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.

Since 1994, the Company has used two mechanized mining methods: ramp-and-fill and sub-level stoping. Ramp-and-fill is a mining method in which a series of horizontal cuts are extracted from the ore body using mobile equipment. Access to the ore body is from ramps driven within or adjacent to the ore body, allowing the use of hydraulic drills and load-haul-dump equipment. Sub-level stoping is a mining method in which blocks of the reef approximately 50 feet high and up to 75 feet in length are extracted in 30 foot intervals utilizing mobile long-hole drills and remote-control rubber tired load-haul-dump equipment. The reef is mined in a retreat sequence and mined out areas are filled with development waste or sand backfill as appropriate. Traditionally, captive cut-and-fill has been viewed as being more selective in nature than either ramp-and-fill or sub-level stoping, but it also requires miners with specialized skills and is generally less productive. Other factors considered in determining the most appropriate mining method for each area include the amount of ancillary development required, as well as the ore grade and ground conditions expected. The Company determines the appropriate mining method to be used on a stope-by-stope basis utilizing engineering and economic analysis.

The Company processes ore from the Stillwater Mine through a surface concentrator facility (mill) adjacent to the Stillwater Mine shaft. Crushed mine ore is fed into the concentrator, mixed with water and ground to slurry in the concentrator’s mill circuits to liberate the PGM-bearing sulfide minerals from the rock matrix. Various reagents are added to the slurry, which then is agitated in a froth flotation circuit to separate the valuable sulfides from the waste rock. In this circuit, the sulfide minerals are successively floated, recycled, reground and refloated to produce a concentrate suitable for further processing. The final flotation concentrate, which represents approximately 2.0% of the original ore weight, is filtered, placed in large bins and then transported by truck to the Company’s metallurgical complex in Columbus, Montana.