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ASX:RMX
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100% RMX

Thompson Falls Antimony

Red Mountain Mining (ASX: RMX) has acquired the Thompson Falls Antimony Project on the Montana-Idaho border, USA.

Lithic Lithium
AUS Projects
100% RMX

Thompson Falls
Antimony Project.

The project is located just 4.2km from United States Antimony Corporation’s (NYSE: UAMY) operations, which host the only operating antimony smelter in the United States. This strategic location in the “Antimony Heartland” positions RMX to leverage critical mineral supply chains outside of China as the US government moves to secure domestic supply.

Highlights

  • Strategic Proximity: Located 4.2km from the USA’s only operating antimony smelter and near the Stibnite Hill Mine, the second-largest stibnite vein deposit in the US.
  • Superb High-Grade Results: Initial first-pass assay results have returned outstanding grades of up to 36.5% Antimony (Sb) and 0.65g/t Gold (Au).
  • Historical Mines Identified: RMX has immediately identified three historical underground mines (including the Eastern Star) and an open pit within the project area.
  • Silver District Potential: The project lies at the eastern end of Idaho’s Coeur d’Alene mineral district, which has produced over 1.25 billion ounces of silver.
  • Federal Alignment: Both Antimony and Silver are federally designated Critical Minerals; the project is well-positioned to benefit from the new $12 billion US strategic minerals stockpile initiative.
  • Immediate Fieldwork: Geological mapping and sampling are already underway, with further results expected this quarter

Antimony Mineralisation at Thompson Falls

The Thompson Falls Project is located within a globally significant mineralized belt. Mineralisation in this region typically occurs as fault-controlled siderite-quartz-sulfide veins hosted in Middle Proterozoic metasedimentary rocks. These veins can be laterally and vertically extensive, extending to depths of up to 1.5km.

Within the project area, RMX has identified several key geological indicators:

  • Host Stratigraphy: The project encompasses the Upper Prichard Formation, the same host rock for mineralisation at the nearby Stibnite Hill mine.
  • Eastern Star Mine: Samples from the historical Eastern Star underground mine returned high antimony results of up to 36.5% Sb and 0.65g/t Au.
  • Massive Stibnite: Initial sampling has identified massive stibnite veining in iron oxide altered argillite, closely resembling the ore at the neighboring UAMY deposit.

Strategic Advantage: The UAMY Connection

The project’s development pathway is fundamentally de-risked by its proximity to the Thompson Falls Smelter and the Stibnite Hill Mine. UAMY (Market Cap ~A$2.2b) restarted mining at Stibnite Hill in late 2025 using modern “cut and cover” surface mining to address severe US supply shortages. With total historical production at Stibnite Hill estimated at 15.4kt of Antimony, RMX is leveraging its 6km proximity to this active operation to accelerate its own exploration of similar high-grade veins.