The majority of gold deposits on the West African Craton are orogenic-style deposits (Robertson and Peters, 2016). Orogenic-style deposits are sited proximal to major accretionary structures within, or at the boundaries of, composite metamorphosed volcanic- plutonic or sedimentary terrains. The majority of orogenic gold deposits occur within rocks that have been metamorphosed to greenschist facies, within a metamorphic pressure-temperature regime broadly corresponding to the brittle-ductile transition (McCuaig and Kerrich, 1998; Moritz, 2000).

The Markoye Fault is interpreted to be the main conduit for the gold mineralisation found at Sanbrado, accommodating large scale fluid migration. Sanbrado hosts shear zone type quartz vein gold mineralisation, similar to that found elsewhere in late Proterozoic Birimian terrains of West Africa. These orogenic type deposits exhibit strong relationships with regional arrays of major shear zones.

The structural environment can be described as brittle-ductile, coincident with a greenschist-amphibolite metamorphic facies. Arrays of transtensional features are apparent within a preferred S2 shear corridor and a strong shear controlled folding component is evident. Gold mineralisation is hosted in the sheared diorite and in the deformed mylonitic metasediment, and there is also a spatial relationship with the contact zones. It appears that the competency contrast caused by more brittle bodies of diorite within zones of more highly strained metasediment has played a role in partitioning the brittle deformation and dilation which hosts gold mineralisation.

Typically, the high-grade gold is observed within foliation parallel quartz lenses/veinlets often as short thin stringers of pure gold parallel to the foliation. In places gold can be seen along fractures in the quartz and in some cases the “vein material” that hosts the gold is observed to have relict rock fabric (i.e. foliation) indicating almost complete replacement of the protolith by silica.

Gold mineralisation is associated with the main hydrothermal event which produced strong silicification of the surrounding rock during opening and reactivation of the pre-existing foliation. Veinlets and lenses of quartz formed at this stage are mostly parallel to the foliation and can be observed as the siliceous bands in some of the banded mylonite. Subsequent local remobilisation would explain how much of the coarser free gold is in late fractures within the quartz.

This interpretation places gold mineralisation as post peak metamorphism after the bulk of the deformation, which means late during the regional D2 Birimian deformation within a roughly WNW-ESE (to NW-SE) stress field. Deformation and shearing along the high strain corridors has resulted in a pressure shadow, south of the main northern granitoid as the M1 and M5 high strain zones peel away (trending SE and SW respectively) from the same granitoid body. Conjugate movement along these two corridors - sinistral along M1 and dextral along M5 - is consistent with the late D2 stress field and has resulted in dilational opening and high grade steeply plunging ore shoots - along left-hand flexures at M1 and right-hand flexures at M5.

Late D3 deformation is orthogonal to D2 and reactivates D2 structures with an opposite sense of shear. While D3 reactivates, and in some cases offsets D2 structure and mineralisation, it is generally not associated with gold mineralisation, except possibly at M3, where the geometry of mineralisation is consistent with dextral shear across a north- west trending shear couple.

The mineralised structures occasionally display syn to post mineralisation displacement due to plunging fold noses and minor late faulting. Known mineralisation at M1 extends along strike for approximately 1km, is up to 50m wide and 550m in depth in M1S. The M5 mineralisation extends along strike for approximately 3km, is up to 200m wide and up to 300m in depth. The M3 mineralisation extends along strike for 750m, is up to 50m wide and 75m in depth. Mineralisation at all prospects remains open at depth.

Open pit mining
Mining occurred at the M1 South, M1 North, M5 South and M5 North open pits during 2021.

The Sanbrado open-pits employ conventional open-pit mining techniques using drill and blast with material movement by hydraulic excavator and trucks. The project scale and selectivity suit the operating mining fleet of 150 t class excavators in a backhoe configuration matched to 90 t class mine haul trucks. The Sanbrado operation is a multi-pit operation with ore being mined from the M5, M1 South, M1 North and M3 pits. All pits are within 2 km of the primary crusher location.

To suit this sized equipment a bench height of 5 m has been adopted. The benches will be excavated on 2 x 2.5 m high flitches, for blasted material this will be 2 x 3 m high flitches when swell is accounted for.

Underground mining
For most of 2020, M1 South underground ore was sourced from development headings with first underground stoping ore mined from panel-1 in late September 2020. Mining from panel-2 commenced in late Q2 2021, which enabled more continuous stope production from two ore panels for the remainder of 2021. Some 3.2 km of development was completed in 2021. At the end of 2021 the decline was 385 metres vertically below surface.

The M1 South underground mine is a decline access mine using diesel powered loaders and trucks; and electric powered drilling equipment. A long hole open stoping method with cemented rock fill is used to mine the ore. Mining of stopes commenced in September 2020 and since March 2021 the M1 South underground mine has sustained its target production rate, averaging 34,000 ore tonnes per month.

Conventional underground mining methods of long hole open stoping on 25 m levels with stope filling uses a combination of cemented aggregate fill, cemented rock fill and development waste rock depending on whether or not the fill needs to be exposed to mine adjacent stopes. Access is via a 1 in 7 decline designed to accommodate 50 t trucks.

For underground mining, the stope recovery has been estimated to account for irregular geometry, grade control errors and ore/waste misallocations. A mining recovery of 92 % has been applied to all long hole open stopes.

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commissioning of the process plant commencing in March of the prior year (2020).

The plant design proposed is simple, but robust, and broadly comprises the following:
1 Primary Crushing;
2 Crushed Ore Stockpile;
3 Grinding and Classification;
4 Gravity Recovery;
5 Leaching and Adsorption;
6 Elution;
7 Electrowinning; and
8 Smelting.

The gravity scalping screen (134-SC-001) will receive a portion of the cyclone underflow slurry. The gravity screen underflow will fall into the single centrifugal gravity concentrator (134-GC-001) which will remove concentrate into the Inline leach Reactor (ILR) storage feed hopper (134 HP-001). This hopper is part of the intensive leach reactor (134-ZM-001) which is located in a secure area adjacent to the milling building. The tailings from the gravity concentrator will be fed back into the cyclone feed hopper. The oversized stream from the scalping screen will be gravitate back to the ........