Red Mountain Mining picks up antimony project next door to only US operating antimony smelter

Special Report: Red Mountain Mining has acquired the high-grade Thompson Falls antimony project, which is adjacent to America’s only smelter of the critical mineral owned by United States Antimony Corporation (NYSE:UAMY).

Thompson Falls is on the Montana-Idaho border, within the same host stratigraphy and near UAMY’s Stibnite Hill Mine, the second largest known stibnite vein deposit in the US, where high-grade antimony has been mined since 1884.

UAMY restarted operations in late 2025 in response to the severe supply shortage in the US and its smelter plays a key role in the country’s critical minerals supply chain.

The project hosts three historical underground mines and a pit within the project area, plus initial assays have returned high-grade results including:

• 36.5% antimony and 0.48g/t gold;

• 21.0% Sb and 0.65g/t Au; and

• 13.7% Sb and 0.14g/t Au.

The project is also highly prospective for silver as it lies at the eastern end of Idaho’s Coeur d’Alene mineral district.

This region has accounted for ~18% of the USA’s total accumulated silver production – more than 1.25 billion ounces between 1884 and 2020, along with 7.8Mt lead, 3Mt zinc, 1.1Moz Au, 191,000t copper and over 160,000t Sb.

Red Mountain Mining (ASX:RMX) said the acquisition further strengthened its Utah, Idaho and NSW critical minerals portfolio, creating a unique Western asset base positioned to benefit from unprecedented US and Australian government support as both nations seek to secure supply.

Red Mountain’s Thompson Falls antimony project showing the proximity to United States Antimony Corporation’s Thompson Falls smelter and Stibnite Hill mine. Pic: RMX.

Watch: Rock chips return strong antimony

Fifth antimony project in portfolio

Just this week, the US Government launched a $12 billion strategic minerals stockpile initiative, aimed at securing critical mineral supply chains, and the new project is well positioned to potentially supply antimony and silver.

And while most assays to date returned those minerals, the company says Thompson Falls has potential to also host the silver-rich polymetallic vein mineralisation typical of the rich Coeur d’Alene mineral district.

Additional assays are expected this quarter, after which the company plans to undertake further reconnaissance exploration and sampling to locate any additional undocumented historical mine workings and potential mineralised exposures.

Red Mountain’s US team also plans to further inspect and sample underground mines to better understand the nature of mineralisation at these prospects, prior to assessing and finalising plans for drill targets.

In addition to Thompson Falls, Red Mountain holds three additional high-quality antimony projects in the USA – the Utah antimony project in southern Utah and the Yellow Pine and Silver Dollar projects in central Idaho.

It also has the Armidale antimony-gold project in northeast NSW, Australia, where initial drilling of the high-grade Oaky Creek antimony target is expected in the first half of 2026.