Exceptionally low δ 18O values of primary uraninite and
pitchblende (i.e. -32 per mil to -19.5 per mil) from Proterozoic
unconformity-type uranium deposits in Saskatchewan, Canada, in conjunction
with theoretical uraninite-water oxygen isotope fractionation factors
suggest that primary uranium mineralization is not in oxygen isotopie
equilibrium with clays and silicates. The low δ 18O values have
been interpreted to have resulted from the recrystallization of primary
uranium mineralization in the presence of modern meteoric fluids having low
δ 18O values of ca. -18 per mil. The absence of apparent
alteration in many of the uraninite and pitchblende samples requires that
the uranium minerals exchange oxygen isotopes with fluids, with only minor
disturbances to their original chemical compositions and textures. However,
experiments on the interaction between water and natural uraninites, from
these deposits, and detailed electron micro-probe analyses of natural
uraninite and pitchblende indicate that, in the presence of water, old
uraninite rapidly alters to curite
(Pb2U5O174H2O). The hydration
of uraninite to curite releases uranium and calcium into solution and
becquerelite
(Ca(UO2)6O4(OH)6H2O)
is precipitated. In the presence of Si-saturated waters, uranium silicate
minerals, soddyite
((UO2)2(SiO4)2H2O) and
kasolite (Pb(UO2)SiO4H2O are precipitated
in addition to, curite and schoepite
((UO2)8O2(OH)12(H2O)12).
The mineral paragenesis observed in these experiments is similar to
sequences observed in oxidized zones in uranium deposits and
UO2-water experiments. Therefore, it is unlikely that natural
uraninite and pitchblende can simply exchange oxygen with an oxidizing fluid
without a concomitant change in phase chemistry or structure, nor will
oxidation of uraninite lead to the formation of U3O7.
as predicted by theoretical calculations used in natural analogue studies
for the disposal of high level nuclear waste.