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).

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