Differentiated hydrothermal and meteoric alterations of the Lueshe carbonatite complex (Democratic Republic of Congo) identified by a REE study combined with a sequential acid-leaching experiment

The behavior of race-earth elements (REE) during hydrothermal alteration an d progressive weathering has been studied by acid-leaching experiments on m inerals and whole-rock samples of the Lueshe carbonatite complex (NE of Dem ocratic Republic of Congo). The fresh and hydrothermally altered samples we re successively leached with acetic acid (HAc, 1 N) and hydrochloric acid ( HCl, 1 N). An additional leaching step with nitric acid (HNO3, 1 N) was app lied on meteoric weathered material. After this sequential extraction proce dure, the residues were dissolved in a fluorhydric-nitric solution (HF + HN O3). The Lueshe carbonatite complex contains a suite of REE-carriers identi fied as Ca-bearing minerals including carbonates, apatite and pyrochlore as primary minerals that are accompanied by hydrothermal ancylite-(Cr), paris ite-(Ce), synchysite-(Ce) and monazite-(Ce). The hydrothermal alteration in duces an enrichment in the Light REE, but the authigenic minerals (except m onazite) do not, in fact, play an important part in the pre-lateritic REE s tock, because of their low amounts. During the early stage of weathering, t he residual minerals retain the bulk of the REE, the distribution being onl y slightly modified, During intense weathering, the leaching experiments in dicate that the REE are redistributed among secondary REE-phosphate mineral s and bonded to Fe-hydroxides, in the lateritic profile. (C) 2000 Elsevier Science B.V. All rights reserved.