Sedimentary Exhalative (SEDEX) Potential of the Cuyuna North Range, Cuyuna South Range, Emily District, and Portions of Aitkin County - Old Drill Log Searches

Abstract

Many SEDEX deposits, specifically those in Canada and Australia, are hosted by Proterozoic sedimentary rocks that were deposited in intracratonic or intra-rift tectonic settings. Rocks of similar age and tectonic settings are also found within east-central Minnesota. However, exploration for SEDEX deposits has been very limited in Minnesota due to an almost complete lack of outcrop. Also, while extensive drilling for iron-formation took place in these same areas, little drill core exists for examination. In lieu of this absence of information, all of the old drill log descriptions, from areas in Crow Wing and Aitkin Counties, were searched for any mention of “unique” features that may be indicative of SEDEX mineralization. This report summarizes the results of those drill log re-checks. In addition, core samples were also collected from select drill holes in the Cuyuna North Range for thin-section inspection. The samples were collected from the Mahnomen Formation in order to more fully document the spatial distribution of areas with, and without, tourmaline-bearing rocks. Previous studies (Cleland et al., 1996) found that tourmaline-bearing rocks were present in four widely-spaced holes and a large SEDEX hydrothermal system was envisioned to be present somewhere in the Cuyuna North Range. The lithologic drill log searches found some holes that contain descriptions of sulfide-bearing rocks, and in a very few instances, descriptions were found that are suggestive of tourmaline, but these are vague and uncertain (in all but a few instances, no drill core remains to conduct a further check of these vague descriptions). Attempts to outline further areas with tourmaline-bearing rocks in the Cuyuna North Range, by sampling and thin section inspection, were not successful as only trace amounts of tourmaline were found in only one, out of the eleven, drill holes that were studied. In essence, the thin section study, though limited in extent, indicates that tourmaline is not abundantly present everywhere in the footwall rocks of the North Range; rather, tourmaline-bearing rocks are present in at least three much smaller areas. All of the data assembled for this investigation, when viewed collectively, suggests that some SEDEX potential may be present in the Glen Township Sulfide - South Body of Aitkin County, and in the Cuyuna North Range and Emily District of Crow Wing County. SEDEX potential may also be present in the Cuyuna South Range, but review of the old lithologic drill logs was not instrumental in uncovering any useful data.