Metallogenic Implications of Granite Tectonics: The Lachlan Fold Belt as a Case Study

Phillip L. Blevin, David J. Ellis, Rosalyn G. Warren

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

The Lachlan Fold Belt (LFB) is a tectonic regime which has not been deeply eroded since it became a stable tectonic regime in the Devonian. The oldest units comprise extensive Ordovician turbidites and igneous units, which were followed by elongate Silurian basins filled by marine sediments and volcanics, and separated by highs of granite and subaerial felsic volcanics of Silurian to Early Devonian age. Silurian and Devonian mineralisation in the LFB can be classified into three groups: granite-related deposits of Sn, W, Mo, Cu and Au; VHMS (Ag-Pb-Zn-Cu-Au) deposits in the Silurian basins; and "turbidite-hosted" gold-quartz vein deposits. In addition, the Ordovician "basement" to the LFB is host to several large porphyry Cu-Au systems, which have been preserved despite extensive Siluro-Devonian plutonism now exposed at the same erosional level. We propose that unusual tectonic character of the LFB generally and the unusual association of metallogenic styles present in the LFB can all be explained as a response to a single cycle of crustal overturn driven by granite tectonics.

The cycle of crustal overturn that makes up granite tectonics (Sorgenfrei, 1971; Warren & Ellis, 1996) begins with an influx of hot mantle-derived material into the base of the crust causing widespread anatexis. Felsic melts coalesce at depth and rise into diapiric structures. Removal of material into the rising diapirs is balanced by downwarping between the diapirs, creating sag-ducted basins (marginal or rim- synclines). Felsic volcanics may break through to the surface, either ahead of the rising granites or as sills and eruptives into the sag-ducted basins. Basement beneath the sag-ducted basins will move down into higher temperature regions in a stress field that may change from vertical to horizontal, and may begin to melt. At high crustal levels, outward expansion of granite plutons over the rim synclines and underlying crust cause local compression. The net effect is to move older deeply-buried crustal material back towards the surface in the form of granites and in some cases aureoles, while moving younger sediments, and their basement, in the sag-ducted basins downwards. Granite tectonics results in massive crustal reworking but no crustal thickening and/or mountain building.

The model allows for the penecontemporaneous production of fluids from diverse sources. These include: the mantle or deep crust, progressive dewatering of the sag-ducted sedimentary pile, magmatic fluids derived from igneous intrusions and "local" waters generated in the upper crust by shortening. Seawater incursions into sag-ducted basins, as well as meteoric water provide additional fluid sources. Sulfur isotope data for the LFB VHMS systems require variable proportions of magmatic and seawater inputs (Solomon & Groves, 1994), but recycled Ordovician Pb is also present (Carr et al., 1995). Sag-ducted Ordovician ore deposits beneath Silurian basins may act as metal sources in this regard. Isotopic and fluid inclusion data for the gold-quartz vein deposits suggest that the auriferous fluids were exotic relative to the immediate environment of the deposit and derived from the dewatering of pelitic sediments at depth during amphibolite grade metamorphism. Direct magmatic fluid contributions to some shear hosted Au deposits also occurs. These fluids were channelled into the upper crust where they were structurally trapped, Au precipitation being driven by processes such as cooling, mixing with locally derived fluids or via wallrock reactions. The high temperature stages of granite-related mineralisation are dominantly magmatic, with ore metal ratios in these systems being simple functions of magmatic source, process and magmatic intensive variables.

Granite tectonics provides a unified model linking the production of felsic melts, vertical and lateral crustal movements, high-T low-P metamorphic P-T-t paths, fluid production and circulation, and source and depositional environments for significant accumulation and preservation of metals. It explains the occurrence of diverse mineralisation styles within one metallogenic epoch. Large porphyry Cu-Au systems in the Ordovician volcanics and related intrusives owe their preservation to granite tectonics, these systems would have been eroded away if subsequent crustal thickening, mountain building and erosion had occurred in the LFB (Ellis, 1987).

REFERENCES

Carr, G. R., et al., 1995. Precise lead isotope fingerprinting of hydrothermal activity associated with Ordovician to Carboniferous metallogenic events in the Lachlan Fold Belt of New South Wales. Economic Geology 90, 1467-1505.

Ellis, D. J., 1987. Origin and evolution of granulites in normal and thickened crusts. Geology 15, 167-170.

Sorgenfrei, T., 1971. On the granite problem and the similarity of salt and granite structures. Geologiska Foreningens i Stockholm Forhandlingar 93, 371-435.

Solomon, M., & Groves, D. I., 1994. The geology and origin of Australia's mineral deposits. Clarendon Press, Oxford 951p.

Warren, R. G., & Ellis, D. J., 1996. Mantle-underplating, granite tectonics, and metamorphic P-T-t paths. Geology 24, 663-666.

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