Stillwater and East Boulder are shallow to intermediate level underground PGM mines located approximately 85 miles southwest of Billings and in Sweet Grass County approximately 32 miles south of the town of Big Timber, in the state of Montana, in the US.
As at 31 December 2020, the US PGM operations have 26.9 million oz of proven and probable 2E PGM mineral reserves and 86.9 million oz of 2E PGM mineral resources.
Both Stillwater and East Boulder operations were previously operated by the Stillwater Mining Company, which was acquired by Sibanye-Stillwater for US$2.2 billion in a transaction finalised in May 2017. This transaction constituted the largest PGM transaction globally in over a decade.
Both the Stillwater and East Boulder mines extract PGM ore from the J-M Reef – Stillwater from the east and East Boulder from the west. The J-M Reef is a geological formation which is the only known significant source of PGM’s in the US and the highest grade PGM deposit known in the world.
PGM production commenced at the Stillwater mine in 1986 and at the East Boulder mine in 2002. Mechanised ramp-and-fill stoping is the primary mining method at both mines. Mined ore is crushed and concentrated at processing facilities located on surface at both operations; concentrates are then transported to the Company’s Columbus Metallurgical complex in Montana, for further processing, smelting and refining. The US PGM operations primarily produce palladium and platinum (78% Pd and 22% Pt).
The Stillwater operation has two principal mining sections. The current section, which has been in operation since 1986, produces approximately 380koz per annum of platinum and palladium in concentrate. The Blitz section, a major expansion project currently under development, started ore production in 2017 and is currently producing 75koz per annum. The current section of the operation is accessed by a 580m deep shaft and five portals. The Blitz section is accessed by two portals.
East Boulder has been in operation since 2002, and currently produces approximately 230Koz per annum of platinum and palladium in concentrate. The East Boulder operation is accessed via twin 5,800m long decline tunnels. “Fill the Mill” is a project to add all development and infrastructure necessary to bring into production a seventh ramp system, from the current six. This will allow East Builder to fill its mill capacity, which will increase the East Boulder ounce production to 275koz annually. This production rate increase is planned for the fourth quarter of 2020.
The Stillwater concentrator has a design capacity of 3,100 tonnes per day, and currently operates at an 92% utilisation rate, this rate will increase to meet design capacity in 2021 as production from the Blitz section reaches steady state. The East Boulder concentrator has a design capacity of 1,800 tonnes per day and currently operates at an 91% utilisation rate.
Currently, at the Stillwater mine 55% and at the East Boulder mine 50% of mill tailings go back underground for backfill. The remaining tailings are piped to tailings storage facilities adjacent to each mine. With current storage and planned expansion, both facilities have sufficient tailings storage for all of the known proven and probable reserves.
It is estimated that current Mineral Reserves will sustain Stillwater until 2046 and the Blitz project has the potential to significantly expand the Mineral Reserves beyond 2055. It is estimated that current mineral reserves will sustain the East Boulder mine until 2054.