Uranium ore-forming systems of the - Geoscience Australia

Uranium ore-forming systems of the Lake Frome regionU, P and REE may have been precipitated simultaneously from the ore fluid(s). This is supportedby the high U contents of some of the phosphate minerals. The PO 4 3- ligand is highly effective intransporting U 6+ at a range of temperatures and pH values provided that the fluids are relativelyoxidised (see Cuney and Kyser, 2008 and references therein). Thus, P-rich fluids potentially maytransport U 6+ at relatively high concentrations, although other ligands such as carbonate ions andhydroxyl ions also may be important (see reviews by Cuney and Kyser, 2008; Skirrow et al.,2009). Deposition may have occurred via reduction, for example through reaction with reducedcarbon or iron already present in the sediment, or in mobile form (e.g. aqueous or gaseous CH 4 ,H 2 S). Alternatively, the formation of phosphate minerals may destabilise U in the fluid. Thesource of P in the fluids is unknown, but may have been either local organic matter, apatite in thesediments, or from distal sources carried by groundwaters or hydrothermal fluids. On the otherhand the source of the REE is unlikely to be the immediate host sediments of the NambaFormation, which are typically composed of quartz, feldspar, kaolinite, and minor heavy minerals.The presence of REE-rich minerals would appear to demand REE transport from sources beyondthe immediate host sequence, such as the REE-rich igneous rocks of the Willyama Supergroupand/or Mt Painter Inlier.The ubiquitous presence of high levels of organic matter in mineralised samples of the EyreFormation is most likely the primary control on U deposition in the Four Mile East deposit.Reduced Fe minerals, such as pyrite and ilmenite, are also probable sites of uranium reduction anddeposition. It is interesting to note that the REE-rich rhabdophane contains Ba, an elementnormally transported only in reduced fluids. The possibility of fluid mixing between oxidised U-rich waters and a reduced fluid warrants further investigation as a U-depositional mechanism atthe Four Mile deposit. This process has potential to result in high grades of uraniummineralisation, perhaps higher than simply through fluid-rock reaction.6.6 CONCLUSIONSBased on reconnaissance studies of a limited number of samples, uranium mineralisation at theFour Mile East deposit is overwhelmingly composed of uraninite. This is in contrast to the nearbyBeverley uranium deposit, where coffinite is the dominant uranium phase. A precise parageneticsequence is difficult to constrain for the fine grained secondary minerals including uraninite.However, it is inferred that uraninite formed contemporaneously with a second phase of pyrite(Py2) and REE phosphate minerals. Other uranium minerals present in the Four Mile deposit(carnotite and uranopilite) are oxidised uranium minerals, and probably represent a later stage ofuranium remobilisation.Although the Four Mile deposit is high grade, the composition of uraninite is unusual. Inparticular, high Fe and Ti contents are anomalous, and may be related to the association betweenTi-oxides and uraninite. The uraninite also contains elevated levels of P and REE including Dy.These observations will assist in future geochronological studies. In particular, the differencesbetween Beverley and Four Mile are curious given the proximity of the two deposits and previousassumptions of similar origin.Page 79 of 151