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. At the known extremities of the mineralisation, interpreted as a pepperitic apron, the porphyry host is less continuous and interchanges with mafic volcaniclastics.

The main gold related hydrothermal alteration assemblage comprises quartz-ankerite-arsenopyrite- pyrrhotite-galena and gold. This assemblage has been overprinted by a retrograde chlorite- epidote-white mica-biotite-quartz and pyrite assemblage. Synmineralisation veins have a continuum of vein textures ranging from laminated to pseudo-breccias.

Throughout the Thunderbox Deposit, elevated grades occur within southerly plunging ore shoots that are more evident in the lateral pepperitic margins of the orebody. Whilst the shoots persist centrally, the gold distribution is far more uniform and ubiquitous than in other areas. On a local scale, internal to the mineralised porphyry, are lenses of non-mineralised andesite.

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.

In late FY2018, underground mining commenced at Thunderbox with the cutting of two portals in the southern end of the A Zone pit.

Beneath the mined C Zone pit is a portion of the mineral resource that will be extracted by a bulk underground method.

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.

Development of the Thunderbox underground “Thunderground” continued with 1,475m of horizontal development completed. A second portal access was established at C Zone, plus associated infrastructure for the large scale underground mine including primary fans and substations. Commercial production anticipated towards the end of FY21.

Mining Contractor:
Byrnecut: Declining, lateral and vertical development, longhole stoping, production & UG paste-fill activities.

Single stage crushing:
- SAG and ball mill grinding circuit.

Thunderbox mill to be expanded to 3.5Mtpa during FY22. The secondary crushing circuit will deliver a finer product size to the grinding circuit. Saracen has purchased two Sandvik crushers that will be utilised (currently in storage).

The Thunderbox mill employs a conventional crushing, grinding and CIL leaching process to extract the gold. The mill operated successfully between 2002 and 2007, processing in excess of 9Mt of ore. The conventional plant displayed excellent performance with gold recoveries between 93.4 to 96.6 % over the life of the mine. It has since been upgraded to include a gravity recovery circuit facilitating continued high recoveries with increased throughput rates. Ongoing supply of Thunderbox ore to the processing facility will continue to be sourced from a combination of open pit oxide and fresh rock from the underground operations.