Explained by Michael P. Foose and William F. Cannon
On the choice of deposit models
A number of large magmatic sulfide deposits are known in the Duluth Complex in Minnesota. The copper sulfide deposits occur as irregular disseminations near the base of various mafic intrusions. Deposit grades range up to about 0.6 percent Cu and 0.2 percent Ni; Cu/Ni ratios typically are between 2.5 and 3.0 and cutoff grades used for resource estimation are typically about 0.25 percent combined Cu and Ni. Similar mineralization is known on a smaller scale in the Mellen Intrusive Complex in northern Wisconsin.
On the delineation of permissive tracts
Known deposits in Minnesota occur along the basal (northwestern) edge of the Duluth Complex, a Middle Proterozoic intrusive complex composed predominantly of mafic to anorthositic intrusive bodies. The complex extends for about 250 km along a northeast trend and dips gently to the southeast. Footwall rocks of granite and metasedimentary rocks are exposed along the northern and western edge of the complex, whereas flood basalts form the roof of the complex along its southeastern margin.
Isotopic studies have shown that a significant part of the sulfur in these deposits was assimilated from adjacent sedimentary footwall rocks. Conversely, there are no known deposits in locations removed from this footwall where country rock sulfides would not have been available for assimilation. For this reason, the permissive tract within the Duluth Complex has been limited to the basal parts of the complex along the northwestern footwall. It encompasses the lower troctolitic series of the complex which is host to all known deposits.
The permissive tract is extended to eastern Iowa and southern Minnesota where aeromagnetic and gravity data define anomalies interpreted by Sims (1990) as a large mafic to ultramafic body, probably correlative with the Duluth Complex. Recent geophysical studies suggest that the anomalies represent a series of individual intrusions, rather than a single body (Raymond Anderson, personal commun., 1995). These bodies intrude granitic rocks of the Early Proterozoic terranes wich form the eastern and southern margins of the permissive tract. Middle Proterozoic clastic sedimentary rocks unconformably overlie the intrusions on their western margin. No drill data or other evidence relating to lithology, age, or possible mineralization are available, and correlation with the Duluth Complex must be viewed as highly speculative at this time. If the intrusions are similar to the Duluth Complex, a critical factor for evaluating potential for Cu-Ni sulfides would be whether the country rocks contain a source of sulfur that could have been assimilated during emplacement.
A third part of the tract encompasses the Mellen Intrusive Complex in northern Wisconsin. This intrusive complex ranges from peridotite to granite but is dominantly a variety of gabbroic and anorthositic rocks. It intrudes Middle Proterozoic basalts and Early Proterozoic metasedimentary rocks. Mineralization has been identified at several localities and is very similar to that in the Duluth Complex (Bakheit, 1981).
On the numerical estimates made
The lack of a grade-tonnage model for this type of deposit precludes making a quantitative estimate of undiscovered resources. Estimates, controlled by drilling, of known Cu resources made for part of the Duluth Complex indicate approximately 4 billion tons of mineralized rock grading 0.6 percent Cu and 0.2 percent Ni (Listerud and Meineke, 1977). These estimates make the Duluth Complex the largest Ni resource in the United States. At least equally large Ni-Cu resources can be inferred for the remaining permissive tract in the Duluth Complex for which no estimates have been published.
No resources are known in the part of the tract in Iowa and Minnesota but it is virtually unexplored. If parts of the country rock contain abundant sulfur which could have been assimilated during intrusion of the complex, very substantial mineralization could be expected. If not, then the potential for mineralization is much less.
The Mellen Complex is known to be mineralized but insufficient exploration has been done to outline and measure deposits. It is likely, however, to contain at least one, and possibly several small deposits with grades comparable to the Duluth Complex.
Bakheit, A.K., 1981, Petrography of Cu-Ni mineralization in Mineral Lake area, Ashland County, Wisconsin: Madison, University of Wisconsin, M.S. thesis, 104 p.
Listerud, W.H., and Meineke, D.G., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northern Minnesota: Minnesota Department of Natural Resources, Division of Minerals Report 93, 74 p.
Sims, P.K., 1990, Precambrian baement map of the northern Midcontinent, USA: U.S. Geological Survey Miscellaneous Investigations Map I-1853-A, scale 1:1,000,000.