Stillwater & East Boulder

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 2018, the US PGM operations had 25.6 million oz of proven and probable 2E PGM mineral reserves and 79.9 million oz of 2E PGM mineral resources.

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  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, which closed 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 mine consists of two principal mining sections: Stillwater West and Blitz, a major expansion project currently under development. The Stillwater West section, which has been in operation since 1986, produces approximately 330,000 2E PGM oz per annum with the Blitz section which started production in October 2017 building up to full production of approximately 300,000oz 2E per annum by 2022. The Stillwater West section is currently accessed by a shaft and five portals. The Blitz section is accessed by one portal.

  The East Boulder mine has been in operation since 2002 and currently produces approximately 230,000oz 2E per annum and is accessed via twin decline tunnels. A new incremental project at the East Boulder mine, the Fill the Mill Project (FTMP), was recently approved by the Board. This project is forecast to deliver approximately 40,000oz of annual 2E PGM production from late 2020, over a 10 year period, through an incremental expansion of mining and certain support facilities at the East Boulder Mine and Columbus Metallurgical Complex. The incremental cost of the project approximates US$29 million (capital expenditure of US$19 million and additional operating costs of US$10 million).

  The Stillwater concentrator has a design capacity of 3,100 tonnes per day. While it currently operates at an 81% utilisation rate in line with mine production, the design capacity is projected to be reached by 2021/22 as production from the Blitz section reaches steady state. The East Boulder concentrator, which has a design capacity of 1,800 tonnes per day, currently operates at an 89% utilisation rate, with excess capacity to process additional tonnage if required. The FTMP will utilise some of available capacity going forward.

  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 the Stillwater West section until 2044 while the Blitz section has sufficient mineral resources to sustain production beyond 2055. It is estimated that current mineral reserves will sustain the East Boulder mine until 2056.