Bennet Well Uranium Deposit Technical Detail
Bennet Well Uranium Deposit
The Bennet Well Uranium Resource is a large, shallow accumulation of uranium oxide comprised of four spatially separated deposits – Bennet Well East, Bennet Well Central, Bennet Well South and Bennet Well Channel.
Cauldron has reported a total Indicated plus Inferred Mineral Resource (JORC 2012) of 38.9 million tonnes @ 360 ppm eU3O8 for 30.9 million pounds (13,990 tonnes) of contained uranium oxide, using a cut-off of 150 ppm eU3O8.
The geology is summarised here and the mineralisation and physical characteristics is summarised below.
- Quaternary – Recent – unconsolidated sands, sand dunes and alluvium;
- Tertiary – partly consolidated sandstone and sands, local calcrete-silcrete development, minor conglomerate marks the base of the unit;
- Cretaceous – Mardi Greensand, intensely bioturbated, glauconitic, interbedded sand and silts to massive greensand; very low permeability due to bioturbatic destruction of grain sorting;
- Cretaceous – Nanutarra Formation – unit 4 and 5, alternating intervals, each 5-10 m thick, of sand and silts, with broad upward fining cyclicity;
- Cretaceous – Nanutarra Formation – unit 1,2 and 3, fluvial sequence
- Weathered Basement – Saprock of mainly granite and gneiss,
- Basement – Archaean/Lower Proterozoic Granite with minor granitoids and metasedimentary basement.
Prospective sediment-filled palaeochannels of Mesozoic age occur on incised Proterozoic-aged granite and metamorphic basement comprising the Gascoyne Province of the Capricorn Orogen. The sediments of the channels are sourced from the east and enter into a deep north to south trending depression that was probably caused by regional faulting and may represent an ancient coastline. Most of the channel sediments are limonite-oxidised, quartz-dominated, sub-rounded sand and pebbles with occasional occurrences of a reduced variant. The channels have an erosional base between 50 to 100 m below surface, with only the lower portion comprising channel sediments. During the Cretaceous, sea levels rose flooding the channel depressions of the ancient coastline, depositing thick marine sediments (sands, clays, lignitic clays and carbonaceous sands) of the Nanutarra Formation, Mardi Greensand, Birdrong Sandstone and the Muderong Shale that conformably overlay the channel sediments. In this low energy setting, marine clays (which are glauconitic) and lignitic units were deposited along and across into the overbank regions of the channel sands.
In cross-section, the channels have an asymmetric shape with a steep eastern margin and a very shallow non-demarcated western margin, resembling a series of unconfined valley systems. Bennet Well Central occupies the broadest of the palaeovalley systems, with mineralisation located above the palaeovalley floor (and above the channel basal sands) at the level of the shoulder of the channel, and mostly to the west of the steeper valley edge (refer to Figure 2).
Most uranium mineralisation occurs in three sedimentary settings, with the majority hosted near the top of the Nanutarra Formation, particularly adjacent to organic-rich sands and lignites beneath intensely bioturbated Mardi Greensand acting as an aquiclude. A less laterally extensive range of uranium mineralisation occurs at the top of the lowermost sedimentary cycle of the Nanutarra Formation.
Ravensgate Mining Industry Consultants (Ravensgate) were commissioned to complete an upgrade to the Mineral Resource for the Bennet Well uranium deposit, after the completion of a mud rotary drilling program in late 2015. The report was prepared in accordance with the Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves 2012 (JORC Code).
Ravensgate updated the Mineral Resource (JORC 2012) estimate using a newly generated three-dimensional resource block model. This resource modelling followed on from a comprehensive revision of the stratigraphic setting completed in-house, following information provided by the 2013 and 2014 diamond drilling program and the mud rotary drill programs of 2014 and 2015, along with a reassessment of all previous drilling results, as summarised in Table 1.
The Mineral Resource estimate for Bennet Well and its classification is shown in Table 5-1 and summarised as a total Indicated plus Inferred Resource (JORC 2012) of 38.9 million tonnes @ 360 ppm eU3O8 for 30.9 million pounds (13,990 tonnes) of contained uranium oxide, using a cut-off of 150 ppm eU3O8.
Figures 3, 4, 5 and 6 show sectional views through the Bennet Well deposit model, displaying the sedimentary geological units modelled from the recent core drilling. These units were modelled into three-dimensional wireframe solids, used to constrain grade in block model generation, and subsequently for estimation of the Mineral Resource.
Table 1: Mineral Resource Estimate (JORC 2012) for Bennet Well, at various cut-off grades
RESEARCHING AMENABILITY TO IN SITU LEACHING
Permeability, porosity and density testing was completed by independent reservoir optimisation specialists, Core Laboratories Australia Pty Ltd using core obtained from Bennet Well East and Bennet Well Central. Core was sampled on depth increments of 15 cm, commencing from about 5 m above mineralisation (in the overlying aquiclude sequence), through the mineralisation zone and 5 m into the lowermost sequence, in order to obtain measured permeability. In addition, porosity and density measurements were obtained using a porosimeter and balance.
This physical characterisation testing shows the porosity of the uranium bearing lithologies is suitable for mining via the In-situ Recovery (ISR) method, having values ranging from 27% to 42%, with an average of 34% porosity. A tightly-bound, highly bioturbated greensand unit overlies mineralisation, having low to extremely low permeability of 0.07 to 10 millidarcies (md), showing that this unit is impermeable and will provide the confining pressure required to contain the mining fluids of a potential ISR operation.
A composite photograph of drill core from hole BW0070 (Figure 7), illustrates the varying permeability returned from Air-Permeability (Ka) <10 md (impermeable) measurements taken from the hanging wall, which then increase to Ka 750 md (permeable) within the mineralised zone, and then returning to Ka 60 md (low permeability) in the footwall to mineralisation. A summary of permeability profiles from the permeametry measurements of four core holes in Bennet Well Central and Bennet Well East is shown in Figure 8. There is generally an order of magnitude increase in the permeability of the sands that are host to mineralisation compared to the sediments of the adjacent hangingwall and footwall.
CSIRO Metallurgical Characterisation Research Program
In late 2016, Cauldron successfully secured funding from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the Minerals Research Institute of Western Australia (MRIWA) to initiate a deposit-focused investigation into the amenability of Bennet Well for uranium extraction by the In-Situ Recovery (ISR) method of mining. Phase 1 involved ten column leach tests on five mineralised zones which had been sampled by diamond drill core from the Bennet Well East and Bennet Well Central deposits. Both acid and alkali leaching solutions were tested, with oxidant added to each leachate mid-way through the leaching cycles. The ion exchange method of extraction was also tested, using nine commercially available ion exchange pellets to strip the uranium mineralisation from the pregnant liquor solution.
The second phase of the investigation (Phase 2) is aimed to support the activities of the proposed Field Leach Trials for which the approvals process was also commenced in 2016 and 2017. However, due to the change in State government and subsequent changes in policy towards uranium mining, these trials and Phase 2 activities are yet to be commenced.
Figure 9 illustrates the resulting column leach test recovery curves which show:
- Acid leach achieves higher uranium extraction than alkali leach;
- Use of oxidant improves uranium extraction in acid leachate;
- Oxidant may not be required because very high extraction rates are achieved by acid leaching solutions that do not contain oxidant;
- Column test results concur with bottle roll recoveries measured by ANSTO in a previous study completed in 2014.