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.

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The compositions of fluid inclusions hosted in ore and gangue minerals from Mississippi Valley-type (MVT) Pb-Zn-Ba deposits of the Ozark Plateau region were measured to develop a regional hydro-geochemical conceptual model for ore emplacement. This model may explain the diverse compositions of fluids involved in mineral precipitation, the ore precipitation mechanism, and the temporal change in composition of fluids invading the ore districts. The conceptual model additionally provides evidence for what factors may have controlled deposit size, stratigraphic location, and Zn/Pb ratio. High Pb concentrations up to 1,000's of ppm were identified in sphalerite-hosted fluid inclusions from all of the region's districts. If these high Pb concentrations were transported in the same fluid with sulfide, then total sulfur concentration in the fluid must have been low. Mass balance calculations demonstrate that the Arkoma Basin, a presumed source basin for the mineralizing fluids, is too small to have contained enough fluid to precipitate the observed masses of sulfur in the larger MVT districts, given the low sulfide concentrations that could coexist in the fluid with such high concentrations of Pb. High methane concentrations in sphalerite-hosted fluid inclusions from all of the region's districts, in dolomite-hosted fluid inclusions from the Tri-State and Northern Arkansas districts, and quartz-hosted fluid inclusions from the Northern Arkansas district suggest that the prevailing redox conditions during MVT mineralization were reducing, making it unlikely that sulfate was transported in large concentrations in the fluids. During sphalerite precipitation, assuming saturation with respect to carbon dioxide, measured methane concentrations in sphalerite-hosted fluid inclusions would have required oxygen fugacity to have been at least two log units below the sulfate predominance field boundary. Fluid inclusion methane concentrations can also be used to estimate minimum burial depths of mineralization of about 0.08 to 1.2 km. Available evidence indicates that sulfide mineral precipitation in the Ozark Plateau MVT districts most likely occurred primarily as a result of the introduction of sulfide into a Pb-and Zn-rich ore fluid. In the two larger MVT districts, sulfide may have been supplied by local organic-and sulfur-rich carbonate facies. An apparent mixing line between high Ca/Na ratio and low Ca/Na ratio fluids hosted predominantly in main stage sulfide minerals and paragenetically late minerals, respectively, indicates the ore fluid was relatively Ca enriched. The lack of continuity in high Pb concentrations in fluid inclusions in sulfide and nonsulfide minerals from across the mineral parageneses suggests that the ore fluids entered the districts intermittently and/or had variable metal contents over time.

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