Uranium-bearing hematite from the Olympic Dam Cu-U-Au deposit, South Australia: A geochemical tracer and reconnaissance Pb-Pb geochronometer

Uranium-bearing hematite (containing up to several wt.% U), also containing Al, Mo. W and radiogenic Pb, is described from Olympic Dam, South Australia. These elements are present in grains that display both oscillatory zonation and porous structures. Laser-ablation inductively coupled mass spectrometry (LA-ICP-MS) element mapping confirms oscillatory and sectorial zoned domains in hematite grains are enriched in U-238- and Pb-266, and are distinct from those of W and Mo. The crystal structure and absence of inclusions within zoned hematite was assessed by transmission electron microscopy on foils obtained by in situ slicing across zonation patterns using the scanning electron microscope-focused ion beam technique. Satellite reflections on the electron diffraction patterns obtained from banded zones in hematite are attributable to long-range superstructure ordering, that is inferred to assist metal incorporation via the substitution 2Fe(3+) <-> Me6+ + vacancy, where Me = U, W, Mo. The suitability of U-bearing hematite for Pb-Pb geochronology as a first pass was tested on both zoned and porous hematite grains via LA-ICP-MS, using the GJ-1 zircon as the primary external standard. Only Pb-Pb ages were considered and resulted in (207)pb-Pb-206 ages of 1590 +/- 8 Ma and 1577 +/- 5 Ma for oscillatory and sector zoned hematite from two samples. Although reconnaissance in nature, these ages potentially support the supposition that mineralization is coeval with emplacement of the Gawler Range Volcanics and associated Hiltaba Intrusive Suite. The geochronological application utilizing an abundant refractory mineral represents a new tool for dating iron-bearing ores. (C) 2013 Elsevier B.V. All rights reserved.