RADIOELEMENT DISTRIBUTION IN THE TERTIARY LUNDY GRANITE (BRISTOL CHANNEL, UK)

The radioelement distribution and content of the Lundy granite, a coar se-grained megacrystic granite of Tertiary age, has been measured usin g a portable gamma-ray spectrometer in order to assess fractionation a nd alteration processes in the granite. Results indicate a systematic variation of K, Th and U (with a few notable exceptions) that follows a partially concentric distribution to lower concentrations inland. Th e plateau region of the island (particularly the southern half) is rel atively depleted in all radioelements. Over the island, measurements o f K vary from 1.3-4.9 wt %, Th varies from 5.0-20.3 ppm and U varies f rom 2.0-12.5 ppm. A petrographic, electron microprobe and autoradiogra phy examination of the granite indicates that the radioelements mainly reside in discrete major and accessory minerals, of which K-feldspar (K), biotite (K), monazite (Th), xenotime (U), tungsteniferous columbi te (U) and uraninite (U) are the most important. Uraninite is rare, be ing preserved only in fresh samples which come mainly from abandoned q uarries. Mass balance modelling indicates that up to 76.6% of uranium could reside in uraninite and where this has been leached by secondary processes such as hydrothermal alteration or weathering then the pres ent radioelement content no longer reflects the original rock composit ion. Fission track evidence is presented to show the pathways along wh ich uranium has been mobilized from or within the granite. Secondary s ites of radioelements include fractures cross-cutting all major minera ls (but especially quartz), gain boundaries, altered cores of plagiocl ase feldspar and occasionally yellowy brown mixed chlorite/smectite re placement product after biotite. Biotite itself may exhibit secondary tracks along cleavage traces. Combined effects of crystal fractionatio n (primary variation) and secondary alteration best explain the distri bution of radioelements, with K controlled by fractionation of the maj or phases K-feldspar and biotite, Th by fractionation of the accessory mineral monazite (+/-xenotime and uraninite) and U contents by uranin ite and tungsteniferous columbite. Secondary processes have removed mu ch of the uraninite leaving behind indeterminate Fe-U material along f ractures and residual U (and Th) enrichment within altered major miner als. There is some evidence to suggest that late radioelement-bearing fluids precipitated monazite and uraniferous zircon along fractures du ring the waning stages of magmatic activity.