Thunderbox is a mesothermal lode gold deposit located at the southern end of the Yandal greenstone belt in an area where several major shear zones converge and join with the Perseverance Fault. The shear zone dips at 30° to 60° WSW, with the exception in the vicinity of the mineralisation, where the shear is vertical to steeply dipping. Mineralisation is hosted by strongly deformed, silicified and carbonate altered albite-quartz porphyry in the hangingwall of the shear zone. The shear juxtaposes foliated basalts and intrusive porphyries in the hangingwall against sedimentary rocks in the footwall. The zone of shearing is over 200m wide. An ultramafic unit occurs within the shear, in the footwall of the deposit and is attenuated along the shear. The main gold related hydrothermal alteration assemblage comprises quartz-ankerite arsenopyritepyrrhotite-galena and gold. This assemblage has been overprinted by a retrograde chlorite-epidote-white mica-biotite-quartz and pyrite assemblage. Syn-mineralisation veins have a continuum of vein textures ranging from laminated to pseudo-breccias.

Thunderbox mineralisation extends from 6879000mN to 6881000mN, 304000mE to 304400mE and 500 meters below surface. The Thunderbox shear generally strike NNW and dips 60° towards the WSW. In the vicinity of the strongest gold mineralisation the shear is vertical to steeply west dipping. The shear and mineralisation is offset across a series of dextral, NE trending faults.

The Thunderbox gold deposit consists of free milling gold which occurs as inclusions within, and at the rims of arsenopyrite crystals, and as free gold clusters within quartz-carbonate veins.

Sub Level Open Stoping (SLOS) has been selected for the wider sections of the A Zone and C Zone orebodies utilising a bottom-up primary-secondary mining sequence. The use of cemented paste fill in primary and secondary stopes has been applied to maximise recovery of the orebody and optimise capital development. Production down holes will be utilised over the 35m sublevel spacing.

SLOS was selected over the sublevel caving method contemplated in the 2017 Pre-Feasibility Study (PFS) due to lower technical risk, lower execution risk, lower capital costs and less mining dilution (lower grade risk). SLOS also provides more selectivity / optionality to future changes to gold prices and / or operating costs.

Long Hole Open Stoping (LHOS) has been selected for the upper section of the A Zone orebody. This mining method is safe, efficient and widely employed by the Western Australian mining industry (including at Carosue Dam). Recovery of the orebody is maximised, and mining dilution is minimised with this flexible method. A level spacing of 25m (floor to floor) has been applied in a top-down mining sequence retreating to the level access.

A sublevel spacing of 35m has been applied within the primary-secondary mining areas, with stoping spans limited to 25m along strike. Stope thickness (across the strike of the orebody) will be limited to 30m and mined in two passes where the orebody thickness exceeds this threshold.

Several independent stoping panels have been created within the larger A Zone to provide greater productivity and flexibility for production. Stoping will be transverse or longitudinal, and generally in a bottom-up sequence (an opportunity afforded by the investment in accelerated decline development during FY19-20).

The currently operating Thunderbox CIL processing facility will treat the underground ore. The assumed metallurgical recovery of 91.5% is consistent with the performance of the A Zone and C Zone ore currently being treated through the facility.

The Thunderbox Process Plant is a CIL cyanide leach plant incorporating a gravity circuit which is appropriate for the extraction of gold from free milling gold ores.