b'Don Emersons best of Exploration GeophysicsFeatureThe Tuolumne intrusion, California, represents a classic zoned pluton that grades from quartz diorite at the margin through progressively more felsic hornblende- and biotite bearing granodiorite phases, to a core of biotite monzogranite porphyry (Bateman and Chappell 1979). The normal zoning pattern, from mafic margin to felsic core, represents fractional crystallisation within the magma body as it cooled from the outside in. Figure19 shows the compositional variations across the Tuolumne intrusion. Note the general slow decrease in opaque mineral (mainly magnetite) content from the margins towards the centre, with a pronounced dip in modal magnetite within the felsic core. The expected magnetic signature of fractional crystallisation is gradation or zoning of susceptibility, where the most fractionated phase, which is most likely to be intimately associated with the mineralisation, has the lowest susceptibility. Figure 19 provides an example of this pattern. In the case of an oxidised comagmatic suite, the susceptibility contrast between less evolved and more evolved phases should be large, whereas the effect will be subtle for a reduced paramagnetic suite. Large increases in radio element concentrations and changes in radioelement ratios in the most fractionated rocks may also be detectable radiometrically. Airborne radiometric data can complement magnetic survey data in this environment, Figure 18.Oxygen fugacity versus temperature fields for typical 1-, S and because the radiometric signal is best developed over the most A-type granitoids and for porphyry Cu, W-Mo-Cu skarn, W-Sn-F skarn andfelsic and fractionated intrusive phases, which are the phases porphyry Sn mineralisation, together with a number of standard oxygenthat have the most subdued magnetic signature.fugacity buffers (after Kwak and White 1982). Mineral and fluid oxygen fugacity buffers that may be important controls on the magnetic mineralogy of theThe Tuolumne pluton is unmineralised, probably because igneous intrusions and their associated mineralisation include (CO 2 -CH 4 ;this intrusion, at least at the current level of exposure, was a PMP = pyrite magnetite-pyrrhotite; AQMH = andradite-quartz-magnetite- sealed system during emplacement and cooling, precluding hedenbergite; MSIP = magnetite-sphene-ilmenite-pyroxene) as well as the FMQescape of late metal-bearing hydrothermal fluids, and because and HM buffers. fractionation of the magma did not quite proceed to the stage have undergone fractional crystallisation. Such granites containrequired to concentrate metals into an ore-bearing fluid. negligible magnetite and are paramagnetic. On the otherHowever, a slightly more evolved variant of the Tuolumne hand, Cu and Au mineralisation is associated with oxidised,intrusion, emplaced at a shallower depth or in a more favourable magnetite- and/or sphene-bearing, intermediate I-type suites.structural setting for tapping off hydro thermal fluids should be Mo is associated with similar granites that are more fractionatedquite favourable for development of Cu-Au mineralisation.and oxidised. W does not appear to show a close relationship toEffects of sulphur saturation and halogen contents of magmasgranitoid type and is an opportunistic ore element, occurring in association with a number of other metals. Within mineralisedWyborn and Sun (1994) suggested that most magma types are granitoid suites, ore element ratios are simply related to relativegenerally sulphur-saturated and are unlikely to produce gold oxidation state and degree of fractionation. or copper-rich fluids after fractional crystallisation. Au and Cu partition strongly to sulphide phases and sulphur saturation The importance of fractional crystallisation leads to precipitation of sulphides and early removal of these Blevin and Chappell (1992, 1995) point out that fractionalmetals from the melt. For development of a magmatic Cu-Au crystallisation of magmas is a powerful mechanism fordeposit, the magma must remain sulphur undersaturated concentration of ore elements into the residual melt, which is athroughout most or all of its magmatic evolution. Oxygen prerequisite for formation of intrusive-related mineralisation. Latefugacity has a large effect on sulphur saturation. Under stage fractional crystallisation leads to quasi exponential increasesoxidising conditions, sulphur becomes more soluble in the of concentration for incompatible elements in the residual melt,magma, dissolving as an anhydrite component. Thus oxidised which may then partition the ore elements into late stage fluidsmagmas are more likely to be sulphur-undersaturated and are and ultimately deposit them in a suitable trap to form economicmore likely to generate Cu Au mineralisation.mineralisation. Fractionated granitoids can be recognised, forThe most favourable magma source for formation of high goldexample, by high Rb content and high Rb/Sr, which are sensitivesulphur-undersaturated magmas is lithospheric mantle that indicators of fractional crystallisation. Other causes of chemicalhas already been depleted in sulphur by removal of sulphur-variation within granitoid suites, such as restite unmixing, magmasaturated basaltic melt, leaving behind small amounts of mingling or crustal contamination cannot produce the enormoussulphide enriched in Cu, Au and other precious metals. If this concentration factors required to form an ore deposit. Magmaticrefractory mantle is metasomatised, its liquidus temperature differentiation by fractional crystallisation is characteristic of melt- is lowered and it can subsequently undergo partial melting rich magmas. Thus mineralisation is associated with granitoidsmore readily, in appropriate tectonic conditions. The magmas that are derived from hot magmas (very hot magmas if the sourcegenerated often have shoshonitic affinities and have region is relatively anhydrous) or with felsic granitoids that havecharacteristics that are favourable for generation of magmatic-undergone extensive fractionation after all restite has separatedhydrothermal mineralisation. Less potassic magmas (i.e. those from the melt. within the normal K-SiO 2 field for calc-alkaline magmas) that APRIL 2020 PREVIEW 62'