b'Don Emersons best of Exploration GeophysicsFeatureTable 1.Petrological and geological characteristics of ferromagnetic and paramagnetic igneous intrusions FERROMAGNETIC INTRUSIONS PARAMAGNETIC INTRUSIONSSource RockMantle Metasediments (particularly pelites) Mafic crustal underplate Reduced igneous rocksOxidised intermediate-felsic igneous rocksLithologyGabbrodioritetonalitegranodioritegranite Hornblendebiotite granitoidsPredominantly granite and granodioriteand biotite granites with high Mg, low Al biotite Muscovite and two mica granitoids, most leucogranites,Pyroxenehornblende granitoids biotite-rich granitoidsMany monzonites, quartz monzonites, syenites, quartz syenites and miaskiticCordierite, corundum or aluminosilicate-bearing granitoids nepheline syenites Peralkaline granites, syenites or nepheline syenitesMost alkali gabbros, essexites, ijolites etc.Ferrogabbros, ferrodiorites and granophyres, within upperLower gabbros in layered mafic complexeslevels of layered mafic complexesMineralogyGenerally higher colour index Generally lower colour indexBiotitehornblende in felsic calc-alkaline granitoids; hornblende pyroxeneBiotite + muscovite, cordierite, garnet or aluminosilicate in calc-alkaline olivine (except fayalite) in more mafic varieties granitoids.Fe3+ and Mg-rich biotite and hornblende; biotite colour Fe2+ and Al-rich (annite/siderophyllite-rich) biotite, often withis brown, black or olive green "foxy red" colour; occasionally Fe2+-rich hornblende orfayaliteSphene ( 0.1 vol %)hemoilmenite (8 mole % to 20 mole % Fe 2 O 3) or Mn-rich"Reduced" ilmenite( 8 mole% Fe 2 O 3 , usuaJly Mn-poor)ilmenite (up to 30 mole % MnTiO 3 )epidoteallanitepyrite as accessories pyrrhotite (predominantly hex po)spinelgraphiteWhite plagioclase + pink K-feldspar as accessories, primary sphene absentWhite plagioclase + white K-feldspar or sometimes brick red K-feldspar; Intermediate to Fe-rich olivine and pyroxenes + intermediate to sodic plagioclasesometimes green plagioclase + pink K-feldspar Mg-rich olivine and pyroxenes,hornblendeapatite in uppermiddle levels of tholeiitic layered intrusionscalcic plagioclasechromite in lower to middle zones of tholeiiticZoned plagioclase ( 60 %) + quartz + biotite and/or hornblende in M-typelayered intrusionsoceanic/ophiolitic plagiogranitesNepheline + alkali feldspar + plagioclase + calcic pyroxene Quartz + alkali feldspar + sodic pyroxene and/or + hastingsite + biotite in silica-undersaturated metaluminous (miaskitic) rocks sodic amphiboleaenigmatiteastrophyllitebiotite inNepheline + alkali feldspar + sodic pyroxene and amphibole oversaturated peralkaline (ekeritic) rocksbiotite without aenigmatite or astrophyllite in mildly peralkalineNepheline + sodic pyroxene + aenigmatiteastrophylliteundersaturated rocks in silica-undersaturated peralkaline (agpaitic) rocksChemistryPredominantly metaluminous, but also weakly peraluminous Strongly peraluminous (A/CNK 1.1) and strongly peralkaline (A/NK 0.9) or weakly peralkaline granitoids (0.9A/NK A/CNK1.1) granitoids, some metaluminous granitoidsLow ferric iron (Fe 2 O 3 0.8 wt%) or very low total iron ( 1 wt % FeOT)Moderate-high ferric iron (Fe 2 O 3 0.8 wt%, typically 1-3 wt%) and moderate- Low oxidation ratio (mean Fe3+/Fe2+ - 0.1 at 60% SiO 2 ,high total iron ( 2 wt% FeOT) Moderate oxidation ratio (mean Fe3+/Fe2+ ~ 0.6 atFe3+/Fe2+ - 0.4 at 75% SiO 2 , molar Fe 2 O 3/(FeO+ Fe 2 O 3 )60%SiO 2 , Fe3+/Fe2+ - 0.9 at 75% Si02, i.e. molar = 0.05.-0.2) or very high oxidation ratioFe 2 O 3/(FeO+Fe 2 O 3) -0.2-0.3) Normative corundum or normative acmite+sodium metasilicateNormative (diopside olivine acmite) plus 1 wt% normative magnetite Relatively hydroushaematiteRelatively anhydrousEmplacement depthPredominantly epizonal, particularly subvolcanic, some mesozonal and catazonal Predominantly mesozonal or catazonal, some epizonalAssociated rocksAssociated volcanics commonAssociated volcanics uncommon Gabbro-diorite-trondhjemite associations Syenogranite-monzongranite-granodiorite associations(Gabbro)-diorite-granodiorite-monzogranite associationsQuartz syenite-syenogranite associationsDiorite-monzonite-quartz monzonite-monzogranite associationsSyenite-alkali syenite-alkali granite associationsTectonic settingAndinotype (subduction of oceanic plate beneath continental margin,Hercynotype (continental collision, e.g. Himalayan and Hercynian leucogranites)generating Cordilleran I-type batholiths) Encratonic ductile shear belts with thickened continental crust Island arc plagiogranites, gabbros and quartz dioritesLate tectonic/post tectonic catazonal migmatites and mesozonal granitoids Alpinotype (tectonically emplaced serpentinised peridotites, gabbros andassociated with regional metamorphism plagiogranites) Compressional regimesCaledonian-type post-closure uplift and tensional regimes with major faultingAnorogenic, rifting-associated moderately evolved granitoids Anorogenic, rifting-associated highly evolved granitoidsA/NK = atomic Al/(Na + K); A/CNK = atomic Al/(Ca +Na+ K)around Tertiary plutons generally reduced the susceptibilityincorporates a core potassic zone, surrounded successively of magnetite-series Mesozoic tonalites and granodiorites overby phyllic, argillic and propylitic zones arranged in concentric substantial areas. However, hydrothermal alteration with a lowerbut incomplete shells. This model is typically most applicable water/rock ratio locally produced secondary magnetite withinto calc-alkaline granodiorite-quartz monzonite porphyries ilmenite-series granitoids, enhancing their susceptibility. (often associated with quartz diorite intrusions) with copper and molybdenum mineralisation, but negligible Hollister (1975) distinguished between the Lowell andgold. In the diorite model the phyllic and argillic zones are Guilbert (1970) quartz monzonite model of porphyry copperabsent and the propylitic zone adjoins the core potassic deposits and a diorite model. The Lowell-Guilbert modelzone. Sulphides are less developed in the diorite model APRIL 2020 PREVIEW 64'