Understanding microbially-mediated pyrrhotite dissolution in the Duluth Complex, northern MN

Abstract

The Duluth Complex, located in northern Minnesota, represents the largest undeveloped copper, nickel, and platinum-group element deposit in the world. The majority of previous work on the environmental impacts of sulfide mineral mining has been conducted on the mineral pyrite (FeS2), and the microbial communities that drive pyrite oxidation and generate highly acidic, metal-rich effluent. In contrast, the primary gangue (waste) sulfide in Duluth Complex ores is pyrrhotite (Fe1-xS, 0 ≤ x ≤ 0.125), and the relatively low concentration of sulfide minerals in the rock and the geochemistry of the host silicate minerals mean that waste rock and tailings from these deposits is not expected to generate highly acidic drainage (discussed further in Chapter 2). Studies on abiotic pyrrhotite dissolution are limited, and while dissolution rates in acidic systems are reported as 10 to 100 times higher than pyrite, studies of abiotic pyrrhotite dissolution at more neutral pH frequently present contradictory results. Further, the impact of microorganisms on pyrrhotite dissolution – particularly microorganisms that are endemic to northern Minnesota – is poorly understood. The goal of my dissertation research, therefore, is to better understand the dissolution of pyrrhotite under “Duluth Complex” conditions. This can be further divided into four chapters that each address a facet of this question. Chapter 2 examines the complex relationship between heating, mineral structure, and magnetic properties in the mineral pyrrhotite, Chapter 3 evaluates the effect of microorganisms on pyrrhotite dissolution in laboratory experiments, Chapter 4 compares the microbial communities found on naturally-weathered Duluth Complex outcrops to the microbial communities found on waste rock and tailings, and evaluates potential avenues for management of waste rock and tailings from Duluth Complex mines, and Chapter 5 compares the growth behavior and metabolic capabilities of four different strains of sulfur-oxidizing Sulfuriferula spp. used in the laboratory experiments in Chapter 2.