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The role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern Australia

By
Phillip L. Blevin
Phillip L. Blevin
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Bruce W. Chappell
Bruce W. Chappell
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Published:
January 01, 1992

The ore-element associations of granite-related ore deposits in the eastern Australian Palaeozoic fold belts can be related to the inferred relative oxidation state, halogen content and degree of fractional crystallisation within the associated granite suites. Sn mineralisation is associated with both S- and I-type granites that are reduced and have undergone fractional crystallisation. Cu and Au are associated with magnetite- and/or sphene-bearing, oxidised, intermediate I-type suites. Mo is associated with similar granites that are more fractionated and oxidised. W is associated with a variety of granite types and shows little dependence on inferred magma redox state. The observed ore deposit-granite type distribution in eastern Australia, and the behaviour of ore elements during fractionation, is consistent with models of ore element sequestering by sulphides and Fe–Ti phases (e.g. pyrrhotite, ilmenite, sphene, magnetite) whose stability is nominally fO2 -dependent. Fractional crystallisation acts to amplify this process through the progressive removal of compatible elements and the concentration of incompatible elements into decreasing melt volumes. The halogen content is also important. S-type granites are poorer in CI than I-types. CI decreases and F increases in both S- and I-type granites with fractional crystallisation. Low CI contents combined with low magma f O2 in themselves seem to provide an adequate explanation for the rarity of Mo, Cu, Pb and Zn type mineralisation with S-type granites. Although such properties of granite suites seem adequately to predict the associated ore-element assemblage to be expected in associated mineral deposits, additional factors determine whether or not there is associated economic mineralisation.

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GSA Special Papers

The Second Hutton Symposium on the Origin of Granites and Related Rocks

P. E. Brown
P. E. Brown
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B. W. Chappell
B. W. Chappell
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Geological Society of America
Volume
272
ISBN print:
9780813722726
Publication date:
January 01, 1992

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