Stillwater complex

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The Stillwater Complex, which is up to 7000 meters thick, is a large layered intrusion in the south of the US state of Montana , which was formed in the Neo-Archean . The complex covers an area of ​​around 180 square kilometers and 47 kilometers on the northern flank of the Beartooth Mountains . It has a long mining history , mainly chromium , copper and nickel ores were mined , more recently palladium and platinum ores have also been mined.

Name and location

The Stillwater Complex was named after its type locality on the Stillwater River in Stillwater County , Montana. It's also due in Sweet Grass County and Park Counties .

geology

The Mouat Mine in the Stillwater Complex, Montana

The Stillwater Complex, which belongs to the Wyoming Craton, has a lot in common with the much younger Bushveld Complex in South Africa . The intrusion came in the late Archean by about 2.7 billion years BP ( Methanium ) in upcoming para- and orthogneisses of Long Lake Suite A - up to 3,000 million years old, metasediments and former Granite and granodiorite , which from 2780 to 2740 million years BP at the end a crust formation phase were imposed. After emplacement of the Stillwater Complex even smaller intrusive body then were ( Engl. Stocks ) from quartz monzonite formed. The intrusion is either concordant or with a fault contact on the host rock, which it has changed metamorphically to cordierite-pyroxene-hornfels .

This plutonic phase was replaced around 2500 million years ago ( Archean-Proterozoic boundary ) by a deformation phase ( faults and folds ) with accompanying, low-grade metamorphosis . This was followed by an expansion phase with north-south trending, mafic ducts . After lifting and tilting to the north, the leveled basement in the Central Cambrian was discordantly overlaid by sediments up to 3000 meters thick . During the Laramian mountain formation at the end of the Cretaceous and in the Paleogene , the Stillwater complex was raised again, pushed over in places , tilted and the late Proterozoic leveling surface was exposed.

The elongated, up to 8 kilometers wide intrusion body strikes approximately N 120 to N 135 and dips at 50 to 90 ° to the northeast. Its thickness is 5500 meters. It is believed that an additional 1,500 to 4,500 meters in thickness was lost through erosion during the Proterozoic .

stratigraphy

Chromitite with bronzite phenocrystals from the Stillwater complex

The internal stratigraphy of the cumulative body consists of three zones (from hanging wall to lying ):

  • Banded zone
  • Ultramafic Zone
  • Basal zone

The basal zone , which is up to 210 meters thick , has a chilled, fine-grain gabbro on contact with the adjacent rock. This is followed by gabbro, norite and feldspar-leading pyroxenites . The lower section of the 1100 meter thick ultramafic zone consists of peridotites , which are composed of dunite , chromitite , harzburgite and bronzite-pyroxenite. The upper third of the Ultramafic Zone is occupied by a massive bronzite-pyroxenite. The 4300 meter thick banded zone is made up of alternating layers of norit, gabbro and anorthosite . It is divided into three series, a lower, middle and upper series (Lower Banded series, Middle Banded series and Upper Banded series).

Mineralization

Sulphidic serpentinite (former dunite) from the Lower Banded series. Contains Pt / Pd-rich chalcopyrite and pyrrhotite

The chromium mineralization is limited to the peridotite of the Ultramafic Zone. The platinum group metals appear in the lower portion of the Banded Zone, in a horizon known as the JM reef ( Johns-Manville reef ).

The JM reef is very similar to the Merensky Reef of the Bushveld Complex in South Africa. It forms a 1 to 3 meter thick, enduring layer at the base of the Banded Zone and is made up of pegmatitic peridotite and troctolite . Scattered in the JM reef are sulfide minerals , including pyrrhotite , pentlandite (the main palladium mineral with up to 5% palladium) and chalcopyrite . The sulfide minerals cooperite (Pt, Pd, Ni) S and braggite (Pt, Pd, Ni) S, the telluride monchite (Pt, Pd) (Te, Bi) 2 and cotulskite Pd (Te, Bi) 1-2 are much rarer and other platinum-iron compounds. The sulfide minerals Vysotskite PdS and Cubanite CuFe 2 S 3 , the arsenides sperrylite PtAs 2 and Stillwaterite Pd 8 As 3 as well as the tellurides telluropalladinite Pd 9 Te 4 and Keithconnite Pd 3-x Te (with 0.14 <x <0 , 43). The JM Reef contains 20 to 25 ppm platinum and palladium with a Pd / Pt ratio of 3.6 at an average thickness of 2 meters .

Mineralization

Massive sulfide ore from the JM reef with Pt / Pd-rich chalcopyrite (yellow) and Pt-Pd-rich pyrrhotite (brownish)

Until now it was assumed that the minerals of the platinum group (English platinum group elements or PGE for short ) had settled as heavy, immiscible, sulphidic melts at the bottom of the intrusion. However, the intimate association of PGE with a pegmatite and other water-bearing minerals as well as the extraordinarily high concentration of rare earths in pyroxenes indicate a redistribution of the interstitial melt that was triggered by the compaction of the cumulus minerals.

Petrology

At least two different magma surges were involved in the creation of the Stillwater Complex . The rocks of the ultramafic zone show affinities to boninites , whereas the rocks of the banded zone emerged from a tholeiitic magma . In addition, crustal material was assimilated, either during magma ascent or through incorporation of previously subducted material.

Dating

De Paolo and Wasserburg (1979) established a Sm-Nd crystallization age of 2701 ± 8 million years BP for a gabbronorite. According to Nunes (1981), zircons from the basal zone provided 2713 ± 3 million years BP. Premo et al. (1990) found using the U-Pb method on zirconia for storage ducts in the basal zone 2705 ± 4 million years BP. These warehouse corridors had penetrated at about the same time.

Individual evidence

  1. ↑ Jackson, Everett D .: The Chromite Deposits of the Stillwater Complex, Montana . In: Ore Deposits of the United States, 1933-1967 (The Graton-Sales Volume) . Vol. 2, 1968, pp. 1495-1509 .
  2. ↑ Stanton, RL: Ore Petrology, Ch 11 Ores of Mafic and Ultramafic Association . McGraw Hill, 1972, p. 305-351 .
  3. ↑ Mathez, EA et al .: Petrologic evolution of partially molten cumulate: The Atok section of the Bushveld Complex . In: Contr. Mineral. Petrol. tape 129 , 1997, pp. 20-34 .
  4. ^ DePaolo, DJ and Wasserburg, GJ: Sm-Nd age of the Stillwater Complex and the mantle evolution curve for neodymium . In: Geochim.Cosmochim. Acta . tape 43 , 1979, pp. 999-1008 .
  5. ↑ Nunes, PD: The age of the Stillwater Complex; a comparison of U-Pb zircon and Sm-Nd isochronous systematics . In: Geochim. Cosmochim. Acta . tape 45 , 1981, pp. 1961-1963 .
  6. ↑ Premo, WR, Helz, RT, Zientek, ML and Langston, RB: U-Pb and Sm-Nd ages for the Stillwater Complex and its associated dikes and sills, Beartooth Mountains, Montana: Identification of a parent magma . In: Geology . tape 18 , 1990, pp. 1065-1068 .

Coordinates: 45 ° 23 '10 "  N , 109 ° 54' 11"  W.