The Stillwater Complex

Abstract

The Stillwater Complex was emplaced at 2.7 Ga at a depth of 10-15 km. Laramide deformation and subsequent erosion have exposed the basal hornfels and a thick, but incomplete, section of the complex. The complex is divided into a Basal Series, an Ultramafic Series and a Banded Series. The Basal Series forms an irregular sheet-like mass composed of early-formed cumulate rocks and coeval sills of diabase and sulphide-rich mafic norite. The Ultramafic Series is made up of cycles of harzburgite and bronzitite with conformable layers of chromitite in the lower parts of each cycle. The cyclic units and absence of cryptic variation in the Ultramafic Series indicate repeated influxes of magma and venting of fractionated magma from the chamber. The Lower and Upper Banded Series are composed primarily of norite and gabbronorite while the Middle Banded Series is predominantly anorthosite, troctolite and olivine gabbro. Cryptic variation in the Lower and Upper Banded Series is consistent with crystal fractionation and accumulation. In contrast, plagioclase in the Middle Banded Series has a uniform composition throughout which is consistent with a sorting mechanism in which plagioclase crystals were suspended in a convecting magma for extended periods. In the Lower Banded Series, a PGE-rich sulphide zone (J-M Reef) is associated with the reappearance of olivine, most likely due to a magma influx. Two models have been proposed for the origin of the reef. The magmatic model holds that sulphides became enriched in PGE during batch segregation of an immiscible sulphide melt which came into contact with a large volume of silicate melt during magma mixing. In the hydromagmatic model, it is proposed that PGE enrichments are the result of leaching of underlying cumulates by Cl-rich hydrous fluids exsolved during the late stages of intercumulus crystallization. Crystallization sequences reveal that at least two compositionally distinct magmas were involved in the formation of the complex. The magma that formed the Ultramafic Series was rich in both MgO and SiO2 and appears to be related to the mafic norite sills. The magma that formed the Middle Banded Series was tholeiitic in character. Isotopic and trace element compositions preserve a record of crustal interaction. Addition of older continental crust to the mantle source regions via subduction is the most plausible contamination mechanism.