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ABSTRACT An orientation project to determine variability of glacial till in the Mud Creek area of northeastern Minnesota’s Vermilion Greenstone Belt demonstrates that gold grains and pathfinder elements are present in basal till, and that anomalies stand out in contrast to regional background levels. Clastic and chemical variations within the till sample set are sufficient to consider using the basal till as sampling media for gold dispersal mapping.
Of the thirty-two till samples analyzed, four were highly anomalous for gold, with counts of 88 to 1,282 gold grains per 10 kg of –2mm sample, and pristine gold grain proportions up to 98%; up to 8,050 ppb gold in HMC (nonmagnetic heavy mineral concentrates); and up to 1,050 ppb gold in the –63μm silt/clay fraction of till. A suite of bedrock grab samples collected as reference mineralization returned assays up to 12,247 ppb Au, and silver concentrations up to 42,500 ppb. Analytical results for the till samples support an hypothesis that clastic dispersal trains of mineralized material exist in tills in the area.
Within the project area, particulate gold is more anomalous in basal till samples than in the thin drape of overlying melt-out till. The gold grain counts and morphology add a transport distance value to chemical measurements of gold in soils and till, and suggest that the gold in the samples is locally derived. Analytical results for the present study are comparable to larger, more extensive regional evaluations conducted in neighboring Ontario and further confirm anomalous soil and fine fraction gold values reported in earlier Vermilion Greenstone Belt studies.
Further sampling in the area is warranted for the purposes of detecting additional dispersal trains, delineating potential bedrock source areas and defining the shape of mineralized trains. Additional sampling to evaluate methods for stepping out into areas of deeper glacial drift cover and greater drift complexity would also be useful. As demonstrated in this study, analysis of basal till in this portion of the Vermilion Greenstone Belt, particularly for gold grains, offers a capacity to positively detect local, previously unrecognized mineralization both inside and outside of areas of detailed bedrock geologic mapping.
Report 365: Results of Glacial Till Sampling In the Vermilion Greenstone Belt, Northeastern Minnesota
(85 pages, 8.5 x 11 inches, 1.30 MB)
Author: Dave Dahl
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MN DNR - Lands and Minerals
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