GEOCHEMICAL ALTERATION OF PYROCHLORE GROUP MINERALS - PYROCHLORE SUBGROUP

Primary alteration of uranpyrochlore from granitic pegmatites is chara cterized by the substitutions (A) square(Y) square --> (CaO)-Ca-A-O-Y, (NaF)-Na-A-F-Y, --> (CaO)-Ca-A-O-Y, and (NaOH)-Na-A-O-Y, --> (CaO)-Ca -A-O-Y. Alteration occurred at similar to 450-650 degrees C and 2-4 kb ar with fluid-phase compositions characterized by relatively low a(Na) +, high a(Ca)2+, and high pH. In contrast, primary alteration of pyroc hlore from nepheline syenites and carbonatites follows a different tre nd represented by the substitutions (NaF)-Na-A-F-Y, --> A square(Y) sq uare and (CaO)-Ca-A-O-Y --> (A) square(Y) square. In carbonatites, pri mary alteration of pyrochlore probably took place during and after rep lacement of diopside + forsterite + calcite by tremolite + dolomite +/ - ankerite at similar to 300-550 degrees C and 0-2 kbar under conditio ns of relatively low a(HF), low a(Na)+, low a(Ca)2+, low pH, and eleva ted activities of Fe and Sr. Microscopic observations suggest that som e altered pyrochlores are transitional between primary and secondary a lteration. Alteration paths for these specimens scatter around the tre nd (NaF)-Na-A-F-Y --> (A) square(Y) square. Alteration probably occurr ed at 200-350 degrees C in the presence of a fluid phase similar in co mposition to the fluid present during primary alteration but with elev ated activities of Ba and REEs. Mineral reactions in the system Na-Ca- Fe-Nb-O-H indicate that replacement of pyrochlore by fersmite and colu mbite occurred at similar conditions with fluid conpositions having re latively low a(Na)+, moderate a(Ca)2+, and moderate to high a,ii. Seco ndary alteration (<150 degrees C) is characterized by the substitution s (NaF)-Na-A-F-Y --> (A) square(Y) square, (CaO)-Ca-A-O-Y --> (A) squa re(Y) square, and (CaO)-Ca-A-O-X --> (A) square(X) square together wit h moderate to extreme hydration (10-15 wt% H2O or 2-3 molecules per fo rmula unit). Minor variations in the amounts of Mg, Al, K, Mn, Fe, Sr, Ba, and REEs are commonly observed as a result of secondary alteratio n. Major cation exchange for K, Sr, and Ba is a feature of samples fro m laterite horizons overlying carbonatites. In most cases U, Th, and B -site cations remain relatively constant. Radiogenic Pb is typically l ost via long-term diffusion, but in some grains of uranpyrochlore 25-9 0% of the Pb is lost as a result of alteration.