J.MOUTTE, J.J.GRUFFAT, M.NASRAOUI, Ecole des Mines de Saint Etienne, 158 Cours Fauriel, Saint Etienne, Loire

Abstract

Web Version of the Poster presented at EUG99, Strasbourg
 

The present study is an extension of an EC project conducted in 1991-94 on ore beneficiation processes

Our Department has been involved, in 1992-1995, in a BRITE-EURAM project devoted to the optimization of
beneficiation processes of Niobium ores of the Lueshe mine (North Kivu, at that time Zaire). The precise characterisation of the
ores and associated rocks was carried out through the close cooperation between three teams involved the following tasks:

• systematics of pyrochlore chemistry by British Museum team (UK),
• determination of modal compositions by quantitative X-ray  diffraction at Louvain-la-Neuve University (Belgium),
• bulk chemistry of ores and laterites by our team (Ecole des Mines de Saint Etienne).

The main Tasks of the project were:

• distribution of pyrochlore and  phosphate minerals
• size distribution of the pyrochlore in the Lueshe ore
• chemical variation in the pyrochlore

• Albers K.H., Bilal E., Garcia D., Gruffat J.J., Guy B., Meixner K., Moutte J., Nasraoui M., Philippo S., Schürmann B., Sonnet P., Verkaeren J., Wall F., Williams T., Woolley A.  (1993). "Applied mineralogy  pyrochlore and related minerals inof the weathering zones of the niobium deposits of the Lueshe and Bingo carbonatites, Zaïre". Final Report, Contract CCE MA2M-CT90-0038
• Nasraoui M. (1996) "Le gisement de niobium de Lueshe (nord est du zaire): évolution géochimique et minéralogique d'un complexe carbonatitique en contextes hydrothermal et supergène", Thèse Ecoles des Mines de Paris et Saint Etienne.

Besides the analytical tasks carried out on samples provided by GfE for the EC project itself, we undertook a geological study
aimed at understanding more directly the processes.

The methodology designed for the EC project has been applied to an extended set of samples.

The larger number of fresh rocks analyzed allows for a better charcaterization of the possible protoliths of the Niobium- bearing protoliths.

Different sampling strategy of the laterites

• detail of the 'transition' from apatite- to crandallite-laterite
• High sample density
• fine scale structure of the deposit

Using bulk chemistry of laterite for an intrepretation in terms of phase assemblage evolution
 
 

2. Geological background

Petrography
 

H.v.Maravic (doctoral dissertation, Berlin, 1983): __cancrinite syenite intruded by calcite-carbonatite (aegyrine-bearing and with   feldspathic 'nodules' derived from syenitic inclusions) > primary Nb-enrichment __dolomite-carbonatite (no associated silicates)

Geochemistry

Strong complementarity (chemistry, mineralogy) between cancrinite syenite and soevite (similarity with compositions of conjugate liquids produced by immiscibility of a carbonated silicate melt)

Geomorphology :

reflects two main stages of evolution,
1. formation of a peneplain (cf. Bitobitoba plateau, at 1700 m asl, to the North), with iron crust formations and limnic/ lacustrine deposits
2. strong dissection in relation with regional uplift > extensive supergene alteration in high drainage conditions
 

3. Protoliths

Carbonated framework readily dissolved >> no preserved textures in the laterites, abundance of collapse structures, listric faults, etc.
presence of allochtonous alterites (e.g. fluvio-lacustrine formations) in the upper part of the profile
strong heterogeneity of possible parent rocks (esp. carbonatites with pyroxenitic and feldspathic nodules)

High geochemical variability

>> Statistical approach on interelement ratios using an extended set of geochemical data on fresh rocks (100+ samples) and laterites (250+ samples)

• the laterite population has a consistent and well constrained geochemical signature
• this part of the orebody results from in situ alteration of soevites and associated silicate nodules, without significant mixing of allochtonous laterites

a. Scheme of the Normative Calculation
 

0	Weight percent composition >> Number of Atom
1	Fe,Mn,Ti oxydes
2	Phosphates  Ca:=Ca+Sr IF  (Ca > 5/3 P)  THEN (Calcite + Apatite) __ELSE IF (Ca > 1/2 P)  THEN (Apatite + Crandallite) ____ELSE  (Crandallite + Wavellite) Al:= Al - (Al in Phosphate)
3	Silicates Na,K in Felspars, recalculate Si,Al, Si,Al is recast into 'kaolinite' + quartz or 'kaolinite' + 'montmori'

b. Validation of the Normative Scheme :

Comparison of Normative results with Modal compositions (quantitaive XRD, U.C.Louvain).

• Modal Crandallite correlated with sum of Normative Al-phosphates
• Correlation between Modal and Normative Quartz (quartz-bearing laterites in the upper part of the mine)

Main feature of the alteration profile (Maravic, Albers et al, op.cit.):

• lower part, apatite-bearing
• upper part, crandallite-bearing

leaching of Calcium is the most striking feature of the alteration profile,
it proceeds step by step:
the laterite compositions are distributed along tie lines controlled by the main minerals of each alteration zone
>> two main fronts of decalcification :
1. dissolution of calcite > apatite as the only Ca-bearing phase,
2. destabilization of apatite > crandallite

Gallery 8

• Cross section, at 1500 m asl, 140 metres long, from fresh carbonatite to highly evolved crandallite laterites.

• large oscillations: reflects mainly the protolith heterogenity
• crandallite close to fresh carbonatite, apatite zone further East: indicative of the hydrological structure (strong permeability contrast near fresh rock and steep structure > high drainage)a  zone of lower drainage (montmorillonite)

Projection of normative assemblages:

• High sample density (2 m) > definition of fronts between the main zones
• Zone of lower drainage (montmorillonite) in the Northern part

Although generally used as a descriptive tool, useful only for gross quantification and mass balance calculations, the whole rock chemical approach can be, once the mineral assemblages have been characterized on a limited number of representative samples, a powerful tool for the interpretation of lateritic processes in terms of the evolution of mineral assemblages

> makes possible the systematic analysis of a high density sample set
> interpretation in terms of phase assemblages
> fine scale structure of a supergene deposit

A peculiarity of the Lueshe profile is  the absence of secondary apatite (i.e. no absolute enrichment in P).
It may result from the high drainage condition induced by the steep topography of the site of alteration Lueshe profile develops mostly above the water table, whereas secondary apatite would require high levels of carbonate / calcium activities associated with saturated conditions