The Deposit

Scientific companion to The Water from The Briefing: Tom Emmer (R-MN-06). Licensed under CC BY-NC-ND 4.0.

What is under the ground

About 1.1 billion years ago, North America tried to split apart. A crack 1,200 miles long opened from what is now Kansas through Lake Superior and into Michigan. Magma from deep in the Earth's mantle rose through the crack. The continent did not split. The rift failed, and the magma cooled underground into a formation geologists call the Duluth Complex.

The complex covers more than 2,000 square miles of northeastern Minnesota. In places it is 10 miles thick. It is the second-largest formation of its type on Earth, after the Bushveld Complex in South Africa.

As the magma rose, it intruded into older rocks that were rich in sulfur and absorbed sulfur from them. The added sulfur pushed the magma past a tipping point. A separate liquid — rich in sulfide — separated out inside the magma, like oil separating from water. These sulfide droplets were heavier than the surrounding magma. They sank, and as they sank they collected copper, nickel, platinum, palladium, and cobalt, concentrating them into layers at the bottom of the formation.

The richest layers sit along the northwestern edge of the complex, where the magma first contacted the sulfur-rich rock. That edge runs in an arc from Duluth to the Canadian border, directly through the Ely and Babbitt area — the watershed that drains into the Boundary Waters.

The deposit contains an estimated 4.4 billion tons of ore. It is the world's largest undeveloped copper-nickel deposit, the third-largest accumulation of nickel sulfides, and the second-largest for combined copper and platinum-group elements.

How sulfide mining works

The target metals make up a tiny fraction of the rock — copper and nickel less than 1%, platinum-group metals far less than that. Extracting them requires four stages.

Stage 1: Crush. The ore is brought to the surface and ground into fine powder — roughly the consistency of flour. This maximizes the surface area for chemical separation. For every ton of usable metal, approximately 99 tons of waste rock and powder are produced.

Stage 2: Float. The powder is mixed with water and chemical agents in large tanks. The agents make sulfide minerals repel water. Air is pumped through the mixture, creating bubbles. The sulfide particles attach to the bubbles and float to the surface as foam. This foam — the "concentrate" — is skimmed off. Everything that sinks is waste, called tailings.

Stage 3: Smelt. The concentrate is heated in a furnace until the metals separate from the remaining rock. The waste rock melts and floats to the top, where it is scraped off. The heavier metal mixture sinks to the bottom.

Stage 4: Refine. The crude metal mixture is further cleaned — either by blowing oxygen through the molten material to burn off remaining impurities, or by dissolving it in chemical solutions and using electricity to plate out pure metal. The final product is sheets of 99.9% pure copper or nickel.

The process is energy-intensive and produces two major categories of waste: waste rock (excavated but not processed) and tailings (the powdered leftovers from flotation). Both contain exposed sulfide minerals — the source of the contamination problem described below.

Why it contaminates water

The problem is not the mining itself. The problem is what happens to the waste after the mining stops.

Minnesota has mined iron ore on the Iron Range for over a century. The lakes near those mines are still fishable. The difference is chemistry. Iron ore on the Range is oxide — iron bonded to oxygen. When you crush oxide ore, it does not make acid. Sulfide ore — iron bonded to sulfur — does. That one difference changes everything.

Sulfide minerals are stable underground, where there is no air and limited water. Mining brings them to the surface, crushes them into powder, and exposes them to both. What follows is a self-sustaining chain reaction called acid mine drainage.

Step 1. Sulfide minerals — most commonly pyrite, also called fool's gold — react with oxygen and water to produce sulfuric acid and dissolved iron.

Step 2. The dissolved iron transforms into a more reactive form. Think of it as rust, but supercharged.

Step 3. This reactive iron attacks more sulfide rock, producing more acid and more dissolved iron — which transforms again and attacks more rock. The reaction feeds itself. It is a chemical fire that never burns out.

Step 4. Naturally occurring bacteria thrive in these conditions and speed up the reaction — typically 10 to 1,000 times faster in the field, up to 100,000 times faster under laboratory conditions.

Once this cycle begins, it continues as long as there is air, water, and sulfide mineral. In northern Minnesota, which receives 30 inches of precipitation a year and the ground freezes and thaws every spring, all three conditions are permanent.

What the acid does to water

The sulfuric acid makes nearby water as acidic as lemon juice or vinegar. At the most extreme sites, such as Iron Mountain in California, drainage has been measured at levels more acidic than battery acid. Most aquatic life dies long before water reaches those levels.

The acid also acts as a solvent. As it flows through waste rock and tailings, it dissolves heavy metals — copper, nickel, lead, mercury, arsenic, cadmium — and carries them into groundwater and streams.

When acidic water eventually reaches clean water downstream, the dissolved iron turns back into a solid — a thick orange sludge called "yellow boy." This sludge coats streambeds, smothers fish eggs, clogs fish gills, and kills the bottom-dwelling insects that form the base of the aquatic food chain.

Heavy metals that enter the water are absorbed by algae, eaten by insects, eaten by small fish, eaten by larger fish, and eaten by people. Concentrations increase at each step — a process called biomagnification. What is harmless at the bottom of the food chain becomes dangerous at the top.

Why the Boundary Waters is uniquely vulnerable

Not all landscapes react the same way to acid mine drainage. The Boundary Waters is worse than most, for specific geological reasons:

• No natural buffering. In some regions, limestone in the bedrock neutralizes acid on contact. The rock beneath the Boundary Waters is ancient, hard granite — part of the Canadian Shield, some of the oldest exposed rock on Earth. It contains almost no limestone. Once acid enters the water, nothing in the geology neutralizes it.
• Interconnected watershed. The mine site sits in the Rainy River watershed. Water flows north from the site into the heart of the wilderness, through Voyageurs National Park, down the Rainy River, and into Lake of the Woods — a distance of roughly 370 miles. There is no point in this chain where contamination can be isolated.
• Mercury conversion. The acid-producing process also releases sulfate — a chemical compound of sulfur — into the water. Sulfate feeds bacteria in lake sediments. Those bacteria change the mercury already present in the environment into a form that fish absorb and cannot get rid of. The mercury concentrates as it moves up the food chain — from insects to small fish to walleye to the person who eats the walleye. Fish in some Boundary Waters lakes already carry mercury at 3 to 7 times the level the EPA considers safe. Additional sulfate from mining would make this worse.
• Wild rice. Wild rice — manoomin to the Ojibwe — is extraordinarily sensitive to sulfate. Its roots die when sulfate concentrations rise above 10 milligrams per liter, one of the strictest water quality standards in the country. Minnesota adopted this standard specifically to protect wild rice beds. Acid mine drainage is a sulfate delivery system. The same process that poisons fish also kills the food that has sustained indigenous communities in this watershed for centuries.
• Freeze-thaw cycle. Northern Minnesota winters reach 30 to 40 degrees below zero. Every spring, the ground thaws and refreezes dozens of times. This is the same cycle that destroys Minnesota roads every spring. It cracks concrete, splits liners, and opens seams in any containment structure built to hold waste. Tailings dams, caps, and covers designed for arid climates — where the ground stays dry and stable year-round — face conditions here that stress them every season. The containment does not need to fail catastrophically. It needs to develop one crack, once, and water finds it.
• International boundary. The watershed crosses into Manitoba, Canada. A mine in Minnesota could contaminate Canadian water, creating an international incident governed by the International Joint Commission.

How long it lasts

Acid mine drainage is not a temporary problem. It lasts as long as there is sulfide mineral exposed to air and water — which, in a crushed waste pile, can mean thousands of years. Longer than any government or company has ever existed.

• Roman-era copper mines at Rio Tinto, Spain, have produced acid drainage continuously for over 2,000 years.
• Iron Mountain, California, will produce acid for an estimated 2,500 to 3,000 more years.
• The PolyMet environmental impact study — for the other proposed mine in Minnesota — assumes water treatment will be required for 200 years at the mine site and 500 years at the processing site.

An obvious question: why not put the waste back underground? Because you cannot un-crush powder back into solid rock. The mining process ground it into flour-sized particles, creating an enormous surface area that did not exist before. Even underground, water would find it, air would reach it, and the acid reaction would begin. Backfilling slows the process. It does not stop it.

There is no known method to permanently stop acid mine drainage once it begins at scale. Every operational plan assumes active water treatment — pumping, neutralizing, filtering, monitoring — for centuries, paid for by someone.

The track record

The mining industry describes its proposed methods as "21st-century mining" — underground extraction, dry-stack tailings, real-time monitoring, advanced water treatment. These designs are real. They are also untested at this location and in this climate.

The historical record for sulfide mines operating near water:

From 1998 to 2017, Wisconsin maintained a law called "Prove It First." It required any company proposing a sulfide mine to name a single example of a comparable mine anywhere in the world that had operated for 10 years and been closed for 10 years without polluting surrounding water. In 19 years, no mining company on Earth could meet the standard. The law was repealed — not because an example was found, but because the legislature changed.

What perpetual treatment costs

The EPA currently manages over 40 hardrock mine Superfund sites that collectively produce 17 to 27 billion gallons of polluted water annually. Each requires treatment that will never end. Modern regulations require financial assurance bonds — PolyMet's $544 million bond is listed above. But bonds are sized to estimated costs, and estimates have historically fallen short. When the mining company goes bankrupt — as the operator at Summitville, Colorado, did after posting a $4.5 million bond on a site that has cost over $200 million to clean up — the gap between the bond and the actual cost transfers to taxpayers.

Where else these minerals exist

The argument for mining the Boundary Waters watershed rests on two claims: that these minerals cannot be obtained elsewhere, and that domestic supply is a national security imperative because China and Russia dominate global processing and supply chains for nickel, cobalt, and platinum-group metals. The national security argument is real. The question is whether it requires mining this specific deposit, in this specific watershed, when domestic alternatives exist. The supply claim is partially true for nickel and largely false for copper.

Copper

The United States already produces substantial copper without touching the Duluth Complex. Arizona alone accounts for roughly 74% of domestic output. These deposits are in arid regions with no major freshwater watersheds at risk.

Copper is not the bottleneck. The argument for the Duluth Complex is not primarily about copper.

Nickel

Nickel is the harder case. The United States is approximately 95% import-dependent for nickel, and the Duluth Complex is the largest known domestic deposit. But it is not the only one.

The Tamarack project is the most direct answer. It is a high-grade nickel-copper-cobalt deposit in a different part of Minnesota that drains to a different watershed. It has a supply agreement with Tesla for 75,000 metric tonnes of nickel concentrate and has received Department of Defense funding as a strategic domestic source. It exists. It does not threaten the Boundary Waters.

Platinum group metals

The Stillwater and East Boulder mines in Montana are the only primary source of platinum and palladium in the United States. They operate without threatening a wilderness-scale freshwater system.

What else could work

Several extraction technologies do not require conventional open-pit or underground sulfide mining.

Mining without digging

Instead of digging ore out of the ground, a method called in-situ recovery injects a weak acid solution — comparable in strength to household vinegar — into underground ore through wells. The solution dissolves copper minerals in place. The copper-rich fluid is pumped to the surface and processed into 99.9% pure copper through electroplating.

No open pit. No tailings dam. No blasting, hauling, or crushing of rock. The surface is left largely undisturbed.

The Florence Copper project in Arizona claims a 75% reduction in greenhouse gas emissions, 65% less energy, and 78% less water compared to conventional open-pit mining. Its first commercial copper harvest is expected in early 2026. The technology works for copper oxide deposits. It does not work for nickel sulfides.

Phytomining

Some plants are so hungry for nickel they pull it straight out of the ground and store it in their leaves. The plants are harvested like a crop and processed to recover the metal. No digging. Potentially carbon-negative when combined with rock weathering.

The Department of Energy's ARPA-E program has funded pilot-scale phytomining projects in southern Oregon. The technology is pre-commercial and cannot approach conventional mining volumes yet, but it demonstrates that alternatives are under active development.

Geothermal brine extraction

At the Salton Sea in California, lithium and other minerals are extracted from hot, mineral-rich fluids already being pumped for geothermal power generation. The process has minimal surface footprint and avoids the evaporation ponds and water consumption of conventional lithium mining. The principle — extracting minerals from fluids that are already being handled for another purpose — applies beyond lithium.

Mine tailings reprocessing

The United States has billions of tons of mine waste already sitting on the surface from a century of previous mining. Some of this waste contains minerals that were uneconomical to extract when the mines operated but are now recoverable with modern processing.

• Missouri Cobalt reprocesses old lead-zinc tailings to recover cobalt, nickel, and copper.
• USGS research at Bingham Canyon (Utah) has identified significant tellurium and other critical minerals in existing copper tailings.
• A 2025 study from the Colorado School of Mines found that recovering just 10% of the cobalt already discarded in U.S. mine waste could meet the entire domestic battery market demand.

Reprocessing turns environmental liabilities into resources without digging a single new hole.

Recycling

Recycled copper already provides approximately 33% of U.S. domestic supply — roughly 700,000 metric tons per year. Battery recycling is the fastest-growing segment of critical minerals recovery, with companies processing end-of-life electric vehicle batteries to recover nickel, cobalt, and lithium. Recycled cobalt supplied 25% of U.S. consumption in 2024.

Recycling does not replace mining. These figures reflect current demand. The global energy transition — electric vehicles, wind turbines, solar panels, grid infrastructure — is projected to double or triple copper and nickel requirements by 2040. No single source, including the Duluth Complex, would satisfy that growth alone. But recycling reduces how much new mining is needed and buys time for lower-impact extraction technologies to scale.

The arithmetic

The Duluth Complex is the world's largest undeveloped copper-nickel deposit. That is a geological fact. But "largest undeveloped" is not the same as "only option." It is the most convenient option — the most mineral in one place, for a single mining operation, under a single set of leases.

The convenience is real. So is this:

• Copper is abundant in the American Southwest, in arid regions far from major freshwater systems.
• Nickel exists at Tamarack, in the same state, draining to a different watershed.
• Platinum comes from Montana without threatening a wilderness area.
• Cobalt can be recovered from waste we have already dug up.
• New extraction technologies are producing copper without open pits or tailings.

Every sulfide mine studied — 14 of 14, 25 of 25, 8 of 8 — has contaminated nearby water. No company in 19 years could name a single exception. The company proposing this mine has never operated in the United States. The minerals would be processed overseas. Under the General Mining Act of 1872, zero federal royalties are paid on minerals extracted from public land. If the company fails, taxpayers inherit the treatment obligation — which, based on every comparable site, lasts centuries.

The question is not whether the minerals are valuable. They are. The question is whether this particular deposit — beneath the watershed of 1,100 lakes, in a wet climate with no natural acid buffering, draining across an international boundary into water that indigenous people have harvested for centuries under a treaty the federal government signed — is the right place to extract them, when alternatives exist.

Sources

Duluth Complex geology -- Minnesota Geological Survey (Report of Investigations 58); USGS Mineral Commodity Summaries; Miller et al. (2002). Midcontinent Rift -- USGS; University of Minnesota Digital Conservancy; Sims (1968). Sulfide mining process -- EPA; Minnesota DNR. Acid mine drainage chemistry -- EPA acid mine drainage documentation; Nordstrom and Alpers (1999). Bacterial catalysis -- *Acidithiobacillus ferrooxidans* literature; Fondriest Environmental. Boundary Waters geology and watershed -- USFS Superior National Forest Environmental Assessment (Federal Register 87 FR 38373, June 28, 2022); MPCA; IJC. Mercury methylation -- Dr. Steve Engstrom (2016); MN Department of Health fish consumption advisories. Mining failure studies -- Earthworks (2012); Kuipers and Associates (2006); 2025 peer-reviewed study. Wisconsin Prove It First -- Wisconsin Act 134 (1997). Iron Mountain -- EPA Superfund; DOJ settlement records. Berkeley Pit -- EPA Superfund; Montana DEQ. Summitville -- EPA Superfund; GAO. Rio Tinto (Spain) -- USGS; geological literature. Eagle Mine -- Michigan DEQ/EGLE monitoring reports; Lundin Mining. PolyMet financial assurance -- Minnesota DNR; PolyMet/NewRange EIS. Treatment costs -- EPA; mine closure literature; financial assurance filings. Domestic copper production -- USGS Mineral Commodity Summaries 2025; Freeport-McMoRan; Rio Tinto/BHP (Resolution Copper). Domestic nickel -- USGS; Talon Metals; Eagle Mine reports. Tamarack -- Talon Metals/Rio Tinto filings; DOD critical minerals funding; Tesla supply agreement. Stillwater -- Sibanye-Stillwater; USGS. In-situ copper recovery -- Taseko Mines (Florence Copper); company filings and environmental data. Phytomining -- DOE ARPA-E; Metalplant; University of Oregon. Geothermal brine -- DOE; Salton Sea lithium projects. Tailings reprocessing -- Colorado School of Mines (2025); USGS Earth MRI; Missouri Cobalt. Recycling -- USGS; industry data; Nth Cycle; Redwood Materials. Wild rice sulfate standard -- Minn. R. 7050.0224, subp. 2; MPCA. General Mining Act -- 30 U.S.C. 22-54; GAO; Taxpayers for Common Sense.