The J-M Reef is a magmatic reef type PGM deposit defined as the palladium-platinum rich stratigraphic interval, mainly occurring within a troctolite (OB-I zone) of the Lower Banded Series. The J-M Reef shows a high degree of variability in grades and thickness at a local level and the J-M Reef has PGM grades that are significantly higher than the Merensky Reef. The J-M Reef pinches and swells typically ranging from 0.9m to 2.7m and averaging 1.8m in thickness but can locally form keel-shaped zones which transgress the footwall mafic rocks, commonly reaching thicknesses of 6m and greater. These wider areas are locally referred to as “ballrooms”. There is no correlation of thickness to grade, so as these sulphide zones thicken, they can maintain grade and become an important source of metal. The J-M Reef shows a high degree of variability in grades and thickness at a local level and the J-M Reef has PGM grades that are significantly higher than the Merensky Reef. The J-M Reef pinches and swells typically ranging from 0.9m to 2.7m and averaging 1.8m in thickness but can locally form keel-shaped zones which transgress the footwall mafic rocks, commonly reaching thicknesses of 6m and greater. These wider areas are locally referred to as “ballrooms”. There is no correlation of thickness to grade, so as these sulphide zones thicken, they can maintain grade and become an important source of metal.

Palladium and platinum are the main PGMs, both constituting between 7g/t to 40g/t over a variable thickness with economic mineralised thickness ranging from 0.9m – 2.7m and averaging 1.8m. In situ palladium:platinum ratio of 3.4:1 to 3.6:1. Other associated PGMs, such as rhodium, iridium, ruthenium, osmium and gold occur in low concentrations.

The J-M Reef contains approximately 0.25% to 3% visible disseminated coppernickel sulphide minerals, predominantly chalcopyrite, pyrrhotite, and pentlandite, with microscopic PGM minerals and platinum-iron alloys. Copper, nickel, gold and rhodium are by-products of J-M Reef production. These sulphides are interstitial and will often mimic the host rock in texture. The sulphide can occur as coarse-grained networks in pegmatoidal olivine-bearing rock or as fine-grained disseminations in fine-grained anorthosites. As they transgress into footwall rocks, the hosts can often be medium-grained gabbro and gabbro-norites. A common hangingwall lithology is typically slightly finer-grained and anorthosite-rich. Economic mineralisation is rarely seen above this rock type.

Most of the regional faults affecting the Stillwater Complex have been grouped as west-northwest trending thrust and reverse faults and north-northeast trending steeply dipping transverse faults. Local and regional structure, as well as local mafic intrusive dykes, create disturbances to mining.

The Stillwater (including the Blitz expansion project) and East Boulder operations are located near the towns of Nye and McLeod in Montana, USA. Both are underground PGM operations. PGM production commenced in 1986 and has largely been uninterrupted.

The operations both target the J-M Reef zone, predominantly via mechanised ramp and fill mining method (+80%). All surface infrastructure and tailings management facilities are located within Stillwater and East Boulder operation operating permits, which cover an area measuring 1,396ha.

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.

The three principal mining methods are:
• Mechanised ramp and fill (both overhand and underhand) (80 – 90%);
• Sub-level extraction by long hole, open stoping with hydraulic backfilling (10 – 15%);
• Overhand cut and fill (C&F) stoping, utilising either raisebore or Alimak for access (<5%).

Ore from the two 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.

Mineral processing and capacity:
- Stillwater plant: Flotation with Design capacity of 3,100 tpd;
- East Boulder plant: Flotation with Design capacity of 1,800 tpd.