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Attenuation of uranium (U) and rare earth elements (REEs) has been observed in stream and wetland sediments, but the geochemical and mineralogical processes involved in sequestering these elements in natural systems are not well understood. The decommissioned Bicroft Uranium Mine near Bancroft, ON uses a modified stream and wetland system to reduce the concentrations of U and other metals in tailings pond effluent to levels below the Provincial Water Quality Objectives. The Bicroft Mine was operated from 1957 to 1963, and processed low-grade (~0.17 wt% U3O8), disseminated U hosted by pegmatite dykes in amphibolite gneiss, forming 2,284,421 tonnes of tailings, deposited into two tailings impoundments. The Bicroft site has since operated as a passively attenuating stream and wetland remediation system for 55 years, demonstrating the potential longevity and viability for long-term sequestration of U and REEs through natural attenuation. To identify U and REE hosts and their stability in the natural environment, colloids, tailings, stream and wetland sediments were analyzed using various methods, including tangential flow filtration (TFF), ICP-ES/MS, scanning electron microscopy, and synchrotron techniques (bulk and µXANES, µXRF, and µXRD). The results show that Fe- and Mn-oxyhydroxides, goethite, and birnessite are the main mineral hosts for U and REEs in both the colloids and sediments. In addition, detrital grains of U- and REE-bearing minerals were found >200 m downstream in colloids and wetland sediments, showing the potential for long range transport of colloids and particulates in the stream system. Seasonal influences on the stability of trace metals in sediments were observed, and may demonstrate the limitations of passively attenuating remediation systems as a viable method for attenuation. Changes in the redox state of the stream system were observed to influence the attenuation of U and REEs, however, changes in the redox state with depth in the wetland sediments were not observed, and showed no direct influence on the attenuation of U and REEs. The results of this study will help to develop better monitoring strategies for U tailings sites and should reduce the impacts of future U mining operations. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2013-05-06 07:44:03.327

Supereruptions are rare but giant and violent volcanic eruptions that have the potential to wreak havoc on life and infrastructure. Two key questions surrounding supereruptions are investigated in this work: (1) What are the timescales over which giant magma bodies accumulate and erupt? Three-dimensional x-ray tomography of quartz-hosted melt inclusions and cathodoluminescence imaging of compositional zoning in quartz crystals are used to assess timescales from melt inclusion faceting and diffusion chronometry, respectively. Results from three eruptions (240 ka Ohakuri-Mamaku, 26.5 ka Oruanui Taupo Volcanic Zone, New Zealand; 760 ka Bishop Tuff California, USA) suggest that large to giant systems accumulate over extremely short timescales (10¹-10³ a) and that quartz growth rates are ~10^-12-10^-12.5 m/s. (2) What is the geometry of supereruptive systems in the Earths crust? Phase-equilibria and amphibole geobarometry are used to investigate the residence depth of the Peach Spring (southwest USA) magma body. Results indicate that this magma body resided at a pressure of ~200-250 MPa. The analysis also shows that rhyolite-MELTS phase-equilibria geobarometry is an excellent method for obtaining pressure information and weeding out altered glass analyses. Finally, these questions and methods are linked together in an effort to assess the longevity and geometry of a single supereruptive system the Oruanui, the most recent supereruption in Earths history. Melt inclusion faceting indicates this crystal-poor high-silica rhyolite magma was short lived in the crust (10¹-10² a). Such short timescales are notably different from those derived from some other methods and likely reflect the unstable condition of crystal-poor, buoyant magma parcels residing in the shallow crust, rather than the development of the broad magmatic system and pre-accumulation priming of the crust. Phase-equilibria geobarometry suggests that the typical model of a single magma body is not appropriate to describe the Oruanui magmatic system. Instead, results suggest the Oruanui was comprised of multiple magma batches that resided at different depths in the crust but were erupted contemporaneously. This result fits well with an increasingly common model for supereruptive systems in which several vertically and/or laterally juxtaposed magma bodies are erupted simultaneously.