Composition and Th-U-total Pb ages of huttonite and thorite from Gillespie's Beach, South Island, New Zealand

The average result of 49 electron-microprobe spot analyses of huttonite (monoclinic ThSiO4) from its type locality, Gillespie's Beach, southern Westland, New Zealand, yields the following chemical composition (in wt.% +/- 1 sigma standard deviation in italics in parentheses): P2O5 0.43 (0.13), SiO2 18.78 (0.17), ThO2 79.21 (0.53), UO2 0.66 (0.07), Y2O3 0.35 (0.15), La2O3 0.02 (0.02), Ce2O3 0.06 (0.07), Pr2O3 0.02 (0.02), Nd2O3 0.12 (0.08), Sm2O3 0.06 (0.05), Gd2O3 0.10 (0.05), Dy2O3 0.06 (0.04), CaO 0.01 (0.01), FeO 0.01 (0.01), PbO 0.03 (0.01), total 99.94 (0.32). Huttonite is very close to the theoretical ideal stoichiometry, with the general formula (Th0.96U0.01Y0.01REE0.01)(Sigma 0.99) (Si0.99P0.02)(Sigma 1.01) O-4. It displays little compositional heterogeneity and is near end-member composition (93.2 to 98.4 mol.% ThSiO4). The chondrite-normalized REE patterns are umbrella-like, with Gd being the most prominent rare-earth element. The P, U, Fe, and total REE contents of the grains studied differ strongly from those of the type material published by Pabst & Hutton (1951). Thorite (tetragonal ThSiO4) from Gillespie's Beach displays the following average composition (n = 30): P2O5 0.12 (0.09), SiO2 18.84 (0.18), ThO2 73.46 (1.85), UO2 5.90 (1.46), Y2O3 0.21 (0.11), La2O3 0.00 (0.01), Ce2O3 0.08 (0.05), Pr2O3 0.02 (0.04), Nd2O3 0.07 (0.05), Sm2O3 0.04 (0.04), Gd2O3 0.04 (0.04), Dy2O3 0.04 (0.04), CaO 0.13 (0.08), PbO 0.43 (0.20), total 99.42 (0.44). Like huttonite, it is virtually non-metamict, nearly stoichiometric [(Th0.90U0.07Y+REE0.01Ca0.01Pb0.01)(Sigma 1.00)Si1.01O4] and approaches end-member composition (86.3 to 96.7 mol.% ThSiO4). Th-U - total Pb dating reveals a late Tertiary (Miocene) age for the huttonite. The thorite records older ages, from late Triassic-early Jurassic to early Tertiary. All ages coincide with major metamorphic or magmatic events on South Island. Rocks that formed in the experimentally derived P-T stability field of huttonite are unknown in the area. There thus are suggestions of discrepancies between the experimental data on huttonite-thorite phase relations and those observed in natural systems.