Date of Award
Master of Science (MS)
The Bear Lodge complex is an elongate, dome-shape uplift about 8.5 km long by 4 km wide, 10 km northwest of Sundance, Wyoming. The study area is centered about Warren Peaks and includes approximately 23 km2. The igneous intrusive core of the complex is mostly alkali trachyte but latite is abundant in the southern portion of the complex. Phonolite, pseudoleucite trachyte porphyry, and carbonatite dikes cut across the core. Precambrian granitic rocks and lower Paleozoic sedi mentary rocks flank the intrusion and occur as xenoliths.
The latite is relatively unaltered and consists mostly of oligo clase, andesine, alkali feldspar, salite, and hastingsite phenocrysts in an aphanitic groundmass of alkali feldspar. The alkali trachyte is pervasively altered and exceptionally potassic. It is intensely fractured, brecciated, and stained by limonite. Phenocrysts of sanidine and relict mafic mineral grains are present in an aphanitic groundmass. Altered mafic minerals are replaced by potassium feldspar. The ground mass is also very potassic. Potassium feldspar and limonitic Th-RE veinlets cut the alkali trachyte. Compared with other Tertiary rocks in northeast Wyoming) the alkali trachyte has anomalously low Na2O/K2O ratios. Chemical changes resulting from alteration of the trachyte include increased K2O and FeO + Fe2O3 , and decreased Na2O, CaO, and MgO. Variation in SiO2 is a reflection of the degree of silicification.
Alteration of the alkali trachyte is due to feldspathization, a high level type.of potassic metasomatism found in alkaline carbonatite complexes along the East African rift zone. The Bear Lodge complex is similar to some of the East African complexes and is also feldspathized. The rocks are hypabyssal and represent an intermediate volcanic level.
Rare earth elements are present, primarily in Th-RE veins and carbonatite. Th-RE veins occur throughout the central portion of the intrusion. Carbonatite may be more abundant at depth. Potential for economic rare earth deposits in the Bear Lodge complex is significant.
Wilkinson, Michael, "Petrology and alteration in the core of the Bear Lodge Tertiary intrusive complex, Bear Lodge Mountains, Crook County, Wyoming" (1982). Theses and Dissertations. 323.