Makuutu is an ionic adsorption clay (IAC) deposit. IAC deposits contain rare earth elements (REE) ionically bonded to the clay rather than existing as primary minerals in the ore. IAC REE mineralisation can be summarised as REEs that are mainly adsorbed onto the surfaces of clay minerals in the form of hydrated ions or hydroxylhydrated ions. These REE deposits are hosted within the regolith (laterite profile).

At Makuutu, the mineralisation is contained within the tropical lateritic weathering profile of a basin filled with sedimentary rocks including shales, mudstones and sandstones potentially derived from the surrounding granitic and mafic rocks. These rocks are considered the original source of the REE which were then accumulated in the sediments of the basin as the surrounding rocks have degraded. These sediments then form the protolith that was subjected to prolonged tropical weathering.

The weathering developed a lateritic regolith with a surface indurated hardcap, followed downward by clay rich zones that grade down through saprolite and saprock to unweathered sediments. The thickness of the regolith is between 10 and 20 metres from surface.

The REE mineralisation is concentrated in the weathered profile where it has dissolved from its primary mineral form, such as monazite and xenotime, then ionically bonded (adsorbed) or colloidally bonded on to fine particles of aluminosilicate clays (e.g., kaolinite, illite, smectite). The adsorbed and colloidal REE is the target for extraction and production of REO at Makuutu.

At Makuutu the top of the mineralised zone is defined by a thin surficial soil/hardcap zone that averages 3 m in thickness (cover zone). The base of the mineralised zone is defined by the top of the saprock/fresh rock boundary which extends to an average vertical depth of 9.0 m (Clay zone).

The Study has identified that the most suitable and economical mining methods at Makuutu comprise low unit cost, open pit extraction methods.

The assumed mining method would be ‘free dig’ using truck and shovel using a selective mining approach to target higher grade ore feed in the early years of mine life.

Wall angles have been assumed at 45 degrees in the absence of further geotechnical information, however mining is generally shallow with no permanent high walls.

Mining will involve a pre-strip of 1m of topsoil which will be stockpiled adjacent to the pit. ROM mining will be selective with lower recovered rare earth grade open pit material stockpiled adjacent to the pit, whilst waste will be mined and paddock dumped adjacent to the pit. Open pit material will be hauled to the Run-of-Mine (ROM) pad and either stockpiled or direct fed into the process plant. Once areas (slots) are completely mined out, the mined waste and heap leach residue will be reclaimed and the slot will be backfilled (progressive rehabilitation), prior to the topsoil spread back on the area disturbed.

Approximately 84.5Mt of Mineral Resource in the mining plan will be mined at an average grade of 810 ppm TREO and delivering a mine life of 11 years. Mine scheduling demonstrates that minimal mining capital will be required ahead of the development of the open pit. Mineral resource extraction and project economics are optimal with a single open pit.

The shallow nature of the deposit results in a low strip ratio, with TMM nominal average annualised material movement initially commencing at 10Mtpa then stepping up to 12.5Mtpa in Year 7, then increasing to 15Mtpa in Year 9 prior to increasing to 25Mtpa in Year 10.

Heap leaching has been nominated as the preferred technology for Makuutu.

The process flowsheet is simple and modular in nature, which will enable production capacity expansion via additional modules, with throughput of 2.5 Mtpa each. The heap leach pads are dynamic, meaning an on/off heap leach arrangement. ROM will be agglomerated, stacked approximately three (3) metres high, then irrigated in a counter current fashion using ammonium sulfate and sulfuric acid to desorb and solubilse the REE into solution. The primary and secondary leach pad will target slightly different pH conditions to minimise residual free acidity in the residue plus minimise overall reagent consumption. The secondary leach will generate an intermediate leach solution (ILS), which will then be dosed with minor additional salt addition to target a salt desorption step in the primary stage therefore reducing free acid concentrations and impurity tenors in the in the pregnant leach solution (PLS).
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