File:Sulfidic metatroctolite (platinum-palladium ore) Stillwater Mine MT.jpg

Sulfidic metatroctolite (field of view ~4.3 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. Black = serpentine (altered from olivine). Gray = plagioclase feldspar.

Troctolite is a coarsely-crystalline, intrusive igneous rock dominated by olivine and plagioclase feldspar. This Stillwater troctolite has had the oilvine component metamorphosed to black serpentine, while the gray plagioclase feldspar component is still preserved. The silicate portion of the rock is ~60% black serpentine (from olivine) and ~40% post-cumulate plagioclase, plus a small amount of bronzite pyroxene oikocrysts. Platinum/palladium-rich chalcopyrite and pyrrhotite are also present.

This rock has an ore grade of about 1.0 ounces of Pd-Pt per ton of rock, with a Pd-Pt ratio of ~3.5:1 (= highest grade platinum group metals deposit in the world).

Locality: 5000 West 15600 area in the Stillwater Mine (= 5000' elevation above sea level & 15,600' west of shaft; = ~level with surface mine site; = ~3000' underground at this point), 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.