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

Abstract

Magmatic pyrochlores from the Lueshe syenite–carbonatite complex from the northeastern part of Democratic Republic of Congo (ex-Zaı̈re) are characterized by Ta/Nb ratios in an increasing order from pyroxenite, calcite-carbonatite (sövite), silicate xenoliths (nodules) to syenite. Substitutions involving Nb, Ta, Ti and REE have been precisely described. Hydrothermal alteration 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 after the precipitation of ancylite-(Ce), strontianite, celestite, baryte and fayalite according to a fluid composition of relatively low pH, a Na + , a Ca 2+ and a HF, and high a Sr 2+ and a LREE 3+ . The supergene alteration is characterized by complete leaching of Na, Ca and F and partial incorporation of K, Ba, Sr and Ce resulting in the formation of kali-, bario-, strontio- and ceriopyrochlore respectively. The Na-poor pyrochlore may be an intermediate variety corresponding to an alteration stage between the hydrothermal and weathered pyrochlores. The IR spectroscopic study has indicated that 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 variable chemical compositions of the pyrochlores from the Lueshe complex represent geochemical memories of the different alteration conditions including the variation in the oxidation–reduction environment.