Uranium ore-forming systems of the - Geoscience Australia

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