DNR Reports 236: 1985; Geochemical Survey of East-Central Minnesota

Abstract extracted from Report 236 A reconnaissance scale organic-rich lake sediment geochemical survey was conducted over portions of Cass, Crow Wing, Aitkin and Carlton Counties in east-central Minnesota. The goal of this survey was to examine the usefulness of lake sediment geochemistry over thick, glaciated overburden for mineral potential evaluation. In particular, the east-central Minnesota survey area contains overburden that was deposited by several different glaciations of contrasting till types. The survey area is underlain by both older Archean granite-greenstone-gneisses and younger Proterozoic basin sediments, metavolcanic, igneous, and metamorphic rocks.

Six hundred eighteen samples were collected from 463 lakes encompassing an area of 3,150 mi² (8,160 km²). Unashed samples were analyzed by atomic absorption spectrophotometry for Ag, Co, Cu, Pb, Ni, Zn, Fe, and Mn after being partially digested using a 4M HNO₃/1M HCI mixture. Arsenic was leached by using concentrated HN03130% hydrogen peroxide solution followed by analysis in an arsine generator and uranium was determined by using a 4M HN0₃ solution and a Scintrex uranium analyzer. Finally, organic content was estimated by ashing (Loss-On-Ignition).

Review of statistical analyses revealed that Zn-Cu-Ni and Ni-Co-Cu correlate positively, suggesting an interdependent relationship. Also, as might be expected, Fe and Mn correlate strongly. No meaningful positive associations between the trace elements and LOI were observed, suggesting that scavenging and organic complexing leading to false anomalies is not indicated from statistical trace element analyses. In fact, most LOI correlations are slightly negative, indicating some trace element enrichment in the inorganic fraction of the lake sediment.

Particular attention was addressed to superimposed dispersion trains, drainage patterns, ice directions, trace element mobility, variation of trace element background with bedrock types, and physical and chemical composition of glacial till lithologies. Results of the survey suggest that the chemistry of the bedrock geology is reflected in the organic-rich lacustrine environment. Therefore, under favorable chemical, geologic, and hydrologic conditions, trace element analysis of lake sediments should reflect possible economic mineralization. In the study area at least six locations exhibit multi-element anomalies.

Report 236-2: Reconnaissance Scale Well Water Geochemical Survey of Central Minnesota; portions of Pine, Carlton, Aitkin, & Crow Wing counties.
(42 pages, 8.5 x 11 inches, 1.86 MB)
Authors: D.W. Beckwith and R.C. Clark

Abstract extracted from Report 236-2 A reconnaissance scale well water geochemical survey was conducted over portions of central and east-central Minnesota. The primary objective of the survey was to determine if the chemical analyses of groundwater obtained from domestic water wells would reflect differing rock types located beneath thick glacial overburden, and consequently, could be used to help evaluate the mineral potential of the area.

In the course of the survey, samples were collected from 226 water wells over an area of 2160 mi² (5600 km ²). The survey area is underlain by Archean granites and gneisses, Early Proterozoic sedimentary and volcanic rocks and Late Proterozoic (Keweenawan) volcanic and sedimentary rocks, and includes drift from several phases of Wisconsin glaciation. The water was analyzed by atomic absorption spectrophotometry for Au, Ag, As, Cu, Ni, Pb, Zn, Fe, Ca, Mg, Na and K, and measurements of specific conductance, alkalinity, sulfate content and pH were made.

Statistical analyses were performed on the data in an attempt to identify relationships between elemental concentrations and other parameters that could be useful as indicators of bedrock geology. The effect of bedrock lithology, drift composition, bedrock penetration, plumbing composition and well depth on water chemistry was investigated. It was determined that there are differences in elemental concentrations of water analysis between rock types, and that many of these differences can be related to rock lithology. The effect of drift composition was found to be less prominent; the variation in water chemistry between drift types is only slight. The analysis also showed that pipe composition and whether the well penetrated bedrock or not had little effect on water chemistry. There was a correlation, however, between well depth and the concentration of some elements in the well water.

It was determined from correlation coefficients that Ca, Mg, alkalinity and specific conductance showed a moderate to strong positive relation to each other for each rock and drift subdivision. Other elemental pairs, however, showed little or no correlation.

Two areas of multi-element anomalies, consisting of mainly the major elements, were indicated by this survey, one occurring in the Glen Township Formation near Thor and the other in portions of the Thomson Formation.

This survey indicates that with a few modifications such as increased sampling density, additional analyzed elements and expanded analytical techniques, there may be some potential in using a reconnaissance scale well water sampling program for geologic interpretation purposes.

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Map plates available

Report 236: Plates 1-12: Reside on microfiche at the DNR Hibbing office.
Report 236-2: Plates 1-12: Reside on microfiche at the DNR Hibbing office.
Author: DNR, Lands and Minerals Division

Digital data available

See DNR Report 376, Minnesota Lake Sediment Geochemistry Surveys for compiled GIS geochemistry data from historic DNR lake and stream sediment survey reports including report 236.

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