Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome region5.4 TIMING OF URANIUM MINERALISATION: HYPOTHESES5.4.1 Maximum ages of mineralisationThe depositional ages of host rocks provide maximum ages of mineralisation. The Four Mile Eastzone and Honeymoon deposit are hosted by the Eyre Formation. Sediments of the Eyre Formationwere deposited in two separate episodes (late Paleocene to late Eocene, i.e. ~55 to 49 Ma; andearly middle Eocene to middle Eocene, i.e., ~45 to 37 Ma) interrupted by a period of nondepositionand possibly erosion (Fig. 3.1). Sedimentation of the Eyre Formation was terminatedby a period of uplift and erosion (Middle Eocene to Late Oligocene, i.e. ~37 to 28 Ma).Mineralisation at the Beverley deposit is hosted by the Beverley Sands in the Namba Formation.Palynological studies indicate that the age of the Namba Formation is between ~ 24 Ma and 4-6Ma (Martin, 1990, in Wülser, 2009, page 38).In most sandstone uranium systems the sandstone aquifers hosting mineralisation arehydrologically constrained between aquitards or aquicludes, and it is thought that mineralisationforms after the deposition of the upper aquiclude/aquitard (Dahlkamp, 1993). In the Lake Fromeregion the Eyre Formation contains an alternating series of interlayed aquifer (sand, silt) andaquitard (shale) units (e.g., Fig. 5.5). Hence, the maximum age of uranium mineralisation hostedby the Eyre Formation is that of the shaley/clay intervals overlying mineralisation in thisFormation. These are potentially as old as ~55 Ma. Alternatively, thick regionally distributedaquitards/aquicludes in the hangingwall of mineralisation may have been an important control ontiming of mineralisation. The Alpha Mudstone or Beverley Clay (Namba Formation) both couldhave functioned as regional seals. In this case, uranium mineralisation in the Eyre Formationwould have post-dated deposition of the Alpha Mudstone (late Oligocene to early Miocene, i.e.~28 to 20 Ma) or the Beverley Clay (late Miocene, ~10-6 Ma).Similarly, the Beverley Clay may have been a regional seal during formation of the Beverleydeposit. In this case, mineralisation at Beverley would have formed after ~6 Ma.In summary, based on depositional ages of host rocks and aquitards/aquicludes, the maximumages of mineralisation hosted by the Eyre Formation and Beverley Sands are ~55 Ma and ~6 Ma,respectively.5.4.2 Mineralisation timing constraints from uplift and cooling historyThe periods of uplift and erosion could be significant for the generation of sandstone uraniumsystems by creating highland topography in the region. Uplift of basement potentially can triggergravity-driven shallow-level groundwater flow into aquifer units surrounding the basement uplift,a concept supported by our numerical modelling (Chapter 7). These conditions also may befavourable for the leaching of uranium from uranium-rich source rocks in the basement. Thus, anunderstanding of uplift and cooling history may provide constraints on the timing ofmineralisation.As noted in section 3.2 and illustrated in Figure 3.1, thermochronological and other studies haverevealed three major episodes of regional cooling and corresponding uplift since the late Mesozoicin the Lake Frome region and Mt Painter Inlier in particular (Foster et al., 1994; Mitchell et al.,2002; Célérier et al., 2005; Quigley et al., 2007). Some of this uplift has occurred on the ParalanaFault Zone, located at the eastern margin of the Mount Painter Inlier. The fault system wasinitiated in the Mesoproterozoic and has been repeatedly reactivated. Early in the development ofNeoproterozoic rifting it was active as an extensional or transtensional fault, and may have beenreactivated as a steep reverse fault in the Cenozoic and/or Quaternary (Preiss, 1995), commencingin the early Paleocene or even in the late Cretaceous (Mitchell et al., 2002). Observations of tiltedgravel deposits suggest that major uplift of the northern Flinders Ranges occurred during the LatePliocene or Early Pleistocene (Foster et al., 1994). Using cosmogenic 10 Be erosion-rate estimatesPage 65 of 151
Uranium ore-forming systems of the Lake Frome regionQuigley et al. (2007) suggested that current rugged topographic relief in the northern FlindersRanges could have formed in the past 4 million years. Seismic activity in the area indicates thatthe Paralana Fault is still active with reverse movement (Sprigg, 1984).If three major episodes of uplift and exhumation occurred since the late Mesozoic, as appearslikely from available evidence, then we may infer that there is potential for three periods of fluidflow from uplifted regions into the surrounding basins. Each of these fluid flow episodes mayhave resulted in uranium mineralisation (Fig. 3.1), although this hypothesis remains to be tested.The Beverley deposit is interpreted to have formed during the third episode (i.e., since ~6 Ma),whereas deposits hosted by the Eyre Formation (e.g., Four Mile, Honeymoon) may have formedeither in the second (late Eocene – early Oligocene) or third episodes of uplift and fluid flow. Nosignificant uranium mineralisation has been described in the literature within Mesozoic sedimentsin the region. Our synthesis of available data suggests there is potential for fluid flow driven bylate Cretaceous uplift in the region and hence the possibility of late Mesozoic – Paleoceneuranium mineralisation. However, the presence or absence of other components of a fertilemineral system (Fig. 1.2) will determine whether any such mineralisation is of economicsignificance.Present day groundwater flow in the Beverley Sands, in conjunction with evidence fordisequilibrium of radioactive parent and daughter isotopes (Wülser, 2009), suggests thatmineralisation at Beverley may be still in the process of formation, or at least re-mobilisation.Similarly, flowing groundwaters in the Yarramba Paleochannel may be actively forming orredistributing uranium mineralisation in the Honeymoon deposit.Page 66 of 151
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