Minnesota's geologic framework is moderately well known from a combination of outcrop mapping where exposure permits, and interpretation of high resolution geophysical data and drilling where it does not.
The state is underlain by rocks of Precambrian age that are covered in part by veneers of Paleozoic and Mesozoic (Phanerozoic) marine strata and rather extensively by Quaternary glacial deposits (Figs. 1 and 2). The Late Archean rocks of the Superior Province and the Early Proterozoic rocks of the Penokean orogen are southwestern extensions of counterparts in southern Ontario that are noteworthy for their abundance of metallic mineral deposits. The mafic igneous rocks within the Middle Proterozoic Midcontinent Rift System trend obliquely across the regional east-northeast strike of the older rocks and separate the Minnesota segments of some lithotectonic belts from their equivalents to the east and northeast in Wisconsin, Upper Michigan, and Ontario. Nevertheless, the geological continuity of Late Archean and Early Proterozoic belts from Ontario into Minnesota is well established.
Archean mineral potential
The Superior Province in Minnesota consists of three subprovinces defined and named in Canada (from north to south, the Wabigoon, the Quetico, and the Wawa); and a fourth, the Minnesota River Valley (MRV) subprovince, which lies south of the Wawa subprovince (Fig. 3). The Wabigoon and Wawa subprovinces are volcanoplutonic belts that consist of deformed, relatively low-grade metavolcanic and metasedimentary rock sequences intruded by granitoid plutons. The volcanic-rich portions of both subprovinces possess lithologic and structural attributes broadly similar to those in mineralized greenstone belts in Ontario. The Quetico subprovince consists chiefly of metasedimentary schist, various migmatitic rocks derived primarily from sedimentary protoliths, and granitoid intrusions. The MRV subprovince consists dominantly of quartzofeldspathic gneiss and large granitoid intrusions. Radiometric ages for gneisses in the MRV terrane are as old as 3.5 billion years, substantially older than the 2.9 - 2.65 billion year ages for volcanic, sedimentary, and intrusive rocks in the volcanoplutonic and metasedimentary subprovinces to the north. The major MRV batholiths, on the other hand, yield radiometric ages in the 2.6 - 2.7 billion year old range. These data, together with subhorizontal structural style, high metamorphic grade, and a sparsity of supracrustal protoliths, suggest that the MRV terrane probably was a pre-existing continental fragment that was accreted to the margin of the Superior craton by northward-verging (directed) subduction at the end of late Archean continental growth.
Based on the history of discoveries in Canada, the Archean greenstone belts of Minnesota are intrinsically prospective for diverse mineral-deposit types including shear-zone-hosted lode gold, iron-formation-hosted stratabound gold, volcanic-hosted base-metal sulfides (VMS), Ni-Cu-PGE-Cr deposits hosted by komatiitic volcanic sequences or associated sills, and Cu-Mo-Au deposits hosted by granitoid porphyry (see Fig. 3).
To date, the only commodity successfully mined from Archean rocks in Minnesota has been iron. Successful exploration in Canada has tended to focus on major faults and shear zones that are both marginal to and within the volcanoplutonic subprovinces (Wabigoon and Wawa). Similar fault structures have been identified through geologic and geophysical mapping in Minnesota, but relatively little systematic mineral exploration has been done along them.
Penokean orogen mineral potential
The Penokean orogen records an extended history of continental extension and convergence that affected the southern margin of the Superior craton in the time interval between 2.45 and 1.75 billion years ago. Several tectonic episodes took place, with the earliest activity in the Huronian belt of southern Ontario and the youngest in east-central Minnesota. The strongest collisional pulse apparently occurred at about 1.85 billion years ago, when intense deformation, metamorphism, and plutonism occurred along the entire strike length of the orogen. In Minnesota, the principal features of the Penokean belt are:
- An arcuate, northwest-verging fold and thrust terrane that involves supracrustal volcanic and sedimentary rocks as well as Archean basement;
- A succession of tectonic foredeeps, the youngest, largest, and best-preserved of which is the Animikie basin;
- Abundant syntectonic to post-tectonic granitoid plutons that range in age from about 1.85 to 1.77 billion years ago.
The world-class iron deposits of the Mesabi iron range are localized in sedimentary iron-formation along the north, or cratonic, margin of the Animikie foredeep (Fig. 4).
Major structural breaks and tectonic elements in the Minnesota portion of the Penokean orogen appear to correspond with similar features on the eastern side of the Midcontinent Rift System in Wisconsin and Michigan. On this basis, the volcanic belts of northern Wisconsin that host significant deposits of massive-sulfide base-metal ore at Crandon, Ladysmith (Flambeau Mine), and other sites may have counterparts in Minnesota (see Fig. 4). Furthermore, the period of intense tropical weathering that affected Minnesota's Penokean rocks in the late Mesozoic may have produced supergene caps above massive sulfide deposits, as was the case at Ladysmith. Little systematic exploration has been done in Minnesota to follow up on these possibilities.
It should be noted also that the Penokean rocks of east-central Minnesota are tectonically and lithologically similar to rocks that host SEDEX Zn-Pb-Ag deposits in Australia (McArthur River, Mt. Isa, Broken Hill) and British Columbia (Sullivan). The details of this comparison cannot be fully described here, but they include the general inference of a rift-related depositional setting for iron- and manganese-rich strata and mineralogical indications of submarine hydrothermal contribution to the chemical systems from which the Fe-Mn sediments were precipitated. The formerly active iron-mining district known as the Cuyuna iron range is an inviting regional target for exploration based on the SEDEX model, as is the Glen Township area northeast of Lake Mille Lacs.
Kimberlites in Minnesota
The ancient cratonic terranes of the MRV subprovince and its reworked equivalent in the Penokean orogen are peppered by small, subcircular aeromagnetic anomalies that are known from scattered drilling to reflect small mafic and ultramafic intrusions (Fig. 5). The possibility that kimberlite pipes (possibly diamond-bearing) may lurk among the several hundred potential anomalies remains open to future investigation.
Sioux Quartzite mineral potential
The Sioux Quartzite, inferred on indirect evidence to have been deposited in the time interval between 1.76 and 1.63 billion years ago, rests unconformably on Archean rocks of the MRV subprovince and minor Early Proterozoic intrusions within them (see Fig. 1). The great bulk of the Sioux Quartzite is a supermature quartz arenite that was deposited principally by a southeastward-draining plexus of braided streams. The basal contact zone of the Sioux Quartzite has been considered as a speculative locus for unconformity-related uranium deposits of the Athabaska type, but it never has been seriously explored. In addition, the formation has received speculative consideration as a host for paleoplacer gold deposits.
Duluth Complex mineral potential
The Midcontinent Rift System developed in response to crustal-scale tectonic extension in the Middle Proterozoic, approximately 1.1 billion years ago. The western arm of the rift extends southwestward from Lake Superior—where rift-fill rocks are moderately well exposed—to the subsurface of the Twin Cities metropolitan area, and from there to the subsurface of northeastern Kansas. The fill associated with the active stages of rift development consists mainly of tholeiitic basalt that was erupted under subaerial conditions, together with petrologically-related sills, dikes, and large layered intrusions that cooled beneath or within the cogenetic volcanic pile. The largest and most important of the layered intrusions is the Duluth Complex, a composite intrusion of troctolite and gabbro derived from periodic tapping of an evolving magma source (Fig. 6). In the waning stages of rifting, the principal rock types deposited in the rift shifted gradually from magmatic to sedimentary; among the sedimentary sequences are those for which alluvial-fan, fluvial braid-plain, aeolian, and lacustrine depositional environments may be inferred.
The Duluth Complex hosts four distinct types of magmatic mineral deposit, none of which is economic to mine at the present time (see Fig. 6). The deposit types include:
- Large, low-grade, disseminated Ni-Cu concentrations, some of which contain local zones enriched in platinum-group elements (PGEs);
- Localized high-grade zones of massive Ni-Cu sulfides, some of which are moderately enriched in PGEs;
- Stratabound PGE-enriched "reefs" associated with specific types of phase-layer transitions;
- Oxide-rich ultramafic plugs that in some instances are potential sources of Ti and V.
Deposit types (1) and (2) occur only at or very near the basal contact of the Complex, whereas types (3) and (4) occur in the basal zone and also at higher levels.
Significant quantities of native copper, native silver, bornite, and other copper minerals were mined in the early 19th and 20th century from hydrothermal vein and stockwork deposits in basalts and interflow sediments of the Midcontinent Rift System on the Keweenaw Peninsula of Michigan. In addition, large amounts of finely dispersed native copper and other copper minerals were mined from a "kupferschiefer" type of deposit in lacustrine siltstone and shale at White Pine, Michigan. Although trace occurrences of native copper, native silver, and various other copper minerals have been found in basaltic rocks along the North Shore of Lake Superior in Minnesota, no mineable deposit of the Keweenaw or White Pine type has been discovered.
Paleozoic and Mesozoic mineral potential
In southeastern Minnesota, essentially undeformed Ordovician carbonate rocks (see Fig. 1) host scattered calcite- or dolomite-filled veins and breccia zones in which marcasite, sphalerite, and galena locally occur. These represent the outer fringes of the regional hydrothermal system that produced the Mississippi Valley type (MVT) lead-zinc deposits farther south, in southwestern Wisconsin and eastern Iowa. As a result of recent refinements in the MVT deposit model, some interest has been rekindled in the lead-zinc potential of southeastern Minnesota. Furthermore, there has been recent interest in the silica sand mining potential of the Cambrian and Ordovician sandstone deposits in southeastern Minnesota and east into Wisconsin (Fig. 7).
Undeformed carbonaceous shale and siltstone of Late Cretaceous age lie unconformably on Precambrian rocks in much of southwestern Minnesota (see Fig. 1). Although some Cretaceous strata are currently being mined for industrial clay, greater potential may exist. The rocks have stratigraphic and compositional attributes that suggest the possibility for shallow-marine sedimentary manganese (Mn) concentration according to the Cannon-Force depositional model. Reconnaissance exploration has determined that some manganese enrichment did occur in these rocks, but no economic Mn deposits have been identified.
There is no denying the fact that much of the Precambrian rock in Minnesota is covered rather continuously by glacial deposits (see Fig. 2). Obviously, this makes the job of mineral exploration more challenging. On the other hand, the very fact that highly prospective exploration targets are concealed, and thus have remained unevaluated, presents opportunities for firms willing to invest in the methods and strategies required for subsurface investigations. Recently completed and ongoing mapping of glacial deposits and bedrock; using outcrops, archived drill core and exploration data, detailed geophysical maps and models, test drilling, and geochemical sampling of unconsolidated sediments provides the geologic background necessary to support new exploration efforts. The reward of a major discovery could await a company that is imaginative, technologically capable, and sufficiently patient to invest in a long-term exploration program.