Braggite in sulfidic serpentinite from the Precambrian of Montana, USA. (field of view 1.7 cm across)

Silvery area near center = braggite Brownish bronze = Pt/Pd-rich pyrrhotite Yellow brassy = Pt/Pd-rich chalcopyrite Dull greenish gray = serpentinite host rock (formerly a dunite)

Southern Montana’s Beartooth Mountains has one of only three platinum mines in North America. There, platinum and palladium are mined from the 2.71 billion-year-old Stillwater Complex, a classic example of an LLI (large, layered igneous province). LLIs are large intrusive bodies that display large-scale and small-scale layering, even including cross bedding, ripples, graded bedding, channelforms, and other sedimentary-like features. The Stillwater started out as a large subsurface mass of slowly cooling magma. As various minerals crystallized, they settled to the bottom of the magma chamber. This resulted in layering. Igneous rocks that formed this way have a cumulate texture. Currents in the still-liquid portions of the magma chamber produced the sedimentary structures mentioned above. Most of the Stillwater displays only large-scale layering.

The rocks in the Stillwater are ultramafic & mafic intrusive igneous rocks. Common lithologies include gabbros, norites, harzburgites, anorthosites, troctolites, chromitites, pyroxenites, and dunites. Portions of the Stillwater have been metamorphosed. Olivine is the most commonly altered component, usually metamorphosed to serpentine.

The main platinum & palladium occurrence is in the Johns-Manville Reef (J-M Reef), an interval in the lower part of the Lower Banded Series. There, the Pt & Pd occur in intercumulate sulfides, typically pyrrhotite (Fe1-xS) and chalcopyrite (CuFeS2). Platinum ores in the J-M Reef are principally sulfidic anorthosites, but other lithologies also occur. The J-M Reef is the highest grade deposit known for platinum-group elements (PGEs).

The J-M Reef has other Pt/Pd-rich minerals besides pyrrhotite and chalcopyrite, but they are uncommon to rare. Shown above is a specimen of the very rare sulfide mineral braggite (= silver-colored patch near the center). Braggite is platinum-palladium-nickel sulfide - (Pt,Pd,Ni)S. Macroscopic crystals have been reported from only two localities on Earth - Montana's Stillwater Complex and South Africa's platinum mines.

Stratigraphy: Johns-Manville Reef, Lower Banded Series, Stillwater Complex, Neoarchean, 2.71 Ga