Abstract

Oak Dam East is a large (~560 Mt of 41–56 wt % Fe), Cu-Au-U-bearing, massive Fe oxide deposit in the northeastern Gawler craton, South Australia. It is one of several unmined deposits in the same province as the giant Olympic Dam deposit, and it lies beneath more than 500 m of younger cover rocks. The Oak Dam East breccia complex formed in a near-surface environment and was characterized by an early history of skarnlike albite-calc-silicate ± magnetite formation (from hypersaline, 400°–500°C fluids). Large-scale brecciation was an early feature, and the resulting complex was progressively filled in and replaced by (1) hematite and goethite (hematite stage I), (2) magnetite-apatite-quartz ± actinolite (magnetite stage), (3) hematite-goethite-quartz-chalcedony-pyrite-chlorite-monazite (hematite stage II), and (4) chalcopyrite-pitchblende-illite-hydromus-covite-florencite-carrolite (Cu-U-(Au) mineralization stage). Hematite stage II assemblages are volumetrically most abundant and were formed from fluids of 1 to10 wt percent NaCl equiv that were boiling at 170° to 190°C and had δ18O values more negative than −3.7 to +1.7 per mil, interpreted as meteoric water. The upper 200 m of the breccia complex contains spectacular colloform hematite void fillings, interpreted to have replaced hydrothermal goethite. The Cu-U mineralized zones resemble Olympic Dam ore in terms of geometries, being grossly subhorizontal, 10 to 70 m thick, and crosscutting with respect to the host breccias. The mineralization is zoned vertically from pyrite (δ34S = −6 to 0‰) at depth to shallower chalcopyrite (δ34S = −14 to −7‰). High-grade pitchblende occurs centrally within the chalcopyrite zone (e.g., 10 m of 0.46% Cu and 3,700 ppm U3O8), straddling a boundary between K mica + illite alteration and underlying Fe-Mg chlorite alteration.

The deposit is assigned to the 1570 to 1595 Ma iron oxide Cu-Au-U event in the eastern Gawler craton on the basis of its similarity to other Cu-Au-U deposits in the province. An LA-ICPMS Pb isotope age for monazite in hematite stage II was calculated to be 1455 ± 20 Ma, and possibly as young as 1370 ± 54 Ma, to explain all isotopic data. This is an age of isotopic resetting. Our preferred model is that Cu-U mineralization developed within a stratified hydrothermal brine complex analogous to the Salton Sea geothermal system. Copper, Fe, and S in the deep brine was sourced from an evolving deep reservoir, with local compositional controls on sulfur isotopes including cooling, boiling, and replacement of previous pyrite. Chalcopyrite (±U) veins and disseminations formed as a result of mixing with overlying, steam-heated, U-rich, saline ground water. Mixing-induced cooling and dilution only occurred episodically, following hydrofracturing events that ephemerally linked the brine layers within the substantially cemented upper complex. By contrast, high-grade UO2 formation occurred as deep upflow waned, via reaction of the upper brine with hematite stage II Fe-Mg chlorite along an interface that descended with time. This resulted in an increase of U grades through time caused by leaching and reprecipitation.

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