Sulfidic norite (field of view ~4.5 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. Gray crystals = plagioclase feldspar. Dark crystals = bronzite pyroxene.

Norite is a common intrusive igneous rock having a coarsely-crystalline texture and a plagioclase-pyroxene composition. It is similar to gabbro, except that the pyroxene in gabbro is clinopyroxene and in norite, it is orthopyroxene. In the Stillwater Complex, compared with anorthosite rocks, norites are merely richer in bronzite pyroxene. This rock has ~69% plagioclase feldspar (= gray crystals) and ~29% bronzite pyroxene. The remaining ~2% of the rock is intercumulate platinum/palladium-rich sulfides. About 1% of platinum-palladium ore mined at the Stillwater Mine is norite.

This rock has an ore grade of about 1.0 ounces of Pd-Pt per ton of rock, with a Pd-Pt ratio of ~3:1 (= highest grade platinum group metals deposit in the world).

Locality: Upper West portion of the Stillwater Mine (probably from the 53W13300 D6 area), Beartooth Mountains, southern Montana, USA.

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.