The Feather River ultramafic body is an elongate mass of metamorphosed peridotite and dunite with subordinate lenses of mafic schist. Classification of the body as an alpine-type peridotite is indicated by its mineral chemistry (olivine, Fo88–92; enstatite, En89–92; chromium-rich spinel) and the presence of “flow-layering” and kink bands in olivine and enstatite. The body is located along the northernmost part of the Melones fault zone and is faulted against surrounding slate units derived from thick mudstone-chert and volcaniclastic-carbonate sequences of Paleozoic and Triassic age.
The ultramafic rock underwent an early episode of high-temperature flowage, probably as a crystal mush, and later was metamorphosed in the greenschist facies, at least in part contemporaneously with emplacement. Mafic schist lenses within the ultramafic body were metamorphosed under amphibolite- and then greenschist-facies conditions. The western contact of the northern part of the body is a zone of intense shearing and is bordered by a layer 100 to 600 m thick of complexly deformed schist which is intermediate in metamorphic grade between amphibolite and greenschist facies. Evidence is inconclusive as to whether the ultramafic body was intruded into the Sierran environment at high temperatures or was tectonically emplaced as a cool slab of oceanic upper mantle. The latter origin, however, is supported by the tabular form of the body and the discordance of high-temperature flow features with the northwest trend of the body.