Gold mineralisation occurs as a number of en echelon, moderately east dipping foliation parallel lodes within strongly potassic altered mafic rocks and extends over a strike length of approximately 500 m and to a vertical depth of at least 2,300 m. Four primary lodes (Main Lode, South West Branch, South Gwalia Series and West Lode) have been identified and the geometries summarised below.

Mineralisation domains are defined by abundance of quartz and quartz/carbonate veining, the presence of distinctive laminated veining (quartz/sericite/sulphides +/- au), strong potassic alteration, abundance of sulphides (commonly >3% pyrite) and elevated gold grade (>0.5g/t).

The mineralised zone strikes 15 degrees east of true north over a distance of 500m and plunges 45 degrees to the southeast. The mineralised zone consists of several stepped or en echelon style foliation parallel lodes disposed in plan in a “horse-shoe” shape with the limbs converging at the southern end. The mineralised zone and individual lodes dip east at 35 to 45 degrees and are conformable with the foliation of the Mine Sequence mafic schists. Individual lode widths vary from 2m to 30m true width. Mineralisation has been tested to approximately 2,300m below surface and remains open.

St Barbara’s Leonora Operations comprise almost 300 square kilometres of mineral tenure and consists of the Gwalia underground mine and 1.2 Mtpa processing plant, as well as nearby development opportunities as:
- Gwalia open pit mineralisation between 280 and 500 metres below surface(mbs) was identified as a potential source of open pit mill feed upon completion of underground mining.

- The Harbour Lights Deposit is located approximately 5 kilometres from the Leonora Processing Facility. The deposit was initially mined by underground methods between 1904 and 1935 via a series of shafts to around 40 mbs. More recently, ore was extracted from an open pit by Harbour Lights Mining between 1985 and 1992. Since 1995, mining is not undertaken.

- Tower Hill is located approximately 2 km north of the Leonora Processing Facility. Intermittent mining has occurred in the vicinity of Tower Hill since 1898. Early mining was from a small open cut and underground mine to 66 metres depth. The existing Mineral Resources and Ore Reserves for Tower Hill will remain in place while work continues on the best extraction and processing options for this mineralisation.

- The Jasper Region comprises a series of historic open pits and prospects approximately 25 km north of Gwalia. These include the Jasper Flat, Jasper Hill, Falklands Trend, Hawaii, Greenland and Trevor Bore deposits.

St Barbara’s flagship mine Gwalia is located 3 kilometres south of the Leonora Township. Gold has been extracted from the deposit over time by both open pit and underground methods.

Currently reported Mineral Resources start at approximately 1,100 mbs and extend to depth and are referred to as Gwalia Deeps. The extent of past production within the mine itself was determined by the prevailing gold price at the time and was also a function of the distance from the original shaft access. In particular as the Gwalia Mineral Resource contains four main lode systems (Main Lode, South West Branch, South Gwalia Series and West Lode) there are areas within the mine where only some lodes have been developed and others have been left untouched.

Gwalia is the deepest underground trucking mine in Australia, that employed long hole open stoping with paste backfill.

Having added 1.4 Moz of gold to the Mineral Resource base through Gwalia Open Pit and Harbour Lights and with aspirations to add a further 1.1 Moz of gold from Gwalia and the Jasper Region the next step is to progress studies to convert this material into Ore Reserves and bring it into the life of mine plan.

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Gwalia processing plant consists of a three-stage crushing circuit, single-stage milling circuit and hybrid carbon-in-leach circuit with one designated leach tank and seven adsorption tanks. Gold is recovered from activated carbon into concentrated solution via a split AARL-type elution circuit. Electrowinning and smelting are conducted in an adjacent secure gold room. The tailings from the process are thickened and pumped to a paddock-type tailings storage facility with multi-spigot distribution. Gold doré bars are transported to the Perth Mint for refining.

The technology associated with processing of Gwalia ore is currently in operation and is based on industry standard practices.

Metallurgical recovery is modelled based on the observed relationship between head grade and recovery. The average of the modelled metallurgical recovery over the Ore Reserve mine plan is 96.8%.