The J-M Reef lies under southern Montana's Stillwater, Sweet Grass and Park Counties.
This large mafic/ultramafic layered complex contains layered bands of norite, gabbro and anorthosite. Crystal settling has allowed the heavier mineral ore to settle to the bottom with the lighter siliceous minerals remaining near the tip. Platinum-group metals (PGM's) are found in layer averaging 2.4 meters thick.
Facts About the J-M Reef
• The Reef was originally horizontal
• Approximately 67 million years age it was faulted approximately 20,000 feet to the northeast and tilted up 50 to 90 degrees to the north by what geologists call the "Beartooth Uplift."
• The lenticular-shaped surface exposure of the Stillwater Complex and J-M Reef package evident today are due to erosion.
• The reef is highly homogeneous, as observed over its 28 mile-length. PGMs in the reef are platinum, palladium and minor amounts of rhodium. There are also significant amounts of iron, copper and nickel, and traces of gold and silver.
