THE WORLD'S OLDEST OBSERVED PRIMARY URANINITE

Abstract

Secondary ion mass spectrometry (SIMS), an in situ micro-analytical technique, was used to date uraninite from the Huron Claim pegmatite in southeastern Manitoba. Detailed petrography shows that there are zones in the uraninite grains that have undergone varying degrees of alteration. Altered and relatively unaltered zones were analyzed on the scale of a few micrometers. In situ U–Pb isotopic analysis shows that the uraninite from the Huron Claim pegmatite is highly discordant (>5%) with an upper intercept of 2575 ± 38 Ma. Lead (207Pb/206Pb) isotope ages range from 2514 ± 3 to 2573 ± 18 Ma, with an average age of 2550 ± 17 Ma. Therefore, the crystallization age of the uraninite is interpreted to be 2575 ± 38 Ma. A comparison of this age with uraninite from other deposits suggests that the Huron Claim pegmatite uraninite is currently the oldest known example of primary uraninite. Evidence for the presence of very old primary uraninite is an important part of the global evolution of uranium minerals. Detrital accumulations such as the Witwatersrand and Elliot Lake deposits have inferred ages much greater than 2.2 Ga. However, no primary uraninite approaches the age of these deposits except for the Huron Claim uraninite.