Les minéraux phylliteux dans les gisements hydrothermaux d'uranium. II. Distribution et évolution cristallochimique des illites, interstratifiés, smectites et chlorites
Authors: Cathelineau Michel 
Published: November 2018

Mineralogical studies have been performed on samples of vein filling and alteration zones from five uranium hydrothermal deposits located in/or around leucogranites of the western part of the Variscan range : Chardon, Ecarpière and Commanderie deposits (leucogranite of Mortagne) in Vendée, Penaran deposit (leucogranite of Guérande) in South Britanny, and Pierres Plantées deposit, located in the leucogranite of Grandrieu (Margeride, southern part of the french Massif Central). These studies have indicated the existence of numerous clay minerals in relation with the ore bodies as well as in their alteration zones.

Illites, di or trioctahedral, smectites mixed-layered illite-smectite, vermiculite, chlorites and kaolinite have been encountered ; however the three first quoted groups are the most abundant. These phyllites give evidence of reactions between hydrothermal fluids and host rocks during the evolution of thermodynamical conditions in the deposits. They occur from the earliest phases preceding uranium deposition to the latest phases of reworking of the primary ores in new paragenesis (late pitchblende-coffinite-" sooty pitchblende"...).

Spatial distribution

Clay paragenesis are strictly limited to the veins or to their associated alteration zones, which are poorly developped during all the first stages (3-4-5).

During stages 6 to 8, circulations become more and more diffuse and consequently affect more extensive areas, especially in rocks which were strongly altered previously as episyenites ("dequartzified" granites).

At the scale or the deposits, alteration zones are distributed as a function of jointing ; the numerous juxtaposed microsystems (veins altered wall-rocks) do not define therefore extensive monoparagenetic zones as those defined in porphyry copper deposits for instance. Chronological sequence

Each hydrothermal event has practically induced developments of phyllitic paragenesis. A global sequence of clay minerals has been established in concurrence with metallic sequences, which are based on metallographic, mineralogical and microthermometric datas. The main stages are as following :

1) Phengites and chlorites (ripidolite-brunsvigite) occur during the earliest stages of alteration of granites which precede the hydrothermal stages, strictly related to uranium mobilization.

2) Phengites are the earliest minerals to crystallize in the vicinity of veins.

3) "Mica stage" : the first minerals encountered in the veins, as inclusions in the quartz Q1, or directly crystallized on the wall-rock are illites and celadonites (and rare phengites).

4) During pitchblende deposition mixed-layered illite-smectite are largely dominant in almost all the deposits.

5) Complexe patterns of clays are produced by reactions between fluids which rework the primary uranium ores in new paragenesis, and the wall-rocks. For instance in Commanderie deposit, the following sequence is observed from the inner part (vein) to the outer one : (illite) ; mixed layered illite-smectite, K-smectite and adularia ; K-smectites ; chlorite ; kaolinite.

6) Smectites are systematically associated with coffinite.

7) Kaolinite stage (pyrite-sooty pitchblende) is the last hydrothermal event before meteoric alterations.

Chemical evolution

More than 200 analyses have been performed in the aim to obtain ponctual analysis, and determine exact location of the clay minerals identified by X-ray diffraction. Furthermore detailed datas on clays from different stages and occurrences, permit to define a global evolution.

All changes in composition and structures follow the chronologic evolution paragenesis. This evolution is essentially related to the decrease of temperature of fluids circulating in the veins, and is caracterized by :

— an increase of celadonite content (phengites) and then of the Si/ Al ratio in phyllites to ratio more typical of smectites ;

— a decrease of the interlayer occupancy ;

— a slight increase of calcium content ; however all the terms of illite-smectite series remain dominantly potassic, even the smectites, what is noteworthy.

Chemical evolutions confirm the microscopic and X-ray data :

— for dioctahedral series : phengites, celadonite, illite mixed-layered illite-smectite, K-smectite, K-Ca smectite, kaolinite ;

— for trioctahedral series : biotite, chloritized biotites-ripidolites and brunsvigites, (metavermiculite), smectites, kaolinite.

These sequence are quite similar to those described in geothermal systems, in progressive sediment burry, and sequences of dissolution of groups of minerals obtained by computation studies. Valid for all the studied deposits they support analogies developped between fossil hydrothermal (in uranium deposits) and geothermal activities.