Geochemistry of carbonatites and related rocks: the Lueshe Complex (Kivu, Congo)

Jacques MOUTTE 1, Mohamed NASRAOUI 2

1 Ecole des Mines, 158 Cours Fauriel, 42100 Saint Etienne, France
2 Instituto Tecnologico e Nuclear, Estrada Nac 10, 2686-953 Sacavém, Portugal

In connection with a study of the lateritic Niobium deposit of Lueshe (Kivu, R.D. Congo) [1,2], a comprehensive study has been done
on the bulk chemistry of the Lueshe Complex.  This complex comprises two parts with no direct relationships in the field: a plug of
dolomite carbonatite, and an intrusion where silicate- bearing calcite- carbonatites intrude and surround cancrinite- syenites.

The central part of the dolomite carbonatite plug is essentially composed of dolomite.  Its bulk Mg/(Mg+Fe) is homogeneous (0.92 +-
0.01), and the chemical variability reflects mainly varying degrees of apatite accumulation without fractionation in REE patterns.
Silicate minerals are restricted, mainly in marginal parts of the intrusion, to folded amphibole schlieren representing xenoliths of host
fenites dragged by the carbonatite during emplacement.  The dolomite carbonatite evolves, towards margin, to ferrocarbonatites, with
a concomitant increase in MnO.  There is no sign of involvement of an alkaline silicate phase in the genesis of this carbonatite.
In contrast, the other part of the complex consists of cancrinite-bearing syenites and aegirine- bearing calcite-carbonatites that intrude
them.

The dominant syenite type ('trachytoidal syenite' [3]) has an homogeneous chemistry, suggesting magmatic emplacement, no
mafic precursor, nor internal differentiation;  its composition reflects its simplified mineralogy (>90 % on the alkali felspar - cancrinite
join).  Its trace element signature is dominated by high contents in Nb (900 - 1500 ppm) and Zr (1500 - 3000 ppm).  Strong textural
and mineralogical modifications are induced on this syenite by the emplacement of the calcite-carbonatites [3].  'Nodules' of purely
feldspathic syenite (mostly derived from the intrusive syenite, some from fenitised wall rocks), which are disseminated in the soevite,
are considered as the end product of these fenitic transformations.

The calcite-carbonatites show large compositional variations in relation with cumulative processes (aegirine, apatite, magnetite,
pyrochlore) and incorporation of feldspathic nodules and xenocrysts.  Taking this variability into account, the carbonatite/ syenite
distribution coefficients of key elements (e.g. Nb, Nb/Ta, Zr, REE) are similar to those found in experiments on conjugate immiscible
liquids.  For example, Nb/Ta is 30-50 in the syenite, compared to 200-500 in the calcite carbonatites (and more than 1000 in the
dolomite carbonatite).  This suggests an equilibrium was approached between the respective parental liquids.  There is however no
petrographic evidence for immiscibility to have occurred, and the field and chemical data are more in favour of the idea [3, 4] of
extensive interactions, at depth, of carbonatite melts with crustal wall rocks, leading to the formation of syenite by melting of highly
fenitized walls.

The Lueshe complex appears as representative of a class of carbonatite complexes where a primitive dolomite carbonatite plays a
major role and alkaline silicate rocks are secondary derived.

References
[1] Albers KH et al, 1993, Final Report, Contract CCE MA2M-CT90-0038
[2] Nasraoui M, 1996, Le gisement de Niobium de Lueshe, Thèse Ecoles des Mines de Paris et Saint Etienne
[3] Von Maravic H, 1983, PhD Thesis TU Berlin
[4] Kramm U, Von Maravic, Morteani G, 199, Jour African Earth Sci 25: 55-76 

Lueshe studied for a BRITE-ERAM project (1992-95) aimed at improving the beneficiation processes of Niobium ores [1]
>> Thesis by M.Nasraoui [2] on the mineralogy / geochemistry of the supergene processes
>> systematic database of fresh rock compositions
>> complement to results already published on Lueshe [3,4,5]

= no direct relationships in the field

>> There is no sign of an silicate phase involved in the genesis of this carbonatite.
>> If the carbonatite evolves to Fe-rich composition by crystal fractionation, analyzed composition represents cumulates, then the Mg# in core is the maximal value for the parent carbonatite
>> If the dol-cbtite represents early cumulates from the parental magma of the complex, then genesis of the calcite-carbonatite involves incorporation of a silicate component

Cancrinite- Syenite
the 'trachytoidal' type is the main component

6 samples from different parts
> Homogeneous compositions
> no mafic precursor; no internal differentiation to leucocratic (e.g. constant Mg/Fe)