Regional Till Geochemical Survey of the Western Vermilion Greenstone Belt, Minnesota

Abstract

This report presents results of a reconnaissance-scale fine-fraction (silt and clay; -250 mesh, <63μm) till geochemical survey of the western Vermilion Greenstone Belt (VGB). Survey results provide information on the background concentrations of a suite of precious, base metal, and pathfinder elements in till. The survey has identified a number of areas with elevated to anomalous precious and base metal concentrations in till associated with bedrock mineralization in the greenstone belt. The dataset provides a framework for interpretation of the significance of results of future regional and property-scale till geochemical surveys. The survey has identified a number of areas with anomalous precious and base metal concentrations. A prominent zinc anomaly (up to 266 ppm) is present down-ice of Fivemile Lake, underscoring the potential for significant undiscovered zinc mineralization in this area. Highly anomalous copper (314 ppm), gold (54 ppb), molybdenum (26 ppm), and zinc (368 ppm) are present in till in near Needleboy and Armstrong Lakes, suggesting the presence of significant mineralization in an essentially unexplored area. Anomalous gold concentrations suggest significant unidentified gold mineralization exists within the greenstone belt, most notably in the Bass Lake sequence between the Mud Creek shear zone and Vermilion Fault, where gold values up to 940 ppb were identified. Using the results of this survey as a baseline, higher density follow-up till sampling promises to more clearly define and determine the significance of anomalies, as well as determine the location of potential source rocks. Interpretation of mixing of exotic granitic with local greenstone lithologies indicate that over 50% of the fine-fraction material has been transported less than 3 km. Flow direction, transport distance, and probability sector data indicate most geochemical anomalies identified by this survey are correlative with previously identified areas with high potential for bedrock mineralization. This survey demonstrates that till composition in the VGB systematically reflects both bedrock composition and economic mineralization. Results suggest drift exploration methods in general and fine-fraction till geochemical surveys in particular, can be successfully incorporated into an integrated exploration program in the VGB. Higher density sampling is recommended to determine the significance of anomalies and more precisely define source rocks. Further work is recommended to test the applicability till geochemistry to property-scale investigations and of heavy mineral surveys.