The survey collected 608 samples from an area of approximately 945 mi2 (2448 km2 ) from 253 lakes underlain by rocks of the Duluth Complex, North Shore Volcanic Group, and the Rove Formation. Unashed samples were analyzed by atomic absorption for Ag, Co, Cu, Pb, Ni, Zn, Fe, and Mn by using a 4M HN03/1M HCI leach and for As by using a concentrated HN03/30% 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 organic-rich lake sediment. This suggests that the lake sediment should also reflect mineralization under favorable chemical, geologic, and hydrologic conditions. The observations and suggestions described are based only on the analytical methods used and may differ with other chemical techniques.
A statistical analysis of the data indicates that Fe and Mn demonstrate a positive relation to all trace elements and that As, Co, Fe, and Mn show a negative relation to LOI. Element ratios were not justified due to non-proportional parameter relations, and univariate regression residuals or inorganic concentration conversions based on LOI did not significantly normalize the raw data for the effects of or relation to 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 trace elements in the lake sediments of the survey area. Furthermore, the negative relation of some elements to LOI indicates that these elements may be concentrated in the inorganic fraction of the lake sediment.
A non-proportional and approximately exponential relation was exhibited between the trace elements and Fe, Mn, and LOI. A proportional, approximately exponential relation was demonstrated between Fe and Mn and also between the trace elements. It is suggested that these relations are not related to chemical processes, such as scavenging and coprecipitation, which tend to enhance the trace element concentrations, but rather are a function of the relative abundance of the elements in the glacial and bedrock geologic environment.
A number of anomalous regional trends and significant multi-element anomalies were revealed by this survey including several within the expanded borders of the Boundary Waters Canoe Area. This expansion took place subsequent to the completion of this survey.
Some factors that should be considered in the interpretation of this survey for mineral potential purposes include relative trace element mobility, glacial dispersion, variation of trace element background with bedrock lithology, and increase in lake sediment element concentrations with increase in lake area.
Report 138-2: Lake Sediment Geochemical Survey of Cook County, Minnesota.
(45 pages, 8.5 x 11 inches, 2.4 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, 138-2.
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