Daouda Traoré

Anicet Beauvais
email: anicet.beauvais@noumea.ird.nc

François Chabaux

Chantal Peiffert

Jean-Claude Parisot

Jean-Paul Ambrosi

Fabrice Colin

Chemical and physical transfers in an ultramafic rock weathering profile: Part 1. Supergene dissolution of Pt-bearing chromite

Keywords: Lateritic weathering, ultramafic rocks, mass balance calculation, Pt-chromite, New Caledonia

INTRODUCTION

Lateritic weathering of rocks is one of the major processes that modify the earth's surface and contribute to the geochemical cycle of elements [START_REF] Marker | Control of the distribution of Mn, Co, Zn, Zr, Ti, and REEs during the evolution of lateritic covers above ultramafic complexes[END_REF][START_REF] Compton | Rare earth element behavior in soils and salt pan sediments of a semi-arid granitic terrain in the Western Cape, South Africa[END_REF]. It affects numerous regions of the intertropical zone [START_REF] Pedro | Distribution des principaux type d'altération chimique à la surface du globe. Présentation d'une esquisse géographique[END_REF][START_REF] Nahon | Evolution of iron crusts in tropical landscapes[END_REF][START_REF] Tardy | Geochemistry and evolution of lateritic landscapes[END_REF][START_REF] Tardy | Petrology of laterites and tropical soils[END_REF], where long-term chemical weathering has led to the dissolution of primary minerals including heavy minerals like metal-bearing spinel that results in supergene formation and accumulation of secondary and residual minerals in profiles of 10 to 100 m in thickness. The mobilization and recycling of chemical elements during supergene weathering processes is complex, depending on dissolution kinetics of primary minerals, formation of secondary phases, Eh-pH conditions, mass transfers and co-precipitation and ionic exchanges between various minerals [START_REF] Harris | Geochemical and mineralogical studies on the weathering of granitic rocks[END_REF][START_REF] Nesbitt | Mobility and fractionation of rare earth elements during weathering of granodiorite[END_REF][START_REF] Cramer | Mass-balance relations and trace element mobility during continental weathering of various igneous rocks[END_REF][START_REF] Nahon | Introduction to the Petrology of Soils and Chemical Weathering[END_REF][START_REF] Nesbitt | Recent chemical weathering of basalts[END_REF][START_REF] Islam | Geochemistry and mineralogy of saprolite in Finish Lapland[END_REF]. We study here the chemical weathering of ultramafic rocks of New Caledonia and particularly the supergene dissolution of platinum-bearing chromite grains disseminated in the weathering profile to characterize the weathering processes of the parent rocks and the release mechanism of platinum group minerals (PGM) in the weathering profile.

More than one-third of the area of New Caledonia is covered by ultramafic rocks, which are remnants of a Late Eocene overthrust belt now dissected by erosion [START_REF] Paris | Géologie de la Nouvelle-Calédonie: un essai de synthèse[END_REF]. These ultramafic rocks are deeply weathered, resulting in progressive leaching of Mg and Si and relative concentration of Fe and other metals in thick weathering profiles [START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Latham | Altération et pédogenèse sur roches ultrabasiques en Nouvelle-Calédonie: genèse et évolution des accumulations de fer et de silice en relation avec la formation du modelé[END_REF]). Ultramafic rocks are mainly harzburgite or lherzolite, which can exhibit serpentinization at their base; these rocks are also intercalated with dunite layers [START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF]. Chromium spinel (Mg, Fe 2+ )(Cr, Al, Fe 3+ ) 2 O 4 is present interstitially up to 3% [START_REF] Guillon | Les Massifs péridotitiques de Nouvelle-Calédonie: type d'appareil ultrabasique stratiforme de chaine récente[END_REF] or in pure chromite deposits of the ophiolitic harzburgites [START_REF] Cassard | Structural classification of chromite pods in southern New Caledonia[END_REF][START_REF] Augé | Chromite et minéraux du groupe du platine dans les complexes ophiolitiques[END_REF][START_REF] Leblanc | Chromitite and ultramafic rock compositional zoning through a paleotransform fault, Poum, New Caledonia[END_REF]. This primary chromite is subsequently mobilized in eluvial and alluvial deposits. Significant proportions of this mineral have been previously reported in New Caledonian ophiolitic complexes, but the secondary supergene evolution of chromium spinel is poorly understood [START_REF] Phan | Altération météorique de chromite de Nouvelle-Calédonie: Etude à la microsonde électronique. Contribution à la géochimie supergène du chrome en milieu latéritique. Conséquences pratiques pour la concentration des chromite détritiques[END_REF]. A petrological and geochemical study of lateritic weathering profiles can contribute to improve this knowledge, and more particularly to address the specific metallogenic issue of the supergene weathering processes of platinum-bearing chromite.

GEOLOGICAL SETTING

The study area is located in the downslope part of the Pirogues River drainage basin in Southern New Caledonia (Fig. 1). The lithological units of this area consist of ultramafic rocks as cumulates of dunite, wehrlite and pyroxenite, and also harzburgites, which are serpentinized and cross-cut by pyroxenite dikes (Fig. 1). Platiniferous chromite are primarily concentrated either as stratiform accumulations in dunite and wehrlite or are disseminated in pyroxenite dikes and wehrlite. Most of the PGM are included in chromite [START_REF] Augé | Stratiform and alluvial platinum mineralization in the New Caledonia ophiolite complex[END_REF]. Chromite forms thin schlieren and layers a few centimeters thick and a meter or so long in dunite and wehrlite cumulates. Irregular concentrations of massive chromite form ore in the pyroxenite dikes [START_REF] Augé | Stratiform and alluvial platinum mineralization in the New Caledonia ophiolite complex[END_REF]. Chromite is crystallized in between large grains of pyroxene. Massive chromitite was also observed in the different horizons of the weathering profiles.

The ultramafic rocks are deeply weathered under the influence of a warm and humid climate characterized by a mean annual rainfall of 1700 mm with a wet season occurring from December to August. The ultramafic bedrock is generally overlain by a thick lateritic weathering mantle consisting typically from the base to the top [START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Besset | Localisation et répartitions successives du nickel au cours de l'altération latéritique des péridotites de Nouvelle-Calédonie[END_REF][START_REF] Latham | Altération et pédogenèse sur roches ultrabasiques en Nouvelle-Calédonie: genèse et évolution des accumulations de fer et de silice en relation avec la formation du modelé[END_REF][START_REF] Llorca | Les concentrations cobaltifères supergènes en Nouvelle-Calédonie: géologie, minéralogie[END_REF]) of: a coarse saprolite comprised of parent rock blocks, surrounded by weathering cortices; a fine saprolite where the silicates are totally hydrolyzed but the parent rock structure is preserved; a mottled zone composed of some ferruginous nodules embedded in a goethitic matrix crossed by numerous millimeterscale bioturbations; a soft nodular layer consisting mainly of millimeter to centimeter size nodules and pisoliths, and ferricrete fragments embedded in a red goethitic ferruginous matrix; a ferricrete exhibiting a massive, alveolar, and/or pisolitic structure, which is essentially composed of goethite and hematite. The parental structures are generally blurred in the ferruginous upper horizons.

Natural weathering profiles exposed in deep gullies (also locally called "lavakas") have been described and sampled in situ (Fig. 2) for mineralogical and geochemical studies.

MATERIAL AND METHODS

Fourteen samples of parent rock, laterite and soil were collected in situ for measuring the bulk and grain density (Figs. 2 and3). These samples were analyzed to determine (1) major element composition by Inductively Coupled Plasma -Atomic Emission Spectroscopy (ICP-AES) and (2) trace and precious metals by Inductively Coupled Plasma -Mass Spectrometry (ICP-MS). For ICP-AES analyses, the samples were crushed and finely ground ( 75 μm) after oven drying to be decomposed with 10 times their weight of a composite flux comprising 65% metaborate and 35% tetraborate according to the ME-ICP93 analytical procedure developed by the Australian Laboratory Services (ALS, Chemex). The resulting melts were dissolved in dilute nitric acid. Repeated fusion of finely ground powder was necessary to obtain complete decomposition of chromite, particularly for the samples containing coarser-grained chromite. For ICP-MS analysis of precious metals (Pt and Pd) each ground sample was fused with a mixture of lead oxide, sodium carbonate, borax and silica, inquarted with 6 mg of gold-free silver and then cupelled to yield a precious metal bead according to the PGM-MS23 analytical procedure developed by ALS Chemex, Australia. After cupellation the bead was dissolved in nitric and hydrochloric acids and the solution diluted to volume. Detection limits for Pd and Pt are 1 and 0.5 ppb, respectively. X-ray diffraction (XRD) determinations were also processed on each sample. Semi-quantitative proportions of minerals were deduced by the measurements of the surface of main peaks in XRD spectra.

Three samples of 50 kg were also collected in the friable layers of the weathering profile (fine saprolite, mottled zone and nodular layer). The samples were carefully washed, sifted to 1 mm and panned in order to separate out the heaviest particles. Chromite grains were separated from the other heavy minerals under a binocular microscope for morphological examination by scanning electron microscopy (SEM). After the SEM observation, chromite grains were mounted with resin and polished sections were prepared for electron microprobe analyses. Microanalyses of chromite grains were obtained using a Cameca SX 100 electron microprobe with an acceleration potential of 15 keV, a beam current of 50 nA and counting time of 10 s. The standards used were olivine for Mg, albite for Al, hematite for Fe, orthoclase for Si, Cr 2 O 3 for Cr, MnTiO 2 for Mn and Ti, NiO-466.6 for Ni, ZnO for Zn and synthetic Co for Co.

RESULTS AND DISCUSSION

Physical and geochemical characterization

Density and porosity change. The weathering profile is developed from a wehrlite type ultramafic rock (40-70% olivine, 30-45% antigorite and 10-20% diopside), and its thickness is up to 4 meters. At the base of the profile (Fig. 2), the greenish-black parent rock (-470 to -400 cm) changes to a brownish coarse saprolite (-400 to 290 cm) that preserves parental structure and texture. Above (-290 to -100 cm) a yellow fine saprolite is overlain by a red mottled zone (-100 to -60 cm) and a colluvial nodular horizon close to the ground surface (Fig. 2), which is composed of ferruginous nodules, pisolites and fragments of a previous ferricrete.

Rock porosity is an important physical parameter of weathering mantles as it controls soil, water and air circulation, and thus permeability. Therefore, pores are a first order factor controlling erosion and mass movement processes. In the profile studied, the bulk density decreases from an average 2.37 g.cm -3 in the fresh wehrlite to 1.32 g.cm -3 in the coarse saprolite, 0.93 g.cm -3 and 1.13 g.cm -3 in the fine saprolite and mottled zone, respectively, and 1.71 g.cm -3 in the nodular horizon (Fig. 3a and Table 1). The grain density slightly increases from an average 3.05 g.cm -3 in the parent rock and the coarse saprolite, 3.58 g.cm -3 in the fine saprolite and 3.64 g.cm -3 in the mottled zone to 3.67 g.cm -3 3 in the nodular horizon (Fig. 3b and Table 1). The porosity change is thus very important, from 22% in the fresh wehrlite to 56%, on average, in the coarse saprolite, 74% in the fine saprolite, 69% in the mottled zone, and 53% in the upper nodular layer (Fig. 3c and Table 1).

Petrologic and geochemical patterns. The XRD mineralogical characterization of each horizon composing the weathering profile indicates the presence of olivine, antigorite, diopside and goethite with accessory chromite and enstatite. In the saprolites, the silicates are progressively replaced by goethite, olivine being the first affected by weathering. The coarse saprolite is composed of 30% olivine (up to 50% in the parent rock), 40% antigorite (35% in the parent rock), 15% diopsite, 15% goethite. The base of the fine saprolite is composed of 15% olivine, 5% antigorite, 10% diopside (15% in the parent rock) and 50% goethite. At the top of the fine saprolite olivine, antigorite and diopside are completely replaced by goethite, which reaches 100%. The mottled zone and the nodular layer are essentially composed of goethite with accessory hematite and chromite.

The geochemical analyses of the unweathered wehrlite indicate, on average, 39.6 wt.% SiO 2 , and 34.70 wt.% MgO, and 19.3 wt.% Fe 2 O 3 (Table 1). Silica decreases to 21.2 wt.% at the top of the coarse saprolite and MgO to 7.1 wt.%. Iron (III) oxide contents increase, on average, from 19.3 wt.% to 45.2 wt.%. Aluminum and chromium contents do not significantly change between parent wehrlite and coarse saprolite: on average, 0.6 -0.7 wt.% Al 2 O 3 , 1.5 -2.3 wt.% Cr 2 O 3 . The transition metal contents increase, on average, from 1.97 g.kg -1 to 4.40 g.kg -1 for Mn, 0.23 g.kg -1 to 0.48 g.kg -1 for Co, and 0.34 g.kg -1 to 0.53 g.kg -1 for Ni (Table 1).

The coarse saprolite is progressively transformed into a fine saprolite, in which the texture and the structure of the parent rock are still preserved. On average, the geochemical analyses of the fine saprolite indicate 3.4 wt.% SiO 2 , 0.9 wt.% Al 2 O 3 , 77 wt.% Fe 2 O 3 , 3.3 wt.% Cr 2 O 3 , 0.04 wt.% CaO, 0.85 wt.% MgO, 8.3 g.kg -1 Mn, 2.8 g.kg -1 Co, and 8.2 g.kg -1 Ni (Table 1).

Toward the top of the profile, the fine saprolite changes to a bioturbated-mottled laterite in which the original rock structure is no longer preserved. The chemical composition is similar to the one analyzed in the upper part of the fine saprolite except for decreased Co content (Table 1).

Mass balance calculations

Mass balance calculations related to weathering processes require the application of rigorous means to estimate losses or gains based on a thorough petrological study [START_REF] Millot | Transformations isovolumiques dans les phénomènes de latéritisation et de bauxitisation[END_REF][START_REF] Gresens | Composition-volume relations of metasomatism[END_REF][START_REF] Brimhall | Constitutive mass-balance relations between chemical composition, volume, density, porosity and strain in metasomatic hydrochemical systems: results on weathering and pedogenesis[END_REF][START_REF] Colin | Equatorial rain forest lateritic mantles: A geomembrane filter[END_REF][START_REF] Colin | Quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment[END_REF][START_REF] Beauvais | Formation and transformation processes of iron duricrust systems in tropical humid environment[END_REF]. The parental rock structures are well preserved in the saprolite suggesting that weathering processes have preserved the original volume.

Based on the assumption that the volume has not varied between bedrock (-470 cm) and the top of the fine saprolite (-110 cm), isovolume mass-balance calculations can be applied to quantify mass transfers induced by the transformation of parent-rock into saprolite [START_REF] Millot | Transformations isovolumiques dans les phénomènes de latéritisation et de bauxitisation[END_REF]. Numerous authors have mathematically formalized mass balance calculations [START_REF] Gresens | Composition-volume relations of metasomatism[END_REF][START_REF] Brimhall | Constitutive mass-balance relations between chemical composition, volume, density, porosity and strain in metasomatic hydrochemical systems: results on weathering and pedogenesis[END_REF]) by a functional form of the constitutive relationships between the chemical compositions of parent and weathered material, their bulk densities, and volumes. The net gains and losses of chemical constituents are calculated from the following equation [START_REF] Brimhall | Constitutive mass-balance relations between chemical composition, volume, density, porosity and strain in metasomatic hydrochemical systems: results on weathering and pedogenesis[END_REF]:

K j = C jw jw C jp jp 1 100, (1)
where K j is the gain and/or loss factor of chemical element j ; C jw and C jp are the chemical element j concentrations (expressed in g.kg -1 for Si, Al, Fe, Ca, Mg and Cr, or mg.kg -1 for Mn, Co and Ni) in the weathered layer, w, and the parent material, p, respectively; and jw and jp are bulk densities of the weathered material, w, and the parent material, p, respectively (expressed in g.cm -3 ).

The total mass (m jw ) of each element j gained or lost during the weathering process is calculated from the following equation [START_REF] Colin | Quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment[END_REF]:

m jw = C jp jp V p K jw , (2)
where V p is the volume of the parent rock (V p = 1 m 3 ), K jw is the gain and/or loss factor of chemical element j in the weathered layer, w, calculated from the equation (1), and C jp , jp and m jw are expressed in kg.ton -1 , ton.m -3 and kg.m -3 , respectively.

Mass balance calculations show that 96% Si, 99% Mg, 94% Ca have been depleted in the fine saprolite (Table 2a and Fig. 4) that corresponds, on average, to mass transfers of -426 kg Si, -488 kg Mg and -5.4 kg Ca per cubic meter of weathered parent rock (Table 2b). This is accompanied, on average, by 47% loss of Al and 16% loss of Cr, i.e., -4 kg.m -3 Al and -4 kg.m -3 Cr, the highest values being recorded at the base of the fine saprolite (Table 2 and Figs. 4a and4c). Iron, Mn, and Co contents are enriched at the top of the coarse saprolite only by 4.7% (+17.2 kg.m -3 ), 8% (+0.39 kg.m -3 ), and 9% (+0.05 kg.m -3 ), respectively, while Ni is slightly depleted by -10% (-0.69 kg.m -3 ) (Fig. 4 and Table 2). At the base of the fine saprolite, the enrichment rates strongly increase by 30% (+110 kg.m -3 ) for Fe, 45% (+3.18 kg.m -3 ) for Ni, 54% (+2.61 kg.m -3 ) for Mn, and 458% (+2.35 kg.m -3 ) for Co. At the top of the fine saprolite, the enrichment rates are 38% (+142 kg.m -3 ) for Fe, 81% (+3.9 kg.m -3 )

for Mn and 312% (+1.6 kg.m -3 ) for Co, while Ni is depleted by -20% (-1.38 kg.m -3 ) (Table 2a and2b).

The sharp decrease of Si, Mg and Ca contents (from parent rock to saprolite) reflects the congruent weathering of primary silicates [START_REF] Eggleton | Weathering of basalt: changes in rock chemistry and mineralogy[END_REF][START_REF] Nesbitt | Recent chemical weathering of basalts[END_REF][START_REF] Islam | Geochemistry and mineralogy of saprolite in Finish Lapland[END_REF]. The weathering creates high porosity and resultant improved drainage conditions if pore spaces are interconnected [START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Latham | Altération et pédogenèse sur roches ultrabasiques en Nouvelle-Calédonie: genèse et évolution des accumulations de fer et de silice en relation avec la formation du modelé[END_REF][START_REF] Marker | Control of the distribution of Mn, Co, Zn, Zr, Ti, and REEs during the evolution of lateritic covers above ultramafic complexes[END_REF]). The Fe-enrichment at the transition between coarse and fine saprolite (Fig. 4a and Table 2) is due to relatively short vertical migrations of iron in the weathering profile that has led to goethite formation, which may have trapped transition metals such as Mn, Co and Ni [START_REF] Millot | Transformations isovolumiques dans les phénomènes de latéritisation et de bauxitisation[END_REF][START_REF] Tardy | Geochemistry and evolution of lateritic landscapes[END_REF][START_REF] Beauvais | Formation and transformation processes of iron duricrust systems in tropical humid environment[END_REF][START_REF] Beauvais | Petrological differentiation patterns and geomorphic distribution of ferricretes in Central Africa[END_REF].

The primary host minerals for Ni in the ultramafic rocks are olivine, serpentine and accessory pyroxene [START_REF] Zeissink | The mineralogy and geochemistry of a nickeliferous laterite profile (Greenvale, Queensland, Australia)[END_REF][START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Besset | Localisation et répartitions successives du nickel au cours de l'altération latéritique des péridotites de Nouvelle-Calédonie[END_REF][START_REF] Colin | Lateritic weathering of pyroxenites at Niquelandia, Goias, Brazil: The supergène behavior of nickel[END_REF][START_REF] Gaudin | Cristallochimie des smectites du gisement latéritique nickélifère de Murrin Murrin (Ouest Australie)[END_REF]. During the weathering of these parent minerals in strong oxidizing conditions, Ni is mobilized and enriched by co-precipitation with secondary Fe and Mn oxides at the transition between the coarse and fine saprolite (Fig. 4c and Table 2) as previously shown [START_REF] Schellmann | Formation of nickel silicate ores by weathering of ultramafic rocks[END_REF][START_REF] Golightly | Nickeliferous laterite deposits[END_REF][START_REF] Elias | Geology, mineralogy, and chemistry of lateritic nickel-cobalt deposits near Kalgoorlie, Western Australia. Economic Geology[END_REF][START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Reimann | Chemical elements in the environment[END_REF].

Manganese enrichment in the fine saprolite (on average, +60%), as compared to its content in the parent rock (Table 2a), can be explained by significant vertical and/or lateral transfers of Mn, which is released as Mn 2+ in the saturated zone of the deeper saprolite [START_REF] Golightly | Geology of Soroako nickeliferous laterite deposits[END_REF]. Vertical transfers may be due to the leaching of Mn from the near-surface part of the profile under conditions of low pH and high CO 2 activity. These environmental conditions favor reduction of Mn 4+ to Mn 2+ [START_REF] Golightly | Geology of Soroako nickeliferous laterite deposits[END_REF][START_REF] Beauvais | Pétrologie du gisement latéritique manganesifère d'Azul (Brésil)[END_REF]. Colloidal Mn 2+ complexes are highly mobile and can be transported in the soils over long distances [START_REF] Krauskopf | Introduction to Geochemistry[END_REF][START_REF] Marker | Control of the distribution of Mn, Co, Zn, Zr, Ti, and REEs during the evolution of lateritic covers above ultramafic complexes[END_REF]. As soon as Mn-bearing solutions reach the lower part of the weathering profile, rising pH due to silicate hydrolysis and increased concentration of the solution above the saprolite, are likely to result in the oxidation of Mn 2+ and precipitation of Mn oxides from the colloidal solution [START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Golightly | Geology of Soroako nickeliferous laterite deposits[END_REF][START_REF] Llorca | Les concentrations cobaltifères supergènes en Nouvelle-Calédonie: géologie, minéralogie[END_REF][START_REF] Marker | Control of the distribution of Mn, Co, Zn, Zr, Ti, and REEs during the evolution of lateritic covers above ultramafic complexes[END_REF].

The behavior of Co is also controlled by physical-chemical properties of the downwardmigrating solutions [START_REF] Marker | Control of the distribution of Mn, Co, Zn, Zr, Ti, and REEs during the evolution of lateritic covers above ultramafic complexes[END_REF]. Cobalt is adsorbed by negatively charged Mn 4+ colloids and/or incorporated into Mn oxides [START_REF] Burns | The uptake of cobalt into ferromanganese nodules, soils and synthetic manganese (IV) oxides[END_REF][START_REF] Manceau | Crystal chemistry of cobalt and nickel in lithiophorite and asbolane from New Caledonia[END_REF][START_REF] Llorca | Supergene cobalt minerals from New Caledonia[END_REF][START_REF] Llorca | Metallogeny of supergene cobalt mineralization, New Caledonia[END_REF]. Precipitation of Co at the base of the profile, where pH is higher, is more effective, because the solubility product of Co hydroxide is lower than that of Mn hydroxide [START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Schellmann | Behavior of nickel, cobalt, and chromium in ferruginous lateritic nickel ores[END_REF][START_REF] Golightly | Nickeliferous laterite deposits[END_REF]. This property can explain the higher enrichment factor of Co (+450%) compared to Mn (+54%) or Ni (+45%) in the lower cementation zone (Figs. 4c andd, Table 2) [START_REF] Marker | Control of the distribution of Mn, Co, Zn, Zr, Ti, and REEs during the evolution of lateritic covers above ultramafic complexes[END_REF].

The Cr-bearing minerals in the ultramafic rocks are mainly chromite and accessory silicate minerals. Chromium-bearing silicate minerals are, however, depleted in the upper saprolite.

Some Cr may be leached downward in the form of Cr(OH) 3 in alkaline environments [START_REF] Marker | Control of the distribution of Mn, Co, Zn, Zr, Ti, and REEs during the evolution of lateritic covers above ultramafic complexes[END_REF]. Chromium within chromite is generally considered immobile in supergene environments [START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Schellmann | Behavior of nickel, cobalt, and chromium in ferruginous lateritic nickel ores[END_REF][START_REF] Becquer | Chromium availability in ultramafic soils from New Caledonia[END_REF]. However, recent thermodynamic and field studies have shown that chromite is soluble in lateritic environments [START_REF] Fendorf | Surface reactions of chromium in soils and waters[END_REF], suggesting that Cr in laterites derived from the weathering of ultramafic rocks may be depleted from the profile. As chromite is the main host mineral of aluminum in the wehrlite, its solubility also explains the depletion of Al in the profile.

Supergene dissolution of chromite

The chemical compositions of chromite from different samples of Pt-rich chromitite of the Pirogues site are plotted in Figure 5, together with disseminated chromite from the cumulate series, and chromitite from previously studied mantle deposits (Augé and Maurizot 1995), as, for example, those from the Tiebaghi massif in northwestern New Caledonia, in which the chromite has a very constant composition (high Cr 3+ (55.5-63.5 wt.% Cr 2 O 3 ) and low Fe 3+

(1.5-4.3 wt.% Fe 2 O 3 )). Chromite disseminated in mineralized cumulates is slightly impoverished in Cr (Fig. 5). Platinum-bearing chromite concentrations in the cumulate series of the Pirogues area differ significantly from those of mantle chromitite in having relatively low Cr 3+ (40-46 wt.% Cr 2 O 3 ) and high Fe 3+ (10-17 wt.% Fe 2 O 3 ) (Fig. 5). No significant difference was observed between chromitite of pyroxenite dikes and chromitite of cumulates.

The platinum-bearing chromite in the parent rock and coarse saprolite is euhedral (Fig. 6a), with surfaces that may represent small primary dissolution cracks (Fig. 6b). The progressive weathering of chromite is illustrated in Figure 6. In the fine saprolite, where the lowest bulk densities and the highest porosities were measured (Fig. 3 and Table 1), the dissolution of chromite starts along the primary cracks and increases until forming a network of etching pits of sub-micrometric size (Fig. 6c and6d). This effectively corresponds to the highest losses of Cr (Fig. 4c and Table 2). In the mottled zone, residual chromite grains with smooth edges are embedded in ferruginous aggregates and/or pisolites (Fig. 6e), which are essentially composed of goethite. The dissolution front increases in the chromite by widening of microcracks and fissures hosting weathering solutions (Fig. 6f). The supergene dissolution process leads to the dislocation of chromite crystals in the superficial layers (Fig. 6g and6h).

Chromite grain dissolution in supergene environments is driven by an hydrolysis process that produces Cr(OH) 3 [START_REF] Cooper | Oxidation and toxicity of chromium in ultramafic soils in Zimbabwe[END_REF]) via:

FeCr 2 O 4 + 2H 2 CO 3 + 2H 2 O = Fe 2+ aq + 2Cr(OH) 3 + 2HCO 3 - (3)
The production of Cr(OH) 3 requires moist, slightly reduced conditions to keep Fe(II) ions in solution, such as those prevailing in the fine saprolite, which is characterized by an unconnected fine porosity and poor drainage. The Cr(OH) 3 produced can be adsorbed to the surface of the crystal structure of iron and/or manganese oxyhydroxides [START_REF] Mckenzie | Manganese oxides and hydroxides[END_REF][START_REF] Manceau | X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface: I. Molecular mechanism of Cr(III) oxidation on Mn oxides[END_REF][START_REF] Fendorf | Surface reactions of chromium in soils and waters[END_REF].

The dissolution reaction of Cr(OH) 3 to produce soluble Cr(III) ions is represented by : Cr(OH) 3 + 3H + = Cr 3+ aq + 3H 2 O ( 4 )

This reaction is possible only under strongly acidic conditions with pH values lower than 4 [START_REF] Fendorf | Surface reactions of chromium in soils and waters[END_REF][START_REF] Cooper | Oxidation and toxicity of chromium in ultramafic soils in Zimbabwe[END_REF]. Such conditions prevail at the top of lateritic profiles, which is often rich in organic matter.

The poorly-aerated chromite-rich ferruginous aggregates can also undergo oxidationreduction conditions that favor the hydrolysis of Cr(III) into Cr(OH) 3 [START_REF] Cooper | Oxidation and toxicity of chromium in ultramafic soils in Zimbabwe[END_REF]. The dissolution of chromite ultimately leads to the release of PGM initially included in chromite crystals.

Residual origin of platinum

Decoupling of platinum group elements, Pt and Pd, and variation of the ratio Pt/Pd, are characteristic of the behavior of the PGE in supergene environments [START_REF] Prichard | Evidence for differential mobility of platinum-group elements in the secondary environnement in Shetland ophiolite complex[END_REF]Evans et al. 1994;[START_REF] Oberthür | Geochemistry and mineralogy of platinumgroup elements at Hartley Platinum Mine, Zimbabwe[END_REF]. Between the bedrock and the upper part of the fine saprolite, the Pt/Pd ratio is as high as the chromium is depleted (Fig. 7 and Tables 1 and2).

The increase of Pt/Pd in the weathering profile reflects the preferential release of palladium with regard to platinum [START_REF] Varajao | Early Weathering of palladium gold under lateritic conditions, Maquine Mine, Minas Gerais: Brazil[END_REF].

Figure and Table Captions

Figure 1. (a) Geological map and location of studied area (after [START_REF] Augé | Stratiform and alluvial platinum mineralization in the New Caledonia ophiolite complex[END_REF].

The black star shows the location of the studied weathering profile (Fig. 2). (b) Inset of the main Island of New Caledonia with its ultramafic massifs and location of (a) by the black rectangle.

Figure 2. Weathering profile log from a crosscut into a natural gully ("lavaka") and the location of analyzed samples (white squares), the numbers of which being in italics.

  Figure 7 also suggests that PGM could be released concomitant with chromite dissolution and also, that the supergene dissolution of Ptbearing chromite and/or newly released PGM lead to a preferential depletion of Pd with regard to Pt. The specific behavior of PGM in the lateritic weathering profile is addressed separately (Traore et al. 2007). CONCLUSION Supergene dissolution of platinum-bearing chromite has been characterized in a lateritic weathering profile from New Caledonia. Based on mass balance calculations, the results document the spatial redistribution of chemical elements during chemical weathering processes. Silicon, Mg and Ca are depleted entirely from the weathering profile resulting from the chemical weathering of olivine, pyroxene and serpentine. The residual Fe forms iron hydroxides, often crystallized into goethite, which can eventually trap other transition metals. Manganese, Co and Ni form poorly crystallized secondary minerals such as Mn-asbolanes and/or co-precipitate with iron hydroxides. The Cr-bearing silicates are dissolved and Cr is leached out of the profile. Chromite dissolution induces Cr depletion and Al removal. This investigation illustrates the decoupling of PGE and Cr content during the lateritic weathering of Pt-bearing chromite and suggests that the platinum mineralization is globally residual. The chromite dissolution may effectively contribute to the release of PGM in the profile with a preferential depletion of palladium with regard to platinum.

  Figure 3. (a) Bulk density, (b) Grain density and (c) Porosity variation along the profile.

Figure 6 .

 6 Figure 6. S.E.M. photomicrographs of the progressive weathering of chromite (a) euhedral

Figure 7 .

 7 Figure 7. Comparison between the total mass transfer of Cr and Pt/Pd variation in the

TABLE 1 .

 1 Physical and chemical characteristics of the parent rock, and the different layers of the weathering profile ( w = bulk density; g = grain density). TABLE2. Results of (a) the mass balance factor (K) and (b) the total mass transfer (m) of chemical elements calculated from equation (1) and (2), respectively.

Traore et al., 2007

TABLE 1

 1 Traore et al., 2007 .37 44.37 68.81 73.16 76.30 72.62 72.17 74.86 74.52 72.78 65.22 53.70 53.12

		Uncertainty	Bedrock	Coarse saprolite			Fine saprolite			Mottled zone	Nodular horizon
	Sample n°		PG34 PG35 PG32 PG31 PG36 PG37 PG38 PG39 PG41 PG42 PG43 PG44 PW12 PW13
	Depth (cm)		-470	-420	-380	-300	-270	-240	-210	-180	-140	-110	-95	-70	-25	0
	ρ w (g.cm -3 )	± 0.1	2.38	2.36	1.68	0.97	0.91	0.91	0.95	0.96	0.93	0.93	0.98	1.28	1.69	1.73
	ρ g (g.cm -3 )	± 0.1	3.06	3.04	3.02	3.11	3.39	3.84	3.47	3.45	3.70	3.65	3.60	3.68	3.65	3.69
	Porosity (%) 22.22 22SiO 2 (wt%) ± 0.01 39.48 39.81 34.65 21.27	6.00	4.08	3.32	2.99	2.20	2.07	2.09	1.88	2.10	1.90
	Al 2 O 3 (wt%)	± 0.01	0.68	0.58	0.56	0.97	0.67	1.09	1.02	0.75	1.17	0.80	2.43	0.95	4.30	4.20
	Fe 2 O 3 (wt%)	± 0.01	19.89 18.86 33.78 56.79 75.11 76.14 77.46 77.33 78.36 78.49 75.82 78.01 76.59 76.59
	CaO (wt%)	± 0.01	0.34	0.36	0.14	0.14	0.07	0.02	0.05	0.05	0.03	0.05	0.02	0.03	0.01	0.02
	MgO (wt%)	± 0.01	34.36 35.19 22.97	7.17	1.47	1.26	0.67	0.60	0.65	0.49	0.42	0.38	0.27	0.21
	Cr 2 O 3 (wt%)	± 0.01	1.54	1.45

Traore et al., 2007

ACKNOWLEDGEMENTS

This work is a contribution IRD-UMR161-CEREGE. Dr. P. Maurizot is thanked for introducing us to the study area. Michel Cathelineau, Gaëlle Bouichet, Marc Brouand and Philippe Larqué are gratefully acknowledged for their help during mineralogical investigations. J. Butscher, D. Chardon, V. Chevillotte, V. Combier, A. Paugam, J. Perrier, N.

Perrier and B. Robineau are also thanked for their help during fieldwork. Dr. Giorgio Garuti, Dr. Anthony Velbel and an anonymous reviewer are thanked for their helpful review of the manuscript. et al., 2007

Traore


Daouda Traoré

Anicet Beauvais
email: anicet.beauvais@noumea.ird.nc

Thierry Augé

Jean Claude Parisot

Fabrice Colin

Michel Cathelineau

Revision 1 (including editorial corrections) Chemical and physical transfers in an ultramafic rock weathering profile: 2. Dissolution vs. accumulation of platinum group minerals

Keywords: Platinum group minerals, platinum group elements, lateritic weathering, ultramafic rocks, New Caledonia

The chemical weathering of ultramafic rocks has resulted in eluvial concentration of platinum-group minerals (PGM) in lateritic weathering profiles of southern New Caledonia. The Pt mineralization interpreted as being primary consists of platinum-group minerals included in chromite crystals. The occurrence of PGM as free particles in the weathering profile results from the supergene dissolution of Pt-bearing chromite (Traore et al. 2007). Following their release in the profile, supergene dissolution processes variably affect the PGM particles. The behavior of platinum-group elements in the weathering profile is characterized by significant loss of Pd and relative accumulation of Pt indicating that Pd is more mobile than Pt in the exogenous cycle. Unstable Pt-Fe-Cu-Pd alloys and PGE oxides undergo chemical and mineralogical changes to acquire the chemical configuration of the isoferroplatinum (Pt 3 Fe), which is the most stable Pt-phase in a lateritic environment. The isoferroplatinum phase may also be dispersed throughout the weathering mantle and/or accumulated in the lower parts of profiles according to a translocation mechanism of residual Pt-rich fine particles driven by percolation of water through the connected pore spaces.

INTRODUCTION

Lateritic weathering processes affect numerous regions of the intertropical zone [START_REF] Pedro | Distribution des principaux type d'altération chimique à la surface du globe. Présentation d'une esquisse géographique[END_REF][START_REF] Nahon | Evolution of iron crusts in tropical landscapes[END_REF][START_REF] Tardy | Geochemistry and evolution of lateritic landscapes[END_REF], where long term chemical weathering has led to the supergene accumulation of metals in profiles of 10 to 100 meters in thickness, and among them, precious metals such as Au and Pt. Previous studies concerning the behavior of Au and Pt have proven useful for tracing the geochemical evolution of lateritic weathering covers within which they are concentrated [START_REF] Colin | Quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment[END_REF][START_REF] Colin | Gold: a tracer of the dynamics of tropical laterites[END_REF][START_REF] Varajao | Early Weathering of palladium gold under lateritic conditions, Maquine Mine, Minas Gerais: Brazil[END_REF].

Although hypotheses about the chemical mobilization of platinum-group elements (PGE) under supergene conditions are not new (Wagner 1929), the platinum-group minerals (PGM) are generally considered as resistant phases during weathering [START_REF] Cabri | Zoning in Os-Ir alloys and the relation of the geological and tectonic environment of the source rocks to the bulk Pt: Pt+Ir+Os ratio for placers[END_REF][START_REF] Hattori | Origin of platinum-group-mineral nuggets inferred from an Osmium-Isotope study[END_REF]. Much controversy remains, however, about the mobility of PGE, i.e., the geochemical and physical processes controlling their redistribution as PGM partial dissolution versus neo-formation processes in the surfaceweathering environment. For example, [START_REF] Augustithis | Mineralogical and geochemical studies of the platiniferrous dunitebirbirite-pyroxénite complex of Yubdo, Birnir, West Ethiopia[END_REF] hypothesized the supergene formation of PGM in lateritic weathering materials from ultramafic rocks of Ethiopia.

Two years later, [START_REF] Otteman | geochemistry and origin of "platinum-nugget" in lateritic covers from ultrabasic rocks and birbirites of West Ethiopia[END_REF] suggested that PGM formed during hydrothermal serpentinization. Over the last decades, numerous field and geochemical studies have shown the solubility of PGE in supergene environments [START_REF] Bowles | The development of platinum-group minerals in laterites[END_REF]Middleworth and Wood 1999;[START_REF] Varajao | Early Weathering of palladium gold under lateritic conditions, Maquine Mine, Minas Gerais: Brazil[END_REF][START_REF] Azaroual | Solubility of platinum in aqueous solutions at 25°C and pHs 4 to 10 under oxidizing conditions[END_REF][START_REF] Oberthür | Geochemistry and mineralogy of platinumgroup elements at Hartley Platinum Mine, Zimbabwe[END_REF]. We investigate the geochemical behavior of the PGE and the physical-chemical processes controlling the PGM mineralogy under the influence of supergene conditions in a lateritic weathering profile from New Caledonia to characterize the dissolution and/or the accumulation of those minerals.

Previous studies at this site have shown that a lateritic mantle has developed at the expense of the primary mineralization and that it is systematically characterized by higher Pt and Pd contents than the bedrock (Augé et al. 1995). The most common PGM identified in the secondary mineralization are the Pt minerals, mainly isoferroplatinum, Pt 3 Fe, which represents about 60 percent of PGM [START_REF] Augé | Platinum-group element oxides from the Pirogues ophiolitic mineralization, New Caledonia: Origin and Significance[END_REF]. The other major alloys identified are tulameenite (Pt 2 FeCu) and tetraferroplatinum (PtFe). Accessory cooperite and Rh-Ir-Pt-Fe oxides were also found [START_REF] Augé | Platinum-group element oxides from the Pirogues ophiolitic mineralization, New Caledonia: Origin and Significance[END_REF]. Previous studies at other sites have shown that PGM were, for the most part, included in chromite crystals (Augé and Maurizot 1995). A detailed mineralogical and micro chemical study of the PGM present in the weathering profile may be useful to characterize the physical and geochemical processes underlying the evolution of this profile.

GEOLOGICAL AND GEOMORPHOLOGICAL SETTING

The Pirogues River dunite-gabbro unit of the New Caledonian Southern Massif (Fig. 1) exhibits stratiform chromite-bearing rocks containing PGE at the base of a cumulate series. A lateritic weathering mantle is developed at the expense of the platinum mineralization under the influence of a warm and humid climate, which is characterized by a mean annual rainfall of 1700 mm with a wet season occurring from December to August, and a mean annual temperature of 22°C [START_REF] Jaffré | Végétation des roches ultrabasiques en Nouvelle-Calédonie[END_REF].

The Pirogues River PGE mineralization (Fig. 1) is specifically related to chromitite in pyroxenite dikes crosscutting the basal ultramafic cumulate series, and in rare cases in decimetric scale chromite schlierens in the host wehrlite (Augé and Maurizot 1995).

Chromitite occurs in the dikes as many local decimeter to meter scale segregations. Chromitefree facies are not mineralized in PGE. Platinum-group elements (Pt dominant) take the form of PGM that are, for the most part, included in chromite crystals (Augé and Maurizot 1995).

The parent rock underwent strong lateritic weathering processes that has led to the dissolution of chromite, and thus, to the release of platinum-group minerals in the weathering mantle (Traore et al. 2007). Previous studies have also shown a contrast in behavior between the PGE and chromite in the lateritic mineralization (Augé et al. 1995).

Natural weathering profiles exposed in deep gullies incising hillslopes. One of these profiles (also locally called "lavakas") has been described and sampled in situ (Fig. 2a) for a thorough study of the PGM released in the weathering matrices. A companion paper addresses the mineralogical and geochemical nature of the profile (Traore et al. 2007). The weathering profile is developed from a wehrlite type ultramafic rock, and its thickness (to 4 m) has been divided into four main layers (Fig. 2a). At the base of the profile, the greenish-black parent rock changes to the brownish coarse saprolite that preserves the structure and the texture of bedrock. The mineralogical characterization indicates olivine, antigorite, diopside and goethite, with accessory chromite and enstatite. The coarse saprolite is progressively transformed into a fine saprolite, in which the texture and the structure of the parent rock are still preserved. The chromite grains are progressively dissolved that results in the depletion of chromium and the release of PGM (Traore et al. 2007). At the top of the fine saprolite, the silicates are completely weathered into goethite.

Toward the top of the profile, the fine saprolite changes to reddish laterites in which the original rock structures are no longer preserved. The mineral assemblage is dominated by goethite, with accessory hematite and chromite.

MATERIAL AND METHODS

Fourteen samples of parent rock, laterite and soil were crushed and finely ground.

According to the PGM-MS23 analytical procedure developed by the Australian Laboratory Services (ALS, Chemex), each prepared sample was fused with a mixture of lead oxide, sodium carbonate, borax and silica, inquarted with 6 mg of gold-free silver and then cupelled to yield a precious metal bead. The bead was digested for 2 minutes at high power by microwave in dilute nitric acid. The solution was cooled and hydrochloric acid was added.

The solution was digested for an additional 2 minutes at half power by microwave. The digested solution was then cooled, diluted to 4 ml with 2 % hydrochloric acid, homogenized and then analyzed for platinum and palladium by inductively coupled plasma -mass spectrometry (ICP-MS). Detection limits for Pd and Pt are 1 and 0.5 ppb, respectively.

Five hundred particles of PGM were extracted from three samples (1 sample = 50 kg) in different parts of the profiles (fine saprolite, mottled zone and nodular layer), before being examined by scanning electron microscopy (SEM) for morphological analysis, and by electron microprobe for micro-chemical analysis.

The samples were damped in a 20-liter drum, and then put in a concrete mixer to disaggregate the non-indurated nodules. The remaining non-indurated nodules were manually crushed and the resulting mix was sifted to 1 mm for eliminating fragments of rock and ferricrete prior to sorting with an automatic pan to separate out the heaviest particles. For each sample, heavy-mineral concentrates were separated into three grain-size fractions (< 65 μm, 65-200 μm and > 200 μm). Platinum-group mineral grains were isolated from the other heavy minerals. Because the PGM size ranges between 10 and 40 μm [START_REF] Augé | Platinum-group element oxides from the Pirogues ophiolitic mineralization, New Caledonia: Origin and Significance[END_REF]Augé and Maurizot 1995), the particle morphologies of the smaller grain-size fractions (< 65 μm) were directly examined using SEM. The fraction between 65 and 200 μm was separated with a hand magnet into ferromagnetic and para-and non-magnetic fractions. The collected PGM grains were mounted on aluminum stubs for SEM morphoscopic examination and EDS semi-quantitative analysis.

Micro-chemical analyses of the PGM were made using a Cameca SX 50 electron microprobe under multiple analytical conditions in a single run. The analytical procedure was designed for routine analysis of PGM [START_REF] Augé | Pt-Fe nuggets from alluvial deposits in eastern Madagascar[END_REF] including the quantitative analysis of oxygen. The analytical conditions (except for O) included an acceleration voltage of 25 keV, a beam current of 20 nA, and counting time of 6 s. The standards used are Cr 2 O 3 for Cr, AsGa for As, pyrite for Fe and S, stibnite for Sb, and pure metals for all other elements. The X-ray lines used were K for Ni, Cu, Cr and Mn, K for Fe, L for Rh, Ir, Pt, and Ru, and L for Os, Pd, and As. Oxygen was analyzed using the K line with an acceleration voltage of 10 kV. The change of acceleration voltage was made automatically during the analytical cycle. The Cameca PAP correction program was used.

RESULTS AND DISCUSSION

Chemical and physical characterization of PGE and PGM PGE bulk chemistry. Figure 2 shows the variation of Pt and Pd contents and of the Pt/Pd ratio along the profile. Palladium concentration significantly increases in the upper fine saprolite (Fig. 2b), while Pt reaches a maximum of 647 μg.kg -1 in the coarse saprolite, with a mean value of 512 μg.kg -1 in the fine saprolite and decreases to 240 μg.kg -1 in the mottled zone and to 196 μg.kg -1 in the soft nodular layer (Fig. 2c). The Pt/Pd ratio increases from 14 in the fresh wehrlite to 29 in the coarse saprolite, and decreases to 14 in the fine saprolite and to 10 in the mottled zone and soft nodular layer (Fig. 2d).

Although the absolute concentrations of Pt and Pd may not be directly comparable between the weathered profile and the parent rock, the increase of Pt/Pd ratio from the fresh rock to the saprolite and to the mottled zone (soft nodular layer being mostly a material transported mechanically on the hillslope) suggests that Pd is more mobile than Pt and dispersed chemically in the supergene environment [START_REF] Fuchs | The geochemical behavior of platinum and palladium in the weathering cycle in the Sillwater complex[END_REF][START_REF] Evans | Dispersion of platinum, palladium and gold from the Main Sulphide Zone, Great Dike, Zimbabwe[END_REF][START_REF] Prichard | Evidence for differential mobility of platinum-group elements in the secondary environnement in Shetland ophiolite complex[END_REF][START_REF] Oberthür | Geochemistry and mineralogy of platinumgroup elements at Hartley Platinum Mine, Zimbabwe[END_REF].

PGM distribution and micro morphology. Platinum-group metal distribution in the weathering profile is heterogeneous. Ninety five percent of the collected particles are in the fine saprolite, 4% in the mottled zone and only 1% in the nodular layer. Although the residual PGM particles present in the lateritic weathering profiles have various shapes and surface aspects, the examination of PGM under electronic microscope allows for differentiation of three main morphological groups (Fig. 3). Euhedral PGM particles, with smooth surfaces and perfect crystal faces (Fig. 3a) are the least abundant and they are only found in the fine saprolite and the mottled zone. Rounded PGM particles are found in the fine saprolite and the mottled zone. Their crystal edges are smooth and their surfaces exhibit micrometric cracks and/or etching pits (Fig. 3b). The roughest PGM particles (Fig. 3c) were collected in the soft nodular layer and are strongly weathered. They have also been found in the fine saprolite and the mottled zone.. The size of etching pits on those particles increases from the lower to the upper horizons of the profile.

Physical vs. chemical weathering signatures of PGM micromorphology.

The heterogeneous PGM distribution in the weathering profile and the significant concentration of round and rough PGM particles in the fine saprolite horizon can be explained by mechanical transfer and accumulation similar to the vertical translocation of small gold particles through relatively large porosity and higher groundwater hydrodynamic conditions [START_REF] Michel | Concentration of gold in in situ laterites from Mato Grosso[END_REF][START_REF] Butt | A basis for geochemical exploration models for tropical terrains[END_REF][START_REF] Colin | Quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment[END_REF][START_REF] Hanlie | Behaviour of gold in the weathered mantle at Shewushan, Hubei, China[END_REF].

The size and shape of the cavities and/or etching pits in the PGM particles suggest that they result from a dissolution process. Deep in the saprolite, the pits are round and small; their sizes gradually increased upward in the weathering profile.

An additional change in PGM micromorphology consists of increasing rounding upward, from the saprolite to the soil surface. The rounding is most obvious in the mottled zone (Fig. 3b) and soft nodular layer (Fig. 3c). The absence of striations and impact marks precludes smoothing by a transport mechanism. The roundness is therefore attributed entirely to a dissolution process.

The large cracks affecting the grains of PGM are interpreted to be a result of a change in volume due to chemical leaching in the oxidizing environment [START_REF] Augé | Platinum-group element oxides from the Pirogues ophiolitic mineralization, New Caledonia: Origin and Significance[END_REF][START_REF] Oberthür | Geochemistry and mineralogy of platinumgroup elements at Hartley Platinum Mine, Zimbabwe[END_REF]. Our observations support this interpretation but the composite grains exhibit differential weathering patterns according to the PGM types. The Pt-Fe-(Cu-Pd) alloys and PGE oxides are effectively more weathered than isoferroplatinum in the lateritic weathering profiles as shown by microchemical analyses and SEM photographs (Figs. 4 and5).

PGM micro chemical analysis. Platinum-group elements of the Pirogues River lateritic mineralization zones are associated with platinum-iron alloys and oxides. A nomenclature of the PGM is proposed, based on their chemical composition and previous works [START_REF] Augé | Platinum-group element oxides from the Pirogues ophiolitic mineralization, New Caledonia: Origin and Significance[END_REF]Augé and Maurizot 1995). Three species of PGM recognized are isoferroplatinum (Pt 3 Fe) that represents the majority of the PGM studied, and undetermined Pt-Fe-(Cu-Pd) alloys and PGE-oxides. See Tables 1 and2 in the American Mineralogist Online Background Dataset for the microchemical composition of the different species of PGM.

The microchemical composition of isoferroplatinum is shown in Figure 4. Its Pt content varies between 58 and 69 at.%; Fe content ranges between 21 and 26 at.%; and Pd varies only from 0.2 to 2.7 at.%. While Cu does not enter in significant amounts into the isoferroplatinum composition, its content ranges from 0.2 to 4 at.%. The isoferroplatinum generally occurs as euhedral or subhedral grains (Fig. 5a), which are chemically homogeneous and form free particles, or is associated with other PGM (Fig. 5b and5c). They may also constitute weathered rims around porous and rough particles (Fig. 6d).

The undetermined Pt-Fe-(Cu-Pd) alloys form euhedral and subhedral grains, and are sometimes characterized by an internal structure with concentric zoning (Fig. 6a). Another morphological type of the Pt-Fe-(Cu) system is a symplectic intergrowth with a phase that has not been preserved (Fig. 6b and6c). The grains are isolated or associated with isoferroplatinum and/or PGM oxides in composites particles (Fig. 5b). Their reflectance is lower than that of the isoferroplatinum. In many cases, the Pt-Fe-(Cu) grains present large cracks and/or weathered rims.

Microprobe analyses of Pt-Fe-(Cu) particles indicate a relatively wide range of compositions (Fig. 4). Platinum contents vary from 38 to 50 at.%; Fe from 20 to 28 wt.%; Pd from 1 to 12 at.%; and Cu from 12 to 26 at.%.

The existence of PGE-oxides has been widely established [START_REF] Legendre | Présence d'un oxyde d'iridium naturel dans les concentrations platinifères de la région d'Antambao-Manampotsy, Madagascar[END_REF][START_REF] Prichard | Evidence for differential mobility of platinum-group elements in the secondary environnement in Shetland ophiolite complex[END_REF][START_REF] Augé | Platinum-group element oxides from the Pirogues ophiolitic mineralization, New Caledonia: Origin and Significance[END_REF]Augé and Maurizot 1995;[START_REF] Jedwab | Oxygenated platinum-group-element and transition-metal (Ti, Cr, Mn, Fe, Co, Ni) compounds in the supergene domain[END_REF][START_REF] Salpeteur | Pt and Pd mobility in ferralitic soils of the west Andramana area (Madagascar). Evidence of a supergene origin of some Pt and Pd minerals[END_REF]Garuti et al. 1997;[START_REF] Moreno | Formation of a secondary platinum-group mineral assemblage in chromitites from the Herbeira ultramafic massif in Cabo Ortegal, NW Spain[END_REF][START_REF] Hey | The effects of weathering on the UG2 Chromitite reef of the Bushveld complex, with special reference to the platinum-group minerals[END_REF][START_REF] Oberthür | Geochemistry and mineralogy of platinumgroup elements at Hartley Platinum Mine, Zimbabwe[END_REF].

Platinum-group elements oxides are exceptionally abundant in the Pirogues River area where they form subhedral or rounded grains, which appear chemically heterogeneous (Fig. 7 and8). The oxidized compounds of PGE are associated with the other PGM in composites particles, or form free particles in the lateritic weathering profiles. Their reflectance is significantly lower than that of isoferroplatinum, and slightly lower than that of Pt-Fe-(Cu-Pd) alloys as shown in Figure 5b. PGE oxide particles are often cracked and/or present weathered rims (Fig. 5c and6d).

Based on microprobe analyses, two compositional types of PGE oxides can be distinguished:

(1) Pt-Fe (Cu-Pd) oxides with Pt contents varying between 20 at.% and 46 at.%; Fe varies from 16 at.% to 30 at.%; Pd from 0 to 7 at.%; and Cu varies from 0 to 10 at.%. Representative analyses of the grains including oxygen are given in Figure 7. Oxygen contents indicate a relatively wide compositional range from 16 to 61 at.%, but with a relatively constant atomic ratio of (Pt+Pd)/(Fe+Cu).

(2) Rh-Ir-Pt-Fe oxides constitute small grains associated with Pt-Fe alloys and/or other PGE oxides (Fig. 5b, and5c). Their main chemical components are Rh (12 to 36 at.%); Pt (0 to 18 at.%); Ir (0 to 3 at.%); Fe (11 to 53 at.%); Pd (0 to 9 at.%). The oxygen content varies from 14 to 51 at. % (Fig. 8).

Detailed electron microprobe profiles have been performed on specific PGE oxides grains.

The porous core of grain PGB7_4h shows the presence of S (up to 12 at.%) (Fig. 9). The rest of the grains show significant amount of Si and Mg with 0.2 to 4 at.% and traces of Pd.

The frequent association of a PGE alloy and a PGE oxide suggests that the oxide does not derive from the Pt-Fe alloy. The systematic amount of S in the oxide (Fig. 9), though minor, suggests that the oxide could have been derived from a sulfide. A serpentinization process could have led to the desulfurization of primary PGE sulfides and contributed to the formation of an unstable oxidized phase [START_REF] Stockman | Platinum-group minerals in Alpine chromitites from Southwestern Oregon[END_REF]Garuti and Zaccarini 1997). It is very likely that oxidation occurs at an early stage, as oxides are abundant in weakly weathered chromitite.

Selective dissolution of PGM

The rough PGM grains exhibit weathering rims developed around a primary mineral. This is particularly evident in PGE oxides as well as for the undetermined Pt-Fe-(Cu-Pd) alloys, which can exhibit a weathered cortex that appears silver-white and porous on the SEM and backscattered electron imaging (Fig. 10). Microprobe analyses from the core to the weathered edge indicate a significant compositional change. The weathered rim has lower Cu content, which varies on average from 18.51 at. % in the core to 6.98 at. % in the weathered zone. Iron decreases on average from 23.32 at. % for the parent PGM to 19.96 at. % for the weathered external part. Platinum is relatively enriched and increases from 47.7 at. % to 68.18 at. % from the core to the weathered edges of the grains. Palladium varies from 0.88 at. % in the core to 0.3 at. % in the weathered rim (Fig. 10).

Isoferroplatinum accumulation

The Pt-Fe-(Cu-Pd) alloys and PGE oxides affected by the weathering process develop weathering rims, which are depleted in Cu, Pd and Fe, and relatively enriched in Pt. These Ptminerals are thus impoverished in Cu, Pd and Fe and tend to acquire the chemical composition of isoferroplatinum (Pt 3 Fe), which is a more stable mineralogical phase in the supergene environment. The weathered particles have generally preserved both the structure and the chemical signature (in the core of the grains) of the fresh PGM particles (Fig. 10). This clearly demonstrates that the PGM particles are residual but are affected by dissolution.

The evidence of a supergene accumulation of isoferroplatinum is not new. Several field studies have reported the formation of Pt-Fe alloys of isoferroplatinum type from pre-existing unstable PGM under low-temperature conditions [START_REF] Cousins | Some observations on textures and inclusions[END_REF][START_REF] Evans | Dispersion of platinum, palladium and gold from the Main Sulphide Zone, Great Dike, Zimbabwe[END_REF][START_REF] Salpeteur | Pt and Pd mobility in ferralitic soils of the west Andramana area (Madagascar). Evidence of a supergene origin of some Pt and Pd minerals[END_REF][START_REF] Oberthür | Geochemistry and mineralogy of platinumgroup elements at Hartley Platinum Mine, Zimbabwe[END_REF]. Our observations indicate also that mineralogical changes occur in the supergene environment by the transformation of unstable PGM to porous and friable grains enriched in platinum as isoferroplatinum small particles that can be mechanically redistributed in the lateritic weathering profiles. Our results further indicate that Pt is dispersed mainly in particulate form by mechanical processes, whereas Pd is released mainly in solution (Wood and Vlassopoulos 1990;[START_REF] Cook | Platinum distribution in soil profiles of the Tulameen ultramafic complex, southern British Columbia[END_REF][START_REF] Prichard | Evidence for differential mobility of platinum-group elements in the secondary environnement in Shetland ophiolite complex[END_REF][START_REF] Evans | Dispersion of platinum, palladium and gold from the Main Sulphide Zone, Great Dike, Zimbabwe[END_REF][START_REF] Oberthür | Geochemistry and mineralogy of platinumgroup elements at Hartley Platinum Mine, Zimbabwe[END_REF].

CONCLUSION

An eluvial concentration of platinum-group minerals was studied in a lateritic weathering profile, which developed at the expense of ultramafic rocks in southern New Caledonia. The PGE in the weathering profile take the form of PGM, which are mostly concentrated in the fine saprolite horizon. Our study confirms the abundance of PGE oxides in the Pirogues mineralized system and shows that they can derive from desulfurization of pre-existing PGE sulfides. Our results demonstrate that the PGE alloys of the Pt-Fe-(Cu) system and PGE oxides are weathered in a supergene environment. These PGM undergo selective partial dissolution according to a chemical eluviation gradient (S > Cu > Pd > Fe > Pt) that leads to the formation of richer Pt-phases such as Pt 3 Fe. The resulting isoferroplatinum grains are relatively stable regarding the supergene dissolution processes and Traore et al., 2007 13 mostly transferred mechanically through the connected porosity lower in the weathering profiles, where they accumulate in a cementation zone. Wagner, P.A. (1929) The platinum deposits and mines of South Africa, 326 p. Oliver and Boyd, Edinburgh. Wood, S.A., and Vlassopoulos, D. (1990) The Dispersion of Pt, Pd, and Au in surficial media about two PGE-Cu-Ni prospects in Quebec. Canadian Mineralogist, 28, 649-663.

Figures Caption

Figure 1. (a) geological map and location of studied area (after Augé and Maurizot, 1995).

The black star shows the location of the studied weathering profile (Fig. 2). (b) Inset of the main Island of New Caledonia with its ultramafic massifs and location of (a) by the black rectangle. Figure 2. (a) Description of the weathered profile from a crosscut in a natural gully ("lavaka") and the location of studied samples (white squares). Vertical variations of contents of (b) Pd, (c) Pt and of (d) ratio Pt/Pd along that profile are illustrated. Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 TABLE 2. Traore et al., 2007

Figure 3 .

 3 Figure 3. SEM photomicrographs of PGM grains with (a) euhedral, (b) smooth and (c) rough

Figure 4 .

 4 Figure 4. Compositional variations in the Pt-Fe-Cu system of isoferroplatinum (black circles),

Figure 5 .

 5 Figure 5. SEM photomicrographs of (a) automorphous isoferroplatinum, (b) composite PGM

Figure 7 .

 7 Figure 7. Compositional variation of Pt-Fe(Cu-Pd) oxides. The analyses are plotted as atomic

Figure 8 .

 8 Figure 8. Compositional variation of Rh-Ir-Pt-Fe oxides shown in Figures 5b and 5c. The

Figure 9 .

 9 Figure9. Compositional variation through (a) a zoned particle with a porous core. The

Table captions (for depository)TABLE 1 .

 captions1 Microchemical composition of isoferroplatinum, tetraferroplatinum and tulameenite.

TABLE 2 .

 2 Microchemical composition of platinum oxides.

TABLE 1

 1 Traore et al., 2007 .00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

	Isoferroplatinum Pt 3 Fe

ACKNOWLEDGEMENTS

This work is a contribution IRD-UMR161-CEREGE. Dr. P. Maurizot is thanked for introducing us to the study area. J. Butscher, D. Chardon, V. Chevillotte, V. Combier, A. Paugam, J. Perrier, N. Perrier and B. Robineau are thanked for their help during fieldwork.

Dr. Giorgio Garuti and Dr. Anthony Velbel are gratefully acknowledged for their stimulating reviews of the manuscript.


A M Lombardi

E Z Sargsyan

S Lanzone

J.-B Lallement

CERN G Bellodi

M Baylac

END-TO-END BEAM DYNAMICS FOR CERN LINAC4

LINAC 4 is a normal conducting H -linac which aims to intensify the proton flux available for the CERN accelerator complex. This injector is designed to accelerate a 65 mA beam of H -ions up to 160 MeV for injection into the CERN Proton Synchrotron Booster. The acceleration is done in three stages : up to 3 MeV with a Radio Frequency Quadrupole (the IPHI RFQ) operating at 352 MHz, then continued to 90 MeV with drift-tube structures at 352 MHz (conventional Alvarez and Cell Coupled Drift Tube Linac) and, finally with a Side Coupled Linac at 704 MHz. The accelerator is completed by a chopper line at 3 MeV and a transport and matching line to the PS booster. After the overall layout was determined based on general consideration of beam dynamics and RF, a global optimisation based on end-toend simulation has refined some design choices. The results and lessons learned from the end-to-end simulations are reported in this paper.

LINAC4 LAYOUT

The layout of LINAC4 is sketched in Figure 1. It consists of a RF volume source (identical to the one in DESY) which provides an H -beam at 35 kV further postaccelerated to 95 keV. The first RF acceleration (from 95 keV to 3 MeV) is done by a Radio Frequency Quadrupole (the IPHI RFQ from CEA [START_REF] Beauvais | Recent evolutions in the design of the french high intensity proton injector (IPHI)[END_REF]). The RFQ resonates at 352 MHz, is 6 m long and it is powered by a 1 MW Klystron. At 3 MeV the beam enters a 3.6 meter long chopper line, consisting of 11 quadrupoles, 3 bunchers and two sets of deflecting plates. This system has the capability of removing micro-bunches on the RF scale and rematch the beam to the subsequent system of accelerators. A rudimentary collimation is also performed in this line. The beam is then further accelerated to 40 MeV in a conventional Drift Tube Linac at 352 MHz. The DTL, subdivided in 3 tanks, is 13.4 meters long and it is powered by 5 klystrons for a total power of 4 MW. Each of the 82 drift tubes is equipped with a Permanet Magnet Quadrupole. At 40 MeV the velocity of the beam is such as to allow the transition to structures which don't follow cell-by-cell the beam velocity profile. In LINAC4 the acceleration from 40 to 90 MeV is provided by a Cell-Coupled Drift Tube Linac at 352 MHz. The CCDTL is made of 24 tanks of 3 cells each for a total length of 25.3 meters. Three tanks are powered by the same klystron, for a total of 8 klystrons delivering 6.5 MW. The focusing is provided by electromagnetic quadrupoles placed outside each tank. The acceleration from 90 to 160 MeV is done in a Side Coupled Linac resonating at 704 MHz. The SCL is made of 20 tanks of 11 cells each for a total of 28 m, powered by 4 klystrons delivering 12 MW. Focusing is provided by 20 Electromagnetic Quadrupoles.

This brings the total length of the linac to 80 m, for a total of 18 klystrons. The duty cycle of LINAC4 is 0.1% when used as injector to the PS booster but it grows to 3-4% if we consider its potential use as front-end of a high power proton driver like the SPL [START_REF] Gerigk | Conceptual design of the SPL II, a high-power superconducting H-linac at CERN[END_REF]. During the design phase we have decided to take as effective duty cycle the value of 15%: this value is used through the paper unless otherwise indicated.

END-TO-END SIMULATIONS

The H -current from the source is 80 mA, reduced to 65 mA in each micro-pulse after the chopper line and 40 mA average in the pulse after chopping. The micro-bunch current, 65 mA, is such that space-charge effects are dominating at low energy and therefore some beam degradation can be expected. In particular the unavoidable transition to a slow phase advance in the chopper line (1 FODO per 10 βλ) is the weakest point as far as emittance growth and halo development is concerned.

After an initial phase of optimization of each section standalone to produce the layout of the accelerator, a campaign of end-to-end simulations with the purpose of identifying bottlenecks, weak points, and acceptance limitations allowed a fine tuning of the layout. The codes PATH [START_REF] Perrin | [END_REF], TOUTATIS [START_REF] Duperrier | CEA Saclay codes review[END_REF] and TRACEWIN [START_REF] Duperrier | Toutatis, a radio-frequency quadrupole code[END_REF] have been made read/write compatible for the purpose of tracking beam from the low energy to the high energy end without regenerating a distribution at any point along the line.

RFQ

The beam dynamics in the IPHI RFQ has been extensively presented in [START_REF] Ferdinand | STATUS REPORT ON THE 5 MeV IPHI RFQ[END_REF]. The RFQ is capable of accelerating with an efficiency of more than 99% a beam of currents from 20 to 100 mA with limited emittance growth. Simulations show that, also for a perfectly matched beam, halo develops in the RFQ at the level of 10 -4 and that transverse emittance grows by 8% for a 70 mA beam. The emittance growth happens in the RFQ coupling gaps, placed every two meter along the structure. Figure 2 shows very clearly this effect.

The RFQ is a very good transmission channel and therefore it doesn't filter the halo (also coming from the source) which must be dealt with in the transfer line at 3 MeV.

Chopper Line

The chopper line houses a fast-switching electrostatic device able to remove 3/8 micro-bunches and a conicalshaped dump to dispose of the chopped micro-bunches. Both items are relatively bulky (the chopper is 800 mm long and the dump 120 mm) and therefore the focusing structure of the RFQ, one FODO period per βλ, i.e. 70 mm at 352 MHz and 3 MeV, must be interrupted. In order to keep the line compact and in order to break as little as possible the FODO structure, the chopper plates are mounted inside a quadrupole. In general all the elements are compacted to the maximum. The chopper plates are driven with an effective voltage of ±400 V for a total deflection of 5.4 mrad. This allows separating the chopped and un-chopped beam at the output of the chopper plates in phase space but not in real space. The choice of the appropriate phase advance between chopper and dump allows for a separation in physical space at the dump position. The quadrupole between the chopper and the dump plays a key role in the process. Moreover this configuration minimises the voltage required from the chopper driver as the beam divergence in the chopper is minimised at the expenses of beam size. Unfortunately this trick entails some losses on the chopper plate, which are limited to 4% of the incoming beam, i.e. 1.3 kW. The picture of the transverse phase space of the chopped and unchopped beam at the output of the dump is shown in Figure 3. Some 0.2% of the chopped beam is not stopped at the dump. This beam populates bunches that are supposed to be empty but it fits inside the transverse acceptance of the accelerator and it is therefore transmitted up to the high energy end. After the measurement campaign of 2008, which should confirm the presence of these unwanted particles, provision for eliminating them at the lowest possible energy should be studied [START_REF] Jb Lallement | Measurement strategy for the CERN Linac4 Chopper-line[END_REF].

After the dump, where the unwanted bunches have been disposed of, the beam is matched to the DTL focusing structure, which is a FFDD system in the first tank. Space-charge induced emittance growth is very severe when the beam is compressed back in volume to fit a fast phase advance focusing channel. In our case it amounts to almost 20 %.

DTL -CCDTL and SCL

After the operation of chopping, the beam is accelerated to 40 MeV in three DTL tanks equipped with Permanent Magnets. In the first tank the reference focusing scheme is FFDD whereas in the following tanks FODO is preferred. The reason for this choice is purely technical as it was not sure whereas the higher integrated gradient needed for a FODO at 3 MeV and 352 MHz was reliably achievable. The transition between the two focusing schemes is smooth and no emittance growth is observed provided the matching is done adequately.

The DTL is fully equipped with Permanent Magnet Quadrupoles, i.e. there isn't any possibility of adjustment at a later stage. We have verified that with the chosen At 40 MeV the beam is energetic enough to allow the transition to a structure which doesn't follow the beam velocity profile cell-by-cell. Acceleration to 90 MeV happens in a CCDTL composed of 3-gap tanklets. The average phase in each tanklet is -20 degrees and the focusing period is 3 βλ. At 90 MeV the structure employed is a SCL with 11 cells per tank. The variation of longitudinal phase advance due to the frequency jump at 90 MeV is controlled by adjusting the phase in the modules at the transition. The resulting phase advances are varying smoothly and do not give rise to emittance growth. Figure 4, Figure 5 and Figure 6 show the relevant parameters.

Towards the booster

At 160 MeV the beam phase space looks like in Figure 7. It presents some halo, which is very well transmitted throughout the machine and the energy spread must be reduced to match the acceptance of the PS booster. The extra complication in the design of this part arises from the difficulty of integrating the transfer line in the complex of CERN accelerators. Existing buildings and other accelerators make it necessary to have the line split in three sections with bending magnets in between. The most critical part from the beam dynamics point of view is the initial part, just after the SCL, where the beam has a big energy spread, and it is very compressed in phase. The combination of dispersion and space charge effects are difficult to handle without allowing for some emittance growth. The evolution of the emittance along the first part of the line is shown in Figure 8 : the effect of the dispersion, only partly compensated by the following bending is clearly visible.

Chopper-DTL-CCDTL-SCL

The IPHI-RFQ has been designed several years ago and it is now in the phase of manufacturing, therefore during the end-to-end simulation the output distribution of the RFQ has been taken as a fixed distribution as modification to the hardware were not possible anymore. In this section we present the results of the simulations from 3 MeV to 160 MeV.

The evolution of the rms emittance in the three planes as calculated with TRACEWIN and PATH are presented in Figure 9. The two codes have not been used, on purpose, in the same conditions: the space charge is computed with a 2D model in PATH and a 3D model in TRACEWIN. The maximum difference in the emittance is less than 10% and this gives us an idea of the accuracy of our calculations. When the two programs are run in the same conditions, the results differ by fraction of percent [START_REF] Lallement | End-toend simulations of LINAC4[END_REF]. Besides the code comparison considerations, another important information can be gathered from Figure 9 : the majority of the emittance growth happens in the chopper line (3.6 meters) and just few percent in the rest of the accelerator (70 meters). This situation was foreseen and it is unavoidable as explained beforehand. A quality factor of the solidity of the design is the ratio of the rms beam size to the radius of the vacuum chamber. In Figure 10 it is possible to see that the transverse bottleneck of LINAC4 is the chopper and the dump, where the aperture approaches the 2 rms beam size. Losses are localised in this area and the geometry of the chopper defines the minimum transverse acceptance of the whole LINAC. The equivalent acceptance in the longitudinal plane is difficult to define as the concept of "longitudinal losses" is less clearly defined than the transverse one. Nonetheless we have identified the bottlenecks or weak points in the longitudinal plane by comparing the phase and energy extension of the linearised bucket with the rms phase and energy extension of the beam. The expression used for the linearised bucket is the following: where φ s is the synchronous phase, β and γ the relativistic parameters, q the charge, m the mass, c the velocity of light , E o T the effective accelerating field and ω the RF frequency.

Figure 11 shows the ratio of the two quantities above to the r.m.s. phase and energy spread of the beam. In LINAC4 the bottleneck for energy acceptance is the DTL input whereas the bottleneck for phase acceptance is the SCL input where there is a frequency jump of a factor of two. In the design of LINAC4 we have not respected the continuity of longitudinal acceptance at the transition between 352 and 704 MHz but we don't see degradation of performance also in presence of machine errors (RF amplitude and phase) [START_REF] Baylac | Error study of LINAC 4 using transport code TraceWin[END_REF].

EMITTANCE BUDGET AND KNOWN BOTTLENECKS

Table 1 shows the rms normalised emittance and transmission along LINAC4. It is evident that most of the emittance growth and beam quality degradation happens before 3 MeV in the first stages of acceleration and during the chopping operations. The known causes of emittance growth are the energy spread from the source, the aberrations of the LEBT solenoids and the RFQ coupling gaps. The slow phase advance in the chopper line is another source of emittance growth as the beam grows 10 times its volume and it needs to be compressed back to be matched to the DTL. The emittance growth amounts to 75% from the source to 160 MeV, and it is acceptable for LINAC4 as injector to the booster as well as for its potential use as injector to a high power superconducting LINAC. Bottlenecks have been identified during the end-to-end simulation. The transverse emittance increase and transmission are defined in the chopper line and the only possible cure would be to have a higher chopper voltage. This in fact would allow for a larger distance between the plates and/or a shorter structure. The limit to the peak current per bunch (70 mA) comes from the first DTL cells. This bottleneck could be removed by adopting a FODO focusing system in the first tank.

The longitudinal acceptance is determined in the first cell of the DTL and the first cell of the SCL. This bottleneck could be removed if the LINAC4 were designed respecting the continuity of longitudinal acceptance, probably at the expenses of a longer machine. Longitudinal acceptance and current limit do not seem to be an issue, so these measures have not been implemented in the design.

LINAC4 VS. LINAC2

If LINAC4 is realised and put in operation as injector to the PS booster, it will substitute the present injector, LINAC2 [START_REF] Warner | Accelerating structure of the CERN new 50 MeV linac[END_REF].

Table 2 contains a comparison of the most important parameters of the two LINACs: the smaller emittance together with the higher energy and the possibility to charge-exchange at injection through a foil allow for a higher intensity and brilliance in the booster.

CONCLUSIONS

Linac4 is fit to inject into the CERN PS booster and improve its performance.

Linac4 is also ready to be the injector of a high power driver (4-5 MW) because the beam dynamics has been designed in view of mastering the losses at a higher energy; it is equipped with a chopper system at 3 MeV capable of removing micro-bunches at 352 MHz and all its components are designed for a 15% duty cycle.

Figure 1

 1 Figure 1 Schematic layout of LINAC4.

Figure 2

 2 Figure 2 Transverse and longitudinal emittance along the IPHI RFQ.

Figure 3

 3 Figure 3 Transverse phase space of the chopped and unchopped beam at the dump.

  settings,-optimised for 65 mA -currents in the range 20 mA to 70 mA could be accepted and that the electromagnetic quadrupoles in the chopper line could match the beam for the varying conditions.

Figure 4

 4 Figure 4 Full current phase advance per meter along the DTL-CCDTL-SCL.

Figure 5

 5 Figure 5 Longitudinal to transverse phase advance ratio along the DTL-CCDTL-SCL. Zero current (top) and full current (bottom).

Figure 6

 6 Figure 6 Tune depressions in the three planes along the DTL-CCDTL-SCL.

Figure 7

 7 Figure 7 Beam phase space at the exit of the SCL: transverse planes (top) and longitudinal plane (bottom).

Figure 8

 8 Figure 8 Transverse emittance along the 160 MeV transfer line.

Figure 9

 9 Figure 9 Rms emittance from 3 to 160 MeV. From top to bottom: x-xp; y-yp and longitudinal.

Figure 10

 10 Figure 10 Aperture over the rms size along the chopper-DTL-CCDTL-SCL.

Figure 11

 11 Figure 11 Ratio of the linearised bucket size to the rms phase and energy spread of the beam.

Table 1

 1

		Emittance along LINAC4
		95 kev	3 MeV	160MeV
		(RFQ in)	(DTL in)	(SCL out)
	Transverse	0.25	0.34	0.35
	emittance
	(rms mm mrad)
	Longitudinal	0.13 (shaper)	0.17	0.18
	emittance
	(rms deg MeV)
	Transmission		90%	90%
	Current limit	20-70	20-70	20-70
	mA

Table 2

 2

	comparison of LINAC2 and LINAC4
		LINAC2	LINAC4
	Particle	protons	H -
	Energy	50 MeV	160 MeV
	Current	160 mA	70 mA
	Duty cycle 0.01%	0.08%
	Εrms,n	1.0mm mrad 0.4mmmrad