EUG, Strasbourg, 1999, Session O07: Mineralogy, Ore Geology, Mineral Resources
Jacques Moutte (moutte@emse.fr), Jean Jacques Gruffat (gruffat@emse.fr) & Mohammed Nasraoui (nasraoui@itn1.itn.pt)
Ecole des Mines, 158 Cours Fauriel, Saint Etienne, France
Abstract
The Lueshe Niobium deposit (Kivu, Congo ex-Zaire) was formed by supergene
alteration of carbonatitic and associated syenitic rocks. The present study
is an extension of an EC project conducted in 1991-94 on ore beneficiation
processes [Albers et al, Applied mineralogy of pyrochlore and related minerals
in the weathering zones of the niobium deposits of the Lueshe and Bingo
carbonatites, Zaire, CCE project MA2 M-CT90-0038]. The geological features
of the laterites are discussed here in terms of whole-rock chemistry. An
extensive analytical work (XRF, ICP-AES) has been done on fresh rocks and
along several profiles in the laterites. These profiles are used to reconstruct
the 3D organization of the different alteration facies. The Lueshe alkaline
complex [H. von Maravic, G. Morteani, G. Roethe, Jour. African Earth Sci,
9, 341-355, 1989] consists of two main units: a complex association of
silicate-rich calcite carbonatite and syenitic rocks, and a plug of pure
dolomite carbonatite. Primary niobium segregations are found associated
with pyroxene/apatite/felspar concentrations within the calcite-carbonatites,
or at its margins. The Nb-bearing laterites have a consistent geochemical
signature, well constrained by a number of conservative interelement ratios
involving Ti, Nb, Zr, REE, Al, Fe. The laterites are formed by in situ
alteration of soevites and associated syenitic inclusions, without significant
mixing of laterites derived from other rocks. Due to the high chemical
heterogeneity of the protoliths, classical mass balance approaches cannot
be used directly. The chemical evolutions of the laterites are interpreted,
in terms of phase assemblages, using a combination of graphic projections
and normative calculations specifically designed for phosphate minerals.
A sharp front separates the two main horizons (lower: apatite-bearing,
upper: crandallite-bearing) encountered in the studied area. In contrast
with other supergene deposits of Nb developed on carbonatites, there is
no secondary apatite enrichment at the lower part. The formation of crandallite
is, at whole rock scale, strongly coupled with the breakdown of the primary
silicates to kaolinite. It is suggested that the peculiarity of the Lueshe
profile (absence of phophate accumulation) reflects to the high drainage
conditions induced by the relatively steep topography of the site. The
study intends to show that the whole rock approach of weathering processes
can be more than merely descriptive, and how it can be used to understand,
in conjunction with XRD and microscopic data, the evolutions of phase relations
in an alteration profile.