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The underground Stillwater mine near Nye, Montana, in the United States, is located approximately 85 miles southwest of Billings, Montana and is accessed by a paved road.

Stillwater accesses, extracts and processes PGM ores from the eastern portion of the J-M Reef using mine openings located in the Stillwater River Valley. All surface structures and tailings management facilities are located within the 2,414 acre operating permit area which is administered collectively by the Montana Department of Environmental Quality and the United States Forest Service.

Following the successful conclusion of the acquisition of Stillwater Mining Company in May 2017, the Stillwater mine is wholly owned by Sibanye-Stillwater.

Operations in Montana - USA

The ore reserves developed at the Stillwater mine are controlled by patented and unpatented mining claims either leased or owned outright. The mine is serviced by adequate water and power supplies from established sources to conduct its current operations.

As at 31 December 2016, Stillwater had ore reserves of 10.5Moz – 8.2Moz of palladium and 2.3Moz of platinum.

PGM-bearing ore is accessed via a 6.8 mile-long underground segment of the J-M Reef that has been developed on various levels between 1,700 and 7,500 feet above sea level. Access to the ore is by means of a 1,950 foot vertical shaft from the valley floor and by a system of horizontal adits and drifts driven parallel to the strike of the J-M Reef at vertical intervals of between 150 feet and 300 feet. There are eight main adits, driven from surface portals on the west and east slopes of the Stillwater Valley at various elevations between 5,000 and 5,900 feet above sea level.

Additional principal levels developed below the 5000 level are accessed from the vertical shaft and associated ramp systems. Extensive underground rail haulage has been installed on this level and is currently being installed on the 2000 level. Ore from below the 5000 level is hauled by truck and/or rail to the shaft, where it is crushed and then hoisted to surface. A decline system and additional principal mining levels are being developed to access the west and deeper areas in the central part of the mine. By the end of 2016, this decline system had extended through the 1900 level and nearly to the 1700 level.

The Stillwater mine’s headgear is situated in the Stillwater Valley, adjacent to the concentrator, at 5,000 feet above sea level. Ore and any waste rock transported to surface from the off-shaft and deeper areas of the mine are crushed prior to being hoisted up the shaft. The production shaft and underground crushing station reduce haulage times and costs, facilitate the handling of ore and waste, and improve grinding capabilities of the concentrator. Ore produced from the west side of the mine is hauled to surface by rail and crushed on surface prior to being fed into the concentrator. Waste material not used for backfilling in underground excavations is transported to surface and placed in permitted waste rock disposal sites.

Two mechanised mining methods are in use: ramp-and-fill and sub-level stoping. Ramp-and-fill mining involves a series of horizontal cuts being extracted from the ore body using mobile equipment. Access to the ore body is from ramps driven within or adjacent to the ore body, allowing the use of hydraulic drills and load-haul-dump equipment. In sub-level stoping, blocks of reef, approximately 30 feet high and of variable length are extracted using mobile long-hole drills and remote-control rubber-tyred load-haul-dump equipment. The reef is mined in a retreat sequence and mined out areas are filled with development waste or sand backfill as appropriate.

Ore mined at Stillwater is first treated at a surface concentrator facility (mill) adjacent to the shaft. The mill has a design capacity of 3,000 tons per day. Crushed mine ore is fed into the concentrator, mixed with water and ground to slurry in the concentrator’s mill circuits to liberate the PGM-bearing sulphide minerals from the rock matrix. Various reagents are added to the slurry, which then is agitated in a froth flotation circuit to separate the valuable sulphides from the waste rock. In this circuit, the sulphide minerals are successively floated, recycled, reground and refloated to produce a concentrate suitable for further processing. The final flotation concentrate, which represents approximately 2.0% of the original ore weight, is filtered, placed in large bins and then transported by truck, approximately 43 miles, to Stillwater’s metallurgical complex in Columbus, Montana.

In 2016, approximately 60% of the tailings material from the mill was returned to the mine and used as underground fill material to provide support for additional mining activities. The balance was placed in permitted tailings containment areas on surface. Tailings are disposed of in line with operating permits. In 1998, Stillwater received an amendment to its operating permit that provided for the construction of a lined surface tailings facility that would serve the mine until around 2035. This facility, located approximately eight miles from the mine and generally referred to as the Hertzler impoundment or tailings facility, became operational in late 2000. During 2015, construction of the third and final permitted expansion of this facility was completed. Given current operating parameters, this will extend the Hertzler tailings facility’s functional life by two years to 2037.


(For information only, prior to acquisition in May 2017. Data is provided in imperial units.)

During 2016, ore and reef waste production at Stillwater averaged 1,954 tons per day (2015: 2,049 tons per day).

Historically, mill recovery of the PGMs contained in the ore has been approximately 92.9%. Stillwater produced 327,000oz of palladium and platinum in 2016, an increase of 2.3% on the 319,800oz produced in 2015.

Total costs of revenue in 2016 were $164.0 million and costs of metals sold per PGM ounce mined was $486/oz. Costs of metals sold per PGM ounce mined, a GAAP financial measure, is most directly comparable to total cash costs, net of credits, per PGM ounce mined, a non-GAAP financial measure. Total cash costs, net of by-product and recycling credits, (a non-GAAP financial measure) averaged $437/oz in 2016 compared to $487/oz in 2015. The lower costs for the year ended 31 December 2016 were principally the result of a reduced labour force, higher production volumes, which resulted from improved mining practices, higher grades and higher mill recoveries, as well as the implementation of various cost reduction initiatives.

Before taking the benefit of recycling and by-product sales credits into account, Stillwater’s total cash costs averaged $498/oz in 2016 (2015: $545/oz).


(For information only, prior to acquisition in May 2017. Data is provided in imperial units.)

Ore milled (underground) tons/day 1,954 2,049
Plant head grade oz/t 0.49 0.46
Recoveries % 93 93
2E PGM produced 000oz 327 320
Average PGM basket price US$/oz 694 774
Total cash costs1 $/PGM oz mined 437 487
Total cash costs2   498 545
Total cost of revenues $m 164.0 178.0
Sustaining capital expenditure3 – total $m 38.6 58.7
  1. 1Net of recycling and by-product credits
  2. 2Before recycling and by-product credits
  3. 3Includes expansion of the Hertzler tailings facility