Physics
Scientific paper
Apr 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51..869w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 51, Issue 4, pp.869-878
Physics
3
Scientific paper
K-Ar ages of illite alteration associated with Middle Proterozoic Athabasca unconformity-type U deposits in Saskatchewan range from 414 to 1493 Ma. The K-Ar ages correlate with water contents and D values such that illites with young K-Ar ages have D values as low as -169 and water contents as high as 7.7 wt.% whereas illites with older ages have D values near -70 and water contents near 4 wt.%. Water extracted at 400°C from illites with low D values and high water contents has low D and 18 O values similar to those of modern meteoric water suggesting that some of the illites associated with the original deposition of the ore underwent varying degrees of retrograde alteration. The alteration is initiated by hydration of sites in the interlayer region of the illite which results in the partial resetting of the K-Ar ages and introduction of excess structural water in the form of interlamellar water. The interlamellar water is enriched in 18 O by about 7 per mil relative to the water that physically surrounded the clay particle. Further alteration decreases the D value and increases the 18 O value of the illite by isotopic exchange between the mineral and the interlamellar water. Although the chemical compositions and XRD patterns of the altered illites indicate that no detectable smectite component is present in the samples, the isotopic results suggest that the altered illites may be an early precursor in the formation of mixed-layer illite/smectite by retrograde alteration of pure illite. The wide variation of D values of chlorite and kaolinite from these U deposits is analogous to that of the illite suggesting that retrograde alteration of clays by meteoric water can be substantial. The general association of altered clays with areas containing the highest concentrations of U is probably related to localized permeability within the ore zone.
Hoeve J.
Kyser Kurt T.
Mehnert Herbert
Wilson Ryan M.
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