Abstract

Thirteen concentrates of uraniferous material were prepared from uranium 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 analyses define 207Pb/206Pb ages ranging from 1805 to 1697 Ma for all but one sample, with a mean age of 1752 ± 27 Ma (1 σ). Ages calculated 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 ± 23 Ma and a Tera-Wasserburg curve at 1740 ± 21 Ma, respectively. These data suggest that the occurrences are epigenetic with respect to host rocks and possibly related to a common metallogenic event, therefore resolving a long-standing controversy about the timing and mode of occurrence of the widespread uranium mineralization in this part of the belt. These ages broadly correlate with a period of migmatization, metamorphism, and granitoid plutonism, as defined by U–Pb zircon geochronological data for regional units. Rare earth element data for uraninite from all concentrates resemble those of uraninite in granite-related deposits. One sample has a distinctly different calculated 207Pb/206Pb age of 495 Ma, indicative of a later remobilization of the ca. 1741 Ma mineralization. The geochemical and geochronological data collectively suggest that the Central Mineral Belt uranium occurrences were related to posttectonic granite magmatism and have no direct genetic relationships with nongranitoid host rocks.

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