Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome regionFurther differences between the Four Mile and Beverley deposits are evident in the minor mineralassemblage. The samples studied from Beverley contain malachite, arsenopyrite and probablesphalerite. In addition, unidentified Fe-Ni-Cr and Cu-Zn minerals were also observed. Barite ispresent as inclusions in quartz grains at Beverley. Pyrite is not as prevalent at Beverley comparedwith Four Mile, and no phosphate minerals were observed.Figure 6.13: SEM micrograph (backscattered electron image) of a typical occurrence ofcoffinite at the Beverley deposit. Ilm = ilmenite, Qtz = quartz.6.5 DISCUSSIONThe paragenetic context of uranium mineralisation at the Four Mile East deposit has yet to beestablished in detail. Secondary processes affecting the unconsolidated to weakly consolidatedsediments of the host Eyre Formation may include early diagenesis, weathering, and lowtemperaturehydrothermal alteration. In this reconnaissance study it has not been possible todistinguish the effects of these possible secondary processes.Nevertheless, a working hypothesis for the paragenetic sequence has been developed (Fig. 6.14).The major detrital phases are quartz, feldspar, and ilmenite. The timing of very fine kaoliniteinterstitial to framework grains appears to be early in the paragenetic sequence, as it is replaced byuraninite and phosphate minerals. Kaolinite may have been detrital or washed in, partially fillinginterstices.Page 77 of 151
Uranium ore-forming systems of the Lake Frome regionFigure 6.14: Provisional paragenetic sequence for the Four Mile East deposit, based onreconnaissance petrography. Heavy lines – abundant; thin lines – minor; dashed lines – traceamounts.Our observations indicate that uraninite replaced and possibly overgrew early framboidal pyrite(Py1) of presumably bacterial origin. Uraninite also appears to have replaced some kaolinitealong its cleavages, in association with phosphate minerals. Whereas euhedral pyrite (Py2) in thematrix of the sediment appears to have overgrown uraninite pseudomorphs of framboidal pyrite(Fig. 6.11), other Py2 hosts small inclusions of uraninite. This suggests either that (a) there weretwo separate phases of uraninite growth, or (b) that uraninite and Py2 were broadly coeval, andthat only some Py2 included U minerals. For simplicity, scenario (b) is favoured. The timing ofvery fine overgrowths of Py3 on Py2 euhedra relative to extremely fine Py4 in the matrix isunclear.Detrital grains of ilmenite with low U contents are replaced by U-V-rich Ti-oxide alterationproducts (brookite and/or anatase). Whereas the V may have been sourced from the ilmenite, theelevated levels of U in the Ti-oxide alteration products suggest that the alteration of ilmeniteoccurred in the presence of a U-bearing fluid.Textures of relict detrital quartz suggest that quartz was corroded, particularly where the organicmatter content is higher, although the timing of this process is unclear. Quartz dissolution, as wellas the apparent lack of significant amounts of the U-silicate coffinite, and replacement of kaoliniteby uraninite and phosphate minerals, together point to the presence of silica-undersaturated fluids,at least locally. This would appear to contrast with the Beverley deposit where coffinite iscommon, there is little evidence for quartz dissolution in the few samples studied, and silcreteoccurs only a few metres or tens of metres above ore zones.The intimate association of uraninite with REE phosphate minerals, both replacing kaolinite,together with the high P content of uraninite as shown by electron microprobe data, suggest thatPage 78 of 151
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