In southern Montana’s Beartooth Mountains is 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).
Chromitites are intrusive igneous rocks dominated by the mineral chromite (FeCr2O4 - iron chromium oxide). The mineral chromite has a metallic luster, a blackish color, and is moderately heavy for its size. Chromitite rocks are typically masses of granular chromite. The Stillwater Complex has economic concentrations of chromitite, but Cr mining was only done in the mid-20th century. Today, the chromitites are not the targets of active mining. The chromitite shown below is not from the J-M Reef.
Chromitite (field of view ~5.3 cm across) from the Stillwater Complex (Neoarchean, 2.71 b.y.) in the Beartooth Mountains, Montana, USA.
STILLWATER SULFIDIC SERPENTINITE
Here’s an altered pegmatitic dunite very richly infused with intercumulate Pt/Pd-rich chalcopyrite & pyrrhotite. Dunites are >90% olivine peridotites (ultramafic, intrusive igneous rocks). During metamorphism, olivine typically converts to serpentine, if water is present. The resulting rock is serpentinite. In the sample below, the blackish areas are serpentine masses (formerly large olivine crystals). Some magnetite is also mixed in with the serpentine.
This ore sample grades to about 2.5 ounces of Pd-Pt per ton of rock, with a Pd-Pt ratio of about 3:1.
Locality: 55W15100 D1 area (5500’ elevation above sea level & 15,100’ west of shaft), Stillwater Mine, underground & west of the Stillwater River, southwestern Stillwater County, Beartooth Moutains, southern Montana, USA.
Sulfidic serpentinite (above & below) from the Johns-Manville Reef, Lower Banded Series, Stillwater Complex (Neoarchean, 2.71 b.y.) in the Stillwater Mine, Beartooth Mountains, Montana, USA.
Black: serpentine mixed with a little magnetite (formerly dunite).
Metallic-lustered areas: Pt/Pd-rich chalcopyrite & pyrrhotite.
Above: field of view ~9.5 cm across. Below: field of view ~4.1 cm across.
STILLWATER SULFIDIC TREMOLITITE
An exceedingly rare rock in the J-M Reef is tremolitite. Tremolitite is a rock dominated by tremolite (Ca2Mg5Si8O22(OH)2), a whitish amphibole that typically forms needle-like crystals. This was originally a dunite, composed of olivine crystals. Metamorphic alteration has transformed the olivine into tremolite. This rock is from a very localized alteration spot in the J-M Reef. Typically, dunites get altered to serpentinites (serpentine with a little magnetite), and this tremolitite was observed to be in close association with such rocks.
This ore sample grades out to ~2.0 ounces of Pd & Pt per ton of rock. The Pd-Pt ratio is about 3:1.
Locality: 41W1500 stope (4100’ elevation above sea level & 1500’ west of shaft), Stillwater Mine, underground & west of the Stillwater River, southwestern Stillwater County, Beartooth Mountains, southern Montana, USA.
Sulfidic tremolitite (above & below; field of view ~3.0 cm across) from the Johns-Manville Reef, Lower Banded Seriers, Stillwater Complex (Neoarchean, 2.71 b.y.) in the Stillwater Mine, Beartooth Mountains, Montana, USA.
White = tremolite. Yellowish-gold colored masses = chalcopyrite. Silvery-bronze colored masses = pyrrhotite.
STILLWATER MASSIVE SULFIDE
Platinum- and palladium-bearing pyrrhotite & chalcopyrite in the Stillwater Complex usually occur as intercumulate fills between crystals of plagioclase or pyroxene or olivine/serpentine. Occasionally, these sulfide minerals occur in a massive state. Below is the front & back of a fragment of massive sulfide from the Stillwater Complex’s J-M Reef. The yellowish-gold colored material is Pt/Pd-rich chalcopyrite, and the brownish-gold colored material is Pt/Pd-rich pyrrhotite. There are other minerals present, including bornite (Cu5FeS4) (see multicolored areas in 2nd pic below), and small patches of some silvery-colored mineral (what?). Several rare sulfide and element and element-alloy minerals have been reported from the Stillwater, including hollingworthite ((Rh,Pt,Pd)AsS), gold (Au), tetraferroplatinum (PtFe), palladobismutharsenide (Pd2(Bi,As)), braggite ((Pt,Pd,Ni)S), keithconnite (Pd3-xTe), moncheite (Pt(Te,Bi)2), vysotskite ((Pd,Ni)S), etc.
Locality: 46W500 stope (4600’ elevation above sea level & 500’ west of shaft), Stillwater Mine, underground & west of the Stillwater River, southwestern Stillwater Coutny, Beartooth Mountains, southern Montana, USA.
Massive sulfide (above & below; 4.6 cm across) - front & back of a piece of Pt/Pd-rich massive sulfide from the Johns-Manville Reef, Lower Banded Seriers, Stillwater Complex (Neoarchean, 2.71 b.y.) in the Stillwater Mine, Beartooth Mountains, Montana, USA.
The J-M Reef has some Pt/Pd-rich minerals besides just pyrrhotite and chalcopyrite. The other Pt/Pd-bearing minerals tend to be uncommon to rare, however. Here’s a nice example of the very rare sulfide mineral braggite (silver-colored patch near the center). Braggite is a platinum-palladium-nickel sulfide - (Pt,Pd,Ni)S. It’s an extremely rare mineral. Macroscopic crystals have been reported from only two localities on Earth - Montana's Stillwater Complex and South Africa's platinum mines.
Locality: 50W141 D7 West in the Stillwater Mine (= western side of the D7 level, ~98’ below the 5000’ elevation level, 141’ west of shaft), Beartooth Mountains, southern Montana, USA.
Braggite (silver area near center) in sulfidic serpentinite from the Johns-Manville Reef, Lower Banded Seriers, Stillwater Complex (Neoarchean, 2.71 b.y.) in the Stillwater Mine, Beartooth Mountains, Montana, USA. Field of view: 1.7 cm across.
Silvery = braggite. Brownish bronze = Pt/Pd-rich pyrrhotite. Yellow brassy = Pt/Pd-rich chalcopyrite. Dull greenish gray = serpentinite host rock (formerly a dunite).
Most info. provided by Keith Wood (former geologist at Stillwater Mine).