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 sulfidebearing 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.

The quality of drainage and the release of sulfate, calcium, and magnesium from five field test piles of Duluth Complex rock with sulfur contents of 0.63% (three piles), 0.79%, and 1.41 % were determined for periods of 12 to 14 yr. The pH of drainage from the piles decreased over time and with increasing solid-phase sulfur content. The ultimate drainage pH values for the 0.63% S piles ranged from 4.8 to 5, similar to values observed in the laboratory. Ultimate drainage pH values for sulfur contents of0.79% (pH 4) and 1.41 % (pH 3.5) were about one unit lower than the corresponding laboratory values for these sulfur contents. Over the entire period of record, average rates of sulfate release (2.1 to 10.5 mmol/mt/d) and magnesium release (0.55 to 3.7 mmol/mt/d) increased as the solid-phase sulfur content of the pile increased, while those of calcium release (1.2 to 2.4 mmol/mt/d) were relatively constant with respect to solid-phase sulfur content. Annual release rates for sulfate and calcium were relatively constant over time, while those for magnesium tended to increase over time. Annual release rates for all three parameters increased with increased annual drainage volume, a variable not examined in the laboratory. Release rates in the field were typically 10% to 30% (and in one case, nearly 40%) of those observed in the laboratory. Empirical neutralization potentials in the field decreased as solid-phase sulfur content decreased and were typically 10% to 50% of the corresponding average laboratory values. The empirical neutralization potentials to maintain drainage pH above 6.0 were approximately 0.5% to 8% of ABA NP values for Duluth Complex rock.

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Ten Duluth Complex drill core samples, with sulfur contents ranging from 0.47 to 2.17 percent, were leached in a laboratory experiment. The variation in sulfur content was due largely to fluctuations in iron sulfide content. Solids containing 0.8 percent sulfur or less produced neutral drainage while, with one exception, solids with sulfur contents of 0.92 percent or more produced acidic drainage. The results indicated that acid production due to oxidation of sulfide minerals containing iron, such as pyrrhotite, increased linearly as the sulfur content increased. The acid consumption due to silicate mineral dissolution was relatively constant despite minor variations in the silicate mineralogy. A sample containing 1.17 percent sulfur produced neutral drainage due to the buffering provided by a calcium carbonate content of three percent. Leaching of a sample from a test stockpile indicated that the fine fraction of stockpile solids exerted a major influence on the drainage chemistry. The pH range and temporal variation of drainage from solids containing 0.92 percent sulfur was similar to that from a test stockpile with a bulk sulfur content of 0.6 percent. Analysis of the test stockpile solids indicated that particles smaller than 2.0 mm in diameter comprised about 12 percent of the stockpile mass and had a sulfur content of 1 percent. The laboratory results suggest these small particles exert a major influence on stockpile drainage quality, most likely due to their elevated sulfur content and specific surface area.