METALLOGENIC SIGNIFICANCE OF TRACE-ELEMENT AND U-PB ISOTOPE DATA FOR URANINITE-RICH MINERAL SEPARATES FROM THE LABRADOR CENTRAL MINERAL BELT

Thirteen concentrates of uraniferous material were prepared from urani um occurrences in the Central Mineral Belt of Labrador. Host rocks to these occurrences include granitoid rocks of the Archean basement, ca. 2000 Ma metasedimentary rocks of the Lower Aillik Group, and 1860 Ma felsic volcanic rocks of the Upper Aillik Group. Common lead corrected Pb isotope data from inductively coupled plasma mass spectrometry ana lyses define Pb-207/Pb-206 ages ranging from 1805 to 1697 Ma for all b ut one sample, with a mean age of 1752 +/- 27 Ma (1 sigma). Ages calcu lated for individual samples are similar to those derived by previous workers using standard analytical techniques. Eleven of these samples define linear trends that intersect the U - Pb concordia at 1741 +/- 2 3 Ma and a Tera-Wasserburg curve at 1740 +/- 21 Ma, respectively. Thes e data suggest that the occurrences are epigenetic with respect to hos t rocks and possibly related to a common metallogenic event, therefore resolving a long-standing controversy about the timing and mode of oc currence of the widespread uranium mineralization in this part of the belt. These ages broadly correlate with a period of migmatization, met amorphism, and granitoid plutonism, as defined by U - Pb zircon geochr onological data for regional units. Rare earth element data for uranin ite from all concentrates resemble those of uraninite in granite-relat ed deposits. One sample has a distinctly different calculated Pb-207/P b-206 age of 495 Ma, indicative of a later remobilization of the ca. 1 741 Ma mineralization. The geochemical and geochronological data colle ctively suggest that the Central Mineral Belt uranium occurrences were related to posttectonic granite magmatism and have no direct genetic relationships with nongranitoid host rocks.