English: Gabbro from the Precambrian of 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 and 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 (Pt) and palladium (Pd) occurrence is in the Johns-Manville Reef (J-M Reef), an interval in the lower part of the Lower Banded Series. There, the Pt and 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 sample seen here is just a token Stillwater gabbro - it's not a platinum-palladium ore. Gabbro is a coarsely-crystalline (phaneritic-textured), mafic, intrusive igneous rock. The minerals in gabbro are plagioclase feldspar and pyroxene. In this case, the plagioclase component is bytownite. The chemical formula for plagioclase ranges from NaAlSi3O8 to CaAl2Si2O8. Plagioclase can be all-sodium or all calcium or a mix of sodium and calcium. Bytownite has a 10 to 30% sodium component (= 70 to 90% calcium). Mineralogists often use this nomenclature to designate types of plagioclase feldspar (these are for bytownite): Ab10 to Ab30 or An70 to An90. "Ab" means albite, the sodium end member. "An" means anorthite, the calcium end member. Other types of plagioclase include oligoclase, andesine, and labradorite.

Stratigraphy: Stillwater Complex, Neoarchean, 2.71 Ga

Locality: unrecorded/undisclosed site in the Stillwater Complex, southwestern Stillwater County, southern Montana, USA