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Abstract

The Charters Towers gold field lies within the mid-Paleozoic Lolworth-Ravenswood block where pre-Ordovician metasedimentary and meta-igneous rocks were intruded by the I-type Hogsflesh Creek and Towers Hill Granodiorites in the Late Ordovician, followed by north- and northeast-trending dike swarms in the Silurian. Early Devonian Millchester Creek Tonalite and Alabama Diorite intruded all these rock types and were in turn intruded by Carboniferous mafic dikes.

East- and northwest-trending mylonites cut the Late Ordovician granitoids and older rocks but are truncated by the undeformed Early Devonian granitoids. Gold-bearing quartz veins are contained within shallow east- and north-dipping brittle shear zones, mainly in the Day Dawn, Brilliant and Queen, and Towers Hill vein systems, with depths up to 1,000 vertical meters. Ore shoots are localized along granitoid contacts and locally in brittle reactivations of mylonite. Alteration selvages consist of an inner phyllic assemblage dominated by muscovite and an outer propylitic assemblage dominated by montmorillonite-illite. No direct relationship between any of the granitoid phases and the mineralization has been established.

Fluid inclusion studies indicate a fluid salinity of 5.3 to 11 equiv wt percent NaCl with local CO2 and formation temperatures of between 240° and 300°C. Oxygen isotope values of quartz (12.3-15.6‰), muscovite (8.3-8.6%,), and calcite (11.0-14.8%,); hydrogen isotope values of muscovite (— 57 to — 42%) indicate a fluid composition at these temperatures of 3.2 to 8.4 per mil for δ18O and —62 to —42 per mil for δδD which overlaps the range of metamorphic and magmatic waters. Sulfide δ34S values (-1.1 to +4.2%) and carbonate δ13C values (-4.7 to -3.6%) are consistent with derivation of sulfur and carbon from juvenile fluids or by dissolution and/or thermal decomposition of magmatic sulfides and carbonates during metamorphism.

The mineralization and alteration style, combined with stable isotope and fluid inclusion data, is interpreted to indicate that the Charters Towers gold field is the product of deep-seated Devonian magmatism and crustal ± mantle outgassing.

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