Tiago José

Santos Moraes

Ngac Ky Nguyen

Fabien Meinguet
email: fabien.meinguet@thalesaleniaspace.com

Eric Semail
email: eric.semail@ensam.eu

Fault Tolerant Dual-Motor Drives: Sizing of Power Electronic

Keywords: drive », « Open-winding », « Fault tolerance », « Inverter dimensioning », «

This paper analyzes two dual-motor fault-tolerant topologies. The first one supplies independently both machines while the second one connects them in series for reducing the number of transistors. For a given DC-link voltage, the converter component sizing is based on the peak current obtained in the normal and degraded modes.

Introduction

Capability of degraded functioning is highly required in several specific applications as aeronautics and aerospace [START_REF] Bennett | Fault tolerant design considerations and control strategies for aerospace drives[END_REF]. When a fault occurs, it is necessary to have some supplementary degrees of freedoms (DoF) for ensuring an acceptable functioning. For short-duration faults, the power-converter components are generally the more sensitive devices since the thermal time constants of a machine are larger. Nevertheless the torque pulsations of the motor must be acceptable for the mechanical load. For long-duration faults, machine thermal constraints must be added. In inverter-machine drives, the case analyzed in this paper, the supplementary DoF are achieved by a high number of inverter legs and/or multiphase machines [START_REF] De Lillo | Multiphase power converter drive for fault-tolerant machine development in aerospace applications[END_REF] [START_REF] Villani | High reliability permanent magnet brushless motor drive for aircraft application[END_REF]. Numerous papers have studied only the impact of open-circuit fault [START_REF] Baudart | Torque control strategy of polyphase permanent-magnet synchronous machines with minimal controller reconfiguration under open-circuit fault of one phase[END_REF], considering either that the short-circuit fault of inverter switch can lead to an open-phase circuit using additional power, or that the short-circuit currents are acceptable thanks to a proper design of the machine with sufficiently high values of inductances [START_REF] Meinguet | Fault-tolerant operation of an open-end winding five-phase PMSM drive with inverter faults[END_REF].

The particular application considered in the paper is an aerospace Thrust Vector Control (TVC) system. It is composed of two actuators that orientate the thruster in order to control the rocket trajectory (Fig. 1 and2). In the past these actuators were hydraulic while they are replaced by electromechanic motors in this study. The electrical structure must withstand open-circuit fault and shortcircuit fault of inverter switches, without increasing too much the system cost and weight. In the presented paper, a structure composed of two open-winding six-phase machines connected in series (Fig. 4.) is compared with another more classical solution using two open-winding motors being each one independently supplied by an H-bridge inverter (Fig. 3).

The series-connection between the two machines (Fig. 4) halves the number of transistors and consequently also the number of drivers comparing to the solution using one H-bridge structure (Fig. 3). Consequently, it is expected that global price of the inverters and the mass can be reduced if the constraints on each power converter component do not increase too much. Furthermore, one fundamental advantage of the se inductive component leading thu On the contrary, the global coppe if only one machine is driven, th of the proposed solution depends also on the impact of the mass. F global efficiency can be negligib mass.

However, to ensure the indepen between them is required. Som independent currents (DoF) has t the back-electromotive forces (b sinusoidal, a more complex form proven that the concept can be pr In this paper, a design of the inv account the product of the numbe The paper is organized as follow are proposed and analyzed. Secti Section V ends the paper with a c

TVC System

Thrust Vector Control (TVC) sy This system is composed of two turbine (Fig. 1), for the moveme movement of the turbine is measu axis (θ -between 0° and 6°) ( actuation is represented as a ci inclination is on its own axis. Fig. 1: Up view with actuators' a ries winding connection is that it increases autom us to reduced amplitudes of high frequency curren er losses and, consequently, the global consumpti he currents are crossing both machines. As a cons s on the ratio between the running costs and the For aerospace applications with short time mission ble in comparison with the investment cost and t ndent control of two series-connected machines me papers, like [START_REF] Levi | A Novel Concept of a Multiphase, Multimotor Vector Controlled Drive System Supplied From a Single Voltage Source Inverter[END_REF], demonstrated theoretically to be higher than twice the number of machines back-EMFs) are sinusoidal. In the case where th mulation is required [START_REF] Semail | Multi-machine modelling of two series connected 5 phase synchronous machines: effect of harmonics on control[END_REF]. Different experimental ractically achieved for induction and synchronous verters for a TVC system is presented, based on er of components and their highest peak current in ws. Section II describes the TVC system. In sectio ion IV specifies the simulation conditions and so conclusion.

ystem controls the turbine orientation to guide t mechanical actuators, placed spatially 90° from e ent in two axes, assuming that the actuators are ured with the inclination angle between the turbin (Fig. 2) and its direction (φ -between 0° and ircle. So the highest effort of an actuator is w axis and the direction of inclination angle (φ) matically the value of nt components.

ion rise because even sequence, the interest investment costs and n such as thruster, the the impact of a lower s, a specific coupling that the number of connected in series if he machines are nonset-ups, as [START_REF] Mekri | An efficient control of a series connected two-synchronous motor 5-phase with non-sinusoidal EMF supplied by a single 5-leg VSI: Experimental and theoretical investigations[END_REF], have s machines. n an index taking into n degraded mode. on III, two topologies ome results are given. the rocket trajectory. each other around the e independent. As the ne axis and the rocket 360°), the region of when the direction of Fig. 2: Torque generation representation for 3 different thrust inclinations Up to now, the actuators are mostly hydraulic cylinders having some drawbacks as the high tank volume and the liquid's high pressure which can damage the whole launcher. One of solution is to use electro-mechanical systems to replace the hydraulic cylinders. The tank is replaced by a battery, which can be recharged during the actuator's braking period. However, electronic inverters are less reliable than mechanical systems. Consequently, fault-tolerant capability of the machine drive is mandatory to ensure a functioning when a fault occurs.

Modeling of Fault-Tolerance Topologies

Both topologies analyzed in this study have two six-phase open-end winding motors. In a high power application, having 6 phases is better than 3 since the power per phase is reduced and the DoF of a sixphase machine allow a faulty operation. In open-end winding machines, impacts created by the faulty phase to the others are less important than with a classical wye-coupling machine [START_REF] Bennett | Fault tolerant design considerations and control strategies for aerospace drives[END_REF]. However, openend winding drives need a higher number of components because two isolated-inverters are needed.

The first topology shown in Fig. 3, named as H-bridge, is composed of two machines supplied independently. This structure presents a high number of transistors (48 in 24 inverter legs) and the two machines can of course be independently controlled. This structure is already found in some applications as naval and aeronautics.

Fig. 3: H-bridge structure

The RIMM (Redundant Inverter Multiple Machines) topology, shown in Fig. 4, connects the two motors in series to half the number of transistors, in comparison with the previous topology. The advantage of this reduction of transistors is, beside the investment cost reduction, a mass reduction which is a major problem in aerospace industry.

Two batteries supply the structure ensuring thus also a redundancy in terms of energy source. The voltage between the two negative points of the batteries v n1n2 can be controlled to increase the system performance in case of short-circuit inverter fault [START_REF] Villani | High reliability permanent magnet brushless motor drive for aircraft application[END_REF].

Applying a specific 6-phase Concordia transformation to equation (1), a 6-phase machine is decomposed into two homopolar machines and two diphase machines. Each fictitious machine interacts with some back-EMF harmonics to generate a torque, as shown in Tab. 1 for a 6-phase symmetrical machine.

Tab. 1: Harmonic EMF for each fictitious machine

Inverter Sizing

In order to compare different top (IS) is elaborated. It's considered of transistors and the transistor determined by the topology, the t current which are required in ord this paper are dimensioned to the the highest peak-current in steady [START_REF] Meinguet | Fault-tolerant operation of an open-end winding five-phase PMSM drive with inverter faults[END_REF]. As this index is only used for is the higher value current p that the system might be able to speed reference of the actuators a 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 1 0 0 which lead to torque production, equation (2) lea

I I I I
epresented graphically in Fig 5 . as a series con with the secondary machine of M 2 ( current torques (or speeds) of the two machines are inde sses, but no torque if even harmonics of Back-EM fictitious machines pologies based on its cost and weight, a formula c d that those parameters are proportional to two v power sizing. While the number of transistors transistor power sizing depends on, the values of der to achieve the cycle imposed by the payload e same DC bus, the transistor power sizing is dir y state ( ). Inverter sizing is, in this case, calcu r comparison, a per unit analysis is more conveni peak obtained by comparing the most constrainin achieve. For the TVC system, these operational and the faulty mode. (

) 5
ng operational points points depend on the

Simulation Results

A. Drive model

Machine's parameters are not the same for both topologies because of the different voltages applied to each machine. Each machine is designed for a given DC-link voltage by changing its number of coils leading to different phase resistances, inductances and magnetic flux.

B. Speed reference

For both topologies, two different speed references are simulated: one machine (M1) at Ω ref-1 = 100 rad/s and both machines at Ω ref-2 = 70.7 rad/s.

Both speed references represent the speed needed to achieve a maximum turbine inclination (θ = 6°). But the first speed reference represents the displacement of an actuator in its own axis (φ = 0° or 90°), as the second one represents the maximum displacement of the turbine in the axis between the two actuator axes (φ = 45°). As both actuators act together in the second case, the speed reference of each one is √2 lower in comparison to the first case.

C. Faults simulation

As already mentioned, the most common faults will be simulated. For the studied system, these faults are: a short-circuit inverter switch and an open phase. For simulation, the top switch of the phase A of the inverter VSI1 in both topologies is short-circuited. To avoid short-circuiting the DC-bus, the bottom switch of the same leg has to be 'open', this protection is standardly found in industrialized

components. An open-circuit of one phase (the phase A in our study) is also carried out in simulation.

It can be noticed that no reconfiguration of the control strategy or reduction of the speed or torque references in degraded mode have been made. So the speed and torque generated by the motors will be the same in normal and degraded functioning mode. Otherwise, considering a functioning without reconfiguration may be a simpler solution because the faulty detection and the reconfiguration implementation are sources of fault as well. The objective is to compare in term of peak currents within two topologies in order to have a pre-sizing of the inverters.

D. Results

Fig. 6 presents the simulation results for the two different topologies presented in section III under healthy and faulty modes at two different speeds.

Fig. 6: Peak currents obtained under healthy and faulty operations with two proposed topologies at two speeds.

Firstly, the graphic shows different peak currents for each topology under three modes of functioning.

In the H-bridge topology, the dual-motors are independently controlled. The highest amplitude of

H-bridge (Ωref-1) H-bridge (Ωref-2) RIMM (Ωref-1) RIMM (Ωref-2)
currents is generated when only one actuator is working at Ω ref-1 . On the other hand, the seriescoupling of the RIMM topology leads to a higher current peak when two machines are running since the currents are crossing the dual-motors.

As mentioned above, the criteria of dimensioning is based on the highest peak current among the ones obtained for three operation modes (healthy, short-circuit and open phase). As shown in Fig. 6, the normal mode is no constraining for dimensioning of the drive. The currents obtained in the shortcircuit fault are taken for the H-bridge topology (about 215A). For the RIMM structure, the highest amplitude of current is obtained when two machines are controlled at Ω ref-2 under open-circuit fault of one phase. This amplitude is thus taken for dimensioning of the RIMM topology (about 175A).

It is obvious that the peak currents in faulty mode are higher than the ones in healthy mode because the currents in the healthy phases have to increase to reach the required speed and torque as before the fault. Furthermore, harmonic currents, which are equal to zero for both topologies in healthy mode, become non negligible in degraded mode because of the asymmetrical distribution of the currents in the machine. Fig. 7 to Fig. 10 give the obtained currents of each topology during a switch short-circuit or open phase fault. In fact, in H-bridge structure, the current homopolar is very important in short-circuit or open-circuit fault since there is a path for its circulation. In the RIMM structure, the voltage between two negative points of two DC-buses v N1N2 is equal to zero in healthy mode and this voltage increases significantly in faulty mode. However there is no path for zero sequence current to circulate because of the two-isolated DC-sources, the current controllers try to balance as much as possible the voltage system. Finally, the phase currents of the H-bridge in faulty modes are more unbalanced comparing to the ones obtained in the RIMM topology.

Because of the coupling, an open-circuit affects both machines in RIMM topology. So the healthy phases' currents increase more for this structure in comparison to the H-bridge one topology. Fig. 7: H-bridge topology's currents before and after a half-leg short-circuit occurs in phase A at 0.05s, when driving only one machine. Fig. 8: H-bridge topology's currents before and after phase A opens at 0.05s, when driving only one machine. Fig. 9: RIMM topology's currents before and after a half-leg short-circuit occurs in phase A at 0.05s, when driving both machines. Fig. 10: RIMM topology's currents before and after phase A opens at 0.05s, when driving both machines.

For an inverter sizing (IS) comparison, the H-bridge has double of transistors with a peak current of phase being 20% higher than the one of a RIMM structure. Then, IS, calculated by the equation (5), is 140% higher for the H-bridge topology than the RIMM, as shown in Fig. 11. That means that H-bridge inverter is at least twice more expensive and heavier in the conditions imposed in this paper, even if its current's amplitude is lower in the normal mode. Besides the inverter sizing, which covers an economical view of the comparison, some other technical aspects must be also verified. Concerning the degraded-mode, the torque ripple should respect some limits to be considered acceptable in degraded mode. Fig. 12 and Fig. 13 present the generated torque in degraded mode for both topologies.

Among the four operational point analyzed, the torque ripple depends mostly of the faulty mode. The higher torque ripples, similar to the peak-current analysis, is during a short-circuit fault for the Hbridge topology and for an open phase for the RIMM topology.

RIMM topology presents a higher reliability because the torque ripple is 166% in the worst case, while H-bridge topology may generate torque ripples 2,7 times higher during a short-circuit fault.

Conclusion

This paper analyzes the inverter sizing of a dual 6-phase machines fault-tolerant system. As the system needs at least two machines, a series connection topology is a solution to reduce the number of transistors. Beside this, this system can be cheaper regardless the operational constraints (higher cooper losses).

Degraded mode has to be taken for dimensioning of the system because the currents of the healthy phases have to be increased to compensate the degraded one. Comparing the two proposed topologies, the RIMM structure has more advantages in term of power electronic components and power rate of inverters and lower torque ripple in degraded mode. Although there is not a series connection machine system in the industry for now, this innovated structure may find its place in high performance faulttolerant systems for specific applications like the aerospace TVC.

Fig. 4 :

 4 Fig. 4: RIMM structure

Fig. 5 :

 5 Fig. 5: Series connection of the fi

Fig. 11 :

 11 Fig. 11: Inverter sizing of both topologies

Fig. 12 :

 12 Fig. 12: Torque generated by M1 of the H-bridge topology


"Fertilization" of barren clays in supergene non-sulphide zinc deposits

Keywords: supergene zinc deposits, karst, clay minerals, Morocco 1) Introduction

The nature and the origin of zinc clays are poorly understood. Based on the example of the Bou Arhous Zn-Pb ore deposit in the Moroccan High Atlas, this study presents new advances for the characterization of barren and mineralised clays associated to zinc non-sulphide ores. In the field, white to ochre granular clays are associated with willemite, whilst red smooth clays fill karst-related cavities cutting across the non-sulphide ore bodies. Red clays (kaolinite chlorite, illite and smectite) present evidence of stratification that reflects internal sedimentation processes during karst evolution. White clays contain 7Å-clay mineral/smectite irregular mixed layers with less than 20% of smectite layers.

The willemite crystals are partially dissolved and are surrounded by authigenic zinc clay minerals.

WDS analyses on newly formed clay aggregates have revealed that this mixed layered mineral is composed by fraipontite and sauconite. CEC measurements support that zinc is only located within the octahedral sheets. These new results support the following process: 1) dissolution of willemite, leading to releasing Si and Zn, 2) interaction between Zn-Si rich solutions and residual-detrital clays, 3) neogenesis of mixed-layered zinc clay minerals that grow over detrital micas, representing the source for Al.

mineralurgical tests (Boni et al. 2009b). Besides, the ability of clays to sorb or catch Zn 2+ cations located within the interlayer space of swelling clays is well known [START_REF] Bradbury | Modelling the sorption of Zn and Ni on Ca-montmorillonite[END_REF][START_REF] Churakov | Zinc Adsorption on Clays Inferred from Atomistic Simulations and EXAFS Spectroscopy[END_REF]), implying natural clays may represent an economic potential of zinc recovery without much efforts. Although zinc enriched clays are frequently described in supergene deposits, clear identification of the nature of clay minerals remains rare and the classical sauconite assessment is highly speculative. In addition, the origin of zinc clays and its place within the ore evolution remain badly understood.

This study investigates the characterisation of zinc clays of the Bou Arhous deposit in the Moroccan High Atlas. This deposit is hosted in Lower Jurassic limestone, and ore is mainly willemite, whose origin is not clearly understood [START_REF] Choulet | Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin[END_REF]. Willemite is associated to various types of clayey material, presenting differences in mineralogy, texture and Zn content. The Bou Arhous ore deposit represents a key natural example to understand the origin of zinc-bearing clays and their place in the ore deposition. Using microscopic observations and classical techniques for investigating clay minerals, we aim at characterizing the nature of clays recorded at Bou Arhous and debate on their origin. Further quantification of the cation exchange capacity (CEC) allows us to discuss the potential of zinc recovery.

2) Geological setting of the Bou Arhous ore deposit

The Bou Arhous Pb-Zn ore deposit (32.27°N, 3.76°W) is located in the Moroccan High Atlas range, 60 km to the north-east of Errachidia city (Fig. 1A). The local geology of this deposit has been extensively studied by Leblanc (1968) and recently revisited in [START_REF] Choulet | Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin[END_REF].

The formation of the High Atlas intra-continental fold-and-thrust belt [START_REF] Mattauer | Sur les mecanismes de formation des chaines intracontinentales; l'exemple des chaines atlasiques du Maroc[END_REF] results from the far-field effect of the Alpine collision events since mid-Tertiary (ca. 40 Ma) The Jbel Bou Arhous range is one of these ENE-WSW trending faulted anticlines that exhume the Triassic and Lower Jurassic strata (Fig. 1B). The fold-related "Grand Accident" thrust is rooted in the Triassic substratum (Figs. 1B and 1C;[START_REF] Teixell | Tectonic shortening and topography in the central High Atlas (Morocco)[END_REF]) and represents a reactivated normal fault that delimitated contrasting paleogeographic domains of the former Tethys Sea (Warme 1998). To the north of the fault, deep-seated dolomitic limestone is almost barren, whereas massive limestone of the southern domain originally deposited in reef or para-reef environments includes lenses of base metal sulphides (Figs. 1B and1C). At site of the Bou-Arhous mine located in the footwall of the "Grand Accident" thrust, the two limbs of the anticline are vertical (Fig. 2A) or slightly recumbent and the fold hinge is missing (Fig. 1C). Host rocks of mineralisation include Sinemurian black limestone, locally interbedded with thin layers of greyish marls (Fig. 2B), which have experienced slight dolomitisation during diagenesis and emplacement of base metals sulphide lenses (Leblanc 1968). In depth, mineralisation is tectonically juxtaposed along the "Grand Accident" thrust to the Triassic variegated shales associated to gypsum (Fig. 2C).

Following exploration campaigns in the 1960's, the Bou Arhous mine has been exploited by Managem since the early 2000's and reserves have been estimated at 250 kt grading 4 % Pb and 16 % Zn. Mineralisation consists of an intricate network of various-sized veins, extending over 18000 square metres. Veins up to 10 metres in width parallel the stratification and dip steeply to the north (Fig. 2A).

Non-sulphide zinc-bearing minerals include willemite, hydrozincite, hemimorphite and smithsonite.

Gangue minerals include automorphous white carbonates, such as calcite and dolomite (Fig. 2D), emplaced during the early sulphide stage [START_REF] Choulet | Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin[END_REF]. Willemite is finely associated to galena remnant and cerussite, forming a well-defined black ore. Black ore generally occurs as disseminated patches enclosed in very fine-grained clay-rich material (Fig. 2E), showing a variety of colours. Hence, different types of clayey material have been distinguished and they are detailed in the following section.

3) The different types of clay deposits at Bou Arhous

Previous investigation of the clayey material associated to the mineralisation (Leblanc 1968) suggested the existence of two types of clays. Reddish and smooth clays occur as infilling material of the karstic cavities and were considered of detrital origin as evidenced by the layering structures. These clays are barren, but they locally contain some fragments of the non-sulphide mineralisation. In contrast, white and granular clays are frequently associated to willemite and Leblanc (1968) suggested that these clays might correspond to newly formed clay minerals with up to 23 % of Zn (Gaudefroy

1953).

Based on a recent detailed field study, a new examination of the clay materials has been carried out and we now distinguish three types of clayey material, which may coexist in a same area (Fig. 3A).

1) Red clayey material fills a network of connected cavities, sinkholes and tunnels, related to the karst of Bou Arhous (Fig. 3B). A sub-horizontal bedding (Fig. 3C) with evidence of grain size sorting is frequently observed, as well as various-sized clastic fragments of ore and host limestone. The aspect of the red clayey material may be locally smooth, but is often silty or sandy due to millimetre-sized quartz grains. This red clayey material corresponds to the reddish and smooth clays described by Leblanc (1968).

2) White clayey material occurs as a fine-grained matrix embedding disseminated patches of willemite associated with carbonates and galena. It forms either a thin and partial rim between the black ore and the red clays (Fig. 3D), or a widespread impregnation in the mineralisation (Fig. 3E). The white colour is not systematic and tends be locally yellowish, indicating a progressive transition to ochre clays. This type partly encompasses the white granular type observed by Leblanc (1968).

3) A third type, namely ochre clays has been found as cavity-infilling material (Fig. 3F) or as a mixture of white and red clayey materials (Fig. 3A). Ochre clays share similar features with white clays, such as their association with black ore but this latter one is less preserved than in the case of white clays.

Ochre clays may also occur as isolated material without any spatial connection with white and red clays.

4) Sampling and methodology

4.1) Bulk chemical composition

Fifty-eight rock samples have been collected at Bou Arhous mine and include host-rocks (Triassic shale and Jurassic limestone and marl) and the three types of clayey material (red clays, white clays and ochre clays), listed in Table 1. Among them, thirty-four samples have been duplicated for analysing the chemical composition. The chemical composition measurements using acid digestion procedure have been carried out in Managem Group lab, at Marrakech, Morocco. For major elements, 0.5 g of a crushed sample (< 200 µm) has been dried and dissolved by fusion at 500°C during 45 minutes with 2.5 g of sodium peroxide in a zirconium crucible. The residue was then dissolved in HCl and the solution has been analysed by ICP-AES ULTIMA 2C. For minor elements, a 0.25 g split has been heated (220 °C during 45 minutes) in HCl (50%) -HNO 3 (50%) to fuming and the solution was analysed by ICP-MS Thermo X'Serie 2.

4.2) Clay fraction study

In addition, for twenty-two samples, the clay fraction (<2 µm) has been separated by decantation using the following procedure. A rock sample has been crushed using a Retsch PM200 ball mill and manually ground in agate mortar. The rock powder has been dissolved in distilled water and dispersed with a magnetic stirrer. Progressive addition of HCl (0.2M) monitored by continuous pH measurements was used to remove carbonate at room temperature. Then, the solution has been washed several times, dispersed and placed in a jar for decantation. After about 4 hours, the first five centimetres of the solution containing the clay suspension (<2 µm) have been withdrawn using a syringe. After removing the supernatant by centrifugation, the residue has been dried and used for further X-ray diffraction (XRD) and cation-exchange capacity measurements.

XRD patterns of the whole rock samples and clay fractions have been obtained using a D8 Advance Brucker diffractometer equipped with a LinxEye detector, hosted at Utinam lab, Besancon, France. Parallel beam geometry and CuKα radiation at 1.54184 Å has been used. For whole rock sample, the powder was placed in the specimen holder. Acquisition conditions (divergent slit) included a step-scan of 0.5 ° in the 2-52 ° interval, with a step time of 0.05 s. For the clay fraction, the powder has been diluted in distilled water and a drop of the solution has been deposited on a glass lamella for dewatering under air-dried conditions. The orientated-aggregate mount (along c axis) was then placed in holder and a step-scan of 0.02 ° in the 2-30 ° interval, with a step time of 0.5 s has been used for XRD measurements. A duplicate of the orientated-aggregate mount has been treated for ethylene glycol (EG) saturation, by placing the lamella in a pressurised desiccator overnight. For several samples, a second duplicate has been heated at 550 °C during one hour in a furnace before XRD measurements. EVA software package has been used for data processing and phase identification. The position of 060 reflexion has been determined on clay fraction powders at Institut des Sciences de la Terre d'Orléans, Orléans, France, using an ARL X'TRA diffractometer equipped with Cu anode and a Si solid detector operating at 40 kV and 40 mA. Experimental measurements were carried out at room temperature, with 10 s counting time per 0.02 ° step.

Cation-exchange capacity (CEC) measurements have been carried out at the Institut des Sciences de la Terre d'Orléans, Orléans, France following the procedures described in [START_REF] Meier | Determination of the Cation Exchange Capacity (CEC) of Clay Minerals Using the Complexes of Copper(II) Ion with Triethylenetetramine and Tetraethylenepentamine[END_REF] and [START_REF] Ammann | Determination of the cation exchange capacity of clays with copper complexes revisited[END_REF]. A solution of 0.01M Cu(II)-trien sulphate has been prepared from 1.596 g of copper sulphate and 1.508 g of triethylenetetramine. 100+/-25 mg of clay sample were added to 25 mL of distilled water and to 8 mL of Cu(II)-trien sulphate. The solution has been dispersed using tube roller overnight and was further centrifuged (10 minutes at 6000 rpm) to removed 3 mL of the supernatant. Measurement of [Cu 2+ ] was performed using Atomic Absorption Spectroscopy (AAS) with 2-8100 Polarized Zeeman Hitachi instrumentation and colorimetric techniques. The CEC was then calculated with reference to the dry clay weight, to the water content and to the amount of captured Cu 2+ ions deduced from AAS measurements. A similar procedure is used for calculating the Zn content in the CEC. Standardisation curves of absorbance for [Cu 2+ ] and [Zn 2+ ] gave coefficients of determination of 0.999 and 0.987, respectively.

4.3) Textural and mineralogical observations

Textural and mineralogical observations have been carried out using optical microscopy (OM) and FEI Quanta 450 Scanning Electron Microscope (SEM) hosted at Femto-ST lab, Besançon, France.

The images have been acquired in low vacuum mode (0.7-0.8 mbar) on polished thin sections, with acceleration voltage at 15 kV. An Energy-Dispersive X-Ray Spectrometer (EDS) was coupled to the SEM system providing qualitative determination of the mineral composition using Edax Genesis software package.

4.4) Chemical composition of clay minerals

We have performed quantitative analyses using Cameca SX100 electron microprobe (EPMA), equipped with five Wave-Dispersive X-Ray Spectrometers (WDS) and hosted at Géosciences Montpellier, Montpellier, France. On carbon-coated polished thin-section, we have operated at 20 kV with a beam current at 10 nA. The spot resolution was few micrometres. Quantification of the Na content was not possible due to the interference of the Zn Lβ1 emission line that fall close to the Na Kα emission line.

4) Results

4.1) Whole rock geochemical and mineralogical composition

4.1.1) Host rocks

The Triassic shales are composed of a mixture of quartz, clay minerals, hematite, goethite, and carbonates including both calcite and dolomite (Table 1). Gypsum is also present in some samples. The major elements content strongly varies due to a variable proportion of carbonates and iron oxihydroxides. Variable enrichment in Zn, Pb, Cu and As can be noticed suggesting the local occurrence of sphalerite, galena, and/or other minor metal-bearing minerals.

Jurassic rocks including marl and limestone contain clay minerals, calcite, dolomite and quartz (Table 1). Goethite is common. The SiO 2 and CaO contents are variable and probably mirror the variable abundance in quartz and carbonate in the samples. Minor elements (Zn, Pb, Cu and As) content remains low except for one sample that contains galena, sphalerite and their respective alteration products.

4.1.2) Clayey material

The mineralogy of the red clayey material is characterized by a mixture of quartz, clay minerals, calcite with goethite and rare dolomite (Table 1). Despite important variations of CaO and SiO 2 related to carbonate enrichment of some samples (e.g. BA101), the major elements content remains constant. The red clay samples do not display any significant variations of the minor elements content except for Pb and Zn, which display highly variable amounts (e.g. between 830 ppm and 47300 ppm for Zn); this could be related to the occurrence of willemite and cerussite impurities in these samples.

The white clayey material is composed of a mixture of willemite, galena, quartz, clay minerals and carbonate (dolomite, calcite, and cerussite). Dolomite is almost always observed in white clays, marking a noteworthy difference with red clayey material. The MgO content can therefore reach up to 10 %wt (Table 1). The occurrence of Zn and Pb ore in white clays is responsible for high Zn and Pb grade (up to 320000 ppm for Zn and 37100 ppm for Pb).

In ochre clayey material, clay minerals, quartz, calcite and dolomite are always present, with a common occurrence of cerussite. Chemical compositions of the ochre and white clayey materials are relatively close; indeed they have similar MgO and CaO contents (Table 1). Despite the absence of Znbearing phases (sulphide or carbonate) detected from whole-rock XRD analysis, the Zn content varies between 6275 ppm and 237000 ppm.

The extreme variations of the Zn content in the three types of clayey material cannot be correlated straightforwardly to the mineralogical composition deduced from whole rock XRD measurement, it may be partly due to the presence of willemite impurities, but, for several samples (ochre clays), Zn-bearing clays must be involved to explain tremendous Zn contents, requiring a specific study of the clay fraction.

4.2) Mineralogical characterisation of the clay fraction

4.2.1) Red clays

XRD patterns obtained on the clay fraction of the red clays show a non-expandable phase around 7.1 Å (Fig. 4A), which mainly corresponds to the 001 reflexion of kaolinite as this Bragg peak disappears after heating to 550 °C. Chlorite is significantly present in the mixture as evidenced by the 003 peak at 4.74 Å, and the slight offset of 004 reflection (3.55 Å) relative to the 002 peak of kaolinite (3.58 Å). The non-expendable peak at 10 Å may suggest the presence of illite, whereas the 14.6 Å peak that expands to 16.8 Å for EG-treated preparations corresponds to swelling smectite. The broad shape in the domain between smectite and illite 001 reflections suggests the presence of interstratified illite/smectite. By focusing on the position of the 060 reflection (Fig. 4B), a main broad peak at 1.49-1.50 Å is consistent with the predominance of dioctahedral species like kaolinite and illite, whilst the secondary peak at 1.541-1.543 Å corresponds to a trioctahedral type like chlorite. Quartz and goethite are also present.

4.2.2) White clays

XRD patterns of white clays display two identified Bragg peaks at 7.25 Å and 3.61 Å that expand to 7.39 Å and 3.53 Å, respectively, for samples treated with EG (Fig. 5A). This indicates the presence of randomly interstratified smectite and of another clay mineral, characterized at 7 Å such as kaolinite or serpentine. This mixed-layered (ML) clay mineral, dominated by the 7 Å phase, coexists with illite and occasional chlorite. Smectite was detected in samples BA130F and BA134F. Associated phases such as cerussite, willemite, goethite and goethite commonly occur. Whilst the broad peak around 1.50 Å, typical of dioctahedral clays corresponds to the position of 060 reflexion of illite, the thin well-defined peak at 1.54 Å suggests a significant proportion of trioctahedral type clay minerals (Fig. 5B). Although this may be confused with the 1.541 Å peak due to the 211 reflection of quartz, the 060 reflexion patterns are similar between samples which may (e.g. sample BA130F) or may not (e.g. sample BA124F) contain quartz (Fig. 5A). For sample BA124, the offset of the broad peak to 1.51 Å is probably an artefact due to a parasite reflexion of zinc silicate (e.g. 11 ̅ for willemite or 530 for hemimorphite).

4.2.3) Ochre clays

XRD patterns of the ochre clays do not strongly differ to those of the white clays, with the presence of randomly interstratified smectite and 7 Å mineral (Fig. 6A). But the shift of this Bragg peak to lower angular value after EG treatment (up to 7.72 Å and 3.45 Å) suggests a higher smectite proportion in the ML. In addition, illite and smectite are also displayed, as well as cerussite and goethite. The predominance of the trioctahedral type over the dioctahedral type is obviously shown on the 060 reflexion pattern (Fig. 6B), with a well-defined thin peak around 1.542 Å and a broad and unclear peak around 1.51 Å, probably polluted by the reflection of minor zinc minerals.

4.3) Cation-exchange capacity (CEC) of the different types of clays

Although previous observations obtained through XRD measurements have stressed out a possible trend between the type of clays and the amount of Zn content, it does not provide any information on the localization of Zn within the phyllosilicate structure (i.e. in the interlayer space as an exchangeable cations or included in the phyllosilicate sheets). The CEC measurements represent an easy and convenient method allowing one to determine the proportion of exchangeable zinc in the three kinds of clay samples (Table 2). The CEC value of the red clays is between 30 and 38 meq/100g, consistent with a mixture of illite, kaolinite and smectite. For 3 out of 4 samples, the amount of exchangeable Zn represents less than 1% of the CEC and is therefore not significant. In contrast, for sample BA121, which may be considered as an exception, more than 40% of the CEC contains exchangeable Zn; this may partly explain the high Zn content (30000 ppm) of this sample, but this assumption is not valid for other mineralized samples such as BA127 (Table 1). For the white clays, the CEC values are bracketed between 9 and 27 meq/100g; it may be related to the high content of ML dominated by a non-swelling 7 Å mineral, with a typical CEC value of 5 to 15 meq/100g (Ma and Eggleton 1999). In the ochre clays, the CEC values may reach 41 meq/100g, as the smectite proportion in the ML is probably higher than in the white clays (Fig. 6A). The amount of exchangeable zinc in the CEC is very low for both white and ochre clays, suggesting that zinc is mainly included within the octahedral sites of the clay mineral structure.

4.4) Textural observations (OM and SEM)

SEM observations of the red clays confirm the presence of a mixture of clay minerals, with dense aggregates of clay platelets (Fig. 4C). EDS analyses show that clay minerals are composed of Si, Al, and K, in accordance with the occurrence of illite/kaolinite mixture as identified on XRD patterns (Fig. 4A). Isolated laths or sub-euhedral lamella of chlorite are associated to K-rich clays and quartz sub-rounded clasts too (Fig. 4C). The bulk clayey matrix displays lighter and porous aggregates of lamella-shaped minerals (Fig. 4D); EDS analyses of these patches show Zn enrichment and a low K content relative to the mixture of kaolinite and illite. Descloizite, a zinc vanadate that precipitates under high oxidizing weathering conditions has been locally found (Fig. 4E) and may explain, together with the zinc clays, the anomalous high Zn content of some red clayey material samples (e.g. BA121 and BA127; Table 1). It is worth to notice that no particular phase corresponding to zinc clays was detected by XRD analyses suggesting a low abundance of these zinc-bearing minerals.

Optical microscope observations of the white clays show that clay minerals occur in cavity within the ore formed by roundish aggregates of willemite and disseminated galena coated by cerussite (Fig. 5C). SEM imaging of the clayey material in these cavities reveals a roughly honeycomb texture of the newly formed clay minerals (Fig. 5D). Locally, the micrometre-sized clay sheets are densely aggregated and form patches disseminated in the cavity. EDS analyses reveal that these clays are rich in Zn and poor in K, but sometimes K-rich fragments are found, suggesting an intricate association of both clay types. In most cases, the electronic contrasts between dark K-rich clays and light Zn-rich clays are so clear that it is easy to separate to the related domains, which present a very different textural arrangement (Fig. 5E). In the K-rich zone, the illite lamellae coexist with sub-euhedral clasts of quartz. Thin to large bands of Zn-rich clays exposing a honeycomb texture pervade in the cavity and connect to the willemite crystals.

Comparable observations can be made in the ochre clayey material, which exposes cavities filled by clay minerals within a primary association of willemite, dolomite and cerussite (Fig. 6C). A honeycomb texture of Zn-rich clays analysed by EDS is frequently observed (Fig. 6D). In the weakly dense aggregates, the micrometre-sized clay crystals can be individualised, exposing a typical sheeted aspect. Relics of illite lamellae and clasts made of quartz are found isolated in the Zn-rich clay matrix (Fig. 6E).

The clay minerals and especially the zinc-bearing type are closely associated to willemite; euhedral willemite crystals present evidence of partial dissolution that preferably affects edges or sites of crystalline defect (Fig. 7A). Zinc clays may precipitate in this newly formed porosity (Fig. 7B), but are mainly found as flakes or platelets either within the K-rich clayey material composed by illite, kaolinite and chlorite (Fig. 7C) or along cleavage cracks to form mineralized ribbons (Fig. 7D) that fossilized the paths of reactive waters. The honeycomb texture of newly formed zinc clays is frequent in the core of the domain, whereas the clay sheets appear squeezed at the interface with willemite (Figs. 5E and7E). Together with the euhedral shape of the willemite crystal and the evidence of successive rims of willemite, this supports the crystallization of a second generation of willemite (willemite II in Fig. 7F) after the formation of the Zn-rich clays.

4.4) Microprobe analyses of clay minerals

WDS quantitative measurements have been carried out on clay aggregates from white and ochre types (samples BA02, BA05 and BA05) and the results are reported in Table 3. Only data with a total over 80 %wt, consistent with the range of expected zinc clays minerals (fraipontite, sauconite) have been retained for discussion. The ZnO content, between 1.86 % and 49.39 % appears negatively correlated to the K 2 O content varying from 10.01 % to 0.05 %. Similarly, the SiO 2 and Al 2 O 3 contents decrease with K 2 O. Ca, Fe, Mg and Pb cations are present in minor amount and we observe show slight variations without any relationships with other major elements. Several groups depending on their ZnO and K 2 O content and their total oxide content have been distinguished (Table 3).

For K-rich clayey material, structural formulae were calculated for 22 negative charges (Table 3). For analyses 10, 57 and 58, the number of octahedral cations is around 2 and the number of interlayered cations reaches 0.8, consistent with the occurrence of illite. Analyses 19, 20, 29 and 38 show a significant K 2 O content (around 2 %) and Zn enrichment up to 30 %; they present a total of analysed oxides around 84 % consistent with pure Zn smectite, i.e. sauconite (Table 3). For seven individuals analyses that present a total of analysed oxides around 93 % and a K 2 O content between 0.2 % and 0.6 %, structural formulae were calculated for 22 negative charges assuming a T-O clay mineral structure. The number of cations in octahedral sites is around 5.5 attesting that the analysed particles are trioctahedral Zn-rich T-O minerals such as fraipontite. The value of M + (M + =K+2Ca) is significantly high, and may mirror surface adsorption of alkaline cations. The 17 remaining analyses display a total oxide between 84 % and 93 % and/or a variable content in K 2 O; they probably correspond to a mixture of the above-mentioned end-member clay minerals. The structural formulae calculations were based on 22 negative charges. One can notice the number of cations in the octahedral sites (3.4-3.5) largely exceed the ideal value of 3 (Table 3).

5) Discussion

5.1) Identification of the nature of Zn clays

The results obtained from microscopic observation and XRD analysis, suggest the presence of Zn-bearing clays in the three types of clayey material at Bou Arhous mine, especially in the white and ochre types. By comparing the results given by the two methods, we can deduce that the zinc-rich sheeted aggregates disseminated in the samples correspond to a 7 Å mineral locally interstratified with smectite sheets. In the literature, several types of zinc clays have been described. Sauconite, a trioctahedral smectite close to saponite is the most described clay type and was reported in several deposits in Northern America [START_REF] Ross | Sauconite -a clay mineral of the montmorillonite group[END_REF] In this study, the WDS compositions of the clay crystals show a negative correlation between K (as well as Al and Si) and Zn, suggesting phase mixing at site of analysis (Table 3). This is in agreement with the microscopic observation that supports an intricate association between the K-rich and the Zn-rich clays. In the M + -4Si -3R 2+ ternary plot, classically used for distinguishing the dioctahedral and the trioctahedral types [START_REF] Meunier | Clays[END_REF], our data are scattered along a mixing line that joins the mica pole and the most frequent Zn-clays, namely sauconite and fraipontite (Fig. 8). The data corresponding to the purest Zn-clays do not fall into fields of natural sauconite or fraipontite collected in Peru (Boni et al. 2009a;2009b) 3), this indicates that our zinc clayey material is dominantly a mixture of fraipontite and sauconite, in agreement with the occurrence of interstratified smectite and 7 Å mineral.

Numerical modelling of XRD patterns using the Newmod © program [START_REF] Reynolds | NEWMOD a computer program for the calculation of one-dimensional X-Ray diffraction patterns of mixed-layered clays[END_REF] have been carried out to determine the relative proportions of fraipontite and sauconite in the ML mineral.

Assuming 1) a mixture of a two-water-layer tri-smectite and a 7 Å mineral, 2) the 00l reflexions given by [START_REF] Faust | Thermal analysis and X-ray studies of sauconite and of some zinc minerals of the same paragenetic association[END_REF] and [START_REF] Fransolet | Données nouvelles sur la fraipontite de Moresnet (Belgique)[END_REF], and 3) a Reichweite value of 0, we have produced the XRD patterns presented in Figure 9. In this calculation, we have neglected the minor effects of Zn 2+ on the geometry of ML structure and therefore on the theoretical positions of 00l reflexions. The best simulation to describe the measured XRD pattern is obtained for a mixture containing 80% of fraipontite and 20% of sauconite, namely R0 fraipontite (0.8)/sauconite (Fig. 9). To validate this composition, we have recalculated the structural formula assuming a mixing of sauconite and fraipontite at a ratio of 1:4. Based on 39O, the sum of cations in the octahedral sheets is between 14.2 and 15 for low K 2 O individual data; this value, slightly lower than the ideal value of 15 for the trioctahedral type corroborates the hypothesis of R0 fraipontite(0.8)/sauconite mixed-layered mineral as the zinc-bearing clay mineral. Although such composition of interstratified mineral has never been reported in the literature, the occurrence of the sauconite-fraipontite association has been described in several natural examples of supergene non-sulphide zinc deposits [START_REF] Foord | Cuprian fraipontite and sauconite from the Defiance-Silver Bill mines, Gleeson, Arizona[END_REF]Young et al. 1992; Maltsev and Korshunov 1988), or even found in experimental works (Esquivin 1957;1960).

5.2) Genetic processes accounting for Zn-clays formation

Results obtained on whole-rock samples show the difficulty of identifying significant differences between the three types of clayey material to understand their genetic link with the ore and the host rock. Despite these difficulties, the following implications may be deduced from the above-mentioned analysis:

-Dolomite is present in the Jurassic limestone and in the gangue of Zn-Pb ore. Its occurrence in the ochre and white clayey materials and its relative absence in the red one may indicate different genetic processes accounting for their respective origin.

-Microscope observations have shown that Zn-clays are very frequently associated to detrital illite, which is dominant in the red clayey material. Zinc enrichment of clay minerals in the white and ochre clayey materials is consistent with the neogenesis of mixed layered fraipontite(0.8)/sauconite minerals, requiring sources for zinc, silica and aluminium.

-The main zinc-bearing mineral at Bou Arhous is willemite, whose conditions of formation remain unclear [START_REF] Choulet | Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin[END_REF]). It may have been formed under supergene conditions as exemplified by the deposits in Nevada [START_REF] Takahashi | Supergene alteration of zinc and lead deposits in limestone[END_REF], or in Zambia (Terraciano 2008).

Alternatively, a hydrothermal or hypogene origin for willemite [START_REF] Brugger | Formation of Willemite in Hydrothermal Environments[END_REF]Hitzmann et al. 2003[START_REF] Boni | The carbonate-hosted willemite prospects of the Zambezi Metamorphic Belt (Zambia)[END_REF]) could be envisaged, but no field and mineralogical observations are currently supporting this hypothesis.

The origin of willemite in such supergene environment is not clearly understood as this mineral is mostly described in hypogene or hydrothermal deposits [START_REF] Hitzman | Classification, Genesis, and Exploration Guides for Nonsulfide Zinc Deposits[END_REF][START_REF] Coppola | The "calamine" nonsulfide Zn-Pb deposits of Belgium: Petrographical, mineralogical and geochemical characterization[END_REF]. Its occurrence at Bou Arhous may suggest either a low-temperature hydrothermal stage or exceptional physico-chemical conditions characterised by a combination of high silica activity, low P(CO 2 ) g , alkaline conditions and oxidizing environment [START_REF] Choulet | Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin[END_REF]. At 25°C, willemite may be partly dissolved by meteoric waters rich in Ca and poor in Si [START_REF] Brugger | Formation of Willemite in Hydrothermal Environments[END_REF]; such conditions are reached in karst environment because of water-host rock interaction. This can explain the evidence for supergene dissolution of willemite, releasing Zn and Si in the solution (Figs. 7A and7B). This dissolution is probably congruent as no quartz or silica gel, incorporating the silicon in excess, remains or precipitates in the secondary porosity. The addition of Zn and Si ions in the solution changes equilibrium and the solubility of the Zn-bearing phases strongly depends on the pH conditions [START_REF] Takahashi | Supergene alteration of zinc and lead deposits in limestone[END_REF][START_REF] Brugger | Formation of Willemite in Hydrothermal Environments[END_REF]). As no smithsonite or hydrozincite precipitation has been observed in our samples despite high P(CO 2 ) g due to karst buffering, slightly acid (ca. 5) conditions must be envisaged to transport the Zn 2+ ions as chlorozinc(II) complexes [START_REF] Ruaya | The stability of chlorozinc(II) complexes in hydrothermal solutions up to 350°C[END_REF] and to enable a further precipitation of zinc clays. The absence of widespread hemimorphite is difficult to understand, as this phase is supposed to precipitate under acid pH conditions and relatively high silica activity [START_REF] Takahashi | Supergene alteration of zinc and lead deposits in limestone[END_REF][START_REF] Mcphail | The geochemistry and mobility of zinc in the regolith[END_REF]); however, this may be explained by the limited field of hemimorphite stability and its sensibility to strong P(CO 2 ) g variations (Reichert and Borg

2008).

The availability of Al is also critical for the neogenesis of the Zn clays, and requires a local source because of the low mobility of Al in meteoric waters. The Zn clays are often associated to the K-rich clays, mainly illite as evidenced by the XRD patterns of the white and ochre clayey materials.

At the scale of the cavity, the pervasion of the Zn-rich clays in the material dominated by the K-rich clays shall suggest a pseudomorphosis of primary illite and smectite and their replacement by the mixed-layered clay minerals (Figs. 5D or 6E). The very close association of illite and Zn clays suggests that partial dissolution of external illite layers, due to hydrolysis and the break up of Al-O and Si-O bonds, contributes to releasing Al and Si, further incorporated in the newly formed ML clay minerals. The Zn clays may also pseudomorph after dissolved willemite (Fig. 7B), but this requires a close source for Al.

In karstic environments, the physico-chemical conditions of meteoric waters continuously change due to carbonate buffering. A rapid increase of pH causes tremendous variations of the solubility of zinc silicates like willemite that can further precipitate even at surface temperature and under neutral conditions [START_REF] Brugger | Formation of Willemite in Hydrothermal Environments[END_REF]. This precipitation of secondary willemite (Willemite II) forming euhedral rim around primary willemite (willemite I) leads to squeezing of newly formed zinc clays (Fig. 7E and7F). As a consequence, the honeycomb texture is locally erased, and jamming contributes to emphasize the very fine mixing between illite and ML clay minerals (Fig. 7E).

Secondary willemite also occurs as idiomorphic crystals in the open space of the clayey cavities (Fig. 7D) or as infilling material within fractures of primary willemite (Fig. 7F).

5.3) Conditions for Zn mineralisation of clays and relation to the ore evolution

The mechanisms of Zn-rich clays formation have been rarely reported in the literature. In Zn ore deposits of Peru, Boni et al. (2009a) have identified sauconite and altered feldspars and they proposed that sauconite in silicoclastic host rocks may result from interaction between feldspar weathering and Zn-rich solution derived from supergene dissolution of zinc sulphide. At Abenab West ore deposit (Namibia), hosted in limestone, a progressive enrichment in Zn and Mg of barren clays is suggested, but details of the nature of zinc clays and the processes related to their neogenesis are presently not available (Terraciano 2008).

Like Abenab West ore deposit, zinc ore at Bou Arhous is hosted in limestone that has developed a large karst network since its exhumation at surface (Leblanc 1968). The Bou Arhous karst presents some non-dissolved material concentrated in various-sized cavities. Field observations have shown that this red clayey material is composed by a fine succession of silt and clay horizons that present sedimentation features. The red clay type may correspond to "residual" clays, which generally show mixing components between detrital sources and insoluble parts of the dissolved limestone [START_REF] Chamley | Clay sedimentology[END_REF]). The residual red clays are composed of clay minerals like illite, chlorite, kaolinite and smectite, iron oxi-hydroxide and quartz. This composition is somehow similar to that of the classical peri-Mediterranean karst-related clayey infilling, known as terra rossa, that shows stratiform accumulation of red clayey material in cavity formed after limestone dissolution (e.g. The red clays are originally barren (Table 1), but may be fertilized by several complementary processes that includes:

-Sedimentary recycling of zinc ore fragments, such as willemite (Figs. 3C and3D).

-Precipitation of newly formed zinc-bearing minerals like descloizite (Fig. 4E), which is typical of high oxidising supergene environment [START_REF] Boni | Genesis of Vanadium Ores in the Otavi Mountainland, Namibia[END_REF]).

-Adsorption of the Zn 2+ cations at the clay surface or fixing of the Zn 2+ cations in the interlayer, as indicated by the amount of [Zn 2+ ] released after the CEC measurements (Table 2). This process is well known in the literature as many studies have focused on the capacity of clays to trap heavy metals like Zn (e.g. Bradbury and Baeyens 1999; [START_REF] Dähn | Investigation of the different binding edge sites for Zn on montmorillonite using P-EXAFS -The strong/weak site concept in the 2SPNE SC/CE sorption model[END_REF]).

-Precipitation of the Zn-rich clays in small-disseminated patches (Fig. 4D), up to large pervasion (Figs. 5D and6E) in the red detrital clayey material. This process is able to progressively form the white or the ochre clays found in the Bou Arhous deposit. As seen earlier, this process is characterized by the neogenesis of authigenic clay minerals that nucleate on micas. It may be compared to the epitaxial growth of Zn clays on smectites, which interacts with a solution enriched in Zn and Si [START_REF] Schlegel | Sorption of metal ions on clay minerals. III. Nucleation and epitaxial growth of Zn phyllosilicate on the edges of hectorite[END_REF][START_REF] Schlegel | Evidence for the nucleation and epitaxial growth of Zn phyllosilicate on montmorillonite[END_REF]. These authors reported newly formed clays as a mixture of Zn-saturated TO and TOT trioctahedral phyllosilicates, thereby presenting lot of similarities with the R0 fraipontite(0.8)/sauconite mixed-layered mineral identified in this study. The presence of a clay precursor for nucleation and Al supply is required for the neogenesis of Zn-clays, which also depends on the silica activity [START_REF] Schlegel | Sorption of metal ions on clay minerals. III. Nucleation and epitaxial growth of Zn phyllosilicate on the edges of hectorite[END_REF]. At low Si concentration, Zn may be fixed in polymer chains in structural continuity to the smectite. High Si concentration rather facilitates the neogenesis of authigenic clays and especially the formation of Zn-rich TO phyllosilicates, such as fraipontite. Other experimental studies at supergene conditions have showed that the presence of Al may play a role for the type of clay that precipitates (Tiller and Pickering 1974), with 2:1 layer silicate in Al-free conditions and, in the presence of aluminium, formation of 1:1 layer silicate similar to Zn berthierine, a synthetic product equivalent to fraipontite [START_REF] Esquevin | Les Silicates de zinc, étude de produits de synthèse et des minéraux naturels[END_REF]. Such chemical controls could be hypothesised to spell out the different types of Zn clays met throughout the world.

The climate dependence classically evoked for the variability of clay types [START_REF] Chamley | Clay sedimentology[END_REF]) may also be taken into account, but its role is currently difficult to understand, given the scarcity of reliable data on Zn clay occurrences.

The formation of zinc clays at Bou Arhous is a continuous process during the late supergene evolution of the ore deposit. Initially, zinc was included in sulphide lenses hosted in the Lower Jurassic limestone (Leblanc 1968). The regional uplift and the folding episode related to the Atlas tectonics during the Tertiary have led to an exhumation of the sulphide ore and a verticalisation of the limestone strata [START_REF] Choulet | Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin[END_REF]). As a consequence, the meteoric water has percolated in the fractures and has provoked the dissolution of the limestone and the oxidation of the sulphides. For some unclear reasons, zinc has first re-precipitated as willemite, forming a "black ore" due to its association with preserved galena and cerussite. Willemite has further interacted with karstic waters, probably poor in Si and slightly acid, leading to its partial dissolution and to releasing of Zn and Si in the solution (Fig. 10). In parallel, the development of a karst was characterised by the opening of various-sized dissolution cavities filled by detrital supplies and insoluble materials (Fig. 10). At surface conditions, this red clayey material has been pervaded by Zn-Si-rich solutions resulting in the neogenesis of Znclays (mixed-layered clay minerals) that nucleate on detrital micas. At site of zinc ore concentration, red clays were progressively transformed into white and ocher clays. The further crystallisation of late generations of willemite indicates that this process is continuous in time, highly dependant on the chemistry and dynamics of karstic solutions, and therefore difficult to timely constrain in the ore deposit evolution.

6) Conclusion

This study has revealed the nature of the zinc clays at Bou Arhous deposit and the mechanisms called for their genesis. Although several processes are involved for the metal enrichment of barren clays, the crystallisation of Zn clays appears to be the major one. This neogenesis is related to the interaction between Zn-rich meteoric water and residual clays related to karst evolution. This model has strong implication for the mineral industry, as Zn-rich clayey material grading up to 30% Zn is a non-negligible resource that may be easily identified. The target zones will be the karst cavities located around the original ore body (potentially dissolved); residual clays must fill these cavities, representing the necessary condition for the nucleation of the newly formed clay crystals. The soft nature of the clayey material could probably facilitate mining, at least in case of open-pit extraction, and especially contributes to costs reduction in the early operations.

Conversely, by deciphering the mineralogical nature of the Zn-rich clayey material, this study has showed the mixture of authigenic mixed-layered clays and detrital phyllosilicates. Except for rare samples, zinc cannot be recovered easily as it is located within the octahedral sites in each component of the interstratified mineral. Consequently, Zn can only be released by leaching techniques such as solvent extraction, which is relatively efficient for a given clay species like sauconite (Boni et al. 2009b). But, in the presence of interstratified minerals finely disseminated within detrital barren clays, ore liberation may be more difficult and requires advances for 1) an efficient separation between barren and mineralised clays, and 2) a better selectivity of solvents used. Another consequence of this intricate mineralogical association is the excess of barren materials that need to be handled and processed, thereby representing a major limitation to the economic potential of zinc clays.

Figure Captions

  and up to Pleistocene (Frizon de Lamotte et al. 2009). As a consequence, the Mesozoic sedimentary basins formed during the opening of Tethys and Atlantic Oceans have been affected by inversion tectonics accompanied by regional uplift and localized exhumation of Lower Jurassic and Triassic strata in the core of narrow anticlines. Thereafter, exhumed strata experienced surface weathering leading to the karst development in the Jurassic limestone (Choulet et al. 2014; Charles et al. submitted).

,

  Peru (Boni et al. 2009a), Southern Africa (Kärner 2006; Terraciano 2008; Boni et al. 2011) and Iran (Daliran et al. 2009). Fraipontite (also known as zinalcite), a zincian berthierine (7 Å mineral) has been found in supergene zinc deposits in Belgium (Fransolet and Bourguignon 1975), Iberia (Calvo et al. 2007; Will et al. 2014), Italy (Merlino and Orlandi 2001) and Kazakhstan (Chukhrov 1968). Other rare zinc clays are Zn-rich beidellite (Mondillo et al. 2014), baileychlore, a zinc chlorite (Rule and Radke 1988) and hendricksite, a zinc mica[START_REF] Frondel | Hendricksite, a new species of mica[END_REF] Robert and Gaspérin 1985), the latter two occurring in hypogene environments. Besides, moresnetite[START_REF] Ross | Sauconite -a clay mineral of the montmorillonite group[END_REF]) and vanuxemite[START_REF] Frondel | The minerals of Franklin and Sterling Hill, a checklist[END_REF] have been described as a mixture of zinciferous clays and hemimorphite. No zinc-bearing mixed-layered (ML) minerals have been described, except for illite-zincian smectite ML in Peru[START_REF] Mondillo | Zn-rich clay minerals in supergene ores (Cuzco area, Peru); a TEM study with inferences for genesis and processing IMA meeting[END_REF]).

  , in Namibia (Kärner et al. 2006), in the USA (Ross 1946), in UK (Young et al. 1992) and in Belgium (Fransolet and Bourguignon 1975); data rather fall into an intermediate domain between the theoretical compositions of sauconite and fraipontite. Together with the difficulty of calculating pure sauconite or fraipontite structural formula from our analyses (Table

Fig. 1 Fig. 7

 17 Fig. 1 a: Simplified tectonic map of Morocco. b: Geological map of the Bou Arhous range. c: Cross-Section across the Bou Arhous range, showing the structural and stratigraphic location of the ore deposit (modified after Choulet et al. 2014)

Figure 1

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 10 Figure 10 Click here to download Figure: Fig10.tif

Table 1

 1 Sampling details of the host rocks and clayey material, with bulk mineralogy deduced from XRD and geochemical content obtained by ICPMS analyses. Abbreviations: cc: calcite, ce: cerussite, cl: clays, do: dolomite, ga: galena, goe: goethite, gy: gypsum, he: hematite, qtz: quartz

Table 2

 2 Values of the CEC and zinc content of the CEC for the three types of clay materials reported in this study. The clay content is deduced from XRD analyses and is illustrated in figs 4 to 6

Table 3

 3 WDS results and calculated structural formulae for the different types of clay minerals and aggregates. See text for explanation about calculation and data sorting

Table 3

 3

Young B, Livingstone A, Thomson N (1992) Fraipontite from Wensleydale, North Yorkshire. Proc

Yorkshire Geol Soc 49:125-127. doi: 10.1144/pygs.49.2.125

Acknowledgments

The authors are indebted to Managem Company for providing access to the Bou Arhous Mine and for having supported bulk rock analyses in their labs. We also thank Goka Katrawi for CEC measurements and Didier Convert-Gaubier for thin-section preparation. This study has benefited from a research grant given by the Région Centre for the 2011-2014 CALAMINES project and from a financial support of Région Franche-Comté with the 2014 "Franco-Suisse" call. Access to SEM facilities of the MIMENTO centre was made in the frame of cooperative projects in the Renatech network.