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.

Mining method currently employed at Thunderbox is conventional open pit mining with a hydraulic excavator, dump truck fleet, and drill and blast activity.

Thunderbox Open Pit includes A Zone (mining was completed during FY2018), C Zone (mining was completed during Q1 FY2021) and D Zone. Mining at the D Zone open pit commenced towards the end of FY2020.

The Kailis deposit has successfully been mined by open pit in the past and in Stage 1 and Stage 2.

Open pit mining at the Thunderbox A Zone pit and Kailis Stage 1 pit were completed during FY2018.

The Kailis Stage 2 open pit commenced production in December 2018. Mining at Kailis Stage 2 continued during 2019 with the pit completed in March 2020 and significant post-mining stockpiles available.

During Q1 FY2021, after successfully completing C Zone open pit, open pit mining for the quarter focused on ramping up D Zone development. Mining at the D Zone open pit commenced towards the end of FY2020.

D Zone is the northern extension of C Zone, and will similarly be a simple, robust open pit. Characteristics include:
- High productivity / low cost open pit mining
- Top-down, perimeter-to-perimeter sequence (no staging)
- ~94% metallurgical recoveries
- ~2.5 years of soft oxide feed blend (benefiting mill throughput / unit costs)
- Progressively lower strip ratio and higher grade.

At 30 June 2020, a record stockpile of 2.7Mt @ 1.3g/t for ~111koz had accumulated during the mining of C and A Zone open pits, representing ~12 months of mill feed. C and A Zone project to date reconciliations of ~100% bode well for D Zone.

Mining Contractor:
SRG Global: Drill and blast contract.

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.