Stillwater Mining Company’s current mining operations are in south-central Montana where it operates two underground mines along the J-M Reef, the world’s richest known deposit of platinum group metals (PGMs).
The Stillwater Complex, which hosts the J-M Reef ore deposit, is located in the Beartooth Mountains in south-central Montana. It is situated along the northern edge of the Beartooth Uplift and Plateau, which rise to elevations in excess of 10,000 feet above sea level. The plateau and Stillwater Complex have been deeply incised by the major drainages and tributaries of the Stillwater and Boulder Rivers down to elevations at the valley floor of approximately 5,000 feet.
Geologically, the Stillwater Layered Igneous Complex is composed of a succession of ultramafic to mafic rocks derived from a large complex magma body emplaced deep in the Earth’s crust an estimated 2.7 billion years ago. The molten mass was sufficiently large and fluid at the time of emplacement to allow its chemical constituents to crystallize slowly and sequentially, with the heavier mafic minerals settling more rapidly toward the base of the cooling complex. The lighter, more siliceous suites crystallized more slowly and also settled into layered successions of norite, gabbroic and anorthosite suites. This systematic process resulted in mineral segregations being deposited into extensive and uniform layers of varied mineral concentrations.
The uniquely PGM-enriched J-M Reef and its characteristic host rock package represent one such layered sequence. The geosciences community believes that the PGM-enriched suite and other minerals characterizing the J-M Reef accumulated at the same time and by the same mechanisms of formation as the rocks enclosing them. Over time, the orientation of a portion of the original horizontal reef and layered igneous complex was faulted an estimated 20,000 feet to the northeast and was tilted upward at angles of 45 to 90 degrees to the north by the Beartooth Uplift. Localized faulting and intrusive mafic dikes are also evident along the 28-mile strike length of exposed Stillwater Complex. The upper portion and exposed edge of the reef complex were eroded, forming the lenticular-shaped surface exposure of the Stillwater Complex and J-M Reef package evident today.
The J-M Reef package has been traced at its predictable geologic position and with unusual overall uniformity over considerable distances within the Stillwater Complex. The surface outcrops of the reef have been examined, mapped and sampled for approximately 28 miles along its east-southeasterly course and over a known expression of over 8,200 feet vertically. That predictability of the J-M Reef has been further confirmed in subsurface mine workings of the Stillwater and East Boulder Mines and by over 38,000 drill hole penetrations.
The PGMs in the J-M Reef consist primarily of palladium, platinum and a minor amount of rhodium. The reef also contains significant amounts of iron, copper and nickel, and trace amounts of gold and silver.