A LA-ICP-MS AND SHRIMP U/PB AGE CONSTRAINT ON THE TIMING OF REE MINERALISATION ASSOCIATED WITH BUSHVELD GRANITES

摘要

Monazite occurs in association with fluorite and polymetallic sulphides in brecciated pegmatites emplaced within discordant pipelike structures, and spatially associated mineralised quartz veins that cut hydrothermally altered Bushveld-age Nebo Granite (Lebowa Granite Suite) on the farm Houtenbek 194 JR. Mineralisation of this style has been viewed as having a high-temperature origin related to a late-stage of Bushveld-age granite emplacement. However, early attempts to date Houtenbek monazite in the 1950 to 1960s, using thermal ionisation mass spectrometry, gave a range of very imprecise ~(207)Pb/~(206)Pb ages from isotopically moderately to strongly discordant grains that cannot be used to confidently make this link. Here we present new high spatial resolution SHRIMP and LA-ICP-MS weighted mean ~(207)Pb/~(206)Pb ages of 2059.2 ± 3.5 Ma and 2046 ± 10 Ma for a museum specimen of Houtenbek monazite. These estimates overlap with the 2054 Ma emplacement age of the main phase of the Lebowa Granite Suite. Together with the monazite REE patterns, which indicate a magmatic origin, the new age determinations reinforce the connection between REE mineralisation and Bushveld-age granites.