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Analyses of 6 mineralized and 4 unmineralized samples from drill holes in the uranium ore deposit at Lilljuthatten, Sweden, yield an apparent Pb-Pb age of 425 ± 6 m.y. (errors at 95% confidence level). This age corresponds with the youngest phase of the Caledonian orogeny in central Sweden, indicating that the ore deposit probably formed during this event.
The age of the host granite has not been determined quantitatively because the Caledonian event has disturbed (and in some places completely reset) all the isotopic systems investigated thus far. Four new and 7 published Rb-Sr analyses approximate an isochron of 1,540 m.y., but two samples from near the ore zone have been reset to much younger apparent ages.
Isotope systematics in the Th-Pb and U-Pb systems are highly complex, but U-Pb data for a few samples suggest a host-rock age of approximately 1,500 m.y. If this age and thorium immobility are assumed, there is a suggestion that the host granite was extremely rich in uranium (30 to 50 ppm) at the time of intrusion and that the ore deposit may have formed by concentration of uranium from the host granite.
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