Sulfidic bronzitite (field of view ~3.65 cm across) from the Johns-Manville Reef, Lower Banded Series, Stillwater Complex (Neoarchean, 2.71 b.y.) in the Stillwater Mine, Beartooth Mountains, Montana, USA.

Brownish bronze = Pt/Pd-rich pyrrhotite.

Yellow brassy = Pt/Pd-rich chalcopyrite.

Brown = bronzite pyroxene.

This is a very high grade platinum ore from the Stillwater Mine's J-M Reef. The platinum/palladium-bearing sulfides are here hosted in bronzitite rock, the rarest known host rock in the mine. Bronzitite is a coarsely-crystalline, intrusive igneous rock composed almost entirely of bronzite pyroxene. An alternate name for bronzitite is bronzite pyroxenite (a type of peridotite). What makes this rock so high grade? It is about 25% intercumulate Pt/Pd-rich sulfides.

This rock has an ore grade of about 10 ounces of Pd-Pt per ton of rock (wow!), with a Pd-Pt ratio of ~3.5:1 (= highest grade platinum group metals deposit in the world). This is the rarest host rock & highest-grade type of platinum ore known at the Stillwater Mine.

Locality: small bronzitite lens in the 5300 West 13300 D6 West area of the Stillwater Mine (= western side of the D6 level, ~84' below the 5300' elevation datum & 13,300' west of shaft), Beartooth Mountains, southern Montana, USA.

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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.