The target deposit type at Songwe Hill is a high-level, REE-enriched carbonatite intrusive complex. Carbonatites are traditionally defined as intrusive and extrusive igneous rocks that contain in excess of 50 % modal carbonate minerals (Woolley and Kempe, 1989). Mitchell (2005) defines carbonatites as “containing greater than an arbitrary 30 % by volume of primary igneous carbonate regardless of silica content”. Carbonatites can be named according to their carbonate mineralogy (e.g. calcite carbonatite, dolomite carbonatite and ankerite carbonatite), and chemically they can be divided into the three main varieties: calcio-, magnesio- and ferro-carbonatite.

Carbonatites usually occur as plugs or pipe-like bodies within zoned alkalic intrusive complexes, or as dykes, sills, breccias, and veins, and are almost exclusively associated with continental rift-related tectonic settings. They are characterised by an aureole of metasomatically altered country rocks which are usually referred to as fenites. Carbonatite magmas are typically low-viscosity and volatile-rich, and when intruded at a high crustal level, produce widespread intrusion- and explosion-breccias incorporating country rocks, carbonatites and associated fenites. Carbonatites are typically associated with silicate rocks of which the seven key carbonatite-silicate rock associations are, in decreasing order of abundance, 1) nephelinite-ijolite, 2) phonolite-feldspathoidal syenite, 3) trachyte-syenite, 4) melilitite-melilitolite, 5) lamprophyre, 6) kimberlite, and 7) basanite-alkali gabbro (Woolley & Kjarsgaard, 2008). The carbonatite deposit at Songwe Hill is spatially associated with the large nepheline syenite intrusion of Mauze, and the Songwe Hill carbonatite is intimately associated with intrusive phonolites and their fenitised equivalents. It is therefore interpreted to belong to the phonolite-feldspathoidal syenite association.

Mineralisation in the Songwe Hill complex occurs in all three geological domains, but the highest grades and most consistent mineralisation are found in the carbonatites. The mixed lithologies locally carry a high enough grade over significant widths to be included in the Mineral Resource but are not consistently mineralised throughout their full extent. Fenites only locally contain economically interesting concentrations of REE, and seldom over economic widths.

Mineralogical studies carried out on Songwe Hill samples were reported in detail by The MSA Group (2015) and are summarised here, supplemented by recently published data. Mineralogical studies using a scanning electron microscope (SEM), electron microprobe (EMP) and laser ablation, inductively coupled plasma, mass spectrometry (LA-ICP-MS) showed that the REE mineral assemblage at Songwe Hill, regardless of lithology, is dominated by fluorocarbonates, principally synchysite with minor parisite, apatite and occasional florencite. The synchysite crystals are homogeneous, typically occurring as randomly oriented laths or tabular crystals and/or fibro-radial to plumose aggregates. Crystal size varies, but laths typically range in length from 10 µm to 60 µm, and crystal aggregates can reach up to 400 µm.

Synchysite is invariably associated with strontianite and/or baryte either as inclusions and/or intergrowths, and together they form distinctive vein-like aggregates or segregations. In addition to these two phases, synchysite is locally associated with calcite, fluorite, alkali (K) feldspar, pyrochlore and titanite. The mineral association of synchysite with strontianite and baryte in the Songwe Hill carbonatites and their textural relationships was described by Broom-Fendley et al. (2017) and interpreted on the basis of paragenesis with various states of apatite as representing hydrothermal redistribution of the REE during early hydrothermal activity at 250 °C to 300 °C.

Fluorapatite in the Songwe Hill carbonatites has a complex history and paragenesis. Broom-Fendley et al. (2017) showed that there are five recognisable stages of apatite crystallisation: two early stages representative of crystallisation from a carbonatite magma, and three stages that are texturally atypical of magmatic apatite and progressively enriched in the heavy rare earth oxides (HREO). The HREO enrichment factor of Songwe Hill apatite, defined here as the sum of all HREO from Eu2O3 to Lu2O3 and Y2O3/TREO, ranges from 40 % to 85 % compared to 2 % to 11 % in apatite from other carbonatites (Hornig-Kjarsgaard, 1998). It is rare for apatite in carbonatite deposits to display heavy rare earth enrichment.

The mining method at Songwe Hill will be conventional open-pit mining, making use of relatively small-scale trucks and diesel-hydraulic excavators, selected to match the mining conditions and required production rates.

The pit design was structured around the strategy of Phase 1 accessing high-grade Type 1 ore for the first five years and stockpiling any medium-grade Type 1 ore to feed as run of mine (ROM) at a later stage. The high-grade pit shell from the pit optimisation was used as a guide for the design of the first five-year pit.

The final pit pushback Phase 2 was then designed to the final pit limits, and the final pit shell from the pit optimisation was used as a guide.

The final pit design is approximately 660 m north to south, 550 m east to west, and has a depth of 354 m from the pit rim on the south side (top of Songwe Hill) and 134 m on the north side (bottom of Songwe Hill).

A mining schedule was developed, targeting the design plant throughput of 1,0 Mt/a of ore.

Bench Height
Mining equipment used to drill and blast the rock determines the bench height. Currently, most large mining operations adopt 10 m to 15 m bench heights. For the Songwe Hill open pit, a maximum of 10 m bench heights were adopted based on the PFS mine design criteria.

Bench Face Angles
Based on the SWedge analysis results, a bench face angle of 70° is suggested for both transitional and trans-fresh domains.

Bench Widths
For the weathered and poor-quality domains, 5 m spill bench and 10 m geotechnical bench widths (catch bench) are suggested. Spill benches are implemented after every single bench, while geotechnical benches are wider and are implemented after every stack. For the transitional domain, spill bench widths range from 5 m to 9 m with geotechnical bench widths ranging from 10 m to 12.5 m. For the trans-fresh domain, spill bench widths of 5 m to 8.5 m are suggested, with geotechnical bench widths of 9 m to 15 m, depending on the stack heights.

Overall Slope Angles
Overall slope angles range from 39° to 47°. The mean FOS obtained for the overall slope stability of the final wall designs ranged between 2.05 and 4.09, which were above the threshold criteria of 1.5 as stipulated.

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Concentrator
Songwe Hill is a rare earth element deposit in the south-eastern region of Malawi. It comprises mostly carbonaceous gangue minerals and rare earths associated with oxides. Minor amounts of sulphide minerals are also present. The purpose of the concentrator plant or “front end” is to produce a rare earth oxide concentrate, which is to be treated in a subsequent hydrometallurgical plant.

Flotation of oxide minerals is typically more difficult to achieve than flotation of sulphide minerals, and a defining characteristic of this project has been to develop a flotation circuit and reagent suite that effectively float the rare earth oxides from the other oxidic gangue. The key in achieving flotation recovery lies in the combination of fine grinding, high-intensity conditioning, elevated temperature, and the correct reagent suite and dosages.

The crushing circuit consists of a primary jaw crusher and secondary and tertiary cone crushers. Primary and sec ........