b'Don Emersons best of Exploration GeophysicsFeatureFigure 14.(a) Correlation between opaque mineral content and susceptibility for Japanese granitoids and gabbroids, showing an essentially proportional relationship (after Ishihara 1981). Ilmenite-series granitoids have very low opaque mineral contents ( 0.1 vol %). (b) Range of susceptibilities for magnetite-series and ilmenite-series granitoids versus quartz + K-feldspar (after Ishihara 1981). (c) Relationships between susceptibility, lithology and varietal mineralogy for granitoids from central Australia (after Mutton and Shaw 1979).proportionality of susceptibility and magnetite content forsusceptibilities. White K-feldspar indicates a reduced rock normal ferromagnetic rocks. Figure 14(b) indicates that therewith low susceptibility. Brick red K-feldspars, on the other is a wide range of susceptibilities for mafic magnetite-serieshand, which are most common in very felsic rocks, indicate granitoids, with many strongly ferromagnetic examples, buthydrothermal alteration and are generally associated with the maximum magnetite content, and hence the maximumlower susceptibilities than pink K-feldspars. Green plagioclase susceptibility, decreases linearly with increasing quartz+is generally indicative of alteration that tends to be magnetite-K-feldspar, so that the most felsic members of the seriesdestructive and is correlated with variable, generally lower, (syenogranites) are only weakly to moderately ferromagnetic.susceptibilities that are poorly correlated with rock composition Ilmenite-series granitoids have very low opaque mineraland oxidation state. Yellowish feldspars usually indicate contents ( 0.1 vol %) and there is still a distinctly lower averageweathering and such samples are not representative of the susceptibility for the ilmenite-series syenogranites than for theirfresh rock. Overall, there is a reasonably predictable relationship magnetite-series equivalents. There is also a general trend tobetween susceptibility and the field-observable features: colour decreasing paramagnetic susceptibility in more felsic ilmenite- index (percentage of mafic minerals), which provides a proxy series granitoids, as expected. estimate of silica and iron contents, and K-feldspar colour Figure 14(c) shows the relationships between susceptibility,(provided the plagioclase is white).lithology and varietal mineralogy for granitoids from centralSource rockAustralia (Mutton and Shaw 1979). This confirms the decrease in maximum susceptibility for more felsic rocks, and theWhalen and Chappell (1988) showed that most I-type granitoids association of magnetite with pyroxene and hornblendeof the Lachlan Fold Belt are magnetite-series and most (indicative of M- or I-type affinities) and the apparentS-types are ilmenite-series, although exceptions to the rule antipathetic relationship of magnetite with muscovite, whichare found. Blevin (1994, 1996) has shown that ~80% of I-type is a mineral characteristic of peraluminous, usually S type,granitoids from the Lachlan and New England Fold Belts have granitoids. susceptibilities greater than 1000106 SI (80 G/Oe), mostly greater than 2000106 SI (160 G/Oe), whereas nearly all Blevin (1994, 1996) has established a useful relationshipS-types have susceptibilities less than 1000106SI (80 G/Oe). between feldspar colour and susceptibility for calc-alkalineExceptions to these generalisations occur within specific suites granitoids. For granitoids with white plagioclase there is aor supersuites and are confined to particular basement terranes.distinct increase in average oxidation ratio and susceptibility with increasing pinkness of K-feldspar. Salmon pinkBlevin (1994, 1996) has also shown that within each K-feldspars tend to be most oxidised and have the highestgranitoid suite there is generally a systematic decrease in APRIL 2020 PREVIEW 58'