In the course of the survey, 1096 samples from a 2770 mi2 (7175 km2) area were collected from lakes underlain by bedrock of the Duluth Complex, North Shore Volcanic Group, and Virginia Formation rocks. Unashed samples were analyzed by atomic absorption for Ag, Co, Cu, Ni, Pb, Zn, Fe, and Mn by using a 4M HN03/1M HCl leach, and for As by using a concentrated HNO3/30% hydrogen peroxide leach. Organic content was estimated by loss-on-ignition (LOI). Results of the survey indicate that the chemistry of the bedrock geology is reflected in the element concentrations of the lake sediment, which suggests that the lake sediment should reflect economic mineralization under favorable chemical, geologic, and hydrologic conditions.
Statistical analysis of the data indicates that Fe and Mn demonstrate a positive relation to all trace elements and that As, Co, Cu, Ni, Pb, Fe, and Mn show a negative relation to LOI. Element ratios were not justified because of non-proportional parameter relations, and neither univariate regression residuals nor inorganic concentration conversions based on LOI significantly normalized the data for the effects or relations of Fe, Mn, or LOI.
No positive relations were observed between the trace elements and LOI, suggesting that organic complexing does not playa major role in the concentration of the trace elements in the lake sediment. Furthermore, the negative relation of some elements to LOI indicates that these elements are predominantly concentrated in the inorganic fraction of the sediment.
A non-proportional, approximately exponential relation was found between the trace elements and Fe, Mn, and LOI, while a proportional, approximately exponential relation was demonstrated between Fe and Mn. It is suggested that these relations are not related to chemical processes, which tend to enhance the trace element concentrations. Results from other lake sediment surveys from Minnesota performed by the Division of Minerals are similar to those reported above. The observations and suggestions described are based only on the analytical methods used and may differ with other chemical techniques.
Several anomalous regional trends and significant multi-element anomalies were revealed by this survey; the most striking of which may be the reflection of known Cu-Ni mineralization along the northwestern basal contact of the Duluth Complex.
Factors such as glacial dispersion and variation of trace element background with bedrock lithology should be considered in the interpretation of this survey.
Report 171: Lake Sediment Exploration Geochemical Survey of Portions of Lake and St. Louis Counties, Minnesota.
(35 pages, 8.5 x 11 inches, 1.8 MB)
Authors: M.K. Vadis and D.G. Meineke
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Author: MN DNR, Division of Lands and Minerals
See MN DNR Report 376, Minnesota Lake Sediment Geochemistry Surveys for compiled GIS geochemistry data from historic MN DNR lake and stream sediment survey reports including this report 171.
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