Buffalo Trail and Montana Street Examples of Slag Boulders

Untitled Note the characteristics of these rocks – along with dark coloration (due to metal oxides), the lack of crystals is clear, and the ropy texture of some of the rocks. These important differences separate slag from natural igneous rocks. The ropy textures formed as the slag cooled after melting in the smelter – much like natural lava flows.

Near the intersection of the Buffalo Trail and Montana Street is rubble of true slag from Butte smelting activity. Oxidized ore veins have been confused with slag because of their similarity in color. Blocks of slag have been used for paving stones and some walls in Butte buildings.

Did you know?

Oxidized ore veins

Most ore-bearing veins in Butte had sulfur-bearing minerals, such as pyrite (iron-rich) and other minerals. manganese-bearing rhodochrosite mineral. As the iron and sulfur in these minerals are altered near the land surface (were weathered by oxidation), they produce sulfuric acid which promoted further weathering which resulted in cementation of the veins by iron and manganese. That weathering cementation by oxides of iron and manganese, producing red-brown iron oxides colors and black manganese oxides.

The characteristic red-brown to black coloration indicates they once contained ore minerals, but are now oxidized due to near-ground surface weathering (typically within ~200 ft of the land surface). The cementation of these rocks by the oxides produced hard rocks that were used in building foundations and rock walls around Butte, but especially near the Travona mine.

Slag

During the high-temperature extraction by melting of metals from ore, waste materials typically form a froth as they separated from copper and other metals of interest. This liquid is transported away from the smelter where it cools to form blackish dense material that commonly shows textures that formed as the flowing liquid cooled – much like lava flows.

Images

Untitled Note the characteristics of these rocks – along with dark coloration (due to metal oxides), the lack of crystals is clear, and the ropy texture of some of the rocks. These important differences separate slag from natural igneous rocks. The ropy textures formed as the slag cooled after melting in the smelter – much like natural lava flows.

Location

Location: Latitude 46.02067 Longitude -112.53988

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Metadata

Citation Info

Larry Smith, “Buffalo Trail and Montana Street Examples of Slag Boulders,” Story of Butte, accessed October 2, 2026, https://storyofbutte.org/items/show/3453.

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Environment