West Cobar Metals (ASX:WC1) recalibrates antimony grades at Bulla Park as assay method shifts
West Cobar Metals has moved to reset the analytical baseline at its Bulla Park copper antimony silver deposit in central New South Wales, reporting that revised laboratory methods indicate antimony grades were previously under reported.
The update centres on re analysis of mineralised drill core using peroxide fusion digest rather than four acid digest, with the company stating the new approach delivers more reliable antimony results and will be incorporated into future resource work.
The change affects how investors should read both grade distribution and contained metal across the existing resource and exploration target footprint.
Highlights
- Bulla Park hosts an Inferred Mineral Resource of 20 Mt at 0.58% CuEq at a 0.21% Cu cut off, including 60,000 t copper, 20,000 t antimony and 3 Moz silver
- Re analysis using peroxide fusion digest shows higher antimony grades than four acid digest, with average antimony increases of 6% across re tested mineralised intersections
- Earlier peroxide fusion testing across selected samples showed antimony concentrations averaging 14% higher than four acid digest results
- Mineralisation remains open along strike and down dip, with an Exploration Target of 30 Mt to 50 Mt at 0.23% to 0.33% Cu, 0.08% to 0.12% Sb and 4 g/t to 6 g/t Ag in addition to the current resource
- Further re analysis of remaining mineralised samples is underway with results expected in early Q2 2026
The technical driver behind the revision is assay chemistry rather than new drilling.
Four acid digest, while standard across many base metals, can under report certain elements including antimony due to volatilisation during digestion.
West Cobar re tested pulps and quartered core from diamond holes BPD08 and BPD09 using peroxide fusion digest and reported an overall 6% increase in antimony grades across mineralised intervals, with individual horizons showing differences of up to 16.6%.
Copper grades above the 0.21% Cu cut off showed very similar results between methods, with the company describing acid digest as satisfactory for copper.
West Cobar Managing Director Matt Szwedzicki framed the work as a validation exercise tied to commodity relevance as much as laboratory precision, stating
“The underreporting is seen across a range of antimony values in the two drill holes and indicates that we have more antimony in the system than previously thought.”
He added that remaining mineralised intervals are being analysed with the same method and flagged the possibility of increased contained antimony influencing project economics.
From a resource standpoint, Bulla Park sits west of the Cobar mining hub and spans four granted exploration licences covering 518 km2.
The current Mineral Resource estimate is based on a limited portion of a broader gravity anomaly and constrained by drilling density to date.
The company states that the thick zone of relatively shallow mineralisation, greater than 60 m in places, may allow bulk mining methods, potentially by open pit, although this remains conceptual at the current study level.
Metallurgical testwork reported alongside the assay update outlines recoveries of 94.6% Cu, 82.6% Sb and 84.1% Ag using flotation and selective leaching, producing a copper silver concentrate and an antimony sulphide product graded at 37% Sb.
Testwork is described as preliminary, with further optimisation expected.
The next phase is methodical rather than dramatic, with remaining historical core being quartered for peroxide digest antimony re analysis and those values scheduled to feed into the next resource upgrade.
In a market where antimony has been elevated into critical minerals policy settings in both the USA and Australia, West Cobar Metals is positioning Bulla Park within that framework, but the immediate catalyst remains technical, namely how revised assays translate into updated resource metrics.