Penokee-Gogebic Range: Geology and Iron Mine Impacts

Introduction

This paper focuses on the Penokee Range in Wisconsin, analyzing various aspects of the range including its general geology, formation, surficial geology, and associated geological questions. The paper also addresses the major issues surrounding the proposed construction of an iron mine on the range, discussing how the mine would affect the range's geology and the broader environmental impacts such a project would entail.

The Penokee-Gogebic Range in Wisconsin is an important feature of the region. In recent years, however, it has attracted significant attention due to a proposal to construct a mine in what is regarded as one of the area's ecological hotspots. Environmental agencies, legislators, and local residents have raised strong objections concerning the environmental and social risks that iron mining on the range could cause. The company seeking to build the mine has claimed the support of a "silent majority" of residents in Ashland County who, it argues, back the proposal to construct an extensive iron mine in the Penokee Range (Broman, 2011). This paper presents an analysis of the likely environmental impact of the proposed iron ore mining operation on the Gogebic-Penokee Range.

Formation and Geology of the Gogebic-Penokee Range

The formation of the Gogebic-Penokee Range can be traced to the Precambrian iron formations described by James (1973). The range was formed during the second phase of the Penokean Orogeny, a period of mountain building that took place in the Early Proterozoic period (approximately 1.85–1.84 billion years ago) across what is now Wisconsin, Minnesota, Ontario, and Michigan. The Penokean Orogeny was a major event in the formation of the North American continent. Prior to this event, the area was a passive continental margin occupied by a relatively shallow sea, which created the large sediment deposits — including the banded iron formations that form part of the Iron Ranges. The Penokean Orogeny occurred in two distinct phases. The first phase involved the collision of an island arc known as the Pembine-Wausau terrane with an ancient North American craton, leading to the formation of a back-arc basin. The second phase, which contributed to parts of Wisconsin and Illinois, involved the Marshfield terrane, a microcontinent (Holst, 1989).

The Penokean Orogen

The Penokean orogen is best described as a highly deformed Early Proterozoic belt. It trends toward the southwest and meets the Archean craton margin along its southern edge (Hoffman, 1988). The Penokean orogen is truncated by the Grenville orogen to the east and by the Central Plains orogen to the southwest (Sims and Peterman, 1986).

Early models of the Penokean orogeny in Minnesota emphasized intracratonic tectonism as the main mechanism of formation (Morey and Sims, 1976). These models stressed the role of basement rock that underwent vertical remobilization, as noted by Morey (1979). A similar explanation was applied to formations in Upper Michigan, where Cannon (1973) emphasized the role of vertical movement of basement rocks in the region's structural evolution. Over the last several decades, however, a new theory has emerged presenting plate collisional movement as the primary feature of tectonic models for the Penokean orogeny in Minnesota and Upper Michigan (Van Schmus, 1976; Sims et al., 1987).

The rock formation contains several assemblages. The Wisconsin magmatic terranes are part of the southern assemblage and are primarily composed of volcanic and plutonic rocks. Geochemical analyses have indicated that these rocks have an affinity for island arc settings (Sims et al., 1989; Schulz et al., 1984; Schulz, 1983).

The distribution of lithologies in the Middle Precambrian metasedimentary and metavolcanic rocks includes iron formations, slates, conglomerates, and carbonates. These rocks are least deformed and metamorphosed in regions toward the extreme northwest, while rocks to the south contain kyanite and other metamorphic grade minerals. The metavolcanic rocks are composed of felsic and mafic lavas as well as epiclastic and pyroclastic materials. The granitic rocks include granitic plutons, post-tectonic alkali-rich granites, and epizonal granodiorite (Ostrom, 1979).

A schematic cross-section of the region reveals the lithologic and structural features that developed during the Middle Precambrian "Penokean" event. Wisconsin near Wausau was positioned at the Middle Precambrian period close to 1,900 million years ago. To the north lies a platform containing sediments such as iron formations and carbonate deposits (Ostrom, 1979).

Site Description and Conservation

Martin (2008) noted that the area is composed of a geologic feature known as the Penokee Range, whose rugged topography was formed several million years ago. Since then, the area has weathered and eroded to produce its current configuration, featuring a three-hundred-and-twenty-foot rise in elevation. The underlying rock is highly resistant — four separate glacial events failed to completely pulverize it. Two rivers, the Montreal and the Potato, break through the range and create spectacular classic water gaps in this section (Martin, 2008). To the south of the Penokee Range, Alder Creek runs slowly and parallel to the topography of the range. The area features resistant shale, swamp conifer vegetation, and alder growth, while the bedrock supports forest vegetation including aspen, yellow birch, maple, and basswood, along with patches of other plant communities.

Issue of Conservation

Areas of steep topography within the range have historically not been extensively disturbed. The entire range was previously subject to deep-hole mining activities that ended sometime in the 1960s, leaving it relatively undisturbed thereafter. What remained were extensively forested areas with mature vegetation. Various forms of development on the forested range, combined with forest-harvesting activities, over-browsing by the local deer population, and the spread of invasive species, have degraded habitats extensively and led to a simplification of the forest structure. In response, Iron County Forestry recognized the Penokee Range as an Important Bird Area and designated close to 1,700 acres of County Forest as a biological reserve, appropriately named the Penokee Biological Reserve.

The Proposed Iron Mine and Taconite Ore

Several companies have expressed strong interest in constructing mines to extract iron ore from the iron-rich Penokee-Gogebic Range, an area that extends about twenty-five miles through Ashland and Iron Counties in northern Wisconsin (The Nature Conservancy, 2011). A large portion of the range belongs to RGGS Land and Minerals, Ltd., a company headquartered in Houston, Texas, together with LaPointe Mining Co. from Minnesota. Both companies control close to 22,000 acres along a twelve-mile narrow stretch of the larger Penokee Range — a portion that lies southwest of Hurley and about six miles west of Mellen. The Cline Group, based in Florida, has also secured an option to obtain mineral rights held for the property and has created a subsidiary named Gogebic Taconite (G-TAC) to proceed with iron ore mining.

Any decision to permit extensive mining of iron and other minerals in the region must carefully weigh impacts on the forests, water, and other natural resources that form an important component of the regional economy and livelihoods. Approving mining activities in the area would likely disrupt the environmental and social equilibrium of the region, causing destruction of forest, loss of wildlife, disruption of tourism economies, loss of cultural heritage, and contamination of currently clean water supplies. The result could be disease and death for animals, humans, and plants that depend on the existing biodiversity.

Taconite Iron Ore and Its Source

The iron to be mined from the Gogebic-Penokee Range is taconite iron ore — a low-grade, magnetic iron ore. When high-grade iron ore was still available in large quantities, taconite was considered waste rock and was not extracted. As high-grade supplies dwindled, however, the iron ore mining industry recognized taconite as an important resource and developed processes for concentrating the low-grade ore into taconite pellets that are economically viable. Because taconite iron ore is attracted by magnets, it is classified as magnetite. Magnetite is abundant in the Minnesota Iron Range, the Michigan Iron Range near Marquette, and in the Penokee Range in Wisconsin. Within the Gogebic-Penokee Range, taconite iron ore deposits are concentrated in bands running from the Mellen area in Ashland County to the area near Upson in Iron County.

The Taconite Iron Ore Extraction Process

Mining of taconite iron ore in the Gogebic-Penokee Range would be carried out by open-pit mining methods. The process begins with drilling bore holes to determine the exact location and quality of the iron ore deposit and to reveal the characteristics of surrounding rock. For large modern mines, numerous exploratory and characterization drill holes into the iron formations are required. Once the iron ore body and the overburden have been adequately characterized, the non-ferrous rock lying over and adjacent to the iron deposit is removed. A stripping ratio of waste rock to iron ore of 2:1 or 1:1 is common. If the iron ore deposit has a vertical orientation relative to the ground surface, the amount of overburden to be removed may exceed the amount of iron ore to be extracted. Surrounding rock is hauled out of the open pit and stored as waste rock. The iron ore itself is blasted with dynamite, loaded onto trucks, removed from the pit, and transported to a processing plant where it is crushed using a series of crushers and mills.

The scale of land alteration associated with taconite extraction is enormous. Open-pit mines would create permanent changes to the landscape, and the resources and habitats supported by the Gogebic-Penokee Range would also be permanently removed.

Conclusion

A review of taconite mining experience in Michigan and Minnesota, as documented by the Great Lakes Indian Fish & Wildlife Commission (2011), indicates that the environmental impact of the proposed mining activity on the Penokee Range would at best lead to a permanent alteration of the landscape and at worst result in environmental damages that pose serious human health risks. Taconite mining projects are extremely complex, requiring expansive land areas and significant natural resources. Understanding the true impact of any specific project therefore requires a comprehensive and detailed environmental impact assessment (EIA) containing precise or well-approximated descriptions of the water, land, and other resources that would be affected. No mining activity on the Penokee-Gogebic Range should proceed without such a thorough assessment in place.

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