Surface and Subsurface Geologic Maps of the Soudan Underground Mine State Park, St. Louis County, Northeastern Minnesota

Abstract

These maps were created by Precambrian Research Center field camp students under the guidance of Dr. Dean Peterson of the University of Minnesota, Duluth. The purpose of these maps is to provide Soudan Underground- Mine State Park with an updated geological map of the surface and subsurface geology of the Soudan Mine in northeastern Minnesota for use in geologic tours and scientific research. These maps are the result of five days field mapping by the authors in August 2010. Surface geology was mapped for several hours in the afternoon over a period of five days. The authors worked as individuals to cover the mapping area. The authors recorded notes and observations, collected samples, took structural measurements (bedding orientations, lineation, foliation), and plotted data on a field map. Location was determined using a handheld GPS unit, and structural measurements were taken using a Brunton Pocket Transit. Surface outcrops were exposed by peeling back moss layers and sweeping up remaining soils with wisk brooms. Open pits and other “man-made” exposures were also available within the mapping area, but with limited access. The surface bedrock map (Map 1) displays sheared rocks composed of chlorite (5c) and sericite schists (5s), intrusive rocks such as granite (Gr) and quartz feldspar porphyry (Qfp), and the Upper Sequence rocks which include oxide facies iron formation (4a), lapilli tuff (2e), and greywacke (3a). The surface map also shows two major shear zones which are truncated by the Soudan fault to the east, as well as a minor shear zone. Our assumptions and interpretations of the surface field data were influenced by previously existing map data. Information from Peterson and Patelke’s 2003 surface map, as well as historical drill core data (circa 1954) from the Oliver Mining Division of US Steel Corporation was incorporated into building the final map. The geology of the 27th level west drift was mapped by the authors over a period of five nights. The authors worked as a team to locate contacts, collect samples, take structural measurements (cleavage, lineation, fault orientations, etc.), record notes and observations, and plot important features on a field map. Geologic information from horizontal drill holes were used to interpret the geology adjacent to the drift. Locations in the mine were determined by measuring tape. Structural data were measured using a Brunton Pocket Transit; there was no deviation of local magnetic field observed. In contrast to the surface mapping, the drift provided complete outcrop exposure with rare exceptions where the drift walls and ceilings were obscured by pipes and other man-made structures. The drift map (Map 2) includes rock units that correlate to units on Map 1, however, there are two below ground units, gabbro (Gb) and fragmented schist (5f), that do not show a surface expression. Map 2 includes the geology of the 27th level east drift which was previously mapped by Peterson and Patelke (2003). Map 2 also includes annotations by the authors which indicate the locations of significant drift features that may be relevant for mine employees, visitors, and scientists.