The Costerfield Property lies within a broad gold-antimony province mainly confined to the Siluro-Devonian Melbourne Zone of Victoria. The narrow quartz-stibnite-gold veins of the Melbourne Zone are mesothermal to orogenic in nature and are a product of a 380 Ma to 370 Ma tectonic event. Gold in Central Victoria is believed to have been derived from the Cambrian greenstones that underlie the entire province at depth, however the origin of the associated antimony has been less studied.

The mineralisation at Costerfield occurs as narrow veins or lodes, typically less than 50 cm wide and hosted within unmetamorphosed (anchizone) mudstone and siltstone of the Lower Silurian Costerfield Formation.

Typical lode material at Costerfield consists of gold-bearing quartz and carbonate with massive stibnite either as the latest stage vein fill or segregated to one contact of the vein. The average grade of this material is approximately 9 g/t Au (often greater than 20 g/t) and 4% antimony (Fromhold et. al. 2017). A variety of accessory minerals are associated with the mineralisation, including pyrite, arsenopyrite, aurostibite, pyrrhotite, muscovite, sphalerite and galena within the vein. Wallrock alteration minerals are dominantly pyrite, arsenopyrite and ferroan carbonate spotting, surrounded by a broader, visually cryptic halo of muscovite replacing phengite. Small crystals of barite and bournonite are often seen in chlorite-coated joints near the lodes.

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

The Brunswick Incline development was mined to breakthrough into the Brunswick Open pit, establishing the Brunswick Portal during the second half of 2020. The Brunswick Incline has the dimensions 4.8 m high by 4.5m wide at a gradient of 1:7 up and was mined with a twin-boom jumbo. The Brunswick Open Pit was prepared for the portal breakthrough with a pushback completed by a combination of road-header and drill and blast supported by a twin-boom jumbo. The first 20 m advance of Brunswick Portal was completed by a road-header with the dimensions 5.0 m high by 5.0 m wide at a gradient of 1:25 up. The establishment of the Brunswick Portal provides an additional means of egress from the mine and is the primary material haulage route from underground to the Brunswick Mill for ore processing and waste storage.

Mill feed is produced from three different mining methods: full-face jumbo development, long-hole CRF stopi ........

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