Summary:
The depositional sequence at the Emily deposit records two periods of transgression and regression within the chemical sediments of the Emily Iron Formation bracketed by periods of clastic deposition. The Emily Iron Formation is constructed from a sequence of fine- and coarse-grained iron formation subunits that correspond to rise and fall of sea level during deposition. The sequence of transgressions and regressions observed at Emily is consistent with similar sequences in the Biwabik Iron Formation on the Mesabi Iron Range. The variations in water depth and corresponding grain size, composition, and morphology have previously been linked to changing sediment sources and input due to regional tectonics driven by the Penokean orogen. The observed changes play a critical role in the initial distribution of manganese and subsequent remobilization during supergene processes highlighting the importance in understanding the sequence.
The Emily deposit is hosted by rocks of the Paleoproterozoic Animikie Basin (the Emily Iron Formation). The stratigraphy, structure, and high-grade manganese mineralization within these rocks is the result of long periods of sedimentation, deformation, and erosion along the ancestral southern margin of the Superior Craton. The driving force in the sedimentation and deformation of these rocks occurred during the Paleoproterozoic Penokean Orogeny.
Mineralization
Penokean Orogeny
The Penokean deformation in Minnesota includes a southern intensely and complexly deformed series of thrust panels (Cuyuna North, Cuyuna South, Moose Lake, McGrath-Little Falls panels) that gives way northward to progressively more weakly and simply deformed rocks (Emily District) across a belt about 66 miles (100km) wide. Farther north strata in the Mesabi and Gunflint Iron Ranges are essentially undeformed (Holst, 1991). It should be noted that the “more weakly and simply deformed rocks” of the Emily District have been shortened ~250% into a series of shallowly east-plunging anticlines and synclines. Substantial progress has been made in deciphering the structure of the poorly exposed rocks of the Minnesota foreland through the use of aeromagnetic and gravity data and drillhole information. Southwick and Morey (1991) and Southwick et al. (1988) have presented syntheses of this information.
The complex thrust panels on the south, like comparable structures in Michigan, appear to be thin-skinned slices without Archean basement rocks. However, as in Michigan, this area of thin-skinned thrusting is also the area where Archean-cored gneiss domes developed during post orogenic collapse of the Penokean orogen (Holm and Lux, 1996; Schneider et al., 2004). Farther north, basement-cover relations are not well known except for the Mesabi Iron Range where Paleoproterozoic strata are mostly nearly flat lying above an undisturbed unconformity with Archean basement rocks.
Post Penokean Weathering and Erosion
Perhaps the most important component in the formation of the high-grade manganese resource at the Emily deposit is the vast amount of time (measured in hundreds of millions of years) upon which the newly formed and uplifted Penokean mountains of the southern Laurentia craton weathered and eroded. As plate tectonic forces moved Laurentia across the globe to its current position on planet Earth there were long periods of time when it resided within the tropical weathering zone (+30° to -30° latitude) near the Earth’s equator. It is believed that the supergene enrichment of manganese (to >50 wt.% elemental Mn) at the Emily deposit largely formed during the protracted periods of time that the area resided within the tropical weathering zone.
Animikie Basin Mineral Resources
To gain a true understanding of the geology and mineral resources of the Emily Manganese Deposit, it is best to start with an understanding of the regional-scale geologic setting and its contained ferrous mineral resources. For this Report, a brief description of Minnesota’s Paleoproterozoic Iron Ranges and their contained ferrous mineral resources is included herein. These Paleoproterozoic Iron Ranges include several categories of marine chemocline mineral systems outlined in recent USGS publications (Schulz et al., 2017 and Hofstra and Kreiner, 2020). These categories include:
1) Superior-iron deposits (Mesabi Iron Range, Gunflint Iron Range and the Emily District of the Cuyuna Iron Range) and
2) Algoma-type iron +/- manganese deposits (Cuyuna North and South Iron Ranges, and the Vermilion Iron Range).
Superior Type Iron Resources of the Mesabi Iron Range
Superior type iron formation resources of Minnesota are exemplified by the long-standing mining of iron resources of the Biwabik Iron Formation along the length of the Mesabi Iron Range. The Mesabi Iron Range is largely located in St. Louis and Itasca counties and has been the most important iron ore district in the United States since ~1890s. The Mesabi Iron Range is 120 miles (193km) long, averages one to two miles wide, and is comprised of rocks of the Paleoproterozoic Animikie Group. The Animikie Group on the Mesabi Iron Range consists of three major conformable formations: Pokegama Formation at the base; Biwabik Iron Formation in the middle; and the overlying Virginia Formation. On the Mesabi Iron Range, these three formations generally dip gently to the southeast at angles of 3-15 degrees.
Since the early 20th century, the Biwabik Iron Formation has been subdivided into four informal members referred to as (from bottom to top): Lower Cherty member, Lower Slaty member, Upper Cherty member, and Upper Slaty member (Wolff, 1917). The cherty members are typically characterized by a granular (sand-sized) texture and thick-bedding (beds = several inches thick); whereas the slaty members are typically fine-grained (mud-sized) and thin-bedded (=1 cm thick beds). The cherty members are largely composed of chert and iron oxides (with zones rich in iron silicate minerals), while the slaty members are composed of iron silicates and iron carbonates with local chert beds. Both cherty and slaty iron-formation types are interlayered at all scales, but one rock type or the other predominates in each of the four informal members, and they are so-named for this dominance Severson et. al. (2009).
Mn-Fe Resources of the Cuyuna Iron Range
The Cuyuna Iron Range is about 100 miles (160 km) west-southwest of Duluth in Aitkin, Cass, Crow Wing, and Morrison Counties. It is part of an Early Proterozoic geologic terrane which occupies much of eastcentral Minnesota. The Cuyuna Iron Range is traditionally divided into three districts, the Emily District, the North Range, and the South Range. The Emily District extends from the Mississippi River northward through Crow Wing County and into southern Cass County and comprises an area of about 450 mi2 (1,165 km2). Although exploration drilling was extensive in the Emily District, mining never commenced. The North Range, a much smaller area about 11.8 miles (19 km) long and 5 miles (8 km) wide, is near the cities of Crosby and Ironton, including the former town of Manganese, in Crow Wing County, and the South Range extends approximately 62 miles (100 km) and up to 3 miles (5 km) in width, near Deerwood and Brainerd, in Atkin, Crow Wing and Morrison counties.
Cuyuna Iron Range Manganese Resources
There are additional manganese and manganiferous iron occurrences in the Cuyuna Range. Although attempts have been made, including reports by the U.S. Department of the Interior, the U.S. Geological Survey and the State of Minnesota, there is no credible estimate of the size and potential of the manganese resources withing the Cuyuna Iron Range.