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The East Boulder mine is located in Sweet Grass County, Montana, approximately 32 miles south of the town of Big Timber, and accessed via a public road. East Boulder, which also exploits the J-M Reef, is around 20km to the west of Stillwater.

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

Operations in Montana - USA

East Boulder, which is fully permitted independently of the Stillwater mine, is a second distinct mining operation accessing the western portion of the J-M Reef. As at 31 December 2016, East Boulder had ore reserves of 10.7Moz – 8.4Moz of palladium and 2.3Moz of platinum.

East Boulder actively mines approximately 3.2 horizontal miles underground along the J-M Reef. Mine infrastructure includes underground mine development and surface support facilities, including a concentrator, shop and warehouse, changing facilities, storage facilities, offices and a tailings management facility. Surface facilities are situated on unpatented mill site claims maintained on federal lands located within the Custer Gallatin National Forest and administered by the Montana Department of Environmental Quality and the United States Forest Service.

All surface facilities, including the tailings management complex, are located within a 1,630 acre operating permit area. Proven and probable ore reserves for the mine are controlled by patented mining claims owned by the Company.

The J-M Reef is accessed by two horizontal and parallel rail adits driven into the mountain which intersect the ore body deep within the mountain at an elevation 6,450 feet above sea level. Within the mine, the ore body is accessed from eight levels of horizontal footwall lateral drifts driven parallel to the J-M Reef totalling approximately 85,000 feet in length, and from three primary ramps totalling approximately 37,700 feet of development. The ore body is accessed vertically by ramp systems tying together the footwall laterals and driven approximately every 2,500 feet along the length of the deposit. The mined ore is transported horizontally out of the mine by rail haulage to the mine portal.

Ore from the East Boulder mine is processed at a surface concentrator facility (mill) adjacent to the two portals. The mill has a permitted throughput capacity of 2,000 tons per day. Crushed 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 re-floated to produce a concentrate suitable for further processing. The final flotation concentrate, which represents approximately 2.5% of the original ore weight, is filtered, placed in large bins and then transported by truck to the metallurgical complex in Columbus, Montana.

In 2016, approximately 50% 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. The remaining permitted impoundment area has an estimated staged life of approximately 15 years at the orignila planned production and processing rate of 2,000 tons per day.


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

During 2016, ore and reef waste production at East Boulder averaged 1,755 tons per day, a 10.4% increase on the 1,589 tons per day produced in 2015. Mill recovery was 90% in 2016, consistent with that of 2015. East Boulder produced 218,300oz of palladium and platinum in 2016, an increase of 8.6% on 201,000 returnable ounce in 2015. This increase was a result of improved productivity and the ramp up in production from the Graham Creek area.

For 2016, total cost of revenues was $115.3 million and the cost of metals sold per PGM ounce mined, $545, which is comparable with total cash costs, net of credits, per PGM mined ounce in 2015. Total cash costs per PGM mined ounce, net of by-product and recycling credits, (a non-GAAP financial measure) were $441/oz in 2016 compared with $508/oz in 2015, a decrease of approximately 13.2%. The improvement in cash costs per ounce reflects increased mine output and continuing cost reduction and productivity improvement initiatives.

Excluding the benefit of recycling and by-product credits, total cash costs were $516/oz in 2016 and $581/oz in 2015.


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

Ore milled (underground) tons/day 1,755 1,589
Plant head grade oz/t 0.37 0.38
Recoveries % 90 90
2E PGM produced 000oz 218 201
Average 2E PGM basket price US$/oz 694 774
Total cash costs1 $/PGM oz mined 441 508
Total cash costs2 $/PGM oz mined 516 581
Total cost of revenues $m 115.3 115.9
Sustaining capital expenditure – total $m 16.7 15.1
  1. 1Net of recycling and by-product credits
  2. 2Before recycling and by-product credits