Bedrock Geology Map of the Crocodile Lake Intrusion: Cook County, Northeastern Minnesota

Abstract

This map includes new bedrock geologic mapping by the authors combined with compilation of previous work (see references) under the direction of Dr. Dean Peterson for the 2015 Precambrian Research Center’s field course. The intent of this work was to improve upon previous mapping and to investigate potential economic mineralization near the base of the Crocodile Lake intrusion in Cook county Minnesota. Bedrock outcrop mapping at a scale of 1:20,000 was conducted in the Boundary Waters Canoe Area Wilderness (BWCAW) focusing on the areas immediately surrounding Crocodile Lake. During a period of five days, 233 outcrops were mapped along >130 miles of both land and canoe raverses. In addition, over 45 rock samples were collected from the field to aid the USGS in a regional assessment of mineralization potential. Early reconnaissance mapping was done by Dr. Dean Peterson and Dr. Phil Larson along Shoe Lake Road to the southeast of the map area. Using work done by F.F. Grout from 1925-1927 as a starting point, our intent was to discriminate units initially mapped by Grout as undifferentiated gabbro. After collecting data we were able to discern six units of gabbro, one of which has trace sulfides. Comparisons were made to the Poplar Lake intrusion, a similar Early Gabbro series of the Duluth Complex to the west of the map area. While the Poplar Lake and Crocodile Lake intrusions may be a correlative, a comparison of the lithologies suggests a distinction between the two. However, further mapping of the contact is needed for a definite conclusion. The authors acknowledge and appreciated the support of the University of Minnesota-Duluth, and to the staff of the Precambrian Research Center.