Pyrochlores from the Lueshe carbonatite complex (Democratic Republic of Congo): a geochemical record of different alteration stages

Magmatic pyrochlores from the Lueshe syenite-carbonatite complex from the n ortheastern part of Democratic Republic of Congo (ex-Zaire) are characteriz ed by Ta/Nb ratios in an increasing order from pyroxenite, calcite-carbonat ite (sovite), silicate xenoliths (nodules) to syenite. Substitutions invoki ng Nh, Ta, Ti and REE have been precisely described. Hydrothermal alteratio n of Lueshe pyrochlore involves the substitution of Na+ + F- = VA + VY and Ca + O = VA + VY (VA = A-site vacancy and VY = Y-site vacancy). In calcite carbonatite, hydrothermal alteration of pyrochlore took place during and af ter the precipitation of ancylite-(Ce), strontianite, celestite, baryte and fayalite according to a fluid composition of relatively low pH, a(Na+), a( ca2+) and a(HF), and high a(sr2+) and a(LREE3+). The supergene alteration i s characterized by complete leaching of Na, Ca and F and partial incorporat ion of K, Ba, Sr and Ce resulting in the formation of kali-, bario-, stront io- and ceriopyrochlore respectively. The Na-poor pyrochlore may be an inte rmediate variety corresponding to an alteration stage between the hydrother mal and weathered pyrochlores. The IR spectroscopic study has indicated tha t the weathered pyrochlore is a hydrated variety containing two bands of OH vibration modes at 3413 and 1630 cm(-1). During hydrothermal and supergene alterations, the cations at B-site remain relatively constant. The variabl e chemical compositions of the pyrochlores from the Lueshe complex represen t geochemical memories of the different alteration conditions including the variation in the oxidation-reduction environment. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.