The Stillwater Complex is a layered ultramafic to mafic igneous complex formed by the gradual cooling of an enormous subterranean intrusion. In such intrusions, the gradual cooling leads to the formation of layers of crystals with similar geochemical properties. Some of these layers are essentially monomineralic (anorthosites composed of high-calcium plagioclase), while others have additional minerals such as troctolites, which contain both high-calcium plagioclase and olivine. The layering is very distinct and resembles the layering of sedimentary rocks even though the rocks of the complex are igneous. These layers form over extremely large areas. The fact of this layering is critically important to understanding the deposition of the palladium- and platinum- bearing minerals in the J-M Reef.

The J-M Reef is recognized by the presence of chalcopyrite, pyrrhotite, pentlandite, and, rarely, pyrite within the reef package. Other base-metal sulfides have been identified, but their presence is subordinate. The platinum group minerals can rarely be recognized with a hand lens. SMC (2001) reported that platinum occurs primarily (67%) as sulfide, 25% as the metal alloy, isoferroplatinum, and 8% as the telluride, moncheite. Palladium occurs primarily (80%) as a solid-solution in pentlandite, 15% as sulfides, and 5% in the telluride, moncheite. The platinum:palladium ratio averages 1:3.5 in-situ at the Stillwater Mine, except for the Dow (Upper West) area of the mine, where the platinum:palladium ratio increases to 1:3.9 in- situ, and averages 1:3.6 to 3.7 in-situ at the East Boulder Mine. The currently delineated proven reserves at the East Boulder Mine have a lower PGM grade and higher copper plus nickel content than the Stillwater Mine. At the west end of the Stillwater Mine in the Dow sector, an area of higher copper plus nickel content has been identified that is similar to the East Boulder Mine mineralization.

The East Boulder operation is accessed via twin 5,800m long decline tunnels.

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.

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%).

East Boulder has been in operation since 2002, and currently produces approximately 230Koz per annum of platinum and palladium in concentrate.

Mineral processing - Flotation.