USGS Home
Contact USGS
Search USGS

Environmental Geochemistry to Evaluate Risks Associated with Past and Future Mining in the Lake Superior Region

Generalized map of the western Lake Superior region showing the distribution of major rock types of the Midcontinent rift and locations of known Ni-Cu sulfide mineralization in Minnesota and Wisconsin. Also shown are the locations of favorable new targets for Ni-Cu-PGM exploration -- Mellen Complex, Echo Lake Gabbro, Yellow Dog Peridotite, and an inferred intrusion recognized from geophysical data in western Wisconsin.(From a USGS information handout, 1998)

Objectives:

1. Build a GIS of pertinent geologic, hydrologic, climatic, and other ecosystem data sets to form the scientific basis for future dialogue regarding potential environmental risks associated with future mining;
2. Conduct targeted sampling of surface water and sediment in priority watersheds to fill data gaps in existing data sets from the perspective of pre-mining baseline characterization; and
3. Investigate the environmental characteristics of relevant deposit types to understand specific risks associated with specific deposit types with an emphasis on PGM deposits in the Duluth Complex, sediment-hosted copper deposits, and banded iron formations.

Background:

• mafic intrusion hosted layered Ni-Cu-PGM deposits,
• conduit-type Ni-Cu-PGM deposits,
• sediment-hosted copper deposits,
• banded iron formation, and
• volcanic-hosted massive sulfide deposits.

Collectively, these deposits represent significant potential sources of critical minerals (i.e., Ni-Cu-PGMs) and staple commodities (i.e., iron, copper, nickel, zinc, gold, lead, and titanium) needed to sustain our growing national economy.

The Lake Superior region experiences a climate that is distinctly different from most other areas in the United States with mine development interests. It is affected by both warm, humid air from the Gulf of Mexico and cold, dry air from the Arctic, creating distinct seasons. The landscape is dominated by rivers, lakes, and wetlands.

Insight into the environmental characteristics of the numerous deposits types and potential impacts from future mining is limited. Much of the long history of mining in the region, dating back to the 1800s, proceeded with little concern for the environment. However, proposed new mining in Minnesota, Michigan, and Wisconsin has dramatically raised the level of interest in the environmental characteristics of several deposit types, especially Duluth Complex Ni-Cu-PGM, sediment-hosted copper, and taconite iron ore. A reasonable level of understanding is available for volcanic-hosted massive sulfide deposits and magmatic Ni-Cu deposits because of recent permitting and/or mining of these specific deposits.

However, the Ni-Cu-PGM mineralization style of the Duluth Complex does not fit well within existing deposit classifications for many PGM ores. In the Duluth deposits, the sulfide content is significantly higher than "reef" style ores but significantly lower than many typical magmatic Ni-Cu massive sulfide ores. For sediment-hosted copper deposits, much of the environmental impact predictions are based on deposits in areas with very different environmental drivers than the Lake Superior region.
The White Pine Mine, which began large-scale operations in the 1950s and ceased all mining in 1996, continues to be a site of significant environmental degradation because of the tailing impoundment and smelter impacts. Abandoned mines and active taconite mines in Minnesota and Michigan have ongoing issues with sulfate and selenium discharge from pit waters.
An emerging issue for future mining in the Lake Superior region is the tendency to classify many geologically diverse deposit types as "sulfide mines" (National Wildlife Federation, 2012). This gross oversimplification leads to a number of misleading generalizations about the environmental characteristics and risks associated with an eclectic mix of ore deposits. These generalizations render deliberations surrounding the permitting of future mines cumbersome and inefficient.

A geology-based knowledge of the environmental characteristics of these important deposit types will focus attention of important issues and enhance the planning and permitting process for future mines. For many deposits poised on the edge of permitting and future development, a scientific context to evaluate potential impacts of future mining on the environment is largely lacking. The atypical style of mineralization for the Duluth Complex and ongoing issues from iron and copper mining require an up-to-date assessment of potential environmental impacts from past and future mining.

The major goal of this project is to provide that scientific context, in part to facilitate a scientifically sound dialogue about future mining in the region, with a particular emphasis on

• Ni-Cu-PGM deposits in the Duluth Complex,
• sediment-hosted copper deposits, and
• iron deposits.