b'Don Emersons best of Exploration GeophysicsFeatureparamagnetic. Rift-related granitoids and continental epeirogenic uplift granitoids have an inferred bimodal distribution of susceptibilities, with the mafic compositions tending to be ferromagnetic and the felsic compositions generally paramagnetic.Crustal contamination and contact aureolesIshihara, Terashima, and Tsukimura (1987) presented evidence that local contamination of a I-type tonalitic pluton by sulphur and carbon derived from sedimentary country rocks produced a vertical zonation from low magnetic susceptibility ilmenite series tonalite at lower levels to magnetite-series tonalite at higher levels. There was less contamination at higher levels. Similar effects have also been observed in the Lachlan Fold Belt, but only within a few metres of the granitoid margin (Blevin 1994). More generally, Blevin (1994) argues that crustal contamination effects on oxidation state of Lachlan Fold Belt granitoids are negligible and that in the vast majority of cases the oxidation state of these granitoids is inherited from the source region.On the other hand, Ague and Brimhall (1988) suggest that substantial contamination of granitoid magmas by country rocks has occurred in Californian batholiths. Where strongly contaminated by graphitic pelites, the I-type tonalites are reduced ilmenite-series, otherwise they are magnetite series. Pecherskiy (1965) estimates that the nature of the country rocks has significant effects on the magnetic properties of granitoids in NE Russia in at most 20% of cases (probably much less).As well as the country rocks affecting the magnetic properties of the granitoid, emplacement of granitoids frequently has a pronounced effect on the magnetic properties of country rocks that are metamorphosed and metasomatised by the intrusion. Often the magnetic signature of the contact aureole is more pronounced than that of the granitoid itself. Speer (1981) studied the mineralogical changes, including production of secondary magnetite, within the contact aureole of the Liberty Hill pluton, South Carolina. For that granitoid, there is a very clear relationship between the detailed magnetic signature of the aureole and changes in metamorphic grade of the metapelitic country rocks, as shown in Figure 16. There are smooth variations in magnetiteFigure 16.Magnetic expression of metamorphic magnetite formation in the content, correlated with changes in mineral modes and mineralcontact aureole of the Liberty Hill pluton, South Carolina (after Speer 1981).chemistry, within metamorphic zones, with inflections at metamorphic isograds. The susceptibility of the metamorphicproceeds according the tholeiitic trend discussed above, with magnetite zone is substantially greater than that of theinitial iron-enrichment and late iron-depletion, producing magnetite-series pluton. Outside the aureole, the susceptibilityzoned complexes with basal ultramafic layers, overlain of the country rocks is very low. The magnetic signature of theby paramagnetic mafic rocks, then by increasingly more granite and aureole comprises a relative magnetic high (~ 200magnetic mafic rocks, grading finally to MFM granophyres. nT above regional background) over the granite, rimmed by aCr-mineralisation occurs as chromite bands towards the top strong, narrow high (~ 500 nT above background) centred on theof the ultramafic zone, which contains no primary magnetite, magnetite-rich middle to outer aureole, dropping to the regionalbut may be MFM due to secondary magnetite produced by background level outside the aureole. serpentiriisation of olivine. The paramagnetic mafic rocks that Contact aureoles around granitoids that intrude pyritic sedimentsoverly the ultramafic zone may host platinum group element may exhibit substantial magnetic anomalies due to breakdownand Cu-Ni mineralisation. In the upper portions of the zoned of pyrite to monoclinic pyrrhotite (the ferromagnetic variety ofcomplex, the SFM upper ferrogabbro and ferrodiorite zones pyrrhotite). Monoclinic pyrrhotite generally carries a relativelyhost bands of titanium and vanadium-bearing cumulus strong remanent magnetisation, characterised by Q 1. magnetite, which may constitute economic ore deposits of Ti and V. The Bushveld Complex, which hosts the worlds greatest repository of magmatic ore deposits, may be regarded as the Magnetic petrology and metallogeny of intrusions type example of such mineralised layered mafic/ultramafic intrusions.Mineralisation in layered mafic/ultramafic complexes An idealised model of the magnetic stratigraphy can be Differentiation of reduced mafic magmas within large,developed from this generalised picture of layered mafic/essentially closed system, slowly cooled magma chambersultramafic complexes. Clark et al. (1992a, 1992b) have used APRIL 2020 PREVIEW 60'