b'Don Emersons best of Exploration GeophysicsFeatureFigure 19.Compositional variations across the oxidised I-type Tuolumne intrusion, California, which shows a classic normal zonation from relatively mafic margin to felsic core, produced by fractional crystallisation (after Bateman and Chappell 1979). Modal amounts of major, varietal and accessory minerals are plotted as a function of position along the profile indicated, which passes through all mapped phases of the pluton. Magnetite is the dominant opaque mineral.are derived from less metasomatised mantle can also give risesulphide-complexed Cu and Au preferentially partition into the to large Cu-Au deposits, provided the mantle source is sulphurlow viscosity vapour, which can travel considerable distances undersaturated. Relatively low water and sulphur contents ofbefore deposition, whereas Fe, Mn, Pb and Zn preferentially these mantle-derived magmas produce rather inconspicuouspartition into the brine and tend to be deposited closer to the alteration halos with restricted potassic zones and little ironintrusion from which they emanate.sulphide. However, the oxidised nature of the magmas and high K+, which boosts Fe3+ content, encourages formation of bothEffects of hydrothermal alterationmagmatic and hydrothermal magnetite.Halogens are important complexing agents for metalsStudemeister (1983) pointed out that the redox state of iron in hydrothermal fluids and Cl and F contents of magmasin rocks is a useful indicator of hydrothermal alteration. Large volumes of fluid or high concentrations of exotic reactants, influence development of intrusive-related mineralisation.such as hydrogen or oxygen, are required to shift Fe3+/Fe2+ Cl decreases and F increases with fractional crystallisationratios. When reactions associated with large water/rock ratios in both I- and S-type granitoids (Blevin and Chappell 1992).occur, the change in redox state of the rocks produces large Cl contents of I-type granitoids are higher than for S types.changes in magnetic properties due to creation or destruction When a hydrothermal fluid exsolves from a silicate melt Clof ferromagnetic minerals.partitions strongly to the aqueous phase, accompanied by chloride-complexed metals such as Fe, Mn, Cu, Mo, Pb, Sn and S.Criss and Champion (1984) studied the southern Idaho However, if a sulphur-enriched magmatic vapour phase forms,batholith. They showed that intense hydrothermal alteration 63 PREVIEW APRIL 2020'