The Natural Distribution and Geochemistry of Thallium

Abstract

Thallium is a complex and intriguing heavy element, having both chalcophile and lithophile characteristics, as well as demonstrating a larger range of isotopic fractionation than theoretically predicted. Thallium typically occurs in low concentrations in geologic settings, however, certain environments can concentrate thallium, in particular, ore-forming processes, where thallium can be used as a vector of mineralization. However, elevated thallium concentrations can also pose environmental and human health risks given the toxicity and ability of thallium to bioaccumulate. Therefore, this dissertation contains three studies that evaluate thallium geochemistry and its implications at a range of scales: globally, by characterizing mineralogical thallium distribution and fractionation patterns across a wide range of geologic environments and settings; regionally, by modeling variations in thallium behavior in an area with multiple igneous intrusive centers that have been variably hydrothermally altered; and locally, by quantifying the degree to which biologic systems can concentrate and fractionate thallium and the role of the underlying mineralogy in thallium biosignatures. The analysis of 185 mineral samples, including silicates and sulfides, from over 80 localities reveals a systematic mineralogical distribution and fractionation pattern for thallium, regardless of geologic setting or environment. This is thought to be the result of crystal chemical controls coupled, secondarily, with original source enrichment or depletions. This study documents mineralogical controls on thallium chemical and isotopic fractionation for the first time, which has implications regarding the natural weathering of thallium-bearing substrates, regional cycling of thallium, and the potential bioremediation of thallium contamination. This dissertation also presents mineralogical thallium data from the Battle Mountain district in north-central Nevada. Here, thallium distribution and fractionation are shown to be controlled on a first-order by hydrothermal alteration and on a second-order by previously noted crystal chemistry. These results demonstrate the utility of thallium geochemistry in and around ore-forming settings, yet also highlight the limitations of such applications given the ease with which geochemical signatures can be overprinted during subsequent alteration. Locally, thallium has the ability to easily and readily enter the food chain during uptake and bioaccumulation by plants, particularly those in the Brassicaceae family. Here, plants can partition thallium into edible plant parts and fractionate thallium systematically across plant structures. The final chapter of this dissertation analyzes Brassica juncea grown in various thallium-amended substrates to determine the degree to which biological processes can alter thallium geochemical signatures. This understanding presents opportunities for bioprospecting or bioremediation and adds further insight into the localized redistribution of thallium from one or more sources over time.