How Did American Tungsten Overestimate Antimony Grades at Antimony Canyon?

Background to Visual Estimates

American Tungsten and Antimony Ltd (ASX:AT4) recently responded to an ASX query concerning the accuracy of its visual estimates of stibnite mineralisation from diamond drill holes at the Antimony Canyon Project in Western Australia. The initial January announcements highlighted substantial zones of visible massive to semi-massive stibnite, with visual estimates suggesting antimony grades as high as 12% in some intervals.

The visual estimation process involved on-site geologists logging drill core, estimating sulphide content by volume, and comparing mineral abundance to reference charts. These estimates were reviewed and signed off by the Competent Person, Mr Jonathan King, a qualified geologist with relevant expertise.

Mineralogical Challenges and Discrepancies

However, subsequent assay results released in March revealed a significant discrepancy: the antimony grades in key drill holes ACP26DD001 and ACP26DD003 were below 0.1%, far lower than the visual estimates. The company explained this was due to the presence of fine-grained, amorphous pyrite that visually mimics stibnite in drill core, making reliable discrimination difficult under standard logging conditions.

This mineralogical complexity is a feature of the Antimony Canyon Project’s hydrothermal system, where sulphide assemblages are fine-grained and texturally overprinted, unlike more conventional stibnite deposits with distinctive crystalline forms. While the presence of stibnite in the system has been confirmed by later drilling, the initial visual estimates overestimated its abundance.

Compliance and Remedial Actions

American Tungsten confirmed that at the time of the initial announcements, the visual estimates complied with ASX Listing Rules, the JORC Code, and ASX guidance, with appropriate disclosures about the preliminary nature of the data and pending assays. The company acknowledged that a stronger caveat regarding mineralogical uncertainty would have improved transparency.

In response to the issue, the company has implemented revised core logging protocols requiring detailed documentation of mineral identification, acknowledgment of uncertainty, and mandatory peer review before releasing visual estimates. It also commits to not reporting visual grade estimates without supporting assay confirmation in this geological setting.

Pending Assay Results and Market Impact

Assay results for drill hole ACP26DD002 were not included in the March assay announcement and remain pending. The company has committed to disclosing these results promptly upon receipt. The initial visual estimates had driven a 25% share price increase in mid-January, followed by a 32% decline after the assay results were released, reflecting market sensitivity to the mineralisation data.

American Tungsten’s board has authorised the detailed response to ASX Compliance, affirming the company’s commitment to continuous disclosure and regulatory compliance.