Alkane Resources Replaces 61,000oz Mined at Costerfield With Net Reserve Gain
Key Takeaways
- Alkane Resources achieved net reserve replacement at Costerfield, adding 1,866 oz Au and 3,855 t Sb to reserves after absorbing 61,037 oz Au in depletion and sterilisation — a direct measure of operational health for a producing miner.
- The 2025–2026 drilling program totalled 136,451 m, the largest in the operation's history, and added 213,000 oz AuEq in Measured and Indicated resources from newly defined deposits including Brunswick South and expanded True Blue.
- Total Proven and Probable Mineral Reserves stand at 960,000 t at 5.5 g/t Au and 1.6% Sb, containing 170,000 oz Au and 15,000 t Sb, independently verified by three SRK and Core Resources Qualified Persons who each conducted site inspections.
- The resource cut-off grade was reduced from 4.3 g/t AuEq to 3.0 g/t AuEq, reflecting higher prevailing metal prices and expanding the economic envelope of the deposit.
- Antimony reserve pricing of US$22,000/t sits marginally above mid-2026 spot levels following a sharp price decline from record highs above US$60,000/t in mid-2025, with ongoing demand from defence and flame-retardant applications underpinning the medium-term outlook.
Alkane Resources locks in resource and reserve update at Costerfield gold-antimony operation
Alkane Resources (ASX: ALK) filed an independent NI 43-101 Technical Report on 11 September 2026 for its wholly owned Costerfield Operation in Central Victoria, approximately 10 km northeast of Heathcote. The report, prepared by SRK Consulting (Australasia) Pty Ltd with an effective date of 30 June 2026, delivers updated Mineral Resource and Reserve estimates that demonstrate successful reserve replacement at the producing gold-antimony operation.
The Costerfield Operation is an underground mine with primary production from the Youle and Shepherd lodes, supported by a conventional flotation processing plant (the Brunswick Processing Plant) with a current capacity of approximately 150,000 tpa. Alkane acquired the operation via merger with Mandalay Resources Corporation on 5 August 2025.
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Resource and reserve snapshot as at 30 June 2026
The headline Measured and Indicated Mineral Resource stands at 1,993,000 t at 6.0 g/t Au and 2.1% Sb, containing 387,000 oz Au and 42,000 t Sb. An additional Inferred Mineral Resource of 969,000 t at 3.6 g/t Au and 1.3% Sb contains a further 111,000 oz Au.
The total Proven and Probable Mineral Reserve is 960,000 t at 5.5 g/t Au and 1.6% Sb, for 170,000 oz Au and 15,000 t Sb. The reserve represents a net increase of 1,866 oz Au relative to 2024 after accounting for 61,037 oz Au in depletion and sterilisation — reflecting successful reserve replacement. Antimony reserves recorded a net increase of 3,855 t Sb.
| Category | Tonnes (kt) | Au Grade (g/t) | Sb Grade (%) | Contained Au (koz) |
|---|---|---|---|---|
| Measured (Underground) | 423 | 10.0 | 3.2 | 136 |
| Measured (Stockpile) | 49 | 3.7 | 0.6 | 6 |
| Indicated | 1,521 | 5.0 | 1.8 | 245 |
| Measured + Indicated | 1,993 | 6.0 | 2.1 | 387 |
| Inferred | 969 | 3.6 | 1.3 | 111 |
| Proven Total | 270 | 6.7 | 1.5 | 58 |
| Probable Underground | 690 | 5.1 | 1.6 | 112 |
| Total Proven and Probable | 960 | 5.5 | 1.6 | 170 |
Key pricing assumptions underpinning the estimates:
- Resource: US$4,000/oz Au, US$26,400/t Sb; metal recoveries of 90% Au and 89% Sb
- Reserve: US$3,100/oz Au, US$22,000/t Sb, US$:A$ exchange rate of 0.71
- Resource reported at a cut-off of 3.0 g/t AuEq (reduced from 4.3 g/t at the 2024 estimate, reflecting higher metal prices); Reserve at a sustaining cut-off of 4.4 g/t AuEq
Record drilling program drives exploration success
The scale of the 2025–2026 exploration effort was exceptional. Between 1 January 2025 and 30 June 2026, Alkane drilled 136,451 m of exploration diamond — by far the largest program in the operation’s history, as recorded in Table 10.1 of the Technical Report.
Primary focus areas for the period included:
- Brunswick South (continuous infill and extensional drilling)
- True Blue (Freeman Lode infill and depth extension)
- Kendal (strong infill results, improved vein continuity confidence)
- Sub King Cobra (resource growth testing)
- Youle, Shepherd, Augusta, Cuffley and Alison (production support and infill)
The effort delivered material resource growth. Exploration drilling added 213,000 oz AuEq in Measured and Indicated, with key contributions from Kendal, Brunswick/Brunswick South, Augusta/Cuffley and True Blue. Brunswick South is a newly defined deposit incorporated into the 2026 estimate for the first time; True Blue was discovered in 2024 and expanded during this period.
Understanding NI 43-101 — why this report matters to investors
NI 43-101 is a Canadian securities standard requiring independent Qualified Persons (QPs) to verify mineral resource and reserve estimates. It is one of the most rigorous global disclosure frameworks for mineral projects.
A QP is a qualified mining or geoscience professional whose credentials and independence are defined under the instrument. Three independent QPs signed off this report: Cael Gniel of SRK Consulting (resources), Robert Urie of SRK Consulting (reserves and mining), and Carla Kaboth of Core Resources (processing). Each conducted a personal site inspection prior to sign-off.
The distinction between the two core numbers matters to you as an investor:
- Mineral Resources represent what is in the ground with sufficient geological confidence to be classified as Measured, Indicated, or Inferred.
- Mineral Reserves represent what is economically mineable after applying real-world mining costs, recoveries, and dilution.
Independent QP sign-off reduces investor risk around the credibility of these estimates. Reserve replacement — replacing mined ounces with newly delineated ones — is a key measure of operational health for a producing miner. The fact that Costerfield achieved a net increase in both gold and antimony reserves, after absorbing more than 61,000 oz Au of depletion, tells you the drilling program is keeping pace with production. The report was prepared under CIM Definition Standards (2014), the globally recognised benchmark for resource and reserve reporting.
Metallurgical performance and mine life framework
The Brunswick Processing Plant has operated since 2007, with demonstrated throughput of 10,000–13,000 t/month over recent years. Average gold recovery from non-Brunswick ores (predominantly Youle and Shepherd) was 93.5% using data from January 2022 to February 2026. Reconciled plant recoveries for Q2 FY2026 (October to December 2025) were 93.94% Au and 86.77% Sb, with the lower antimony recovery reflecting a lower antimony head grade rather than any plant underperformance.
For FY2026/2027, the feed blend is expected to include approximately 83,000 dmt of Youle ore and approximately 42,000 dmt of Augusta/Cuffley remnant ore, with the remainder from Brunswick, Kendal, Alison and Brunswick South.
Over the longer-term life of mine, the planned blend shifts toward: 27% Augusta/Cuffley, 22% Youle, 19% Kendal, 19% Brunswick South, 6% Brunswick and 6% Alison. Several recent plant upgrades are expected to support recovery as the feed blend evolves, including an additional centrifugal gravity separator, StackCell flexibility for variable antimony head grades, and a revised concentrate grade target of 40% Sb — down from the prior 51.5% target — which allows additional antimony and associated gold to be recovered to concentrate.
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Antimony’s strategic role in the Costerfield investment case
Antimony is classified as a critical mineral by both Australia and the United States, a designation that reflects its importance to strategic and defence-related applications.
Price volatility has been significant. The Fastmarkets Rotterdam antimony price reached record levels exceeding US$60,000/t in mid-2025, driven in part by China’s introduction of export restrictions in 2024. By mid-2026, a partial easing of those restrictions, combined with increased supply availability and seasonal market factors, had contributed to prices declining to approximately US$20,000/t. The reserve antimony price of US$22,000/t is therefore marginally above the mid-2026 spot price, reflecting prudent economic modelling rather than aggressive assumptions.
Ongoing demand support from strategic and defence procurement, as well as robust consumption in the wire and cable sector where antimony is used in flame-retardant PVC coatings, underpins the medium-term price outlook. China remains the primary destination for antimony-gold concentrate processing, and Costerfield’s established offtake structure provides direct access to that market.
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