b'Don Emersons best of Exploration GeophysicsFeatureBlevin (1994, 1996) has analysed a large collection of samples for relationships between susceptibility, oxidation state, granitoid type and composition. There is a distinct susceptibility gap between two main trends in the susceptibility-SiO 2plot. The ferromagnetic trend, representing magnetite-bearing granitoids, exhibits a gradual decrease in susceptibility in both the maximum and average susceptibility values with increasing SiO 2 , up to ~ 72 wt % SiO 2 , and then plunges rapidly at higher silica contents. The paramagnetic trend lies two to three orders of magnitude below the ferromagnetic trend and also exhibits a gentle systematic decrease in susceptibility with increasing SiO 2 . Oxidised granitoids generally have susceptibilities greater than 2000106 SI (160 G/Oe), with a maximum of 80,00010-6 SI (~ 6000 G/Oe), whereas reduced granitoids have susceptibilities ranging from ~ 50010-6 SI (~ 40 G/Oe) at the low silica end to ~ 13010-6 SI (~ 10 G/Oe) at the highest silica contents.For a given silica content, which generally implies similar total iron contents, there is pronounced increase in susceptibility with increasing oxidation ratio. Granitoids that plot in the gap between the main ferromagnetic and paramagnetic trends are either so felsic (SiO 2 72 weight %) that the iron content is too low to crystallise significant magnetite, irrespective of oxidation state, or show evidence of alteration of magnetite. There is little correlation between susceptibility, SiO 2and Fe 2 O 3/FeO for the latter group of granitoids, indicating that the processes of magnetite alteration are not systematically related to granitoid composition.Geochemical and mineralogical associations with magnetiteThe clearest correlations between geochemical or mineralogical factors in granitoids and magnetite content are the general increase in magnetite abundance with increasing oxidation ratio (except for the most oxidised haematite bearing rocks), for a given iron content, and the increase in maximum magnetite content with increasing total iron, for a given oxidation ratio. Figure 13.(a) Total iron as FeO (wt %), oxidation ratio (%), and percentageHowever, the occurrence and abundance of magnetite is clearly of the total sample for each rock type that is ferromagnetic versus SiO 2forcorrelated with other geochemical characteristics. Metaluminous Finnish gabbros, diorites, granodiorites and granites. (b) Mean susceptibilitygranitoids are much more likely to be ferromagnetic than of paramagnetic subpopulations, mean susceptibility of ferromagneticperaluminous or peralkaline granitoids, and igneous rocks with subpopulations, mean susceptibility of total population, and colourextreme alumina saturation are almost always paramagnetic. index (volume % mafic minerals) versus SiO 2for Finnish gabbros, diorites,Within each Ishihara series, there is a general correlation of granodiorites and granites (data from Puranen 1989). decreasing susceptibility with increasing silica content.Hornblende + pyroxene or olivine (except fayalite) in mafic Data on intrusive rocks from Finland, taken from Puranenvarieties is favourable for the presence of magnetite, as is (1989), are plotted in Figure 13. Figure 13(a) shows the averagehornblende + biotite in more felsic rocks. Fayalitic olivine, total iron, oxidation ratio, and percentage of ferromagnetichowever, indicates reducing conditions and is found only in rocks for compositions ranging from gabbro to granite. Theremagnetite-poor granitoids. Mg-rich hornblende and biotite is a systematic increase in oxidation ratio with silica content,indicate relatively oxidising conditions, with removal of iron offsetting the effect on susceptibility of the decrease in totalinto magnetite and consequent enrichment of the mafic iron. This produces an increased proportion of ferromagneticsilicates in magnesium, particularly when the Mg/Fe ratio rocks at the felsic end of the spectrum, with a slight increase inincreases with increasing rock SiO 2 . When ilmenite is present, its average susceptibility for granites, compared to granodiorites,composition is correlated with magnetite content. Granitoids as a result (see Figure 13(b)). Note that these data, which arewithout magnetite have relatively reduced ilmenite ( 8 mol derived from a very large petrophysical sampling program% Fe 2 O 3 ), whereas magnetite-bearing granitoids have either by the Finnish Geological Survey, refer to all sampled unitsmore oxidised ilmenite, or Mn-rich ilmenite. High Mn-ilmenite within the appropriate QAP field, irrespective of geologicalis favoured by oxidising conditions because Fe is preferentially setting, metamorphic grade, varietal mineralogy etc.incorporated into magnetite rather than ilmenite.Although systematic trends tend to be smoothed out by this geological and petrological variability, a clear correlationFigure 14(a) shows the correlation between opaque mineral between chemistry and magnetic petrology is still evident.content and susceptibility for Japanese granitoids and gabbroic This indicates that there are strong underlying trends, whenrocks, showing an essentially proportional relationship specific geological provinces, tectonic settings, geochemical(Ishihara 1981). This reflects the dominance of magnetite over characteristics or mineralogical varieties are considered. other opaque phases in magnetite-series plutons and the 57 PREVIEW APRIL 2020'