Assessing the influence of natural copper-nickel-bearing bedrocks of the Duluth Complex on water quality in Minnesota, 2013–15

ABSTRACT The Duluth Complex in northeastern Minnesota hosts economically significant deposits of copper, nickel, and platinum group elements (PGEs). The primary sulfide mineralogy of these deposits includes the minerals pyrrhotite, chalcopyrite, pentlandite, and cubanite, and weathering experiments show that most sulfide-bearing rock from the Duluth Complex generates moderately acidic leachate (pH 4 to 6). Microorganisms are important catalysts for metal sulfide oxidation and could influence the quality of water from mines in the Duluth Complex. Nevertheless, compared with that of extremely acidic environments, much less is known about the microbial ecology of moderately acidic sulfide-bearing mine waste, and so existing information may have little relevance to those microorganisms catalyzing oxidation reactions in the Duluth Complex. Here, we characterized the microbial communities in decade-long weathering experiments (kinetic tests) conducted on crushed rock and tailings from the Duluth Complex. Analyses of 16S rRNA genes and transcripts showed that differences among microbial communities correspond to pH, rock type, and experimental treatment. Moreover, microbial communities from the weathered Duluth Complex rock were dominated by taxa that are not typically associated with acidic mine waste. The most abundant operational taxonomic units (OTUs) were from the genera Meiothermus and Sulfuriferula, as well as from diverse clades of uncultivated Chloroflexi, Acidobacteria, and Betaproteobacteria. Specific taxa, including putative sulfur-oxidizing Sulfuriferula spp., appeared to be primarily associated with Duluth Complex rock, but not pyrite-bearing rocks subjected to the same experimental treatment. We discuss the implications of these results for the microbial ecology of moderately acidic mine waste with low sulfide content, as well as for kinetic testing of mine waste. IMPORTANCE Economic sulfide mineral deposits in the Duluth Complex may represent the largest undeveloped source of copper and nickel on Earth. Microorganisms are important catalysts for sulfide mineral oxidation, and research on extreme acidophiles has improved our ability to manage and remediate mine wastes. We found that the microbial assemblages associated with weathered rock from the Duluth Complex are dominated by organisms not widely associated with mine waste or mining-impacted environments, and we describe geochemical and experimental influences on community composition. This report will be a useful foundation for understanding the microbial biogeochemistry of moderately acidic mine waste from these and similar deposits.

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Hydrology and water quality of the copper-nickel study region, northeastern Minnesota

Open-File Report ◽

10.3133/ofr80739 ◽

1980 ◽

Cited By ~ 5

Author(s):

Donald I. Siegel ◽

Donald W. Ericson

Keyword(s):

Water Quality ◽

Study Region ◽

Copper Nickel

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Balancing Socio-Ecological Risks, Politics, and Identity: Sustainability in Minnesota’s Copper-Nickel-Precious Metal Mining Debate

Sustainability ◽

10.3390/su122410286 ◽

2020 ◽

Vol 12 (24) ◽

pp. 10286

Author(s):

Ryan D. Bergstrom ◽

Afton Clarke-Sather

Keyword(s):

Precious Metal ◽

Strong Predictor ◽

Mining Industry ◽

Precious Metals ◽

Economic Benefits ◽

Environmentally Benign ◽

Metal Mining ◽

Duluth Complex ◽

Copper Nickel ◽

Q Sort

In the northeastern corner of Minnesota, two of the state’s most iconic symbols, mining and pristine wilderness, are on a collision course. The Duluth Complex, considered by many to be the world’s largest undeveloped deposit of copper-nickel and precious metals, is the site of mining proposals for several controversial mines. Proponents suggest that mining can be accomplished in an environmentally benign manner, and in the process create nearly 1000 jobs and $500 million in economic benefits annually. Opponents counter that the tourism and recreation industries already provide nearly 18,000 jobs and bring over $900 million in economic benefits annually, and that mining will permanently impair the regions environment. Thus, the copper-nickel and precious metal mining debate has become highly polarized, and serves as an ideal example of how people address national and global sustainability issues at local and regional scales. This study examines this polarization through a Q-sort analysis of subjectivities of residents of the state of Minnesota. Results suggest that partisanship is a strong predictor of attitudes towards mining, and that the strongest differences between respondents were not based on perceptions comparing jobs and the environment, the typical partisan divide, but rather on respondents’ perceived identity with relation to the mining industry or water resources.

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Faulting and fracturing in part of the Duluth complex, northeastern Minnesota

Canadian Journal of Earth Sciences ◽

10.1139/e81-074 ◽

1981 ◽

Vol 18 (4) ◽

pp. 810-814 ◽

Cited By ~ 4

Author(s):

Michael P. Foose ◽

R. W. Cooper

Keyword(s):

Field Studies ◽

Basal Part ◽

Economic Implications ◽

Duluth Complex ◽

Copper Nickel ◽

Major Period

Extensive faulting and fracturing have been documented by detailed field studies of a 48 km2 area in the upper Precambrian Duluth complex, northeastern Minnesota. Faults and fractures dip steeply, trend principally N35°E, N05°W, and N40°W, and were recognized by the displacement of plagioclase and plagioclase–olivine cumulate layers within the so-called South Kawishiwi intrusion. This work represents the first field documentation of extensive faulting in the Duluth complex and shows that fault offsets are an important complicating factor in understanding the geology of the complex. Further, faults of the type mapped are compatible with the structures predicted but not previously recognized by models that genetically link the Duluth complex to a major period of crustal rifting, and these faults have important economic implications, as offsets along them are certain to affect the continuity of important copper–nickel sulfides that occur along the basal part of the Duluth complex.

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