Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome regionFigure 5.6: Bends in the main Yarramba paleochannel (white with red arrow). Note severaltributaries draining in to the main channel. Honeymoon deposit is located at the bend (circled symbolU). Paleochannel is plotted over gravity (0.5 mgal residual gravity contours). NE trending gravity highcoincides with the ridge. Honeymoon deposit is located at the faulted breach in the ridge. N-trendingtributary south of Honeymoon and draining in to the main channel crosses uranium-rich granites(roughly coinciding with purple areas on the gravity). Source: Skidmore (2005).5.3.3 Honeymoon mineralisation, and oxidation-reduction patternsTabular ore zones occur at a depth of 110 m below surface and average 4.3 m in thickness (Fig.5.7). The mineralisation is in the form of microscopic coffinite closely associated with humic andpyritic material (McKay and Miezitis, 2001; Skidmore, 2005).Much of the paleochannel infill sediments appear to have been oxidised with only localoccurrences of reduced sands. A roll-front type geometry of the oxidation-reduction interface hasnot been observed at Honeymoon. The influx of oxidised waters in the aquifer is still activecausing selective dissolution and removal of uranium.Page 63 of 151
Uranium ore-forming systems of the Lake Frome regionFigure 5.7: Cross section showing uranium mineralisation at the Honeymoon deposit. Tabular zonesare located in the basal sandy unit of the Eyre Formation. Locally, mineralisation occupiesdepressions in the paleo-relief at the base of the Eyre Formation. Vertical exaggeration 5 times.Source: Skidmore (2005).5.3.4 Factors controlling uranium ore deposition at HoneymoonThe ore zones at the Honeymoon deposit are located at a prominent bend in the Yarrambapaleochannel. As mentioned above the location of this bend is the result of a combination offactors which may include the structure and composition of the bed rock and the presence of afault zone cutting relatively dense bedrock strata. At this stage it is not clear if the ore zone’slocation is related to changes in the relative permeability of sands in a fining-upward sequence ofpoint bars, or due to selective concentration of detrital organic material in the sediments. Finegrainedoverbank sediments that cap the sandy sequence are typically rich in organic material(Miall, 1992). Migration of channels commonly leads to abandoned channels which are filled withsilt and clay with abundant plant material (LeBlanc, 1972).Studies on the distribution of organic matter in low-energy meandering streams show that thepresence of abundant woody debris at bends enhances the content of organic matter in bedmaterial (Daniels, 2006). Thus the location of ore zones at the bends of paleochannels can beexplained by increased concentration of organic reductant in the sediments at the bends. Thechange in fluid flow rates at bends can also facilitate effective interaction between the fluid andthe organic reductant.An additional factor in localising mineralisation at the Honeymoon deposits may be the closeproximity of uranium-rich granitoids in the basement. A north trending tributary of the YarrambaPaleochannel drains an area of buried granitoid with uranium contents of ~ 75 ppm. Thus acombination of two factors, the presence of beds enriched in organic material and an hydrologicalconnection with uranium-rich source near the bend, may explain the position of ore in theYarramba Paleochannel.Page 64 of 151
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