Uranium ore-forming systems of the - Geoscience Australia
Uranium ore-forming systems of the - Geoscience Australia
Uranium ore-forming systems of the - Geoscience Australia
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
<strong>Uranium</strong> <strong>ore</strong>-<strong>forming</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> 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 <strong>of</strong> 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 <strong>of</strong> 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 <strong>the</strong> original ilmenite (Fig. 6.1). This suggests precipitation <strong>of</strong> a U-V mineral in <strong>the</strong> p<strong>ore</strong> spaces <strong>of</strong> leucoxene, or <strong>the</strong> adsorption <strong>of</strong> <strong>the</strong>se elements onto <strong>the</strong> surface <strong>of</strong>anatase. Iron released during <strong>the</strong> alteration <strong>of</strong> <strong>the</strong> ilmenite grains to Ti-oxides is incorporated intopyrite, which replaced <strong>the</strong> ilmenite along <strong>the</strong> rims and fissures <strong>of</strong> <strong>the</strong> grain, indicating that <strong>the</strong>alteration took place under reducing conditions (Weibel and Friis, 2007). The alteration rim (A)around <strong>the</strong> pyrite is enriched in S and V, and Mg was leached away from <strong>the</strong> alteration site.TiMgVFigure 6.1: (a) SEM micrograph (secondary electron image) <strong>of</strong> an ilmenite grain partiallyreplaced by pyrite and leucoxene (Ti-rich). (b), (c) and (d): Ti, Mg and V distribution maps,respectively, <strong>of</strong> <strong>the</strong> same area as (a), determined by SEM. Ilmenite and leucoxene can bedistinguished by <strong>the</strong>ir anti<strong>the</strong>tic V and Mg distribution: Mg is released during ilmenitealteration and is not retained in <strong>the</strong> leucoxene alteration products which are enriched in Vand U. A = alteration rim.Kaolinite is <strong>the</strong> dominant mineral found in <strong>the</strong> clay matrix. It is interpreted to be <strong>of</strong> generallydetrital origin, and occurs as platy crystals. Illite is also interpreted to be present in <strong>the</strong> matrix,given <strong>the</strong> presence <strong>of</strong> K in energy dispersive spectra (EDS). Much <strong>of</strong> <strong>the</strong> matrix is organic-rich.Typically, this gives <strong>the</strong> samples a brown stained appearance in plane transmitted light, whichobscures much information. Small (1-2 μm) pyrite crystals are also prominent in <strong>the</strong> matrix, andthis will be discussed fur<strong>the</strong>r below.Page 68 <strong>of</strong> 151