Metamict U-rich Pyrochlore of Newania Carbonatite, Udaipur, Rajasthan

ABSTRACT U-rich pyrochlore (UO2 up to 18.63%) occurs along the magmatic bands as well as disseminated grains within rauhaugites of the Newania complex. UO2 appears to be primary as it seems mostly evenly distributed all over the grains, though in one grain it does show concentration in parts. Ta is also an important element in some pyrochlore grains and its concentration reaches up to 17.15%. High U and Th are responsible for bringing metamictization in the Newania pyrochlore. Newania carbonatite has a complex emplacement history; rauhaugite seems to have been replaced at 2200 Ma years followed by emplacement of ankeritic carbonatite at around 1500 Ma. Later during tectonic event pyrochlore was involved in reaction with hydrothermal fluid and at this stage Fe and FeS were deposited on the rims of pyrochlore grains. At the same time pyrochlore was subjected to hydro-thermal alteration resulting in removal of Na, Ca and F leaving large vacancy in its A-site. Very few grains have escaped such alteration.