Summary:
The Costerfield Property hosts narrow, anastomosing, en echelon mineralised vein systems developed along a north–south corridor encompassing the Costerfield, Brunswick, and Augusta zones, extending over at least 4 km of strike. True Blue sits within a second parallel line of mineralisation 2 km to the west.
Narrow vein, antimony-gold and gold-only lodes are the targeted deposit styles at the Costerfield Property. Economic lode material consists of either a ‘typical’ gold-bearing quartz and carbonate with massive stibnite (for example, the Augusta C, D, and E Lodes, N Lode, Cuffley and Youle), or gold-only quartz and carbonate veining as seen in the Shepherd system.
The mineralised shoots are understood to be structurally controlled, typically by the intersection of the lodes with major cross-cutting, gouge filled fault structures and shears. Notable west to northwest dipping thrust faults typically bound the mineralisation packages at the Costerfield Property but can become significantly mineralised themselves along the fault planes. Shallower and dominantly west dipping thrust faults, typically at very low angles or even parallel to bedding with a laminated quartz component, link between the larger order thrust faults. The link faults can also offset the vertical lode structures up to 50 m in an east–west sense. This structural framework leads to the subvertical, north–south extensional veining seen in the Augusta, Brunswick, Kendall and Shepherd systems, along with the moderately west-dipping fault reactivated deposit at Youle.
There are two main types of mineralised lodes found on the Costerfield Property. Typically they consist of stibnite dominant lodes and gold only lodes. The stibnite dominant lodes vary from massive stibnite with microscopic gold to quartz-stibnite, with minor visible gold, pyrite, and arsenopyrite. The stibnite is clearly seen to replace quartz, and gold can also be hosted by quartz. Costerfield’s gold-only veins typically consist of single-generation quartz-carbonate matrix hosting free gold, usually sub-millimetre grains but rarely up to 3–4 mm across with minor pyrite and arsenopyrite.
The best grades in gold-only veins often are associated with antimonybearing sulfosalts such as tetrahedrite, bournonite and boulangerite. Depth of emplacement appears to be the major control on the abundance of stibnite; the gold-only vein systems are generally found below the level of or at the base of stibnite-bearing lodes. Systems considered ‘gold only’ include the bulk of the Shepherd veins (the southwestern portion of Shepherd contains some significant stibnite), and the Sub-King Cobra Fault West veins.
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 predominantly 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.
Mineralisation
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 380–370 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. Significant portions of the local area are obscured by alluvium and colluvium deposits, which have been washed over the surrounding flood plains by braided streams flowing east off the uplifted Heathcote Fault Zone. Some of this alluvial material has been worked for gold but workings are small-scale and limited in extent. Most of the previously mined hard rock deposits were found either outcropping or discovered by trenching within a few metres of the surface.
The mineralised structures in the Costerfield Property, which typically dip steeply east or west (Augusta, Brunswick, Kendall, Shepherd), or moderately west (Youle) are likely to be related to the formation of the Costerfield Dome and the subsequent development of the Moormbool Fault. The main reef system(s) appear to be developed in proximity to the axial planar region of the Costerfield Dome or hosted in reactivated west-dipping thrust faults.
The economic mineralisation at the Costerfield Property occurs in a north–south corridor that includes the Costerfield, Brunswick and Augusta zones. The moderately west to steeply-dipping quartz-stibnite-gold veins have thicknesses ranging from several millimetres to 1 metre, and extend over a strike of at least four kilometres. The vein systems are centred in the core of the doublyplunging Costerfield Anticline and are hosted by unmetamorphosed (anchizone) Costerfield siltstones. Individual veins can persist for up to 800 m along-strike and 300 m down-dip.
The mineralogy of the vein contents and mineral proportions differ from vein to vein throughout the Augusta, Cuffley, Brunswick and Youle lodes. However, the texture and chronological order of each vein/mineral generation remains remarkably consistent across all lodes.
The Costerfield Property lodes are typically anastomosing, en echelon style, narrow-vein systems, which dip from 25–70° west to 70–90° east. 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.
Augusta: N lode, W lode, E lode, Cuffley lodes, SKC lodes
Modern mining operations commenced on the Central Corridor at Augusta in 2006, and includes the major lodes N lode, W lode, E lode, and Cuffley lode which have been partly depleted through mining operations.
Other semi-major lodes include K lode, C lode, Alison and Cuffley Deeps. The SKC lodes (410/420) are significant Inferred lodes sitting directly below the other major lodes in the footwall of the King Cobra Fault.
Costerfield: Youle, Shepherd, Kendal
The Costerfield historical mine was the largest pre-2006 mine in the district. Modern mining below the historical mine includes the major lodes Youle, Shepherd East, and Shepherd West, which have been partly depleted through mining operations and remain the dominant work area. Multiple minor veins and splays occur in the Shepherd area, providing local zones of upside.
Brunswick: Brunswick Main, KR-Brunswick, Brunswick South
Partially west of the Augusta-Costerfield line, the Brunswick shear is >2 km in strike, a sub-vertical stibnite lode in the same Central Corridor host rocks. Brunswick was the focus of mining through 2018 and 2019 and remains a significant resource in the MRE. The Brunswick South lode (320) constitutes a major exploration focus area along strike of Brunswick that contributed to the MRE Inferred inventory growth and has significant results outside of the MRE.
True Blue
The geometry of True Blue is similar to that of the Youle Deposit, consisting of sub-vertical to west-dipping veins hosted by the western limb of an anticline. Like Youle and Shepherd, structural continuity of the True Blue mineralised veins is remarkably consistent over the drill-tested area. This consistency is attributed to being hosted in the same Costerfield Siltstone lithology as the main mining areas of the Central Corridor.
Commodity Production
Mandalay Resources’ FY2024 runs from January 1, 2024 to December 31, 2024, and Alkane Resources’ FY2025 runs from July 1, 2024 to June 30, 2025.
FY2024: Presented using Mandalay Resources’ data for the period January 1, 2024 to December 31, 2024.
FY2025: Presented using Alkane Resources’ data for the half-year period from January 1, 2025 to June 30, 2025.
| Commodity | Product | Units | 2025 | 2024 | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2017 | 2016 | 2015 |
|
Gold
|
Payable metal
|
koz
| ....  | .... | .... | .... | .... | .... | 15 | 22 | 32 | 41 | 42 |
|
Antimony
|
Payable metal
|
t
| .... | .... | .... | .... | .... | .... | 2,032 | 2,173 | 3,115 | 3,597 | 3,712 |
|
Gold Equivalent
|
Payable metal
|
koz
| .... | .... | .... | .... | .... | .... | 25 | 36 | 52 | 60 | 66 |
|
Gold
|
Concentrate
|
kt
| | | | | | | 7.5 | 6.5 | 9.1 | 10 | 11 |