Bedrock Geologic Map of the Duluth Complex in the Northern South Kawishiwi Intrusion and Surrounding Area, Lake and St. Louis Counties, Minnesota

Abstract

This map is the result of numerous investigations by the author and many others over the last 8 years of the South Kawishiwi intrusion (SKI) and it's contained Cu-Ni-PGE mineralization. Detailed geological mapping evolved from a study of the Nickel Lake Macrodike (NLM) into a comprehensive geologic mapping and compilation project (104,000 acres) to answer some of the fundamental questions on the origin of the extensive known and undiscovered Cu-Ni-PGE mineralization in the northern portion of the SKI. Such an increase in scope is needed due to the economic significance of the published resource estimates (>$146 billion in contained metal) from this area. To date, nearly 15,000 outcrops, 1,400 structural measurements, geology and geochemistry from 773 drill holes totaling over 845,000 feet of core, and 12,500,000 meters of elevated contour lines (see Digital Topography image below) have been integrated into the comprehensive GIS database. The map units of the SKI depicted on this map sheet differ from previous maps from the area, in that the author has consulted with numerous company geologists and defined map units based on what industry geologists use to define rock masses encountered in drill core. This new map includes geology from each of the major lithologic units in the area, namely the Late Archean Giants Range batholith, the Paleoproterozoic Biwabik Iron and Virginia Formations, and the Anorthositic Series, Bald Eagle Intrusion, SKI, and the NLM of the Mesoproterozoic Duluth Complex. There are only a few faults depicted on the map, and literally hundreds of kilometers of linear topographic features that remain to be investigated in detail (see Digital Topography image below). However, little obvious offset of rock units have been observed along these features where investigated in detail, thus the author has purposely not drawn many faults on the map. One main new insight of this recently completed compilation is the recognition that the northeastern extent of the SKI is not a shallowly dipping sill but rather a southwest trending, inclined funnel-like body. Such an interpretation leads to the conclusion that the eastern contact of the SKI, which previously was interpreted as the top of the intrusion, is a basal contact, and thus has great potential for hosting Cu-Ni-PGE mineralization at its base. Understanding the origin of mineralized zones is the goal of all economic geologists, and in magmatic ore systems like the SKI, one must try to imagine the magmatic processes that culminated in the formation of the ores and surrounding rocks, i.e. how did the SKI form? Did the magmas intrude as crystal-laden slurries? Are the "Open" and "Confined" styles of mineralization defined by Peterson (2001) true mappable units? Such thoughts are the basis upon which the author began the quest to complete this map sheet. The author has inserted a number of inset maps and figures for the reader to review and ponder about the possible types of magmatic prosesses that occurred in the area (now depicted on this map sheet) 1.1 billion years ago. It is hoped that careful review of the bedrock geologic map and inset figures will give the reader and user of the map new geologic insight and ideas for future mineral exploration programs and scientific study.