b'Don Emersons best of Exploration GeophysicsFeatureFigure 17.(a) Range of CGS mass and volume susceptibilities and opaque mineral contents for granitoids associated with porphyry Cu, granitoid related Mo deposits and granitoid-related Sn-W deposits (Gd = granodiorite, Tn = tonalite, G = granite, Qmd = quartz monzodiorite). (b) Proportions of mineral deposits, of a variety of commodities that occur within magnetite-series and ilmenite-series granitoid belts. Hatched regions represent WFM magnetite-series granitoids. The mineral deposits are inferred to be genetically related to granitoids or to their associated volcanics. Pegmatite refers to stanniferous pegmatite deposits. SI volume susceptibility = CGS volume susceptibility (G/Oe) X 4; mass susceptibility = volume susceptibility/density (after Ishihara 1981).thismodel to predict magnetic anomalies over tilted Bushveld- too oxidised to be associated with tin deposits. Development type complexes. of an exsolved fluid and partitioning of ore elements into hydrothermal liquids or vapour phases also depends strongly Oxidation state and metallogenic associations on the nature and concentrations of volatile species. The ratios More than 30 years ago Pecherskiy (1965) noted anSO 2 /H 2 S and CO 2 /CH 4depend on oxygen fugacity and therefore empirical association of granitoid-related gold deposits withoxygen fugacity exerts a major influence on hydrothermal ferromagnetic granitoids and tin deposits with paramagneticevolution, concentration of ore elements into mineralising granitoids. Ishihara (1981) established the importantfluids, and transport and deposition of ore elements.correlation between his magnetite- and ilmenite-seriesKhitrunov (1985) has attempted a general explanation of the granitoid classification and granitoid-related mineralisation.empirical relationships between oxidation state of granitoids For example, copper and molybdenum porphyries are almostand associated Cu, Mo, W or Sn mineralisation, concluding that always magnetite-series, whereas tin granites are invariablymagmatic conditions are progressively more reduced for Mo, ilmenite-series. It has become apparent in recent times that thisCu, W and Sn mineralisation. Cameron and Carrigan (1987) relationship is not just empirically based, but can be related toand Hattori (1987) have pointed out the association between redox conditions in the magma. Ishiharas data on metallogenicoxidised felsic magmas, magnetic granitoids and Archaean gold associations with granitoid series and with susceptibility aredeposits. They have given a detailed discussion of the factors reproduced in Figure 17. favouring incorporation of gold from sulphide minerals in The compatible or incompatible behaviour of multivalentsource rocks, concentration into a CO 2 -rich melt at mesothermal metals such as Cu, Mo, W and Sn in the melt depends on theirlevels and deposition after development of an immiscible fluid valency, which is a function of redox conditions. For example,phase. Sillitoe (1979) pointed out the association between gold-tin occurs in two oxidation states in magmas: stannous (Sn )rich porphyry copper deposits and oxidised, magnetite-rich 2+ plutons with a magnetite-rich potassic alteration zone. Kwak and stannic (Sn4+). The oxidised stannic species fits easily intoand White (1982) distinguished between reduced porphyry tin the structures of minerals such as magnetite and sphene,and W-Sn-F skarn deposits and more oxidised W-Mo-Cu skarn which are diagnostic of oxidising conditions in the magma,deposits and Cu porphyries (Figure 18).and is therefore dispersed throughout an oxidised granitoid. On the other hand, the reduced stannous ion is too large toBlevin and Chappell (1992, 1995) have published thorough be accommodated readily within mineral structures and isanalyses of the metallogenic implications of granitoid accordingly concentrated in the residual melt. Thus, reduced,chemistry, oxidation state and magmatic differentiation, based and therefore paramagnetic, granitoids are potential sources ofmainly on studies of the Lachlan Fold Belt. Sn mineralisation is tin mineralisation, whereas magnetite-bearing granitoids areassociated with both S- and I-type granites that are reduced and 61 PREVIEW APRIL 2020'