Igneous Metallogeny: Templates, Provinces and the Future

P. L. Blevin, National Key Centre for Geochemical Evolution and Metallogeny of Continents, Department of Geology, Australian National University, Canberra, ACT, 0200

The understanding of magmatic controls on ore element ratios (Cu-Au-Mo-Sn-W-etc; OER) in intrusive-related ore deposits has been significantly advanced in the last few years to the stage where it can be used as a predictive tool. These ratios are dominantly functions of magmatic compositions, process and intensive variable considerations. While the deposit spectrum present in eastern Australia (Au-Cu through Cu-Mo, W, Sn and Mo) correlates with the degree of compositional evolution of associated magmas, it does not correlate with their degree of isotopic evolution. For example, lithophile mineralisation (Sn, Mo, F) in the southern New England Orogen (NEO) is associated with compositionally evolved but isotopically juvenile magmas.

Numerous conceptual advances in recent years have shed new light on some old dogmas. These include:

"Au mineralisation cannot be sourced from felsic magmas". Spatial and genetic relationships, metallogenic associations, and recent experimental evidence (eg. Candela et al., 1996) now demonstrate otherwise. The recognition that Au is an integral part of a diverse range of felsic igneous metallogenies, and is not hostage to Cu, PGEs, mafic rocks, the mantle or some special "magic bullet", has practical implications for where and how Au exploration should be conducted in various terranes.

"A-types are anhydrous and F-rich relative to other granite types". Halogen data from minerals have established that I-, S- and A-type granites have similar F contents and that F/OH and F/Cl ratios vary as a function of composition. Differences in Cl abundance and Cl/OH ratios between I- and S-types, and between different I-type associations are significant - with implications for metal partitioning and transport. A-types are also not anhydrous, indeed volatile exsolution textures are commonly present.

"Sn is an S-type element". In Australia greater than 70% of Sn production has been sourced from I-types, including all major western Tasmanian Sn systems.

"Metallogenic provinces = metal specific provinces". OERs in intrusive-related mineral deposits vary with igneous composition. Thus single supersuites may generate a variety deposits. The Moonbi Supersuite in the NEO for example has been a source for Cu, Au, Sn, W, Mo and Bi mineralisation (Blevin & Chappell, 1996). Igneous metallogenic provinces should be regarded as polymetallic.

"Giant ore deposits require special processes". Experimental and theoretical limitations on magmatic and hydrothermal processes suggest that giant deposits more properly represent systems where "everything went right". Magmas with anomalous metal contents will still be anomalous relative to "normal" magmas if the overall efficiency of metal partitioning between the melt and the exsolving volatile phase is the same in both cases.

The development of igneous metallogenic concepts has lead to the recognition of igneous metallogenic provinces in eastern Australia (Blevin et al., 1996). These provinces define areas of potential for certain elements based on the nature and composition of igneous rock suites. As granites are sourced from the mid to lower crust and mantle, these provinces do not necessarily coincide with upper crustal lithostratigraphic boundaries. Non magmatic-compositional factors are also important in controlling the distribution and preservation of mineralisation in the crust. There is a need to better estimate the pressure (depth) at which crystallisation and volatile exsolution occurred in felsic igneous complexes relative to their current level of exhumation. This will define regions where the epizonal mineralisation window has been preserved within igneous metallogenic provinces. Other advances include: new methods to assess the intrinsic oxidation state of magmas and to recognise the effects of alteration; using element ratios and "proxy" elements (Mo, Re, Ni, Bi) to better understand and track processes in magmas favourable for generating mineralisation; and using textures as a monitor of crystallisation and volatile exsolution mechanisms. The application of these methodologies as a tool for area selection, and their development as a set of field based tools for assessing the metallogenic potential of igneous complexes, provide an intriguing challenge for the future.

REFERENCES

Blevin, P. L. and Chappell, B. W., 1996. Internal evolution and metallogeny of Permo-Triassic high-K granites in the Tenterfield-Stanthorpe region, southern New England Orogen, Australia. Geol. Soc. Aust. Abstracts 43, 94-100.

Blevin, P. L., Chappell, B. W., and Allen C. M., 1996. Intrusive metallogenic provinces in eastern Australia based on granite source and composition. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 281-290.

Candela, P. A., Piccoli, P. M. and Williams, T. J., 1996. Preliminary study of gold partitioning in a low-sulfur, high oxygen fugacity melt/volatile phase system. Geol. Soc. Amer. Abs With Programs 28(7), 402.

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