Shaded Relief Map of the Basal Contact Surface of the South Kawishiwi Intrusion Duluth Complex, Northeastern Minnesota

Abstract

This map depicts a 3-dimensional model of the interpreted shape of the bottom of the western margin of the South Kawishiwi Intrusion (SKI) that is based on ~800 drill hole piercing points into footwall rocks and geophysical data (gravity and aeromagnetic). Severson (1994) defined the igneous stratigraphy of the SKI in the map area based on drill core logging, and the vast majority of all geological information used by the author in the development of this map is taken from that work. Compilation and modeling of geochemical data for all of the mineralized zones within the Partridge River and South Kawishiwi intrusions has been presented by Peterson (1997), and is the source for much of the drill hole assay data within the map area. New interpretations and descriptions of the geology and mineral potential of the whole Duluth Complex have been recently completed (Miller et al., 2001; 2002), and readers interested in the geology of this map area should refer to all these works. A simplified 3-dimensional bedrock geological map of the rocks adjacent to, within, and beneath the western margin of the South Kawishiwi Intrusion is presented in Figure 1. Cross sections of the topographic expression of the basal contact of the SKI are presented in Figure 2. The 3-D model has been instrumental in the development of new ideas on the styles and origin of the Cu- Ni-PGE mineralization within the SKI. Integration of regional geological, geophysical, and geochemical features with the 3-D model has led to new ideas on possible feeder channels for magmas of the northern SKI. The interpreted master magmatic feeder channel of the northern SKI is fed from the central Mid- Continent Rift through the Bald Eagle Intrusion gravity high, into a dike-like body of troctolitic rocks (herein termed the Bald Eagle Trough) cutting older Anorthositic Series rocks (Fig. 3). Integration of the concept of a magmatic feeder channel with assays has led to the development of conceptual models for the formation of the Spruce Road (Peterson, 2002) and Maturi (Peterson, 2001) deposit areas. Early magmas that formed the Spruce Road deposit were deflected to the north by a pillar of older Anorthositic Series rocks that is located at depth within the northern portion of the SKI (Fig. 1). Moreover, PGE-enriched Cu-Ni mineralization of the Maturi Extension deposit is located beneath the pillar, and a conceptual model for the formation of this deposit area is presented in Peterson (2001). 1) Open - (early) vertically extensive (> 450 meters) mineralization with generally low to moderate Cu-Ni grade and low Au+PGE grades; and 2) Confined - (later) vertically restricted (< 150 meters) mineralization with moderate to high Cu-Ni grades and moderate to very high (locally) Au+PGE grades; and 3) Cloud Zone - (latest) Cu-Ni-PGE mineralization seemingly unrelated to the basal contact. Regional geologic and crosscutting relationships (Fig. 3) indicate that the Open-style mineralization preceded the Confined-style.