Decoupling of O and Pb isotope systems of uraninite in the early Proterozoic Conglomerates in the Elliot Lake district

作者：Ono Shuhei^*; Fayek Mostafa

来源：Chemical Geology, 2011, 288(1-2): 1-13.

DOI：10.1016/j.chemgeo.2010.03.015

• Uraninite
• Lead
• Oxygen
• Isotope
• Proterozoic
• Uranothorite

Both isotopic (Pb and O) and chemical compositions were measured by two in-situ techniques, SIMS and EPMA, for similar to 20 mu m-diameter areas of over forty eight individual grains of uraninite in the early Proterozoic quartz pebble conglomerate uranium deposits in the Elliot Lake district, in order to constrain the origin of uraninite and its post-mineralization history. Together with textural observation by SEM, Pb isotope analyses and chemical compositions of brannerite and uranothorite, our data revealed a protracted uranium remobilization history for the uranium deposits in the Elliot Lake district.
All grains of uraninite examined show consistently high Th contents, supporting detrital origin of uraninite derived from pegmatitic rocks. The unimodal distribution of Th concentration of uraninite excludes the possibility that uraninite formed by multiple pathways, where some grains are detrital and some are later diagenetic/hydrothermal in origin. The measured uraninite grains yield U-Pb discordia age of 1.8 Ga, which is much younger than the depositional age of the host Huronian Basin (2.45 to 2.2 Ga). This age closely corresponds with the age of peak metamorphism in the Huronian Basin, suggesting that all uraninite grains completely lost its Pb during this time. The least texturally and chemically altered grains of uraninite yield delta O-18-SMOW values of -10 to -22 parts per thousand. This range of delta O-18 value is lower than that expected for uraninite from granitic/pegmatitic rocks by 10 parts per thousand. It is concluded that the oxygen isotope ratios of uraninite is completely reset during the peak metamorphism and/or by interaction with recent meteoric water. The delta O-18 of uraninite, however, do not show systematic variation with Th/U, Pb/U and Pb-207/Pb-206 ratios, indicating uraninite exchanged its O isotopes without much disturbance in its chemical composition.
Among the various geochemical signatures in uraninite, high Th content is the only original signature preserved since detrital deposition of uraninite. Both Pb and O isotope systems have been disturbed in various degrees by later events. The coupling/decoupling of the composition, Pb and O isotope systematics of uraninite reflect the protracted mineralization/remobilization history of the oldest uraninite.

• 出版日期2011-9-8