PYROCHLORE FROM WEATHERED CARBONATITE AT LUESHE, ZAIRE

A detailed study of weathered pyrochlore in the laterite above carbona tite at Lueshe, NE Zaire, has been made in order to determine its chem ical and textural variations. Pyrochlore in fresh carbonatite at Luesh e is close to an ideal formula of (Ca,Na)(2)Nb2O6(OH,F) (where a gener al formula is A(2-x)B(2)O(6)(OH,F)(1-y). zH(2)O). The first and princi pal change on weathering occurs at the base of the profile and involve s the leaching and partial exchange of A cations together with hydrati on. This change appears common to weathered pyrochlore worldwide. As a result weathered pyrochlore at Lueshe has a large apparent A cation d eficiency with A totals between 0.25 and 0.59. The B cations remain st able. Abundant kalipyrochlore is unique to Lueshe and is thought to be related to the abundance of potassium feldspar in the fresh carbonati te, showing that the actual composition of weathered pyrochlore is a c haracteristic of a particular deposit. Weathered profiles at Lueshe ar e not simple trends from the least to most leached compositions. Furth er factors including variation in whole rock mineralogy and chemistry, and cation exchange and uptake are responsible for local concentratio ns of strontio-, bario- and calcium-rich, sodium-poor pyrochlore in th e ore body, as well as rims of ceriopyrochlore on kalipyrochlore. The most important textural relationship in the Lueshe pyrochlore is the i ntimate intergrowth with crandallite in the most weathered parts of th e laterite. Although pyrochlore persists throughout the weathering pro file, niobium-bearing goethite is thought to represent the final produ ct of pyrochlore breakdown.