The Stillwater Intrusion in Montana is a large Precambrian elongate and layered mass of peridotites and gabbros. The surface exposure is approximately 30 miles long and has a maximum width of 25,000 feet.The country rocks below the intrusion are a complex group of metamorphic rocks and quartz monzonites which intrude the Stillwater mafic massif. Hornfelses, composed mainly of cordierite and orthopyroxene, are the most common metamorphic rock. Bluish quartzite and ironstones are less common.Across 3,000 feet of the contact aureole, substantial depletion of potassium and rubidium can be detected near the contact of the intrusion. However, there is no evidence of a metasomatic introduction of material from the magma into the aureole. The hornfels formed primarily by a breakdown of biotite to form cordierite and hypersthene, releasing potassium and water. Mineral compositions are constant across the aureole. Hornfels inclusions have been partially melted, releasing considerable SiO 2 to the magma, along with minor amounts of Na 2 O, K 2 O, and H 2 O, and trace amounts of ZrO 2 and Rb 2 O.Sulfides in the gabbroic basal zone of the intrusion were formed very late, after some of the mafic silicates in the samples had crystallized. Potassium feldspar inclusions in plagioclase and the development of sulfides are related to abundance of hornfels inclusions in the basal zone. Hornfels inclusions contaminated the surrounding magma. This contamination resulted in rock type changes and concentration of sulfides around the hornfels inclusions, both in the basal zone and in the ultramafic zone of the Still-water Intrusion.

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