Geology Seminar Series - Daniel Ferguson - Electron probe microanalysis and chemical age dating of uraninite and coffinite from the Midwest Main deposit and its implication for uranium mineralization

Wed., Oct. 6, 2021 4:00 a.m.

Location: https://uregina-ca.zoom.us/j/98565104352?pwd=ZUEvQ1dVOGtMaExOdlJGbVVnVFlQQT09

Title: Electron probe microanalysis and chemical age dating of uraninite and coffinite from the Midwest Main deposit and its implication for uranium mineralization

Speaker: Daniel Ferguson, MSc Student, Department of Geology, University of Regina

Abstract:

The Midwest Trend located along the eastern margin of the Athabasca basin is host to two unconformity-related uranium (URU) deposits: the Midwest Main, and the Midwest A. The Midwest Trend can be considered a “modèle réduit” for U mineralization in the Athabasca Basin, because it displays geological features characteristic to URU deposits, but in a more compact deposit size. The deposits were formed by multiple stages of mineralization and remobilization of uranium and nickel-cobalt minerals and are hosted mainly at the unconformity between the sediments of the Athabasca Basin and the underlying Archean and Proterozoic basement.

Determining the age of uranium deposits is complicated and there is much contention regarding the primary age of mineralization in the Athabasca basin, though some studies point to a period between 1600 and 1550 Ma. Uraninite and coffinite, the dominant ore minerals, are chemically active and are generally heterogenous at the micron scale. Additionally, these minerals are susceptible to uranium replacement and lead loss leading to significant error in isotopic U/Pb ages. Although chemical age dating faces similar challenges it has the advantages of generating large quantities of data at a relatively low cost, being performed at analytical facilities that are relatively highly accessible, and the analyses of this data can yield new information not available from isotopic U/Pb dating. I will introduce the chemical age dating method and its application to the Midwest Main uranium deposit in the Athabasca basin.

Electron probe microanalysis (EPMA) of uraninite and coffinite from the Midwest Main deposit has been performed to meet two key objectives: 1) to demonstrate the utility of chemical dating for URU deposits; and 2) to provide a geochronological context to better interpret petrographic observations and fluid inclusion analyses of the current study. A total of eighty-five analytical spots from three petrographically distinct generations of uranium minerals were selected for compositional analysis. Chemical ages have been calculated for all three generations of U minerals. The results of EPMA provide a maximum age for primary mineralization at Midwest, a means to indirectly date the abundant Ni-Co sulphide and sulpharsenide minerals, geochronological constraints on fluid-trapping events, and confirm that uranium mineralization along the Midwest Trend developed and evolved during at least three fluid-circulation events.