Manganese-Rich Pellets in Green Bay, An Exploitable Mineral Resource of the Great Lakes
Authors: Moore J. Robert 
Published: October 2008

Based on the detailed study of 600 grab samples and cores collected in Green Bay, an arm of Lake Michigan, the presence of a major underwater mineral resource of manganese has been established. The manganese occurs combined with iron in pellets, the majority of which range in size from 0.1 inch to 0.6 inch in diameter. While in some areas as much as 30% of the very fine sand fraction is composed of micro-nodules, the very small sizes are usually present in only limited amounts. Manganese content of the pellets, as determined by neutron activation, varies between 1.0% and 25%, with the average for most samples being slightly over 10%. The major areas of Mn-rich pellets are in the central part of Green Bay, principally offshore from Marinette, Wisconsin, and in the Sturgeon Bay area. Over 200 square miles of bay bottom with abundant manganese pellets have been charted in detail, and current efforts are being made to test the depth of mineralization by core drilling. While over most of the area the pellets are devoid of other metals in other than trace amounts, in two areas totaling some 25 square miles, the copper content of the pellets is noticeably high, ranging between 0.5 and 2.0% by weight. The insitu genesis of the manganese pellets is being studied using trace element, petrographic, and electron probe methods, as well as certain water chemistry data. Conclusions at this time suggest that a major mineral resource of manganese, and to a lesser extent copper, is available in exploitable amounts beneath Green Bay, a feature further enhanced by shallow dredging depths, easy separation, local ore loading docks, and nearby markets.

Introduction

Purpose and Scope of Study

In July, 1968, the University of Wisconsin received a Sea Grant Award from the National Science Foundation which included funds for the investigation of and search for potential mineral resources beneath the upper Great Lakes. Further, in order to provide geological and geophysical, information for the benefit of biologists and physical oceanographers also working in Green Bay, it was decided to initiate our mineral resources research in this arm of Lake Michigan. The selection of Green Bay was also a pragmatic measure, for Professor R. P. Meyer, my colleague in charge of acoustic profiling, and I were interested in testing our dual exploration system (profiling and dredging) in waters which provided several bottom sediment types; - were close to our Madison laboratories, and were reasonably shallow. While it is apparent from the foregoing comments that we did not actually plan a search for a specific mineral deposit, we were cognizant of the scattered occurrences of authigenic manganese in fresh water lakes, both large and small, in the region (Henry Nelson, personal communication). Since no previous investigators had sampled, or surveyed, to an extent which would permit delimitation or rigorous assessment of scattered manganese grains or crust deposits, we purposely sampled on a close spacing of stations in order to establish the presence and the size of any potential mineral resource encountered.