Gold within the LEFA Gold Mine is mainly associated with mesothermal fractured and vein style mineralisation, entirely consistent with the majority of Archaean and Proterozoic terrains worldwide, including the Birimian Series of West Africa. This style of mineralisation is generally associated with regionally metamorphosed terrains that have experienced considerable deformation. As such, the deposits are invariably strongly structurally, rather than lithologically controlled. However, the dominance of structural control invariably increases in a manner commensurate with the metamorphic grade.

At the macro scale, mineralisation is preferentially developed in the more permeable, altered, coarser grained sediments, within and/or adjacent to east-northeast oriented structures, and more consistently north to north-northwest trending vein/fracture zones. Mineralisation is localised by a combination of lithological and structural controls and, while the latter is predictably the more dominant, lithology appears to play a greater role, particularly at Fayalala in the 310º orientation. The dip and strike of mineralised zones, and to a lesser extent the style of mineralisation, varies considerably between deposits. Gold mineralisation is dominantly associated with stockwork and sheeted quartz carbonate–sulphide veining, stockworks of albite-carbonate-sulphide veinlets, sulphidic haematitic fracture zones or skarn magnetite epidote-pyrite mineralisation.

Pyrite is the dominant sulphide species, present as discreet poikilitic euhedra, ranging from a fraction to a few millimetres in size, largely confined to vein margins, or disseminated within alteration selvedges. Traces of other sulphides, principally chalcopyrite, galena, pyrrhotite, arsenopyrite, bornite, tennantite, linneite and mackinauwite are present as veins, fracture fillings and localised disseminations adjacent to veins, with the latter minerals explaining the weak antimony, cobalt and tungsten geochemical signature associated with mineralisation.

Gold is largely developed within fractures in pyrite grains, rarely larger than 50 microns, and is non-refractory. Extensive weathering and lateritisation of the mineralisation and surrounding host rocks has resulted in the development of mineralised laterite and saprolite gold deposits. Both transported and residual laterites, up to 15 m thick, host gold mineralisation, which typically averages gold grades of 0.5 to 2.0 g/t, over extensive lateral areas. Saprolite mineralisation tends to be of higher gold grade (1.5 to 5.0 g/t), but is generally developed over more restricted zones 2 to 30 m wide.

The base of oxidation commonly extends to over 100 m depth, more locally depressed within zones of fracturing and brecciation. Oxidation is evident along the Karta Fault at depths exceeding 200 m. However, there is a possibility this is due to deuteric hydrothermal alteration. Supergene enrichment is evident at Lero-Karta with the Lero and Lero South deposits displaying increased gold grades within the saprolite portion. However, most other mineral deposits appear to have remained unaltered by supergene movement of gold within the regolith.

Recent discoveries of the Firifirini and Toume Toume deposits have resulted in the identification of a significant new style of mineralisation not related to the other LEFA Corridor mineral deposits. Both these deposits represent contact metamorphism retrograde skarn styles of mineralisation, characterised by a felsic suite intruding calcareous sediments.

The Lefa gold mine operates a typical modern open pit with three major mining areas and several smaller, higher-grade satellite pits, which will provide supplementary feed for the mill over the life of the project.

Lefa’s processing plant consists of a crushing circuit, two SAG mills, two ball mills, and a CIP circuit. Also there are waste rock dumps, a specially constructed tailing storage facility and a water management system.

Ore from the pit is then transported to a designated stockpile and fed into a crushing circuit. The crushed material is then moved onto a conveyor, which delivers the ore to a grinding circuit consisting of SAG and ball mills. The milled product is then thickened and leached into a CIP circuit.

The cyanidation of the milled ore is carried out in the CIP or CIL circuit. During the CIP or CIL stage, pulp flows through several tanks where sodium cyanide and oxygen have been added to dissolve gold into solution. In the absorption stage, this solution flows through several tanks containing activated carbon. Gold then attaches onto the activated carbon, which flows counter current to the pulp, while screens separate the barren pulp from the gold loaded carbon.

The final gold loaded carbon is removed and washed before undergoing elution. Elution is achieved by introducing the gold loaded carbon to a solution of sodium cyanide at a high temperature (>110°C) and under high pressure. This results in the reversal of the attachment process, with the gold detaching from the carbon back into solution. This produces a small volume of solution with a high gold concentration.

The gold loaded eluate solution that emerges from the elution process is passed through electrowinning cells, where gold is deposited onto the cells’ cathodes.

The cathodes then undergo a smelting process, further refining the gold and leading to the eventual production of gold ingots suitable for transport to a refinery.