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

Abstract

The behavior of rare-earth elements (REE) during hydrothermal alteration and progressive weathering has been studied by acid-leaching experiments on minerals and whole-rock samples of the Lueshe carbonatite complex (NE of Democratic Republic of Congo). The fresh and hydrothermally altered samples were successively leached with acetic acid (HAc, 1 N) and hydrochloric acid (HCl, 1 N). An additional leaching step with nitric acid (HNO 3, 1 N) was applied on meteoric weathered material. After this sequential extraction procedure, the residues were dissolved in a fluorhydric-nitric solution (HF+HNO 3). The Lueshe carbonatite complex contains a suite of REE-carriers identified as Ca-bearing minerals including carbonates, apatite and pyrochlore as primary minerals that are accompanied by hydrothermal ancylite-(Ce), parisite-(Ce), synchysite-(Ce) and monazite-(Ce). The hydrothermal alteration induces an enrichment in the light REE, but the authigenic minerals (except monazite) do not, in fact, play an important part in the pre-lateritic REE stock, because of their low amounts. During the early stage of weathering, the residual minerals retain the bulk of the REE, the distribution being only slightly modified. During intense weathering, the leaching experiments indicate that the REE are redistributed among secondary REE-phosphate minerals and bonded to Fe-hydroxides, in the lateritic profile.