207Pb/206Pb and 238U/230Th dating of uranium migration in carbonate fractures from the Palmottu uranium ore (southern Finland)

Abstract

Precambrian U ore deposits, such as Oklo, Cigar-Lake or Palmottu, constitute invaluable analogues of nuclear waste repositories that provide direct evidence of U mobility or sequestration, over very large time intervals in geological formations. In this study, pervasive millimeter thick calcite veins filling microfractures in gneiss and granite surrounding the Palmottu U ore (Finland) were analysed, as fingerprints of past fluid circulation through the site. Stable Pb isotopes and short halflife 234U-230Th isotopes, all products of the U decay chain, have been chosen to investigate long term (over a Ga) and recent (within the last 500 ka) U migrations, respectively. Lead, U and Th isotopes have been analysed by thermo-ionisation mass spectrometry on mg-size bulk carbonate samples, and ion probe analyses of Pb isotopes performed on microinclusions of pyrite, monazite and coffinite. A striking contrast was found between the preservation of a well defined Pb–Pb isochron (1925±70 Ma, MSDW=2.47, N=17) on the one hand, and fractionated 230Th/238U ratios (230Th/238U<1) on the other hand. This shows that although the veins were formed simultaneously to the ore itself, during the Fennoscandian orogeny, and behaved mainly as a closed system for several billion years, U migrations did occur in the last 0.5 Ma. The recent U mobility is probably related to changes in the groundwater circulation through the bedrock. Nowadays, the dispersion of the U is, however, restricted to within a few meters around the U ore. It is shown that complex processes of dissolution and re-adsorption (or re-crystallisation) took place within these tiny carbonate fracture fillings. Relatively unradiogenic 206Pb/204Pb ratios measured by SIMS in coffinite micro-inclusions demonstrate recent precipitation of this mineral. Coffinite being the main U-bearing phase in the carbonate fracture fillings, its crystallization probably played a major role in U redistribution within the fractures, and in the relatively restricted dispersion of U through the bedrock.