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The Stillwater igneous complex is a large layered mafic intrusion (LMI) located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles (48 km) of the north flank of the Beartooth Mountain Range. The complex has extensive reserves of chromium ore and has a history of being mined for chromium. More recent mining activity has produced palladium and other platinum group elements.
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Geology
The Stillwater complex is a large layered intrusion with many similarities to the Bushveld igneous complex of South Africa. The complex was intruded into existing gneisses during the Archean at about 2700 Mya. The region was subsequently intruded by a quartz monzonite stocks and underwent extensive metamorphism, faulting and folding during the Archean at about 2500 Mya. The area was intruded by north trending mafic dikes before being unconformably covered by a middle-Cambrian sedimentary rock sequence. The intrusion forms a linear body stretching some 30 miles (48 km) and striking roughly N 60 °W and dipping from 50° to near 90° to the northeast. The exposed thickness is around 18,000 feet (5500 m) with an additional estimated 5000 to 15,000 feet having been removed from the top by pre-Cambrian erosion.
The cumulate stratigraphy comprises three distinct zones:
Orebodies
The chromium orebodies are restricted to the peridotite member of the ultramafic zone. The platinum group orebodies are located in the lower part of the banded zone within a horizon referred to as the J-M reef.
The J-M Reef is similar to the Merensky Reef of the Bushveld complex of South Africa. It is a continuous layer near the base of the banded zone. It consists of one to three meter thick pegmatitic peridotite and troctolite with disseminated sulfide minerals. Common sulfides include pyrrhotite, pentlandite (containing up to 5% Pd), and chalcopyrite along with lesser amounts of moncheite ((Pt,Pd)(Te,Bi)2), cooperite ((Pt,Pd,Ni)S), braggite ((Pt,Pd,Ni)S), kotulskite (Pd(Te,Bi)1-2) and platinum-iron alloys. Overall the reef contains an average of 20-25 ppm platinum plus palladium in an average two meter thickness. It has a Pd/Pt ratio of about 3.6.