Emerging Geoenvironmental Issues Related to Proposed Mining in the Lake Superior Region
Science Center Objects
We are studying environmental issues related to mining, and potential mining, in the Great Lakes region, continuing the study of characterizing baseline geochemistry of several watersheds in Minnesota and Michigan, examining the potential for aquatic toxicity from metals, and examining the acid-neutralizing and acid-generating potentials of mine waste, and the environmental, and possible human health, effects of waste generated by past iron mining and steel-making.
Science Issue and Relevance
The Lake Superior region is the focus of widespread mineral exploration and startup mining activity, due to its promising geologic settings for more deposits of minerals critical to the U.S. economy. However, local communities have concern about potential for environmental harm caused by mining, as some past mining activities here have impacted water quality. Concerns are often voiced about the dangers of “sulfide mining". This is an over-simplified perception of mining. This region has a unique climate, and the landscape is dominated by lakes, rivers and wetlands. Preventing environmental impact requires full consideration of the influences of geology, hydrology, climate, mining methods, ore-processing methods, and continued evolution of environmental management practices on environmental risks of unique deposit types. Our science looks beyond acid-generating potential of the ores, and includes the assessment of potential risks to human health and aquatic ecosystems from trace metals.
Methodology to Address Issue
- We are studying environmental issues related to mining, and potential mining, in the Great Lakes region. Our study areas are recently proposed metal mining ventures in the Lake Superior region, including:
- deposits located within the basal zone of the Duluth Complex,
- deposits in the Ironwood Iron Formation,
- and deposits associated with the Nonesuch Shale.
- We are continuing the study of characterizing baseline geochemistry of several watersheds in Minnesota and Michigan, and examining the potential for aquatic toxicity from metals.
- We are examining the acid-neutralizing and acid-generating potentials of mine waste, and the environmental, and possible human health, effects of waste generated by past iron mining and steel-making, a priority brought to light by discussions with our collaborators and stakeholders.
Watershed Baseline Chemistry and Copper Toxicity Study: We will determine the pre-mining chemistry of watersheds near copper-nickel-platinum group element deposits in Minnesota, and copper deposits in the Upper Peninsula of Michigan. Two methods have been used by regulatory agencies in this region to assess copper toxicity to aquatic organisms; these include calculating water quality criteria based on hardness and based on the Biotic Ligand Model. We will compare the predictions of potential toxicity from both methods at sites in Minnesota and Michigan. Next, we will evaluate our predictions, based on the results from toxicity tests done in collaboration with the USGS Columbia Environmental Research Center.
Iron Life Cycle Studies: We will analyze and help predict the source, transport, and fate of products from iron mining in the Gogebic Range of northern Michigan and Wisconsin, and from iron and steelmaking slag in the Chicago region of Illinois and Indiana. Analyses will include:
- Fibrous amphibole minerals – a notable human health risk – which have been identified in parts of the Duluth Complex, and in parts of the Ironwood Iron Formation (Wisconsin)
- Sulfide oxidation, acid generation, acid neutralization, and trace element release from potential mine waste, and rock or soil overlying a mineral deposit, which may influence water quality
- Sulfate sources in watersheds that may be influenced by mining (high concentrations of sulfate may be toxic to wild rice, an important subsistence crop for Native Americans)
- Iron and steel slag is present in large volumes, and is known to be an environmental liability. However, it is also a beneficial resource for road construction and has potential for use in the removal of phosphate and nitrogen from waste water and agricultural water
Mine Waste Prediction: We will determine how potential mine waste from mining deposits in the Duluth Complex (northeastern Minnesota) may release trace elements into the environment, and inform communities and stakeholders of best mine waste management practices. We will also refine our ability to predict (via mineralogical and chemical studies, as well as reaction pathway geochemical modeling) the potential for silicate minerals in the Duluth Complex ore to neutralize acid. Rocks that host the mineralization in the Complex (and will become the waste during mining) are unusual, in that
- they are essentially devoid of carbonate minerals — hence they lack carbonate-neutralizing potential, and
- they have a quantifiable, yet low, acid-generating potential.
However, there is potential for beneficial acid-neutralization from the silicate minerals that make up most of the rock hosting the metals of interest.
Completed Activity - Environmental Geochemistry to Evaluate Risks Associated with Past and Future Mining in the Lake Superior Region
This project was a geoenvironmental assessment to support development of responsible mining methods in the unique Lake Superior region. This benefitted all stakeholders in decision-making regarding future mining: land managers, the mining industry, regulatory agencies, non-government organizations, and citizens. We studied the two kinds of deposits most likely to be developed here:
- nickel, copper, and platinum-group metal deposits in the Duluth Complex of northeastern Minnesota,
- sediment-hosted copper deposits of the Porcupine Mountains area, Michigan.
Examples of environmental problems we addressed:
- Do conditions currently exist from metals in waters of unmined mineralized watersheds, that may be toxic to aquatic organisms?
- Can we make predictions regarding the potential environmental risks from mining and mine waste for these deposits?
Our results make the planning and permitting process for future mines more efficient, and facilitate a scientifically sound dialog about future mining. Project accomplishments include:
- We built a Geographic Information System (GIS) of ecosystem data sets – the basis for dialogue about potential environmental risks associated with future mining. Data layers include geologic, geochemical, hydrologic, climatic, ecological, demographic information.
- We sampled and tested surface water and stream sediment in watersheds affected by the two dominant deposit types, for pre-mining baseline geochemical characterization. We coordinated specific sampling areas with the Midwest Area Mining Initiative, for access to water resource data sets that otherwise would have been less than easy to use, and world-class stream gaging data for the affected watersheds. We analyzed different methods used by regulatory agencies for assessing the surface-water toxicity of copper, as well as methods used for assessing metal toxicity in sediments.
The sulfate budget of these watersheds was of particular interest due to the Minnesota wild rice water-quality standard for sulfate. We compared the stable isotope signatures of dissolved sulfate with known values for mineralized rocks, to fingerprint the sulfate sources.
- We developed geoenvironmental models for various deposit types, customized to the Lake Superior region, which include both pre-mining baseline conditions and the signatures associated with mine waste and drainage. These models enable environmental risks to be ranked and addressed. We evaluated how several factors (such as geology, local hydrology, geochemistry, climate) affect trace elements released to the environment from mining. Deposit-specific details included mineralogic hosts of trace elements, acid-base accounting characteristics, and leaching properties of unmined rock, waste rock, and mine wastes.
Schulte, R.F., Piatak, N.M., Seal, R.R., and Woodruff, L.G., 2017, Major, minor, and trace element compositions of bulk drill core samples from the Duluth Complex, Minnesota: U.S. Geological Survey data release, https://doi.org/10.5066/F7K9362H.