b'Don Emersons best of Exploration GeophysicsFeatureFigure 6.Range of Koenigsberger ratios for common rock types. Stippled portions of bars indicate common ranges.When varietal mineralogy is incorporated into a refined rockGiven the fact that, in extreme cases, up to 90% of the classification, the bimodal susceptibility distribution tends tomineral content of the rock may be ignored in the first-order resolve into a paramagnetic subpopulation and a ferromagneticclassification, it is little wonder that magnetite abundance, subpopulation, each associated with a distinct mineralogy.for instance, is weakly correlated with rock name. It is also Bimodality often also reflects the fact that important geologicalclear that there can be no unique correlation between rock factors, such as geochemical affinity, alteration and metamorphicname and bulk chemistry, given the wide range and variety of grade are not considered in the simple classification schemes usedminor minerals that can be present within any one of the rock for most petrophysical summaries. A truly meaningful magnetictype fields. Of course, the classification is so useful and widely petrological classification scheme must include chemicalaccepted because there are coherent patterns of mineralogical and/or mineralogical data for protoliths, plus information onand chemical variation among plutonic rocks. Figure 9 illustrates metamorphic grade and environment, and/or alteration. some aspects of this coherency. A generalised plot of mineral Similarly, when susceptibility distributions are consideredcomposition for the full range of plutonic rock types is shown on progressively smaller scales, the range of susceptibilitiesin Figure 9(a). Figure 9(b) shows average trends in plagioclase becomes more restricted. Within different geological provinces,composition, mafic mineral contents and homblende/biotite the relative proportions of paramagnetic and ferromagneticratio in granitoid rocks, showing systematic variation with subpopulations differ from those of other provinces. It is oftenposition in the QAP diagram.found that within sufficiently small areas, e.g. within a particularSpatially related plutonic rock series show clear mineralogical geological environment or simply within a single outcrop, alland chemical correlations with tectonic environment and susceptibilities fall exclusively within one of the subpopulations.relative time of emplacement, as shown in Figure 10. These Thus, the distinct susceptibility subpopulations tend to reflectvarious rock associations are characterised by different differing geological conditions, which are not considered in themetallogeny and can be related to magnetic petrology much primary rock classification schemes. more reliably than to the broad IUGS rock names. This has implications for exploration, as use of magnetic methods for Classification of intrusive igneous rocks locating intrusion-hosted or intrusion related mineralisation requires better understanding of the relationship between IUGS classification of plutonic rocks rock magnetisation and the geological factors that influence mineralisation.The internationally accepted IUGS classification of mafic to felsic plutonic rocks (Le Bas and Streckeisen 1991) is simplyChemical classification of plutonic rocksbased upon the relative proportions of three major rockThe following summary of chemical classification schemes for forming minerals: plagioclase ( An5); alkali feldspar (K-feldsparplutonic rocks is largely based on the excellent textbook by and albite); and either quartz (in oversaturated rocks) or aHughes (1982).feldspathoid mineral, most commonly nepheline, in the case of an undersaturated rock. Figure 8 shows the fields and rockFeldspars are the commonest minerals in igneous rocks, names on the QAPF double triangle. Ultramafic rocks, for whichin which they constitute more than 50%, on average. mafic minerals constitute 90% to 100% of the rock, are classifiedAlumina occurs in a 1:1 ratio with oxides of the alkali metals separately. or alkaline earth elements in feldspars. Thus departures APRIL 2020 PREVIEW 50'