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 extents of the orebody. Whilst the shoots persist centrally, the gold ........

Sub-level open stoping (SLOS) with paste fill, and LHOS have been selected as the preferred mining methods.

The TBO UG Ore Reserve is based on sub-level open stoping (SLOS) with paste fill, and long hole open stoping (LHOS) mining methods. SLOS is the dominant mining method, and accounts for 87% of the Ore Reserve estimate by tonnes. The SLOS zone, ranges in width from 3m to 50m, and dips at 70-80 degrees.

Average mining dilution is 11%. 15% dilution has been applied to up hole stopes to account for paste dilution, and 6% dilution assumed for the down hole stopes. All dilution was assumed to be at zero grade.

The average mining recovery is 94%.

Minimum mining widths of 3m, this is based on the current stope performance to date, and mining method considerations.

Inferred material is excluded from the ore reserves. The Life of Mine Plan (LOMP) design includes Inferred Resources representing 6% of the mining inventory, contained within the stopes and development in the periphery of the indicated resource. This amount contributes to a minor amount of metal (<6% of ounces). Ongoing grade control drilling is also part of the LOMP.

The selected mining method requires the development of an access decline, ventilation raises, and associated electrical, paste fill plant and dewatering infrastructure. Paste fill plant is currently operational.

Underground mechanised mining for development, ground support, and ........

The process consists of a double toggle jaw crusher followed by a SAG and Ball milling circuit.

Metso Outotec has been chosen to supply its Premier SAG Mill for Northern Star Resources’ Thunderbox gold mine expansion. The expansion should double the throughput of the Thunderbox operation.

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