The Costerfield field is part of a broad gold-antimony province mainly confined to the Siluro-Devonian Melbourne Zone. Although antimony often occurs in an epithermal setting (in association with silver, bismuth, tellurium, molybdenum and other minerals), the quartz-stibnite-gold narrow veins of the Melbourne Zone are mesothermal-orogenic and are part of a 380–370 Ma tectonic event. Gold in Central Victoria is believed to have been derived from the underlying Cambrian greenstones. The origin of the antimony is less certain.

The mineralisation occurs as narrow veins or lodes, typically 500 mm wide, hosted within low-grade (anchizone) mudstone and siltstone of the Lower Silurian Costerfield Formation. Gold mineralisation of >20 g/t with an average grade of ~9 g/t is typically hosted within and/ or alongside veined stibnite with ~4% antimony (Fromhold et al., 2016). Mineralised shoots in the Costerfield Property are structurally controlled by the intersection of the lodes with major cross-cutting, puggy, sheared fault structures.

The economic mineralisation in the property occurs at the southern end of a system of steeply-dipping quartz-stibnite lodes, with thicknesses ranging from millimetres to 1 m, and extending over a strike of at least 4 km. Individual lodes can persist for up to 800 m strike and 300 m down-dip. The lode system is centred in the core of the doubly-plunging Costerfield Anticline and is hosted by Costerfield Siltstones.

Vein fill mineralogical contents and proportions are found to differ from vein to vein throughout the Augusta, Cuffley and Brunswick lodes. However, the texture and chronological order of each vein mineral generation remains remarkably consistent across all lodes. The overall paragenetic sequence is ordered as follows: laminated quartz, fibrous carbonate (siderite and ankerite), crystalline quartz (rhombic quartz), stibnite, opaline quartz and milky quartz. Acicular stibnite and botryoidal calcite are not generally associated with the main quartzstibnite vein structures and are therefore regarded as a post-mineralisation mineralogical occurrence most likely associated with meteoric events.

The Costerfield lodes are typically anastomosing, en échelon style, narrow-vein systems, dipping from 25° to 70° west to steeply east (70° to 90°). Mineralised shoots are observed to plunge to the north (when structurally controlled) and south (when bedding controlled).

The mineralisation occurs as single lodes and vein stockworks associated with brittle fault zones. These bedding and cleavage parallel faults that influence the lode structures, range from sharp breaks of less than 1 mm to dilated shears up 3 m wide that locally contain fault gouge, quartz, carbonate, and stibnite. Cross faults, such as those seen offsetting other Costerfield lodes, have been identified in both open pit and underground workings.

Mineralised lodes vary from massive stibnite with microscopic gold to quartz-stibnite, with minor visible gold, pyrite and arsenopyrite. Stibnite is clearly seen to replace quartz. Gold can also be hosted by quartz.

The Augusta Lodes occur within NNW-trending shear zones, which dip steeply to the west. They include E and W-Lodes, previously mined; N-Lode, currently being mined; and the smaller C-Lode. The E-Lode vein is approximately 0.4 m thick with a strike length of about 500 m. W-Lode averages approximately 0.4 m thick and has a strike length of approximately 230 m. [2019 AIF, p. 49]

The Cuffley Lode lies approximately 200 m to the west of E-Lode. The lode dips at about 85° to the east and occurs over a strike length of approximately 750 m, with a down-dip extent of approximately 250 m. It has an average true thickness of approximately 0.53 m. At present, the Cuffley Lode is open at depth. [2019 AIF, p. 49]

The Brunswick Lode lies approximately 600 m northwest of the northernmost point of the Cuffley Lode. The lode is sub-vertical and occurs over a strike length of approximately 450 m, with a down-dip extent of approximately 200 m and an average true thickness of approximately 1.28 m. Recent exploration drilling has identified mineralization to the south and at depth below the known Brunswick Lode. Mineralization is broken into two zones of mineralization below the Brunswick Lode, called the P-K domain and Brunswick Deeps. The P-K domain is capped by the shallow west-dipping Penguin Fault,and extends to the Kiwi Fault. The Brunswick Deeps zone is defined by the west-dipping Kiwi Fault and Adder Fault. Similar in nature to Cuffley and N-Lodes, the mineralization in the two domains is generally confined to sub-vertical quartz– stibnite veins. [2019 AIF, p. 49]

The Youle deposit extends below the historical Costerfield, Minerva and Bombay group of mines located approximately 1.2 km northeast of Brunswick. Mineralization was identified in 2011 in drill hole MB012, which struck the down-dip continuation of the vertical Kendall Lode, offset westward over the west-dipping No. 3 thrust fault. In 2016, drill hole BC006W1 revealed the existence of a high-grade north–northwest striking, west-dipping lode structure, Youle. The Youle Lode dips at a shallower angle to the mineralized lodes in Augusta and Cuffley and has been identified as the down-dip continuation of the vertical Kendall Lode offset westward over the west-dipping No.5 thrust fault. [2019 AIF, p. 50]

Youle has thus far demonstrated consistent structural and grade continuity over much of its extent. The Youle Lode has a strike length of 600 m (width of 150 m) and ranges in true thickness between 0.16 m and 1.37 m. Similar to the Augusta and Brunswick Lodes, mineralization is confined to quartz-stibnite veins. [2019 AIF, p. 50]

The Augusta Mine is serviced by a decline haulage system developed from a portal within a box-cut with the majority dimensions of 4.8 m high by 4.5 m wide at a gradient of 1:7 down. Most of the decline development was completed with a twin boom jumbo; however, development of the decline from the portal to 2 Level was completed with a road-header and this section of decline has dimensions of 4.0 m high by 4.0 m wide. The decline provides primary access for personnel, equipment and materials to the underground workings.

Mill feed is produced by three different mining methods: full face development, long-hole CRF stoping and half upper stoping. All mined material is transported to the Augusta box-cut before being hauled to either the Brunswick RoM pad or Augusta waste rock storage facility.

The Cuffley Decline currently extends down to approximately 895 mRL. At 935 mRL, the Cuffley Incline extends off the Cuffley Decline and accesses mineral resources from 945 mRL to 1050 mRL. This incline is used to extract N and NV lodes. Mining of Cuffley Lode on the Incline is complete. A second decline within Cuffley, known as the 4800 Decline, accesses the southern part of the Cuffley Lode, which is positioned south of the East Fault. This decline commences at 960 mRL and extends to 814 mRL.

The Brunswick access is 5.5 m high by 4.5 m wide, starts from 925 mRL on the Cuffley Decline and accesses the Brunswick deposit at 955 mRL. The Youle access is 5.5 m high by 5.5 m wide and extends from the Brunswick Incline at 961 mRL and accesses the Youle deposit at 957 mRL. From this level, the Youle decline, 4.8 m high and 4.5 m wide, is planned to extend down to 700 mRL to access the Youle ore body.

Mining methods
The dominant mining method is longitudinal long-hole stoping with cemented rock fill (CRF), panels generally consisting of three to four operating levels mined bottom-up over CRF with longitudinal retreat to a quasi-central access.

Method description
Mining within the Augusta Mine targets several individual lodes (including the W, NM, E, K and Cuffley Lodes), which vary in width from 0.1 m to 1.5 m and dip between 45° and 85°. This lode geometry is favourable for long-hole cemented rock fill (CRF) stoping using mechanized mining techniques.

Throughout Cuffley, the sub-level spacing of 10 m floor to floor (7 m backs to floor) has predominantly been established to ensure stable spans, acceptable drilling accuracies and blasthole lengths. A sublevel spacing of 15 m has been developed for two select areas. This involved drilling up from the lower level to 8 m and drilling and firing the remainder from the upper level using downholes.

The Brunswick orebody has applied a sub-level spacing of 12 m floor to floor (9 m backs to floor). This has been established due to better drill accuracy and the wider orebody (average diluted stope width of 2.0 m vs 1.5 m in Cuffley and Augusta).

Stoping within the Youle orebody has been designed with a sub-level spacing of 9 m floor to floor (6 m backs to floor vertically, 6–13 m backs to floor along the dip of the orebody). This reduced sub-level spacing has been designed to minimize dilution and improve recovery in the flatter dipping Youle stopes. The orebody dip varies greatly in Youle between 40° and 85°, which is dependent on the influence of the No. 4, No. 3 and Orb Weaver Faults. To optimize the extraction of ore where the dip is shallower than 45°, ore development and stope geometry will be adjusted to steepen the footwall of the stopes.

Brunswick Processing Plant
The Brunswick Processing Plant processes a sulphide gold–antimony ore through a conventional comminution and flotation-style concentrator. Since 2009, several processing plant upgrades have seen production increase to the current average of approximately 12,000 t/month over the 2015–2019 calendar years. The concentrator operates 24 hours per day, 7 days per week. Crushing operates under noise restriction guidelines during extended dayshift hours.

The surface crushing and screening system processes underground feed down to a particle size range suitable for milling through a two-stage, closed-circuit ball milling circuit. Centrifugal-style gravity concentrators are used on the combined primary milling product and secondary mill discharge to recover a gold-rich gravity concentrate. This is tabled and sold as a separate gold concentrate product that is sent to local refineries. Secondary milled products are classified based on size and processe ........