A Second Addendum To: Geology and Sulfide Content of Archean Rocks Along Two Proposed Highway 169 Relocations to the North of Sixmile Lake, St. Louis County, Northeastern Minnesota

Abstract

This second addendum to GEOLOGY AND SULFIDE CONTENT OF ARCHEAN ROCKS ALONG TWO PROPOSED HIGHWAY 169 RELOCATIONS TO THE NORTH OF SIXMILE LAKE, ST. LOUIS COUNTY, NORTHEASTERN MINNESOTA (Severson and Heine, 2010) examines the bedrock outcrops in the western part of the area along the existing Highway 169 alignment. This area is the western part of the identified “Northern Alignment” alternative. Outcrops were examined to determine rock type, sulfide content, and structural features. Samples were collected of both representative rock types and any rock types with elevated sulfides present. Twenty-six samples were collected during this project, and an additional sixteen samples collected during the 2010 work were included from the area where the areas overlapped.

The current map area contains generally higher visual pyrite content and higher sulfur analyzed by Leco technique (AcmeLabs, Canada) than observed along the “Southern Alignment” in Severson and Heine (2010). This elevated sulfur content is due to a number of factors:

1. The presence of the upper black cherty Soudan Iron-Formation, which contains syngenetic disseminated and bedded pyrite (FeS2); 2. The presence of faults in the central part of the area and increased localized shearing; and 3. The new rock units that include the Gray Basalt and Gafvert Lake Sequence, which both contain elevated sulfides.

Observed sulfide mineral contents and chemical concentrations are higher in the Northern Alignment than the Southern Alignment, suggesting that based on sulfide mineral contents and sulfur concentrations alone, the Southern Alignment would be the preferred alignment; however, consideration of other factors beyond the scope of this report are also required for determining the final location for the highway. Additionally, should this route be chosen, additional drilling would be needed to supplement the outcrop information in areas with less exposure.