A RECONNAISSANCE GEOCHEMISTRY OF GROUND WATER SEEPS IN THE SOUDAN, MINE, MINNESOTA

A reconnaissance water-sampling trip to the Soudan Mine to gather data for the Soudan DUSEL proposal produced 12 water samples collected starting on the lowest, 27th level of the mine and working upward. Samples of a surface spring and of nearby Lake Vermillion were also collected for comparison. We observed a variety of coatings, efflorescences, and other deposits on various mine surfaces. There was abundant evidence of microbiological activity such as filamentous, flexible material, foam and gas bubbles. However, no solid or biological samples were collected on this trip. These waters were analyzed by ICP/MS and IC for major cations and anions and trace metals.

The waters on the lowest level were Ca-Na-Mg/Cl solutions about twice as salty as sea water and appear to be related to CaCl2 brines widely reported from Canadian Shield deep ground waters. The waters typically issue as seeps from diamond drill holes extending up to one thousand feet from the mine drift. Major cations were Ca (~ 60 %), Na (~ 30 %) and Mg (~ 10 %) on an equivalent basis. Cl was the dominant anion. The Cl/Br mass ratio in the saltiest samples was 180 indicating that these are residual solutions. The solutions were anoxic and contained up to 150 ppm of dissolved Fe and 10 ppm Mn. When the seeps reach the oxygenated mine tunnels the soluble ferrous Fe begins to oxidize and precipitates as a wide variety of brightly colored secondary deposits.

The water samples higher in the mine yielded much more dilute solutions. The dominant anion shifted from Cl to SO4 presumably from oxidation of sulfides. These waters probably reflect oxygenated surface recharge flowing down into the cone of depression produced by mine pumping. Chemistries included: a pH 7.4, Ca-Mg/SO4 water enriched in Cu, Zn, Ni & Co and depositing bright blue secondary minerals to a pH 3.2 Ca-Mg/SO4 water with 11 ppm dissolved Al.