Stillwater & East Boulder

The Stillwater (including Stillwater east) and East Boulder mines are underground mining operations, located near the towns of Nye and McLeod in Montana, US. The mining assets are located on the front range of the Beartooth Mountains with elevations exceeding 2,700m above sea level.

Statistics as at 31 December 2022

Life of mine

Stillwater

2053

East Boulder

2065

Production 2E PGM

421,133oz

All-in sustaining cost

US$1,586/oz

Workforce

1,530

employees and contractors

The mines both target the J-M Reef zone, predominantly via selective mechanised ramp and fill mining methods. Ore from the operations is milled and treated at integrated concentrator complexes located at each operation. Concentrate smelting and refining takes place at the Columbus smelter complex, situated in the town of Columbus, Montana.

The Stillwater mine has two principal mining sections: the Western section, which has been in operation since 1986, produces approximately 250-300koz 2E per annum of platinum and palladium in concentrate; and the Stillwater East section, which is still in a buildup phase. The Western section of the operation is accessed by a 580m deep shaft and five surface portals, while Stillwater East is accessed via three portal drives.

The East Boulder mine has been in operation since 2002, and currently produces approximately 220 koz 2E per annum of platinum and palladium in concentrate. The East Boulder mine is accessed via twin 5,800m long tunnel bored portal drives.

As at 31 December 2022, the US PGM operations had total 2E PGM Mineral Reserves of 26.3Moz and total 2E PGM Mineral Resources of 84.2Moz.

A closer look

A brief history

The JM reef was discovered in 1974 by Johns-Manville Corporation, and was developed by Manville, Chevron and LAC Minerals Ltd. with production commencing in 1986
By 1990 the smelter was commissioned and in 1994 the Stillwater Mining Company (SMC) was listed
In 1996, the vertical shaft at the Stillwater mine was completed
In 2000, the Hertzler tailings impoundment was constructed
The East Boulder mine was established in 2002
On 23 June 2003, SMC completed a stock purchase transaction with MMC Norilsk Nickel (Norilsk Nickel), whereby a subsidiary of
Norilsk Nickel became a majority stockholder of the company. In December 2010, Norilsk Nickel disposed of its entire ownership interest in SMC through a secondary offering of the SMC shares in the public market
From 2010, SMC operated as a NYSE listed company until May 2017 when it was delisted following its acquisition by Sibanye Gold Limited
The PGM recycling business was established in 2010
Sibanye-Stillwater acquired the Stillwater Mining Company in 2017, the year which also saw the first production from the Stillwater East project

Property and infrastructure

Stillwater Mine

Key infrastructure includes the mining operations and ancillary buildings that contain the concentrator, surface workshop, warehouse, changing facilities, headframe, hoist house, sand, paste plants, water treatment, storage facilities, and offices. Underground development represents 68% of the Stillwater mine property, plant and equipment book value of US$1.15 billion,

The original concentrator plant was built in 1986. A new concentrate handling system was added in 2021 as part of a new plant construction. The balance of the new processing facility will be commissioned in 2023. Currently, the plant and adjoining properties comprise 18% of the asset value. The underground mining fleet ranges in age from 24 years to one year, with the average age of eight years, and contributes 13% of the property’s book value. Overall the physical condition of the asset is improving with the addition of the new concentrator, as well as underground equipment purchases.

East Boulder Mine

Key infrastructure includes the mining operations and ancillary buildings that contain the concentrator, workshop, warehouse, changing facilities, twin tunnels to access mine, sand plant, water treatment, storage facilities and offices. Underground development represents 80% of the East Boulder mine property , plant and equipment book value of US$1.48 billion. The concentrator plant was built in the early 2000’s. Currently, the plant with adjoining properties comprise 16% of the East Boulder book value. The underground mining fleet ranges in age from 22 years to one year, with an average age of 16 years, and represents 4% of the property’s book value. An aggressive underground equipment replacement program is slated for the next few years.

Tailings deposition and capacity

Stillwater Mine

Currently 56% (2022) of all concentrator tailings are returned underground for backfill. The remaining 44% is sent via pipeline to the Hertzler TSF, situated 11km north of Stillwater. The current storage facility has 4,740kt of storage remaining with expansion planned to add an additional 10,200kt of storage in 2029. The Hertzler storage facility, with the planned expansion, will have adequate storage for current Proved and Probable Mineral Reserves.

East boulder Mine

Currently 46% (2022) of all concentrator tailings are returned underground for backfill, with the remaining 54% sent via pipeline to a TSF adjacent to the mine site.The current storage facility has 4,060kt of storage remaining in Stages 4-6. In addition, an expansion is planned to add an additional 5,500kt of storage in 2030 at the Lewis Gulch facility.This facility, including another planned expansion, will accommodate the current Proved and Probable Mineral Reserves.

Site teams are currently evaluating dry stacked tails for the LewisGulch facility which could double the life of that facility.

Key developments in 2022

During 2022 the US PGM operations were repositioned for sustainability and profitability. This was a consequence of a series of disruptions including the Benbow flooding (2018), COVID-19 restrictions (2020), skill and labour shortages (2021/22) and the extreme weather event in mid 2022 which caused regional flooding around the Stillwater mine, all of which impacted operational flexibility and performance. Along with the deteriorating macro environment and outlook for palladium, a review was conducted resulting in a repositioning of the operations including the suspension of further expansion at Stillwater east. To protect margins and to ensure long-term value, a slower build-up in production has been planned to allow for time to improve the mine’s developed state and to focus on a range of interventions underpinning productivity improvements. The repositioned plan incorporates a build-up to +700koz 2E PGM per annum by 2027.