Stillwater Complex

The Morensky-type deposits can be found in very large bodies of basaltic magma, which were intruded into stable continental rock. An example includes the Busheld Complex in South Africa and the Great Dyke of Zimbabwe. Mineralization similar to the above is also found in the Stillwater Complex in Montana, USA. 

The Stillwater Complex consists of a sequence of differential layers of mafic and ultramafic rocks, which extend for a strike length of up to 40 km and has a maximum exposed thickness of about 7.4 m [3], There are several mineralization zones at the Stillwater Complex, including a PGM-rich zone and a low-grade zone. The Stillwater ore that is processed nowadays contains olivine, plagioclase, as well as plagioclase-brauzite, all of which are naturally hydrophobic gangue minerals. 

The type of xanthate has a significant effect on PGM recoveries. Studies conducted on the Stillwater Complex by the US Bureau of Mines [ 12] indicated that the type of xanthate had a significant effect on PGM recovery (Table 18.4). 

It should be noted that in several operating plants from the Morensky Reef and Stillwater Complex, from which plant metallurgical results are available, the total PGM recoveries ranges from 82% to 85% PGM. The grade of concentrate from the Morensky operations ranges from 80 to about 120 g/t (Plants A and B). Most of the contaminants are silicates and talc. 

This flowsheet is used in several operations from the Bushveld Complex (South Africa), Stillwater Complex (USA) and Lac des Illes (Canada). 

Morrice, E., Valkiewicz, J.W., and Casale, G., Pilot Plant Flotation of Serpentinized Platinum-Palladium Ore from Stillwater Complex, Report of Investigation 8885, 1976. 

Layered intrusions. Chlorine isotope studies of the Stillwater Complex of Montana, USA (e.g., Stewart et al. 1996 Boudreau et al. 1997), and the Bushveld Complex in the Republic of South Africa (e.g., Willmore et al. 2002) show that both intrusions are anomalously rich in chlorine (e.g., Boudreau et al. 1986 Willmore et al. 2000), but that, their chlorine-isotopic compositions are distinct from one another (Fig. 1). The 8 Cl values of Stillwater samples range from -0.93%o to 0.27%o with all but one value below 0.1 %o (Fig. 1). In contrast the... 

Boudreau AE, Mathez EA McCallum IS (1986) Halogen geochemistry of the Stillwater and Bushveld complexes evidence for transport of the platinum-group elements by Cl-rich fluids. J Pet 27 967-986 Boudreau AE, Stewart MA, Spivack AJ (1997) Stable Cl isotopes and origin of high-Cl magmas of the Stillwater Complex, Montana. Geology 25 791-794... 

Hatton CJ, Sharpe MR (1989) Significance and origin of boninite-like rocks associated wifii file Bushveld Complex. In Boninite Crawford AJ (ed) Unwin Hyman, Boston, p 174-207 Helz RT (1985) Compositions of fine-grained mafic rocks from sills and dikes associated wifii the Stillwater complex. In Stillwater complex, geology and guide. Special Pub 92. Czamanske GK, Zientek ML (eds) Montana Bureau of Mines and Geology, p 97-117... 

McCallum IS (1996) The Stillwater complex. In Layered Inhusions. Cawthom RG (ed) Elsevier, Amsterdam,... 

Stevenson R, Upton BGJ, Steenfelt A (1997) Crust-Mantle interaction in the evolution of the Ilimaussaq Complex, South Greenland Nd isotopic studies. Lithos 40 189-202 Stewart MA, Boudreau AE, Spivack AJ (1996) Cl isotopes of the Stillwater Complex, Montana, and the formation of high-Cl Bushveld and Stillwater parent liquids during mantle metasomatism by Cl-rich fluids. Eos Trans, AGU 77 F829... 

Fig. 1. Primitive mantle (PM) normalized whole rock PGE profiles for stratiform chromitites from the Stillwater Complex and for podiform chromitites from the mantle section of Thetford Mines Ophiolite (TMO). Note the similarity between IPGE contents of chromitites from these different tectonic settings. D.L.= detection limit.

We analyzed chromites from 1) podiform chromitite deposits from the mantle section of the Thetford Mines Ophiolite (TMO), southern Quebec Appalachians (Canada) 2) stratiform chromitite seams found within the Stillwater Complex, Montana (USA). In addition, we also analyzed chromite in lavas from different tectonic settings in order to 1) verify whether the IPGE solubility in chromite could be related to geological setting and... 

Boudreau AE, McCallum IS (1989) Investigations of the Stillwater Complex Part 5, Apatites as indicators of evolving fluid composition. Contrib Mineral Petrol 102 138-153 Brecevic LJ, Furedi-Milhofer H (1972) Precipitation of calcium phosphates from electrolyte solutions II The formation and transformation of the precipitates. Calc Tissue Res 10 82-90 Brown WE (1966) Crystal Growth of Bone Minerd. Clin Orthopaed 44 205-220... 

Meurer WP, Boudreau AE (1996) An evaluation of models of apatite compositional variabihty using apatite from the Middle Banded Series of the Stillwater Complex, Montana. Contrib Mineral Petrol... 

Figure 16. Plot of apatite fission track age against chlorine concentration for apatite grains in a suite of samples from the Stillwater Complex, Montana. Apatites from these rocks exhibit an unusually wide range of chlorine concentrations. A positive correlation is observed between fission track age and chlorine concentration up to about 4% Cl, reflecting an increasing resistance of the apatite grains to annealing.

The bulk of present world production comes from the Republic of South Africa and, to a lesser extent, Russian deposits, and most presently known reserves are within Merenski-type environments (Bushveld and Stillwater complexes). 

Figure 1. A broken outcrop of fine-scale layering in a norite of the Banded zone of the Stillwater Complex. Hammer at lower right provides scale.

The Stillwater Complex is a large layered intrusion situated in south-central Montana. Its radiometric age has been measured at 2.7 X 10 yrs [5l. The currently exposed portion of the complex, roughly 7 x 50 km, is only a small part of the intrusion s original dimensions. 

Within the Stillwater Complex, fine-scale layering is only locally well developed. It is found associated with chromite-bearing rocks and, in the most extensive developments, with layered norites associated with anorthosites in the middle portions of the complex. Where best developed, as in figure 1, the layering may be present over a stratigraphic distance of several tens of meters. 

Figure 4. Mosaic pattern on bedding surface of a banded troctolite from the Stillwater Complex,...

Boudreau, A.E. 1982, Fine-scale layering in the Stillwater Complex, Montana. Univ. of Oregon, Master s thesis, 67 p. 

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