Geochemical alteration of pyrochlore group minerals: Pyrochlore subgroup GREGORY R. LUMPKIN Advanced Materials Program, Australian Nuclear Science and Technology Organization, Menai 2234, New South Wales, Australia

Primary alteration of uranpyrochlore from granitic pegmatites is characterized by the substitutions ADYD-+ ACaYO, ANaYF -+ ACaYO, and ANaYOI-I --+ ACaYO. Alteration occurred at ""450-650 °C and 2-4 kbar with fluid-phase compositions characterized by relatively low aNa+,high aeaH, and high pH. In contrast, primary alteration of pyrochlore from nepheline syenites and carbonatites follows a different tre:nd represented by the substitutions ANaYF -+ ADYD and ACaYO -+ ADYD. In carbonatites, primary alteration of pyrochlore probably took place during and after replacement of diopside + forsterite + calcite by tremolite + dolomite :t ankerite at ""300-550 °C and 0-2 kbar under conditions of relatively low aHF, low aNa+,low aeaH, low pH, and elevated activities of Fe and Sr. Microscopic observations suggest that some altered pyrochlor1es are transitional between primary and secondary alteration. Alteration paths for these specimens scatter around the trend ANaYF -+ ADYD. Alteration probably occurred at 200-350 °C in the presence of a fluid phase similar in composition to the fluid present during primary alteration but with elevated activities of Ba and REEs. Mineral reactions in the system Na-Ca-Fe-Nb-O-H indicate that replacement of pyrochlore by fersmite and columbite occurred at similar conditions with fluid conpositions having relatively low aNa+,moderate aeaH, and moderate to high aFeH.Secondary alteration « 150 °C) is charactlerized by the substitutions ANaYF -+ ADYD,ACaYO -+ ADYD,and ACaXO -+ ADXDtogether with moderate to extreme hydration (10-15 wt% H20 or 2-3 molecules per formula unit). Minor variations in the amounts of Mg, AI, K, Mn, Fe, Sr, Ba, and REEs are commonly observed as a result of secondary alteration. Major cation exchange for K, Sr, and Ba is a feature of samples from laterite horizons overlying carbonatites. In most cases U, Th, and B-site cations remain relatively constant. Radiogenic Pb is typically lost via long-te:rm diffusion, but in some grains of uranpyrochlore 25-90% of the Pb is lost as a result of alteration. INTRODUCTION are defined by the A-site cation population (see Hogarth, 1977, Table 1). The pyrochlore group consists of a chemically diverse Microlite is mainly restricted to moderately to highly suite of minerals having the general formula fractionated rare-element granitic pegmatites; betafite is A2-mB2X6-wYt-n.pH20,where A = Na, Ca, Mn, Fe2+, found in geocheJmicallymore primitive granitic pegmaSr, Sb, Cs, Ba, rare earth elements (REEs = Sc, Y, lantites and in some carbonatites (Cerny and Ercit, 1989). thanides), Pb, Bi, Th, and U; B = Nb, Ta, Ti, AI, Fe3+, Members of the pyrochlore subgroup predominantly ocZr, Sn, and W; X = 0, OH; and Y = 0, OH, and F. The cur in three major host rock categories: carbonatites, structure type is cubic (space group Fd3m, Z = 8), has a nepheline syenitc~s,and granitic pegmatites. As a result, unit cell dimension of approximately 10.3-10.6 A, and the general chenlistry of the pyrochlore subgroup is extolerates vacancies at the A, X, and Y sites (m = 0-1.7, tremely diverse in relation to the microlite and betafite w = 0-0.7, n = 0-1). Defect pyrochlore may be stabilized subgroups (e.g., Hogarth, 1961; Perrault, 1968; Krivoby the incorporation of H20 molecules (p = 0-2) and koneva and Sidorenko, 1971; Petruk and Owens, 1975). OH groups, with total H20 contents of 10-15 wt% Because of the variety of host rocks encountered as well (Lumpkin, 1989). Hogarth (1977) defined three major as the complex Icompositions of members of the pyrosubgroups of the pyrochlore group on the basis of the chlore subgroup" chemical effects of alteration are exmajor B-site cations: microlite (Nb + Ta > 2Ti, Ta ~ pected to be comlplicated (Lumpkin and Ewing, 1985). Nb), pyrochlore (Nb + Ta > 2Ti, Nb > Ta), and betafite Geochemical alteration of pyrochlore in carbonatites (2Ti ~ Nb + Ta). Individual species in each subgroup to a hydrated, defect pyrochlore enriched in Ba, Sr, or K 0003-004X/95/0708-0732$02.00 732 LUMPKIN AND EWTI'lG: GEOCHEMISTRY OF THE PYROCHLORE MINERALS No. TABLE1. Localities, sources, host rocks, and associated minerals for 18 pyrochlore samples 733