Crystallization and alteration history of britholite in rare-earth-element-enriched pegmatitic segregations associated with the Eden Lake Complex, Manitoba, Canada

Pegmatitic segregations and quartzofeldspathic veins associated with the Ed en Lake Complex, Manitoba, Canada, contain significant abundances of rare-e arth elements (REE), U and Tn, concentrated in minerals such as titanite, a patite, allanite and britholite. Titanite and apatite are typically found a s discrete, zoned, and locally euhedral crystals. Allanite occurs both as d iscrete crystals as well as irregular segregations, whereas britholite occu rs only as irregular masses. The allanite and britholite typically share sh arp but irregular contacts with the host silicate minerals (aegirine-augite , K-feldspar and quartz). Allanite normally occurs between britholte and th e silicate minerals, suggesting that it formed as the result of reaction be tween a REE-bearing fluid rich in P and F and the silicate minerals. Within the irregular masses of britholite, one sees an unusual polygonal mosaic-l ike texture with polygons ranging from 20 to 200 mu m. Polygon boundaries a re straight to slightly curved, intersect at triple junctions, and converge at angles from 60 to 180 degrees. This texture is likely the product of re heating and annealing of the britholite. However, as a result of metamictiz ation, the britholite retains little long-range order. Many of the polygons are altered preferentially along polygon boundaries. Chemical analyses ind icate that U, Th, La, Ce, F and Cl were mobile during the alteration, where as Y seems to have been immobile. Altered britholite has lower La and Ce co ntents than unaltered britholite, but the La/Ce value remains essentially u nchanged. Altered britholite has lower F/Cl values than unaltered britholit e, and U and Th contents show significant scatter that in turn is related t o elevated Cl contents.