Yeomans, Bruce Wyatt2020-04-212020-04-211984https://hdl.handle.net/11299/212410A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Bruce Wyatt Yeomans in partial fulfillment of the requirements for the degree of Master of Science, 1984. There are 3 supplementary files also attached to this record, which contain Plates 1-3 referenced in the thesis. This item has been modified from the original to redact the signatures present.The Boulder Bay area is located one and a half miles west of Ely, Minnesota, near the western shore of Shagawa Lake. This one square mile area is underlain by part of the Archean Newton Lake Formation. Eight rock types were mapped in the area and include metabasalt flows, gabbro intrusions, banded iron formation, muscovite phyllite, schistose metagreywacke, rhyolite intrusions, a granodiorite with associated apophyses, and dacite dikes, in addition to quartz veins. All rocks are metamorphosed to greenschist facies and show varying degrees of cataclasis. Two periods of deformation are recognized in the Boulder Bay area. The first is indicated by a prominent east-northeast strike of steeply dipping volcanic and sedimentary units, which is parallel to the strike of rocks present throughout the Vermilion District. Folds of this deformation are isoclinal. Six high-angle east-northeast trending faults of unknown displacement are recognized. The second period of deformation, with a major axis roughly at right angles to the first deformation, is indicated by west-northwest trending open folds in the limbs of the isoclinal folds and by locally abundant crenulations in rock cleavage. In the central portion of the area, an epizonal syntectonic (Algoman orogeny, 2.7 b.y.) granodiorite stock intrudes metabasalt flows. A narrow contactmetamorphic aureole surrounds the stock. Zoned alteration accompanied the development of gold and sulphide-bearing quartz veins in the granodiorite. Ore minerals occur as fracture fillings in these veins which were subjected to repeated periods of shearing. Similarities between Boulder Bay mineralization and ore in various gold mines in the Canadian Shield suggest a low to moderate temperature mineralizing fluid. Sulphide and gold deposition may have occurred from hydrothermal fluids migrating from depth and along deep seated fault zones. Rock types in the Boulder Bay area are typical of part of an Archean metavolcanic-metasedimentary pile and were probably deposited under subaqueous conditions. Whole rock analyses indicate the majority of the rocks are tholeiitic. The similarity of rock chemistry in the Boulder Bay area to that in other portions of the Superior Province suggests a continental orogenic or island arc origin for the area.enPlan As (thesis-based master's degrees)Department of Earth and Environmental SciencesUniversity of Minnesota DuluthMaster of ScienceMaster of Science in GeologyThesis or Dissertation