Stillwater and East Boulder are shallow to intermediate level underground PGM mines situated 85 miles southwest of Billings 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 had 26.9 million ounces of proven and probable 2E PGM Mineral Reserves and 86.9 million ounces of 2E PGM Mineral Resources.
Both Stillwater and East Boulder operations were previously operated by the Stillwater Mining Company, which was acquired by Sibanye for US$2.2 billion in a transaction concluded 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 Group’s Columbus Metallurgical complex in Montana, for further processing, smelting and refining.
The Stillwater operation has two principal mining sections. The current section, which has been in operation since 1986, produces approximately 380,000oz a year 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 75,000oz a year. The current section of the operation is accessed by a 580m deep shaft and five portals. The Blitz section is accessed by two portals.
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.
In 2020, 64% of all concentrator tailings were returned underground for backfill. The remaining 36% was sent via pipeline to Hertzler tailings storage facility (TSF) situated 11km north of Stillwater. The TSF has 5,180kt of storage remaining with expansion planned to add an additional 10,200kt of storage in 2030. The Hertzler storage facility, with the planned expansion, will have adequate storage for current Proved and Probable Mineral Reserves.
East Boulder currently produces approximately 230,000oz per annum of platinum and palladium in concentrate. The East Boulder operation is accessed via twin 5,800m long decline tunnels. In early 2019 a project called “Fill the Mill” (FTM) was approved to add all development and infrastructure necessary to bring into production a seventh ramp system, from the current six. This project would allow East Builder to fill its mill capacity, thereby increasing annual 2E PGM production by approximately 40,000oz. Critically, the additional production from the FTM is expected to reduce operating costs at the East Boulder mine by approximately 5% over the project’s 10-year life. The project was completed on time at the end of 2020.
The East Boulder concentrator has a design capacity of 1,800 tonnes per day and currently operates at an 91% utilisation rate.
In 2020, 48% of all concentrator tailings were returned underground for backfill, with the remaining 52% sent via pipeline to a TSF adjacent to the mine site. The current storage facility has 5,210kt of storage remaining in Stages 4-6. In addition, an expansion is planned to add an additional 5,460kt of storage in 2030 at the Lewis Gulch facility. This facility, including another planned expansion, will accommodate the current Proved and Probable Mineral Reserves.
It is estimated that current Mineral Reserves will sustain Stillwater until 2045 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 2059.