Stillwater Mining Company’s 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, site of the J-M Reef ore deposit, is located in the Beartooth Mountains, approximately 30 miles from the northern boundary of Yellowstone National Park. It is situated along the northern edge of the Beartooth Uplift and Plateau, which rise to elevations of more than 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 geosciences community believes 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. 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 more than 8,200 feet vertically. The predictability of the J-M Reef has been confirmed in subsurface mine workings of the Stillwater and East Boulder mines.
The PGMs in the J-M Reef consist primarily of palladium, platinum and a minor amount of rhodium. The reef also contains iron, copper, nickel and trace amounts of gold and silver.