxidised 1931). (b) Contrasting trends on the ternary plot of alkalis, total iron and MgOthan ilmenite series granitoids. This is thought to reflect upper (AFM diagram) for differentiating tholeiitic and calc-alkaline magmas. mantle/lower crustal generation of the magnetite-series, involving minimal interaction with carbonaceous material, M- type: metaluminous; calcic gabbros, diorites, quartzwhereas the ilmenite-series is interpreted to have been diorites, tonalites and plagiogranites; 45% to 78% SiO 2 ; littlegenerated in the middle to lower crust and to be significantly or no K-feldspar. contaminated by C-bearing crustal rocks.Refinements of the I-S classification have been suggested. PitcherFershtater, Borodina, and Chashchukhina (1978) and Fershtater (1983) recognised 1-Cordilleran and 1-Caledonian granitoids,and Chashchukhina (1979) have devised a Ferrofacies each with distinctive composition and mineralogy, based on theirClassification of granitoids. The ferrofacies concept has been tectonic setting. His data suggest that the tonalite-dominantapplied to a wide range of granitoids from the former USSR and 1-Cordilleran granitoids tend to be relatively magnetite-rich,can be regarded as an extension of the magnetite-series and whereas the granodiorite dominant 1-Caledonian type granitoidsilmenite-series classification. The categories in that classification tend to have less magnetite. Chappell and Stephens (1988)are: the magnetite ferrofacies; the magnetite bearing ferrofacies; proposed that progressively more felsic and chemically evolvedthe magnetite-"free ferrofacies; and the titanomagnetite I-type granitoids result from successive remelting of older maficferrofacies - each with distinctive mineralogical characteristics. rocks that have underplated the crust. M-types comprise gabbrosThis classification does not appear to have been used by to mafic granites derived directly from the mantle or mantleother workers, but may form the basis for a refined magnetic wedge, I-tonalite types are derived from fusion of M-type materialpetrological classification with metallogenic implications.and I-granodiorite types represent remagmatised products of I-tonalite rocks. This classification allows for both I-tonalite andSuites, supersuites and basement terranesI-granodiorite types to occur in the one tectonic setting, althoughHine et al. (1978) showed that granitoids of the Lachlan Fold typically one subtype will predominate. Belt can be grouped into suites using petrographic, chemical Granitoid classification based on tectonic setting and isotopic criteria. Members of a suite are interpreted to be derived from similar source rocks. Suites with similar character Pitcher (1983) has related other granite classification schemes tocan be grouped into supersuites. Chappell, White, and Hine tectonic environment. Maniar and Piccoli (1989) have proposed(1988) demonstrated that granitoids within specific provinces an independent granitoid classification scheme, based ontend to exhibit common geochemical character. Since the tectonic setting. A first order orogenic category is subdividedcompositions of the granitoids largely reflect compositions of into island arc granitoids (IAG), continental arc granitoidstheir source regions, the distribution of granitoid suites and (CAG), continental collision granitoids (CCG) and post-orogenicsupersuites can be used to define terranes, within each of which 55 PREVIEW APRIL 2020'