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 employed at Thunderbox mine is conventional open pit with hydraulic excavator and dump truck fleet, and drill and blast activity. Thunderbox Reserve pit is currently in operation and designed as successive cutbacks to achieve final life of mine Reserve such that it meets the operation efficiency, safety and production rate. Appropriate mine schedule, operating cost and lead time have been considered to maintain efficient mining operations.

A minimum mining width of 25m has been adopted for the primary excavation fleet. Where ‘pinch-points’ occur or “Good Bye” cuts are considered at the base of the pit, it is assumed that a smaller or more versatile excavator will be employed. The practice is very consistent across all open pit operations and reflects the suitability and efficiency of the mining performance.

The plant is a conventional carbon-in-leach (CIL) gold circuit. It comprises a gravity circuit comprising centrifugal style gravity concentrators (2 × 30” Knelson Concentrators) and intensive leaching of the gravity concentrate. The gravity tailings product is processed through a six-stage leach/CIL circuit, pressure Zadra elution and refinery facility to produce gold doré.

Arsenopyrite is present in the ore and minor levels of arsenic are solubilised in the plant solutions. The arsenic levels are monitored on a regular basis and infrastructure exists for the addition of ferric sulphate where the levels impact the environment/exceed the environmental limits. The ferric sulphate is added to precipitate the free arsenic as ferric arsenate thereby locking the arsenic in the plant tailings for storage.

Tailings are thickened and pumped to a dual cell tailings storage facility.

The processing circuit is designed for a hard rock application but throughput b ........