Red Mountain racks up high-grade antimony hits in the US at Thompson Falls

Special Report: Red Mountain Mining has delivered a series of high-grade antimony and gold assays from sampling at the historical Eastern Star mine located within its Thompson Falls antimony project along the Montana-Idaho border.

Results included 17% antimony and 1.12g/t gold, 16.4% antimony and 0.17g/t gold, and 15.4% antimony and 0.32g/t gold.

Management noted that all samples returned elevated grades reaching as high as 36.5% antimony and 1.12g/t gold.

Overall, they averaged 8.7% antimony and 0.37g/t gold.

A stibnite sample also returned 47.3% antimony through a spot pXRF analysis, with visual logging estimating 45% stibnite across the full rock sample.

Red Mountain Mining (ASX:RMX) only acquired Thompson Falls last month.

The project lies near America’s only operating antimony smelter, owned by United States Antimony Corporation (NYSE:UAMY).

It also sits within the same host stratigraphy as UAMY’s Stibnite Hill mine – the second largest known stibnite vein deposit in the US.

UAMY restarted operations in late 2025 in response to antimony supply shortages in the US, with its smelter playing a key role in the country’s critical minerals supply chain.

High-grade stibnite mineralisation sample 733655. Pic: RMX

Thompson Falls overview

Thompson Falls hosts three historical underground mines.

Eastern Star is recorded as a past producer of silver and lead, with the other two mines producing a combination of antimony, silver, copper, zinc and lead.

Management noted that most of the samples collected at Eastern Star resemble the quartz-stibnite veins mined at UAMY’s Stibnite Hill, located about 7km away.

In addition, only one sample from the Eastern Star tailings showed silver mineralisation, and most samples detected minimal lead mineralisation.

According to the company, these outcomes suggest that the old timers extracted silver and lead but left behind the stibnite-rich material.

In turn, the project is interpreted to be prospective for both silver and antimony mineralisation.

The bigger picture

Management believes that Thompson Falls has the potential to host silver-rich polymetallic vein mineralisation akin to the Coeur d’Alene mineral district immediately west of Red Mountain’s claims.

Located in Idaho, this province accounts for about 18% of total accumulated US silver production. It has also produced significant amounts of lead, zinc, gold, copper and antimony.

Thompson Falls sits within a geological structure known as the Upper Prichard Formation which is known to host mineralisation at Coeur d’Alene and at UAMY’s nearby Stibnite Hill mine.

All up, Coeur d’Alene has produced 1.25 billion ounces of silver, 7.8Mt of lead, 3.0Mt of zinc, 1.1 million ounces of gold and 191,000 tonnes of copper between 1884 and 2020.

 Watch: RMX kicks off antimony testing

Looking ahead

Red Mountain is now eyeing additional sampling and reconnaissance work designed to locate undocumented historical mines or further mineralised exposures across the project area.

Further sampling of existing underground mines could also be on the cards as the company seeks to firm up its geological understanding of the mineralised system.

These works could influence subsequent stages of exploration – including anticipated drilling – as Red Mountain evaluates the resource potential of Thompson Falls.