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Expansion of East Boulder Mine proposed; comments being taken

Expansion of East Boulder Mine proposed; comments being taken

East Boulder Mine

Owners of the East Boulder Mine are seeking approval from the state and Custer Gallatin National Forest to expand the mine's “disturbance area” by 48 acres, which would allow the mine to operate an additional five years.

The request from Sibanye Stillwater Mining Company for its platinum and palladium operation is spelled out in a draft environmental assessment, which the Montana Department of Environmental Quality and forest are taking comments on through June 15.

No expansion

This illustration shows the proposed mine site "no action" alternative.

The proposal also would raise the height of the tailings storage impoundment 14 feet above what was previously authorized; improve and relocate two forest roads; allow construction of a new power line; allow logging to clear the routes for the new roads and power line; and relocate some existing mine facilities including soil stockpiles, underdrains and a guard house. All of the work would be done within the nearly 400-acre mine mill site’s footprint and take seven years to complete.

The road work would require Lewis Gulch Road to be closed to all recreation and outfitter traffic for about six weeks

The mine is located in Sweet Grass County about 23 miles south of Big Timber at the base of the Beartooth Mountains and close to the headwaters of the East Boulder River, a tributary to the Yellowstone River. Treated water from the mine is released into the river, according to the EA, while mine water with high levels of nitrogen is spread on the nearby Boe Ranch.

Mine site expansion

This illustration shows the proposed mine site expansion as proposed.

The 200-page Draft Environmental Assessment, along with Sibanye Stillwater Mining Company application and supporting project documents are available online at: or

At the East Boulder Mine, which has been in operation since 1999, the ore hauled from tunnels under the Beartooth Mountains is crushed, grinded, and filtered to produce an ore concentrate. The concentrate is then filtered and transported to the smelter and refinery in Columbus. For every 100 tons of ore fed to the mill, the mine generates 99 tons of tailings.

“Nitrogen and ammonia residues resulting from ammonium nitrate-based explosives used in underground mining are the primary wastewater contaminants requiring treatment at the site,” according to the EA. “The site is a zero-discharge storm water facility so that all storm water runoff drains internally within the site into collection basins and vegetative buffers, and other BMPs are used to manage storm water runoff.”

The DEQ and the Forest Service hold a joint reclamation bond to ensure renovation of the East Boulder Mine. The bond is based on the costs to cover reclamation activities, monitoring, and maintenance. “It is incrementally increased when the activities that result from revisions or amendments to the Operating Permit indicate increased reclamation expenses would be incurred as a result of the activity,” according to the EA.

The mine is a huge economic driver for Sweet Grass County, contributing more than $1.86 million in property taxes, or about 26% of all county government revenue. As of 2017 the mine employed about 400 people, or about 14% of total employment in the county. Approval of the proposal would allow the mine to extend its operation from the current closing date of 2027 to 2033.

Public comment is encouraged electronically at: or by navigating to, click on Land and Resource Management (left-hand side), then projects and comment on the project. Hard copy is also accepted by mailing: Custer Gallatin National Forest, Attn: Robert Grosvenor, 805 Scott St., Gardiner, MT 59030.

In keeping with precautionary measures related to the COVID-19 pandemic, the office is not open at this time for hand-delivered submissions.


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