Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome region6. Four Mile uranium deposit: mineralogyAnthony Schofield, Mäité Le Gleuher, Andrew Cross, Subhash Jaireth6.1 INTRODUCTIONThe Four Mile U deposit, including the Four Mile East and West zones, is hosted within the latePaleocene to middle Eocene Eyre Formation (Heathgate Resources, 2009). The Eyre Formationconsists of carbonaceous sand, with minor clay lenses and abundant organic material (Callen,1990). Although this deposit represents the most significant recent U discovery in Australia, nopublished mineralogical and petrological information exists. This chapter will discuss preliminaryresults from a petrographic investigation of material from the Four Mile East zone, and will brieflycompare these with observations from the Beverley uranium deposit. It is intended to provide aframework for future geochronological studies by placing the U-hosting mineralogy into itstextural and paragenetic context.6.2 SAMPLES AND ANALYTICAL PROCEDURESThe samples used in this investigation were obtained with permission from the Australian NuclearScience and Technology Organisation (ANSTO). Twenty-eight samples were collected from theFour Mile East zone, spanning both mineralised and non-mineralised zones. The samples aretaken from drill holes AKC026 (1 sample), AKC028 (1 sample), AKC029 (20 samples). AKC030(2 samples), AKC032 (1 sample) and AKC035 (1 sample). Five polished scanning electronmicroscope (SEM) mounts prepared by ANSTO were obtained from the Beverley deposit foranalysis.Optical and scanning electron microscopy (SEM), and electron microprobe analysis wasundertaken on the samples. Electron microprobe analysis was performed using a Cameca SX100instrument at the Research School of Earth Sciences, Australian National University. SEMexamination of the sample thin sections was made using a Cambridge S360 instrument fitted witha Tracor Northern energy dispersive X-ray analyser (EDXA) and a Zeiss UltraPlus analytical fieldemission SEM (FESEM) fitted with an INCA Energy 450 EDXA, both located at the ResearchSchool of Biological Sciences, Australian National University. A JEOL 6490LV SEM fitted witha JEOL JED-200 EDXA located at Geoscience Australia was used for further observations.X-ray diffraction analyses of 21 crushed and unoriented bulk samples were recorded with aSiemens D500 diffractometer operating at 40 mA and 40 kV using Cu Kα radiation, with acounting time of 2 seconds per 0.02º 2θ. Twenty two bulk samples from inside and outside the orezone were selected for XRD analysis. Four extra subsamples consisting of macroscopicallydifferent material in a uranium-rich sample from AKC029 were also analysed.6.3 MINERALOGY6.3.1 Detrital phasesOn XRD patterns, the signal is dominated by quartz and kaolinite. Angular to subrounded detritalquartz grains up to 0.5 mm constitute the most abundant mineral across all of the samples. Somequartz grains display embayed boundaries, suggesting that a degree of quartz dissolution hasoccurred. Quartz dissolution is much more intense in regions of high organic content. No quartzprecipitation was observed however. Minor sericitised, subrounded detrital feldspars of up to 250μm comprise the other framework mineral observed in the samples. Other detrital mineralsPage 67 of 151
Uranium ore-forming systems of the Lake Frome regionobserved as an accessory phase are chalcopyrite, zircon, xenotime, monazite, muscovite andilmenite. It is also likely that some detrital apatite is also present.Detrital ilmenite contains about 3% MgO and traces of Mn, and is mostly altered to a Ti-richproduct and pyrite to a variable extent (Fig. 6.1). The cryptocrystalline and porous Ti-rich (>70%TiO 2 ) alteration product, or leucoxene, consists of Ti-oxides (brookite or anatase) associated withunaltered ilmenite remnants. The Ti-rich alteration product is depleted in Mg and enriched in Vand U, which are not present in the original ilmenite (Fig. 6.1). This suggests precipitation of a U-V mineral in the pore spaces of leucoxene, or the adsorption of these elements onto the surface ofanatase. Iron released during the alteration of the ilmenite grains to Ti-oxides is incorporated intopyrite, which replaced the ilmenite along the rims and fissures of the grain, indicating that thealteration took place under reducing conditions (Weibel and Friis, 2007). The alteration rim (A)around the pyrite is enriched in S and V, and Mg was leached away from the alteration site.TiMgVFigure 6.1: (a) SEM micrograph (secondary electron image) of an ilmenite grain partiallyreplaced by pyrite and leucoxene (Ti-rich). (b), (c) and (d): Ti, Mg and V distribution maps,respectively, of the same area as (a), determined by SEM. Ilmenite and leucoxene can bedistinguished by their antithetic V and Mg distribution: Mg is released during ilmenitealteration and is not retained in the leucoxene alteration products which are enriched in Vand U. A = alteration rim.Kaolinite is the dominant mineral found in the clay matrix. It is interpreted to be of generallydetrital origin, and occurs as platy crystals. Illite is also interpreted to be present in the matrix,given the presence of K in energy dispersive spectra (EDS). Much of the matrix is organic-rich.Typically, this gives the samples a brown stained appearance in plane transmitted light, whichobscures much information. Small (1-2 μm) pyrite crystals are also prominent in the matrix, andthis will be discussed further below.Page 68 of 151
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