Date of Award
Master of Science (MS)
The Bear Lodge Mountains are located in Crook County in north eastern Wyoming. The core of the uplift consists of alkalic igneous rocks that range from Early to Late Eocene in age. A petrologic and geochemical study of surface and subsurface samples from the southeastern and central portions of the core reveals a complex history of multiple alkalic igneous events, potassic metasomatism, carbonatitic magmatism, and hydrothermal alteration.
An early episode of alkalic igneous activity is represented by latite and trachyte porphyries, phonolite and trachyte porphyries, and natrolite-garnet syenites and malignites. Throughout much of the complex, the composition of these older alkalic intrusions has been changed by the metasomatic development of potassium feldspar. This process, termed potassic fenitization, occurred prior to emplacement of carbonatitic dikes and veins. It appears to have involved a volatile rich aqueous fluid that contained potassium, iron, sulfur, carbon dioxide, and fluorine. Rocks associated with potassic fenitization contain significant deposits of copper, lead, zinc, thorium, cerium, lanthanum, and gold.
Two types of carbonatites may be distinguished in the Bear Lodge Mountains, I-type and S-type. I-type carbonatites occur in the central portions of the core and are enriched in strontium, cerium, and lanthanum. The isotopic compositions of carbon and oxygen in calcites from I-type carbonatites indicate a mantle source for carbon dioxide. S-type carbonatites occur in the southeastern portions of the core and have lower concentrations of strontium, cerium, and lanthanum. It is postulated that I-type carbonatites resulted from differentiation of an alkali-rich parent magma, while S-type carbonatites formed by local fusion of Paleozoic sedimentary limestone. Significant deposits of thorium, cerium, lanthanum, copper, lead, and zinc are associated with I-type carbonatites.
Hydrothermal alteration accompanied the episode of early igneous activity and continued through the intrusion of I-type carbonatites. The alteration is especially pervasive in the central portions of the core where it typically extends to depths of several tens of meters in the subsurface. Sodalite-bearing and hauyne-bearing analcime phonolite porphyries crosscut hydrothermally altered rocks and represent the last igneous activity in the region.
The igneous rocks of the Bear Lodge Mountains were probably derived from an alkalic parent magma that formed by partial melting of upper mantle material enriched in potassium, rubidium, iron, titanium, cerium, lanthanum, fluorine, carbon dioxide, and water. Fractional crystallization in a crustal resovoir generated latitic, trachytic, and phonolitic differentiates, while immiscible separation resulted in the formation of an alkali-rich carbonatitic magma. Loss of alkalis accompanied the ascent of the carbonatitic magma from the resovoir, resulting in the potassium-rich fluids responsible for fenitization.
Jenner, Gordon A., "Tertiary alkalic igneous activity, potassic fenitization, carbonatitic magmatism, and hydrothermal activity in the central and southeastern Bear Lodge Mountains, Crook County, Wyoming" (1984). Theses and Dissertations. 148.