Bedrock Geology, Sample Location, and Property Position Maps of the West Birch Lake Area, South Kawishiwi Intrusion, Duluth Complex, Lake and St. Louis Counties, Northeastern Minnesota

Abstract

This map (NRRI/MAP-2002/02) is the outcome of eight field days mapping and sampling in the area by the senior author. The initial impetus for this mapping was to try to define Duluth Complex induced contact-metamorphic zonation in the footwall Giants Range batholith, and to relate this to Cu-rich mineralization in these rocks. Research into footwall Cu-rich mineralization continues, and will be published in the future. However, the discovery of large expanses of Cu-Ni mineralized rock in the basal zone of the South Kawishiwi, in an essentially unmapped area, lead to this preliminary map (Figure 1). The geologic map represents the initial interpretaton of the bedrock geology of the basal zone of the South Kawishiwi Intrusion, based on mapped outcrops, subcrops, and glacial materials (float). In addition, geologic units intersected in drill holes have been projected updip to the surface. The faults depicted on the map are interpreted from aeromagnetic data, steepening of the dip of the basal contact of the Duluth Complex, and topographic lineaments. The location and simplified regional geology encompassing the map area is depicted in Figure 4. The lithologic legend of the geology map is simplified into the intrusive stratigraphy of the South Kawishiwi Intrusion first defined by Severson (1994). Readers interested in detailed descriptions of the regional South Kawishiwi Intrusion stratigraphy are referred to that work. Cu-Ni-PGE mineralization is largely confined to the basal stratigraphic units of the intrusion (units BAN, BH, and U3), and on the ground is largely represented by knob-like outcrops, and large expanses of rusty, gossaneous boulder fields (subcrops). Old test pit dumps (circa 1890 ?) into the Biwabik Iron Formation are common in the southern portion of the map, and occur in areas of anomalous magnetic field properties. Seventy-five rock samples (Figure 2) were collected in the area (described in Table 1), and Dr. Philip Brown and John Marma (Department of Geology, University of Wisconsin - Madison) provided the funding for the base- and precious-metal analyses of twenty of these samples (presented in Table 2). Check assays for anomalous samples were analyzed by ALS Chemex labs from the original pulps and rejects (Table 2). Assay data for the majority of the drill holes in the map area have been compiled by Peterson (1997), which includes > 60,000 geochemical analyses for drill holes throughout the Duluth Complex. The smaller-scale property position map (Figure 3) depicts the current mineral lease holders in the area, and should only be viewed as a "snapshot" of the mineral land positions at the date of this map. Detailed geologic mapping in the area, including additional geochemical analyses, has been approved from the Permanent University Trust Fund, and will be completed during the 2002 field season.