Thierry Augé
email: t.auge@brgm.fr

Laurent Bailly

Jean-Yves Roig

A double Fe-Ti oxide and Fe-sulphide liquid immiscibility in the Itsindro Gabbro Complex, Madagascar June 6, 2017

Keywords: Fe-Ti oxide ores, Fe-(Cu-Ni) sulphides, Fe-Ti-P-rich liquid immiscibility, sulphide liquid immiscibility, mafic-ultramafic intrusion, Itsindro Gabbro, Madagascar

The petrology and mineralogy of the Itsindro complex in south-central Madagascar has been investigated through samples obtained from the 320.7 m-deep Lanjanina borehole. The section consists of a 254 m-thick pyroxenite unit with interbedded gabbro layers that overlies a gabbro unit and is itself overlain by a 19 m-thick granite unit. Most of the structures are sub-horizontal. A weak magmatic layering is locally observed but at the scale of the core, the intrusion does not appear to be a layered complex.

Pyroxenite and gabbro show a systematic disseminated mineralization consisting of Fe-Ti-P oxides and Fe-(Cu-Ni) sulphides that takes the form of ilmenite-titanomagnetite±apatite and pyrrhotite±chalcopyrite±pentlandite. In the upper zone, from 90 to 72 m, sub-massive centimetre-to decimetre-sized layers of oxides and sulphides comprise a total of 16 m of sub-massive sulphide (the main mineralized zone). In this mineralized zone the oxide/sulphide ratio is close to 1/1. The sulphide is strongly dominated by pyrrhotite, which may locally contain inclusions of molybdenite crystals with the Re sulphide rheniite (ReS 2 ).

Oxides are generally euhedral, included in or attached to the Fe-sulphide, and also locally form sub-massive centimetre-sized bands.

Apatite as a cumulus phase is ubiquitous. Locally it may account for 30 % of the ore-rich samples and some samples consist of apatite-Fe-Ti oxides-Fe-Cu-Ni sulphides with virtually no silicate. Apatite is the main REE carrier but the total REE content remains low (<90 ppm).

Mineral compositions and whole rock geochemistry indicate that the rocks are highly differentiated, and in spite of a relatively limited thickness, the differentiation process is observed. Two zones can be distinguished: from the bottom to 162.8 m we see a decrease in the Mg number of olivine and pyroxene, and a drop in TiO 2 and Al 2 O 3 for the latter. A reverse trend is then observed within the pyroxenite unit from the 162.8 m level upwards. The main mineralized zone is located in the upper part of this unit, about 70 m above this discontinuity.

The highly differentiated Fe-Ti-P facies of the Lanjanina series in the Itsindro Gabbro Complex have been interpreted as corresponding to the Fe-Ti-P rich, Si-poor member remaining after an immiscible segregation of an evolved mafic magma. The granite dykes and the overlying granite unit represent the second, Si-rich member of the immiscibility process.

The presence of large amounts of sulphide is attributed to sulphur contamination of the Fe-Ti-rich liquid. Fe-Ti oxides will tend to crystallize on the sulphide droplets and the accumulation of dense Fe-sulphides (liquid) and associated Fe-Ti oxides (solid) will result in this complex and unusual association taking the form of a net texture.

Introduction

The Itsindro massif (south-central Madagascar, Fig. 1) was known to contain an Fe-Cu-Ni sulphide mineralization dominated by pyrrhotite with minor chalcopyrite and rare pentlandite.

Previous work [START_REF] Martel-Jantin | Projet nickel sulfuré Madagascar. Rapport technique de la mission[END_REF]) mentioned a cumulative thickness of 16 m of submassive pyrrhotite between 72 and 90 m in the only drill hole completed in the maficultramafic rocks of the complex. Further work in the area was abandoned because of the lack of Cu-Ni mineralization.

Following a recent 1/1,000,000-scale geologic and metallogenic synthesis of Madagascar geology [START_REF] Roig | Carte Géologique de la République de Madagascar à 1/1 000 000[END_REF][START_REF] Peters | Carte des Substances Métalliques de la République de Madagascar à 1/1 000 000[END_REF], interest in this massif was rekindled and new studies were carried out. As part of a research project aimed at understanding the parameters controlling the composition of magmatic sulphide assemblages (and specifically their Ni-Cu-PGE contents), the sulphide mineralization was reinvestigated by the examination of borehole samples. Selected samples were characterized in terms of petrography, ore paragenesis and geochemistry.

Detailed investigation of borehole samples revealed the presence of a systematic accumulation of Fe-Ti oxides (forming locally dm-sized sub-massive layers), closely associated with the Fe-Cu-Ni sulphides together with--in most samples--magmatic apatite in mafic (ferrogabbro) and ultramafic (ferropyroxenite) cumulates. This type of ore mineral association seems to be very uncommon, although it presents some similarities with mineral occurrences described in the Duluth Complex, Minnesota [START_REF] Ripley | Evidence for sulfide and Fe-Ti-P-rich liquid immiscibility in the Duluth Complex, Minnesota[END_REF].

The aim of this paper is to describe the mineralized drill core and to propose an original genetic model in which by combining textural, mineralogical and chemical evidence, we interpret the coexistence of the Fe-Ti-oxide-apatite mineralization and the Fe-sulphide mineralization (with virtually no Ni-Cu) as a result of a complex immiscibility process preceding a crustal S contamination of the Fe-Ti-(P)-rich magma.

Geological environment and previous work

The Itsindro gabbro massif, located in the Itremo region, belongs to the southern part of a nearly continuous 450 km-long belt of mafic and granitic plutons of Neoproterozoic age [START_REF] Tucker | U-Pb geochronology and isotope geochemistry of the Archean and Proterozoic gneiss belts of Madagascar[END_REF] interpreted as the root of a continental magmatic arc emplaced between 804 and 776 Ma, during or slightly preceding the breakup of the Rodinian supercontinent [START_REF] Archibald | Genesis of the Tonian Imorona-Itsindro magmatic suite in central Madagascar: insights from U-Pb, oxygen and hafnium isotopes in zircon[END_REF]. Mafic-ultramafic bodies belonging to this belt, such as the Itsindro massif or the Ambodilafa complex (located 120 km E of the Itsindro massif) have all the characteristics of anorogenic plutons [START_REF] Tucker | Notice explicative des cartes géologique et métallogéniques de la République de Madagascar à 1/1 000 000[END_REF]. The Itremo region (Fig. 1) is characterized by late undifferentiated Archean migmatitic gneisses and schists [START_REF] Tucker | U-Pb geochronology and isotope geochemistry of the Archean and Proterozoic gneiss belts of Madagascar[END_REF] and metamorphosed Mesoproterozoic sedimentary rocks of the Quarzto-Schisto-Calcaire series [START_REF] Moine | Carte géologique à 1:200,000 du Massif Schisto-Quartzo-Dolomitique, Région d'Ambatofinandrahana[END_REF][START_REF] Moine | Charactères de sédimentation et de métamorphisme de séries précambriennes épizonales à catazonales du centre de Madagascar (Région d'Ambatofinandrahana)[END_REF]. The Itsindro gabbro gives an age of 790.1 ± 0.9 Ma (U-Pb on zircon and baddeleyite) and the age range obtained for other plutons in the region is compatible with the ages obtained in western Madagascar, thus defining a nearly continuous belt of plutonic rocks interpreted as a continental magmatic arc [START_REF] Handke | Neoproterozoic continental arc magmatism in west-central Madagascar[END_REF].

The Itsindro massif is elongated, about 40 x 8 km, with a N-S orientation (Fig. 1), and emplaced in the Itremo "Quarzto-schisto-calcaire" series. Detailed mapping of the massif was carried out in 1968 [START_REF] Coulombeau | Mission roches basiques 1968. Prospection du Massif de l'Itsindro[END_REF] under difficult conditions, owing to the lack of access and outcrop. The massif was interpreted as a series of gabbro-forming sills or laccoliths in the metamorphosed sedimentary series.

Other mafic-ultramafic facies have been sporadically observed but the difficult field conditions did not allow their relationships with the gabbro units to be defined. They include plagioclase lherzolite, clinopyroxenite, orthopyroxenite and dunite. The gabbro units are cut by various felsic facies, such as syenite and various generations of granite, but again, their relationships with the gabbros are not known.

The Itsindro massif was studied in the 1970s for its potential in Cu, after the discovery of malachite and azurite in a gossan developed on a magnetite-rich gabbro. A geochemical exploration survey identified an anomalous zone where the highest Ni and Cu values were superposed. This zone, corresponding to an area of about 0.07 km², includes the mineralized occurrence known as "Lanjanina". A ground geophysical exploration survey conducted in the gossan area gave positive results and the superposition of the geochemical and geophysical anomalies was interpreted as indicating the existence of massive sulphides at depth. A deep vertical borehole was drilled where the geochemical and geophysical anomalies were optimally superposed [START_REF] Fournié | Etude de l'indice de Lanjanina (centre de Madagascar)[END_REF]. The LJ1 hole was stopped at a depth of 320.75 m (Fig. 2). The results were considered to be negative, in spite of the abundance of submassive sulphides: largely dominated by pyrrhotite, they yielded correspondingly low Cu and Ni values.

Borehole description

The drill core recovered in 1969 was well preserved and a detailed log at 1/100 scale was available with macroscopic descriptions [START_REF] Fournié | Etude de l'indice de Lanjanina (centre de Madagascar)[END_REF]. Moreover from 34 to 320.7 m, every one-metre interval had been analyzed for Ni and Cu.

The log is relatively simple at the macroscopic scale and shows five main units (Fig. 2):

-0 -8.2 m: oxidized iron cap ("gossan"), -8.2 -15.35 m: gabbro (saprolite) unit, -15.35 -34.02 m: granite unit (heterogeneous, weathered, and strongly fractured), -34.02 -287.6 m: pyroxenite unit with dm-to m-thick dykes of fine grained granite and syenite above 120 m, and metre-sized gabbro layers throughout the unit, -287.6 -320.73 m: main gabbro unit, with 3-to 7 m-thick pyroxenite layers.

Most of the pyroxenite and gabbro facies are isotropic. In some cases however, a magmatic fabric was observed, suggesting that the complex is not strongly deformed and is preserved in its original position with a sub-horizontal layering. Similarly, when preserved, contacts between pyroxenite and gabbro, and massive ore and mafic-ultramafic rocks, tend to be horizontal. A weak hydrothermal alteration is developed in the first 130 m, with occurrences of secondary amphibole (locally forming amphibolite), biotite, chlorite, and carbonate. Disseminated sulphides are observed throughout the borehole, in all rock types. From 72 to 90 m massive to sub-massive sulphide concentrations form centimetre-to decimetre-thick accumulations corresponding to the equivalent of about 16 m of the "massive sulphides layer" [START_REF] Martel-Jantin | Projet nickel sulfuré Madagascar. Rapport technique de la mission[END_REF]. In this Fe-Cu-Ni sulfide-rich zone (hereafter "the main mineralized zone"), 50-cm intervals were analyzed for Ni and Cu. The maximum Ni value obtained is 2600 ppm, the maximum Cu, 2800, and there is a weak correlation between the two, with a Ni/Cu ratio close to 1/2.

Sampling and petrography

Thirty-three samples were collected throughout the drill core, from 34.8 m to 316.7 m. A systematic sampling was supplemented by a specific sampling of the mineralized facies.

Table 1 summarizes the main characteristics of the samples. Sampling was focused on the mafic-ultramafic rocks; granitic facies were not collected.

All the rock types consist of various proportions of clinopyroxene, plagioclase, olivine, apatite, mica, rare small zircon and secondary amphibole. Orthopyroxene is totally lacking. In the pyroxenite unit; pyroxenite with various proportions of ore minerals is generally coarsegrained, exhibiting a mesocumulate texture (Figs. 3A,D). Olivine was found in four pyroxenite samples (out of 17), and does not exceed 5 % in volume (Figs. 3H,K). In some samples olivine forms a rim around the ore minerals, in contact with clinopyroxene, resembling a reaction texture (Figs. 3I,L). Some pyroxenites contain up to 10 % interstitial plagioclase; plagioclase also locally forms a very thin (20 µm) rim between ore minerals and silicates. Gabbro interlayered in the pyroxenite unit exhibits either a fine-grained equant texture, with a relatively low (20 %) proportion of plagioclase, or a pegmatitic facies in which plagioclase dominates over pyroxene.

In the gabbro unit, gabbro seems to show a more pronounced magmatic mineral layering (Figs. 3B,E), marked by a preferred orientation of the plagioclase laths, and various types of gabbro are observed, from leucogabbro with 60 % plagioclase to mafic gabbro (close to plagioclase wehrlite). Olivine is common in the basal gabbro and may be more abundant than in the pyroxenite unit. Pyroxenite layers in the basal gabbro are olivine bearing. Their contacts with the host gabbro are sharp, and parallel to the magmatic layering.

Throughout the drill core, apatite occurs as a cumulus phase. It is ubiquitous, euhedral, and included in the magmatic silicates, in the oxide phases, in the Fe-(Cu-Ni) sulphides (Figs.

3G, H, J, K), and in the oxide-sulphide association. Locally it forms aggregates of crystals up to 1.5 mm in size. Its volume may locally reach 30 % of the sample in the Fe-Ti oxide-rich facies and some samples consist of apatite-Fe-Ti oxides and Fe-Cu-Ni sulphides with virtually no silicate.

A weak, late hydrothermal alteration affects most of the gabbro and pyroxenite, and is characterized by amphibolization of pyroxene and the local development of carbonates This episode does not affect the ore minerals and is not accompanied by the deposition of metallic minerals. At hand-specimen scale we see no rhythmic layering anywhere in the borehole, and no evidence of a "layered complex" but rather two main units, suggesting that the borehole corresponds to two "mega-cyclic" units. In most cases, disseminated oxides and sulphides tend to be interstitial with respect to silicates. Sulphide and oxide crystals are generally adjacent rather than forming separate patches, and we observe "droplets" of Fe-Cu-Ni sulphides included in oxides and inclusions of euhedral oxide in Fe-Cu-Ni sulphides, suggesting that S immiscibility occurred before or contemporaneously with the oxide crystallization, and that the S liquid was still present when the Fe-Ti oxides crystallized. Rare rounded inclusions of oxides and sulphides are also observed in silicates,.

Mineralization

When the content of ore mineral reaches about 30 %, the ore begins to display a characteristic net texture and the oxide-sulphide association generally appears interstitial with respect to euhedral magmatic silicates and apatite (Figs. 3C,F). This net texture is typical of Fe-Ti oxide rock [START_REF] Dymek | Petrogenesis of apatite-rich rocks (nelsonites and oxideapatite gabbronorites) associates with massive anorthosites[END_REF]. The net and massive textures clearly show euhedral to subhedral oxides enclosed in the Fe-Cu-Ni sulphides, both oxides and sulphides forming a network surrounding isolated cumulus silicate grains (Figs. 4A, B, C, D), very similar to the interconnected network around isolated silicate grains shown by [START_REF] Liu | Using multiphase solid inclusions to constrain the origin of the Baima Fe-Ti-(V) oxide deposit, China[END_REF] for an Fe-Ti mineralization in a context without Fe-Cu-Ni sulphides.

Graphite flakes ubiquitous, but always in very minor proportions (<1 %). Graphite generally occurs as small clusters (<100 µm) of fibres closely associated with the ore minerals.

Sulphides

Pentlandite is sparsely developed, mainly as large crystals and flame-like inclusions found along cracks in pyrrhotite. Chalcopyrite appears as lamellar exsolutions in pyrrhotite as well as larger grains between pyrrhotite crystals. Cobaltite-gersdorffite group minerals form rounded inclusions in pyrrhotite. Tiny crystals of sphalerite were observed in a few samples.

Very small (max. 10 µm long) molybdenite flakes are commonly included in or adjacent to Fe-(Cu-Ni) sulphides. A tiny Re-sulphide (rheniite) is very commonly found next to or partly included in molybdenite (Figs. 4H,I).

Oxides

Most oxide grains are two-phase associations, composed of ilmenite free of exsolution lamellae, and titanomagnetite everywhere exhibiting complex subsolidus exsolution textures.

Titanomagnetite is recrystallized into magnetite containing a complex network of oriented exsolutions and intergrowths of ilmenite and aluminous spinel-group minerals (Figs. 4E,G).

Ilmenite largely dominates over titanomagnetite. We never observed magnetite crystals free of exsolution.

The contact between ilmenite and titanomagnetite is highly irregular and marked by a fringe up to 120 µm in width of tiny Al-spinel crystals, titanite and biotite (Fig. 4F). Lozenge-shaped spinel crystals are also seen in titanomagnetite crystals. Such features have been described elsewhere, for example by [START_REF] Charlier | Ilmenite composition in the Tellnes Fe-Ti deposit, SW Norway: fractional crystallization, postcumulus evolution and ilmenite-zircon relation[END_REF] in the Tellnes deposit in Norway and by [START_REF] Liu | The Shangzhuang Fe-Ti oxide-bearing layered mafic intrusion, northeast of Beijing (North China): Implications for the mantle source of the giant Late Mesozoic magmatic event in the North China Craton[END_REF] in Fe-Ti mineralization in a layered mafic intrusion in China.

Analytical techniques

Among the thirty-two samples collected throughout the section (all prepared in polished thin sections), twenty-two were selected for mineralogical analyses and fifteen for whole rock analyses (Table 1). Major and trace elements were analysed by ALS Minerals (Ireland). Natural in-house minerals were used as standards. Kα lines were used for all the elements. Apatite analyses were performed with a CAMECA SX100, at the GeoRessources Laboratory, Université de Lorraine. For Cl, P, and Ca a standard apatite was used with an acceleration voltage of 15 keV, a beam current of 12 nA and a counting time of 10 sec. All the values given below are above the detection limit.

In each thin section, usually five points were analysed on a given mineral species to test potential zoning and compositional variability. As expected in such a context, no significant differences were observed and average compositions were calculated and used in most of the diagrams below except for olivine, in which large compositional variations were obtained from one crystal to another within a given sample (see below).

Lithogeochemistry

Whole-rock analyses were performed on 15 selected representative samples to characterize their major, trace and rare earth element contents (Table 2). Their geochemistry, and more especially their contents of Ti (reported as TiO 2 ) and Fe (reported as Fe 2 O 3 ), are strongly influenced by their contents of oxides (ilmenite, titanomagnetite) and sulphides (pyrrhotite±chalcopyrite±pentlandite). Note that as the samples are cumulates; the compositions obtained are not representative of liquid compositions.

Major elements

The SiO 2 content of pyroxenite samples ranges between 13.3 and 47.2 wt. %. Highly variable contents of Fe 2 O 3 and TiO 2 are also observed, between 14.25 and 54.4 wt. % and 1.91 and 14.2 wt. % respectively. These variations reflect the varying proportions of Fe-Ti oxides and Fe-sulphides. Some ten samples have an S content lower than 2 %, some showing TiO2 contents greater than 11 % (corresponding to S-poor oxide-rich mineralized samples). This is the case for some gabbros, generally relatively poor in sulphides (less than 1.36 wt. % S) but locally rich in Fe-Ti oxides (TiO 2 up to 9.2 %). Two pyroxenite samples show extremely high S contents (11.4 and 17.9 %) and a TiO 2 content ranging between 4.3 and 9.2 wt. %, corresponding to samples mineralized by oxides and sulphides. Some samples are characterized by TiO 2 contents above 12 % (ilmenite-rich samples) and variable but high sulphide contents between 2.3 and 9.5 % (sulphide-rich samples).

SiO 2 and Fe 2 O 3 are negatively correlated, sulphide-bearing samples showing the highest Fe 2 O 3 contents (Fig. 5G). SiO 2 is also negatively correlated with TiO 2 although the correlation is not perfect (Fig. 5B). TiO 2 and Fe 2 O 3 are positively correlated (reflecting the proportion of Fe-Ti oxides, Fig. 5F). Similarly, and for the same reason, MgO and Fe 2 O 3 are negatively correlated (Fig. 5D).

SiO 2 and MgO are positively correlated and two trends are observed (Fig. 5A). The first one is defined by pyroxenite samples characterized by an SiO 2 /MgO ratio of 3.4, the second is defined by gabbro samples with a higher SiO 2 /MgO ratio (around 6.3). Only one pyroxenite sample (LJ 31) shows Mg enrichment (SiO 2 /MgO = 1.6). A similar evolution is observed for CaO, positively correlated with SiO 2 (Fig. 5C). Both trends reflect an increasing proportion of clinopyroxene (together with a decreasing proportion of ore minerals).

In pyroxenite, Al 2 O 3 remains low, and is not correlated with the SiO 2 variations. High Al 2 O 3 values are seen only in the gabbros (Fig. 5E). Despite the presence of graphite, observed in nearly all thin sections, the total C content remains low, ranging between 0.10 and 0.54 % (Table 2).

Trace elements

Only a few trace elements exhibit significant and distinguishing contents. The V content ranges between 176 and 1440 ppm without significant difference between pyroxenite and gabbro. Its abundance reflects the amount of titanomagnetite, this being the main V carrier (1.12 % V 2 O 3 on average, EPMA data). A positive correlation is observed between V and Fe 2 O 3 (Fig. 5H) except for samples having the highest pyrrhotite contents (corresponding to S contents ranging from 7.43 to 17.95 %). Ba, mainly hosted by plagioclase, is significantly enriched in gabbro (81 to 348 ppm) compared to pyroxenite (2.8 to 68 ppm). The same behaviour is observed for Sr, which ranges between 357 and 425 ppm in gabbro and between 12.8 and 52.4 ppm in pyroxenite. The Sc content ranges between 33 and 100 ppm, and Zr is below 60 ppm.

Base metals are mainly represented by Cu, ranging between 44 and 5460 ppm in correlation with the variable, but always low content of chalcopyrite. Ni (22 to 1350 ppm), mainly carried by rare pentlandite exsolutions in pyrrhotite and the cobaltite-gersdorffite mineral series, is positively correlated with Co (47 to 596 ppm, Fig. 6C), and with Cu (Fig. 6D). Note that the Ni content of olivine is typically low, <0.05 wt.%. Both Cu and Ni are positively correlated with S (Figs. 6A,B).

Few crystals from the cobaltite-gersdorffite mineral series have been identified, but no correlation is observed between Ni, Co and As. Zn, mainly carried by Al spinel (2.75 % on average, EPMA data) and by rare sphalerite crystals, ranges from 57 to 454 ppm. For such an ultramafic environment the Cr content is low, ranging between 10 and 310 ppm (no chromite crystals have been identified and the Cr content in clinopyroxene and titanomagnetite is extremely low. In spite of the presence of molybdenite, the Mo content is low, around 1 ppm, except in the three samples that have the highest S content (LJ4, 17.95 wt.% S, LJ16,11.40 and LJ22,7.43) where it reaches 5-6 ppm, confirming that molybdenite is associated with the sulphide phase.

Rare Earth Elements

The REE content of the 15 analysed samples is low, ranging from 12 to 89.92 ppm in pyroxenite and 15.72 to 32.42 ppm in gabbro (Table 2). Apatite-rich samples are characterized by the highest REE contents and a positive correlation is observed between the amount of P 2 O 5 and the sum of REE, confirming that the majority of the REE are carried by apatite (Fig. 7A). LREE are dominant and the LREE/HREE ratio ranges from 1.56 to 3.74.

The chondrite-normalized REE profiles are reported in Figures 7B,C,D. The REE patterns are slightly fractionated, with a low La/Lu ratio ranging between 1.1 and 6.7. LREE are poorly fractionated (0.53<La/Sm<1.37) compared to HREE (1.84<Gd/Lu<5.25). The Eu anomaly ranges from 0.86 to 1.41 in pyroxenite and 1.79 to 2.45 in gabbro. Sample LJ9 containing about 10 % of interstitial plagioclase also exhibits a positive Eu anomaly (Fig. 7B). The LREE enrichment observed in samples LJ22, 16, 11, 26 and 10 is in accordance with their contents of apatite.

Mineral chemistry

Four main types of sample were distinguished in the log: "barren" pyroxenite and its oxide/sulphide mineralized equivalent, and "barren" gabbro and its oxide/sulphide mineralized equivalent 8.1. Clinopyroxene Clinopyroxene was analyzed in 21 samples (out of 22) and shows a wide range of compositions (Table 3). In the Ca-Mg-Fe triangular diagram (not shown), clinopyroxene plots in the augite field, close to the salite domain, and is marked by large Mg-for-Fe substitution.

The most Mg-rich clinopyroxene is found in gabbro, but there is a large compositional overlap in clinopyroxene compositions from pyroxenite and gabbro (Fig. 8).

As regards minor elements, large variations are observed: Al 2 O 3 content ranges between 3.04 and 1.91 wt.% and TiO 2 between 1.08 and 0.53, whereas Cr 2 O 3 is generally below the detection limit (around 0.01 wt.%). with a maximum of 0.06 wt.%.

There is a significant decrease in the Al 2 O 3 and TiO 2 contents in the pyroxenites and gabbros correlated to an Mg decrease (Fig. 8) but the presence of ore minerals does not seem to influence the mineral composition.

The Mg number (Mg/(Mg+Fe) at. conc.) of the clinopyroxene was plotted as a function of depth (Fig. 9) and two domains appear: the first, from the bottom (316 m) to 162.8 m, is marked by a regular decrease in the Mg number (and a correlated drop in TiO 2 and Al 2 O 3 ); the second, observed from 162.8 m to the top, is characterized by a reverse trend occurring within the pyroxenite unit with a sudden increase in the Mg number, followed by a complex and irregular evolution up to the top of the pyroxenite unit. The first trend cuts across the petrographic and macroscopic units described in the log, and is compatible with an undisturbed fractional crystallization process. In the second domain of the section, the Al 2 O 3 in the pyroxene seems to remain fairly constant whereas TiO 2 appears to vary erratically (Fig. 9).

Olivine

Olivine was analysed in nine samples: four pyroxenites and five gabbros (Table 4). Contrary to the other silicates, olivine exhibits large compositional variations within a given sample.

The maximum range is for the gabbro LJ28 where the Fo values range between 39.80 and 50.40 % (average 46.38 on 6 analytical points). In terms of average values per sample, the Fo content ranges from 62.85 (lowermost olivine gabbro LJ31) to 37.2 (uppermost olivine pyroxenite LJ11). There is no relationship between the nature of the host rock and the olivine composition. Plotted versus depth, the Fo content tends to decrease from the bottom to the marker level (discontinuity) near 163 m. The same trend is shown by the clinopyroxene composition (Fig. 9). Interestingly, as with clinopyroxene, above this point the Mg number increases again, suggesting that the level corresponds to a significant horizon in the evolution of the complex.

Considering minor elements, the NiO content in the olivine appears typically low, <0.05 wt.%, whereas MnO, correlated with Mg number (Fig. 10), decreases from 1.13 (Fo 40.18 %) to 0.36 wt.% (Fo 62.85 %).

The Fe-Mg distribution between olivine and clinopyroxene has been studied experimentally [START_REF] Perkins | Experimental investigation of Fe-Mg distribution between olivine and clinopyroxene: Implications for mixing properties of Fe-Mg in clinopyroxene and garnet-clinopyroxene thermometry[END_REF]. The experimental curve is plotted in Figure 11, together with olivine (average values) coexisting with clinopyroxene from the Lanjanina drill core. The observed trend partly overlaps the experimental one, the former tends to be more Mg-rich than the composition expected from equilibrium with the olivine. This suggests that there is no significant re-equilibration between the silicates and the Fe-Ti oxides.

As mentioned above, olivine is seen either as individual subhedral crystals or as thin rims between clinopyroxene -Fe-Ti-oxide or clinopyroxene -Fe-sulphide (Figs. 3I,L). Heterogeneity in composition is observed within a single crystal and olivine rimming the Fe-rich minerals does not show any specific Fe-enrichment that could be interpreted as subsolidus re-equilibration. Olivine composition may have been locally modified at a late magmatic stage but we do not have any evidence of such a mechanism.

Plagioclase

Plagioclase was analysed in three gabbros from the basal gabbro unit and in three plagioclase pyroxenites and two gabbros from the pyroxenite unit (Table 5). Plagioclase tends to be interstitial in pyroxenite, and to form laths in the gabbro. In some mineralized facies, it also occurs as a thin rim between clinopyroxene and ore minerals.

Plagioclase is relatively homogeneous in composition within a single sample, but at the scale of the drill core variations from An 50.22 to An 98.14 have been recorded. In greater detail, plagioclase from the basal gabbro unit appears very homogeneous (An57-An58) and fairly similar to plagioclase from the gabbro in the pyroxenite unit (An ca. 53), whereas interstitial plagioclases in pyroxenite are more An-rich but also have more variable compositions (An 74.3 for sample LJ14 and An 96-97 for sample LJ12-JL13).

Plotted versus depth (Fig. 9), the An composition seems to reflect changes in the mineral composition already observed around 163 m but there is no obvious correlation between the plagioclase and pyroxene compositions.

Amphiboles

Most amphiboles derive from hydrothermal alteration of clinopyroxene, and locally their proportion may exceed that of clinopyroxene. As a result, they are calcic ferromagnesian amphiboles and various compositions have been obtained in the magnesiohornblende and tschermakite fields, corresponding to various Si/Al ratios. Some are locally enriched in TiO 2 (up to 2.85 wt.%). Rare magmatic amphiboles have been observed, occurring as colourless interstitial crystals, with a composition close to the actinolite-tremolite boundary.

Apatite

Apatite was analysed in the two samples where it is most abundant (Table 6). Their composition is relatively homogeneous, and at the crystal scale no zoning was observed.

The major-element content is similar in the two samples (P 2 O 5 around 42.2 wt.% and CaO 55.9-56.5), with FeO ranging between 0.5 and 0.35 wt.%, and MnO, MgO and SrO remaining close to the detection limits of 0.15, 0.05 and 0.05 wt.% respectively. A slight difference appears in the halogen contents with an X F value of 1.21 for sample LJ19 and 0.90 for LJ22.

The presence of REE (La, Ce, Pr, Nd, Sm, Eu, Gd and Dy) was tested with a detection limit of 0.05 % for the rare-earth oxides; only Pr appears to be above the detection limit (Pr 2 O 3 ranging between 0.15 and 0.2 wt. %).

Fe-Ti oxides

Magnetites were analysed in homogeneous areas devoid of ilmenite and spinel exsolution.

Their compositions are quite homogeneous and no evolution with depth is observed (Table 7). They are characterized by low TiO 2 (less than 1 wt. %), Al 2 O 3 (0 to 1.32 wt. %) and Cr 2 O 3 (0 to 0.87 wt. %) contents, whereas V 2 O 3 ranges between 0.36 and 2.45 wt. %.

Like titanomagnetite, ilmenite (either as exsolution lamellae within titanomagnetite or as discrete grains) exhibits a homogeneous composition characterized by average contents of MgO and MnO of 0.84 and 0.81 wt. % respectively (Table 8). The V 2 O 3 content is low, between 0 and 0.35 wt. %. Cr 2 O 3 is very low, and generally not detected

The spinel-group minerals range in composition between spinel s.s. and hercynite; their Fe 2+ /(Fe 2+ +Mg) ratios ranging between 0.48 and 0.80 (Table 9). Their ZnO contents range between 1.51 and 6.92 % (avg. 2.49 %). No correlation was observed between the composition of the oxides and the depth of sampling.

Phase proportion

Knowing the mineral composition (EPMA data) and the bulk analysis, we have estimated the theoretical proportion of each ore mineral in the analysed samples.

For the oxides, the calculation is based on the following assumptions:

-All TiO 2 is carried by ilmenite (discrete crystals and exsolution in titanomagnetite); the TiO 2 content of associated magnetite, about 0.50 % on average, has been neglected; -The contents of TiO 2 , FeO, MnO and MgO in ilmenite are based on its average composition obtained by EPMA analysis (Table 8); -The proportion of FeO in the bulk analysis due to the titanomagnetite contribution has been estimated on the basis of the relative proportion of V 2 O 3 (magnetite being the only V carrier). The contents of FeO and V 2 O 3 in magnetite are estimated on the basis of its average EPMA analyses (Table 7).

The estimated proportions of ilmenite and magnetite are reported in Table 10.

For the Fe-Cu-Ni sulphides (pyrrhotite, chalcopyrite and pentlandite):

-All sulphur is distributed between these three minerals. The amount of sphalerite and other minerals is considered to be negligible; -The composition of the sulphides corresponds to the theoretical composition (chalcopyrite -34.63 wt. % Cu, 30.43 wt. % Fe, 34.94 wt. % S; pentlandite -32.56 wt. % Fe, 34.21 wt. % Ni, 33.23 wt. % S; pyrrhotite -62.33 wt. % Fe, 37.67 wt. % S).

S was first distributed to chalcopyrite and pentlandite. The remaining sulphur was then attributed to pyrrhotite. The estimated proportions of the main sulphides are reported in Table 10.

Similarly, based on the average composition of apatite (P 2 O 5 : 42.2 wt. % and CaO: 56.2 wt. %, Table 6), and assuming that all P is contained in this mineral, the calculated amount of apatite ranges between 0.05 and 5.7 % in the 15 analysed samples. The silicate proportion is estimated by difference to total 100 %.

Discussion

Emplacement and differentiation

The 320.7 m of the Lanjanina drill core approximately correspond to the true thickness of the cumulate pile sampled, which is very small compared to the whole Itsindro complex, whose outcrop area is about 40 x 8 km. Moreover this drill core, emplaced on geophysical and geochemical anomalies, cannot be representative of the whole complex, although it constitutes a key zone for understanding the mineralizing processes.

The drill core consists of a 254-m-thick pyroxenite unit resting on a gabbro unit (of unknown thickness) and overlain by 19 m of granite, capped by a weathered gabbro unit. Ore minerals are found throughout the drill core but are concentrated in a 19 m-thick zone near the top of the pyroxenite unit. We clearly see that although the dominant facies in the Itsindro complex is a gabbro, the Lanjanina section is dominated by pyroxenite.

Pyroxenite occurrences are interlayered in the basal gabbro, as are gabbro horizons in the pyroxenite unit. The upper third of the pyroxenite unit is cut by granitic dykes, which seem to be feeders for the upper granite unit. A magmatic layering is locally observed, but at the scale of the core mineral layering is generally lacking and most facies are isotropic.

Mineral chemistry indicates that the rocks are highly differentiated, and in spite of their relatively limited thickness, the differentiation process is observed within the core. Two zones can be distinguished: from the bottom to 162.8 m, we see a decrease in the Mg number of olivine (0.63 to 0.40) and pyroxene (0.72 to 0.57), and a drop in TiO 2 and Al 2 O 3 for the latter. This evolution cuts across the petrographic and macroscopic units described in the log and is compatible with an undisturbed fractional crystallization process. This suggests that the basal gabbro unit and the bottom half of the pyroxenite unit belong to the same magmatic system. A reverse trend is then observed above the 162.8 m level, within the pyroxenite unit, followed by a complex and irregular trend in the Mg number of olivine (0.53 to 0.37) and pyroxene (0.68 to 0.61). In the upper part of the drilled section, Al 2 O 3 in pyroxene seems to remain fairly constant whereas TiO 2 seems to vary erratically. This reversed and disturbed trend could reflect physical and/or chemical disturbances in the magma chamber, such as the incorporation of a less differentiated magma and seems to be marked by the local presence of gabbro. Note that the main mineralized zone is located in this upper part of the unit, about 70 m above the marker level. [START_REF] Duchesne | The Fedorivka layered intrusion (Korosten Pluton, Ukraine): an example of highly differentiated ferrobasaltic evolution[END_REF] invoke such an inverse evolution resulting from crystallization downwards from the roof in the case of the Fedorivka layered intrusion.

The silicate composition is not related to the content of ore minerals, Fe-Ti oxides or Fe-Cu-Ni sulphides, suggesting that the magmatic system is relatively independent of the mineralizing systems and that they are all in equilibrium. Thus, the mineral composition suggests that the magmatic pile corresponds to two magmatic cycles in a single magma chamber.

Ore mineral composition

Fe-Ti oxides have an almost constant composition throughout the drill core. Titanomagnetite everywhere contains ilmenite exsolutions resulting from subsolidus re-equilibration. In most cases the ilmenite and titanomagnetite crystals are adjacent, suggesting a genetic link between them. Their contacts are irregular, with complex interfingerings of magnetite into ilmenite (defined as symplectite, see [START_REF] Mcenroe | Chemical and petrographic characterization of ilmenite and magnetite in oxide-rich cumulates of the Sokndal Region, Rogaland, Norway[END_REF], with the systematic presence of Al-Zn spinel surrounding the interfingered magnetite. Al spinel is also common as exsolution lamellae in magnetite. Magnetite is characterized by a high V-content (up to 1.7 % V) which according to [START_REF] Duchesne | The Fedorivka layered intrusion (Korosten Pluton, Ukraine): an example of highly differentiated ferrobasaltic evolution[END_REF] is an indication of a high V 3+ content in the melt, revealing crystallization under low fO 2 conditions Unlike ilmenite in anorthosite-related deposits (where ilmenite is an early liquidus phase crystallizing after plagioclase), which have a high MgO content (2.5 -4.0 wt.% for example in the Tellnes Fe-Ti deposits, [START_REF] Charlier | Ilmenite composition in the Tellnes Fe-Ti deposit, SW Norway: fractional crystallization, postcumulus evolution and ilmenite-zircon relation[END_REF], ilmenite from the Lanjanina area has a low MgO content (< 1.5 wt.%, except in one basal gabbro where it reaches 2.7 wt.%). The low MgO is consistent with the derivation of the Fe-Ti oxide from a fairly evolved Fe-rich Mg-poor liquid, MgO in ilmenite being directly related to the Mg number of the magma [START_REF] Charlier | Ilmenite composition in the Tellnes Fe-Ti deposit, SW Norway: fractional crystallization, postcumulus evolution and ilmenite-zircon relation[END_REF].

Fe-(Cu-Ni) sulphides were not analysed with the EPMA, but only qualitatively characterized using an SEM equipped with an EDS detector. During this checking, we found small molybdenite crystals included in the Fe-Cu-Ni sulphides. In most cases they take the form of elongated particles, 10 x 2 µm. In almost every case, a very small rheniite grain is adjacent to the molybdenite flakes or interlayered in the molybdenite structure. Owing to the size of both minerals, EPMA analyses were not possible. However, qualitative analyses enabled the detection of Re in solid solution in molybdenite. Rheniite and Re-rich molybdenite are typical of certain porphyry-type ore deposits and sublimates derived from volcanic gases. Obviously we are in a very different setting here, and although molybdenite is a common mineral in magmatic Ni-Cu sulphides in mafic-ultramafic environments [START_REF] Garuti | Age, geochemistry and petrogenesis of the ultramafic pipes in the Ivrea Zone, NW Italy[END_REF][START_REF] Helmy | Cu-Ni-PGE mineralization in the Genina Gharbia mafic-ultramafic intrusion, Eastern Desert, Egypt[END_REF], as far as we know rheniite has never been described in a similar context.

In the magmatic sulphide deposit in Wetlegs, Duluth Complex, occurrences of molybdenite associated with Co-Ni arsenides are interpreted as proving the presence of hydrothermal fluids related to country-rock assimilation [START_REF] Raic | Arsenic-rich Cu-Ni-PGE mineralization in Wetlegs Duluth Complex St.Louis County[END_REF]. The molybdenite-rheniite assemblage described here shows some similarities with occurrences described by [START_REF] Voudouris | Rhenium-rich molybdenite and rheniite in the Pagoni Rachi Mo-Cu-Te-Ag-Au prospect, northern Greece: implications for the Re geochemistry of porphyry-style Cu-Mo and Mo mineralization[END_REF] in a porphyry-type Mo-Cu prospect in northern Greece. The authors suggested that rheniite and molybdenite coprecipated from a vapour phase. The close relationships between them could suggest that the Re sulphide may have exsolved from the Mo sulphide, but detailed examination of the SEM images (Figs. 4H,I), and rare occurrences of rheniite without molybdenite preclude this possibility.

Ore mineral texture

A typical net texture characterizes the mineralized samples. This texture, very common in magmatic sulphide ores, has also been recognized in many Fe-Ti oxide deposits. [START_REF] Wang | New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China[END_REF] suggested that net-textured ores formed from immiscible Fe-Ti-(P)-rich melts segregated from an evolved magma. Using 3-D CT reconstruction, [START_REF] Liu | Using multiphase solid inclusions to constrain the origin of the Baima Fe-Ti-(V) oxide deposit, China[END_REF] demonstrated that in the net-textured ores, Fe-Ti oxides form an interconnected network enclosing silicate cumulates, as a result of the crystallization of Fe-Ti oxides from an Fe-Tirich liquid percolating throughout an unconsolidated silicate pile. [START_REF] Wang | New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China[END_REF] underlined the analogy between sulphide and Fe-Ti net-textured ores. There is however a major difference: in the case of sulphide ore, it is accepted that the Fe-Cu-Ni sulphides crystallized directly from an immiscible sulphide liquid, whereas given the existence of an immiscible Fe-Ti-rich melt percolating through the silicates, the Fe-Ti oxides must crystallise from this magmatic liquid and be segregated in the interstices between the silicates; the residual liquid must then be evacuated, probably through a filter-press process.

The special feature of the Lanjanina net-textured ore is its association of Fe-(Cu-Ni) sulphides and Fe-Ti oxides within the same facies.

An alternative hypothesis has been proposed to explain the typical net-texture of the Fe-Ti oxides. This model considers that the Fe-Ti oxides are cumulus minerals crystallized from a Fe-Ti-P-rich magma, the net-texture resulting from sub-solidus reequilibration of the oxides (see [START_REF] Duchesne | Liquid ilmenite or liquidus ilmenite: a comment on the nature of ilmenite vein deposits[END_REF]1999;[START_REF] Charlier | Fe-Ti-V-P ore deposits associated with Proterozoic massif-type anorthosites and related rocks[END_REF]. These authors suggested that high temperature deformation enhances the process. This model was developed for Fe-Ti deposits in anorthosites where deformation is related to the diapiric emplacement of the anorthosite plutons. In the specific case of the Lanjanina mineralisation we do not have evidences of deformation features such as the strongly foliated rocks commonly described in anorthosite bodies [START_REF] Charlier | Fe-Ti-V-P ore deposits associated with Proterozoic massif-type anorthosites and related rocks[END_REF]. Net-textures similar to textures described for Fe-Ti oxides are very common in chromite ore from stratiform [START_REF] Bailly | The mineralization potential of the Burakovsky layered intrusion, Karelia, Russia[END_REF] or ophiolite complexes [START_REF] Cassard | A structural classification of podiform chromite orebodies from southern New[END_REF] where subhedral chromite crystals fill interstices between undeformed olivine crystals. This texture is interpreted as a primary magmatic texture and we favour the same origin for the net texture of the Lajanina mineralization.

10.4. The Fe-Ti-P mineralization and the immiscibility question Horizons of gabbro mineralized by Fe-Ti-(V) oxides and apatite are commonly observed in layered intrusions of various types, such as the Bushveld [START_REF] Eales | The Bushveld Complex[END_REF], Skaergaard (McBirney, 1975), Duluth [START_REF] Ripley | Evidence for sulfide and Fe-Ti-P-rich liquid immiscibility in the Duluth Complex, Minnesota[END_REF], Bjerkreim-Sokndal [START_REF] Duchesne | Geochemistry of cumulates from the Bjerkreim-Sokndal layered intrusion (S. Norway). Part I: constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent[END_REF], Panzhihua [START_REF] Zhou | Geochemistry, petrogenesis and metallogenesis of the Panzhihua gabbroic layered intrusion[END_REF], and Sept-Iles [START_REF] Namur | Dual origin of Fe-Ti-P gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the Sept Iles layered intrusion[END_REF].

Ferrogabbros with large Fe-Ti-(V) deposits are also commonly associated with massif-type anorthosites [START_REF] Duchesne | Fe-Ti deposits in Rogaland anorthosites (South Norway): geochemical characteristics and problems of interpretation[END_REF][START_REF] Duchesne | The Fedorivka layered intrusion (Korosten Pluton, Ukraine): an example of highly differentiated ferrobasaltic evolution[END_REF][START_REF] Charlier | Fe-Ti-V-P ore deposits associated with Proterozoic massif-type anorthosites and related rocks[END_REF].

Several hypotheses have been proposed for their origin. [START_REF] Cawthorn | Origin of anorthosite and magnetitite layers in the Bushveld Complex, constrained by major element compositions of plagioclase[END_REF] listed five possible mechanisms from the literature to account for the formation of the magnetite layers in the Bushveld Complex: liquid immiscibility [START_REF] Reynolds | The nature and origin of titaniferous magnetite rich layers in the Upper Zone of the Bushveld Complex: a review and synthesis[END_REF]; magma addition and/or mixing [START_REF] Harney | Platinum-group element behaviour in the lower part of the Upper Zone, Eastern Bushveld Complex. Implications for the formation of the main magnetite layer[END_REF]; settling and sorting of minerals [START_REF] Wager | Layered Igneous Rocks[END_REF]; change in oxygen content [START_REF] Klemm | The geochemistry of titanomagnetite in magnetite layers and their host rocks of the Eastern Bushveld Complex[END_REF]; and changes in pressure [START_REF] Cawthorn | Incompatible trace element behaviour in the Bushveld Complex[END_REF]. They concluded that liquid immiscibility cannot be the operating process, because this process cannot generate monomineralic layers with a sharp contact to the underlying anorthosite. They favour changes in pressure to explain the origin and characteristics of the layers of magnetite. [START_REF] Charlier | The Grader layered intrusion (Havre-Saint-Pierre Anorthosite, Quebec) and genesis of nelsonite and other Fe-Ti-P ores[END_REF] considered that the Fe-Ti-P ores in the Grader intrusion (Havre-Saint Pierre Anorthosite, Quebec) represent cumulates rather than crystallized Fe-Ti-P-rich immiscible melts and resulted from successive appearances of a liquidus phase in a fractional crystallization process, followed by gravitational segregation. They concluded that there is no evidence for an origin of the oxide ore by liquid immiscibility.

On the other hand, based on mineralogical and textural constraints [START_REF] Wang | New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China[END_REF] considered that the Fe-Ti oxide deposit in the Panxi region (SW China), occurring in plagioclase pyroxenite, had formed from an Fe-Ti-(P)-rich melt segregated from a differentiated ferrobasaltic magma as a result of liquid immiscibility. The mineralization consists of titanomagnetite, ilmenite and apatite interstitial to clinopyroxene and plagioclase, forming a net texture. They emphasized the similarity between this texture and the magmatic sulphide ores.

The question of liquid immiscibility has also been highlighted by the study of melt inclusions.

Fe-and Si-rich liquids are known as inclusions trapped in apatite and olivine in the Upper Zone of the Skaergaard intrusion, showing signs of liquid immiscibility in the late-stage evolution of the magma. Jacobsen et al. ( 2011) conclude that a significant part of the Skaergaard intrusion crystallized from an "emulsion" of Fe-and Si-rich immiscible melts, and explained the heterogeneous composition of the melt inclusions in plagioclase from the Lower Zone by the entrapment of various proportions of Si-rich and Fe-rich liquid. In the Upper Zone, most of the inclusions in apatite are composed of end-member liquids. Metresized pods and layers of melanogranophyre in the upper part of the intrusion are believed to represent pooled bodies of the immiscible Si-rich liquid.

If we accept the existence of liquid immiscibility, the question then becomes the mode of segregation of the two liquids. [START_REF] Charlier | Large-scale silicate liquid immiscibility during differentiation of basalt to granite and the origin of the Daly gap[END_REF] and [START_REF] Jakobsen | Crystallization of the Skaergaard Intrusion from an emulsion of immiscible iron-and silica-rich liquids: Evidence from melt inclusions in plagioclase[END_REF] considered that evolved Fe-Ti-P gabbros from the Skaergaard and Sept-Iles layered intrusions have crystallized from a "liquid emulsion". For [START_REF] Namur | Dual origin of Fe-Ti-P gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the Sept Iles layered intrusion[END_REF] the question concerning the origin of these rocks is not whether they are "cumulates" or "solidified immiscible liquids" but whether they are cumulates formed from a homogenous melt or from a mixture of two immiscible liquids. [START_REF] Charlier | Fe-Ti-V-P ore deposits associated with Proterozoic massif-type anorthosites and related rocks[END_REF] in a review paper describing Fe-Ti-V-P ore deposits in anorthosites listed five possible ore-forming processes: (1) fractional crystallization, crystal sorting and plagioclase buoyancy, (2) immiscibility, (3) magma mixing (4) filter-press compaction and (5) hydrothermal remobilization. They concluded that immiscibility of an Fe-Ti-(P)-rich melt should be more carefully examined. [START_REF] Duchesne | The origin of nelsonite and high-Zr ferrodiorite associated with Proterozoic anorthosite[END_REF] suggested that high REE values in whole rocks and apatites in the Hestnes deposit (Rogaland anorthosite province, Norway) coexisting with high values of Cr in magnetites indicates that the Hestnes cumulates were derived from an Fe-rich liquid resulting from an immiscibility process.

What are the main evidences in the Lanjanina mineralisation in favour of one of these five processes? The very high proportion of Fe-Ti oxide is compatible with fractional crystallization and crystal sorting, but the lack of anorthosite in the Itsindro complex does not permit us to invoke plagioclase buoyancy. Trends in the evolution of the mineral composition in the cumulate pile could suggest magma mixing, but since the ore minerals are distributed along the section, such a process cannot be responsible for the formation of the Fe-Ti oxides. Filter pressing can be invoked as a mechanism causing the removal of trapped interstitial liquid but cannot have a major role in the mineralizing process. Finally we have no evidence of hydrothermal remobilization.

Thus, with reference to some of the literature data we favor the hypothesis of immiscibility between two silicate melts with (1) an Fe-Ti-(P)-rich liquid crystallizing to form the ferrogabbro and the ferropyroxenite, plus the Fe-Ti oxides and apatite, and (2) an Si-rich liquid taking the form of granitic dykes, intruding the top 120 m of the pyroxenite unit (like the granophyre veins in the Skaergaard Intrusion) and forming a sill or layer. The separation of the two immiscible liquids must have been facilitated by the density contrast between the heavier, Fe-rich liquid and the lighter Si-rich viscous one. The Fe-Ti-(P)-rich liquid will undergo fractional crystallization and crystal sorting, leading to the abnormally high concentration of apatite and Fe-Ti oxides. This model also provides an explanation for the presence of Si-rich dykes in the gabbro and pyroxenite units.

Accepting that the Lanjanina sequence corresponds to the two end-members of the immiscibility process, nothing explains the mechanism leading to the formation of Fesulphide. Such a high proportion of disseminated and locally sub-massive sulphides must be the crystallization product of a sulphide liquid, and such a liquid must result from immiscibility between sulphide and silicate liquids. Mechanisms leading to the formation of an immiscible sulphide liquid are well documented e.g., [START_REF] Naldrett | Nickel sulfide deposits: classification, composition, and genesis[END_REF][START_REF] Naldrett | Sulfide melts -crystallization temperatures, solubilities in silicate melts, and Fe, Ni, and Cu partitioning between basaltic magmas and olivine[END_REF][START_REF] Lesher | Komatiite-associated nickel sulfide deposits[END_REF]. In such a context, i.e., an evolved magma, an S enrichment in the magma attributable to crustal contamination must be envisaged but has not so far been demonstrated. 10.5. Sulphide immiscibility in an Fe-Ti-P liquid: evidence from the Duluth Complex as a possible analogue Although occurrences of apatite Fe-Ti-rich oxide rocks ("nelsonites") are common in gabbroic and ultramafic rocks, the complex association of apatite, Fe-Ti oxide, Fe sulphide and mafic silicates observed in the Lanjanina drill hole is fairly uncommon. As far as we know, however, the occurrence of apatite-and Fe-Ti oxide-rich rocks containing sulphide minerals in amounts up to several volume percent has been described only in the Boulder Lake area in the Southern part of the Duluth Complex. In this complex, apatite and Fe-Ti oxide minerals are also found in the massive sulphide mineralization at the Babbitt deposit. These occurrences led [START_REF] Ripley | Evidence for sulfide and Fe-Ti-P-rich liquid immiscibility in the Duluth Complex, Minnesota[END_REF] to compare the two types of mineralization and to speculate about their origin.

The Boulder Lake area is composed of several intrusive bodies in the troctolitic rocks of the Duluth Complex. These intrusions are either apatite-and Fe-Ti oxide-rich ultramafic bodies or apatite-poor oxide-rich ultramafic bodies, both showing the same geometry and position.

The intrusions range from lenses up to 25 m in thickness to large ovoid bodies as much as 500 x 150 x 200 m in size. The ultramafic facies are composed of olivine and pyroxene, in various proportions from dunite to pyroxenite, with local gradations between both. Apatite content ranges from 2 to 30 % and Fe-Ti oxides from 10 to 25 % with a few massive oxide zones up to 40 m thick. Ilmenite is generally the dominant oxide, always accompanied by titanomagnetite. Plagioclase accounts for less than 10 % in most Fe-Ti oxide ultramafic intrusions, but may reach 50 % in oxide-rich troctolitic portions. Sulphide minerals are found as interstitial phases or as inclusions in apatite and silicates. Accessory minerals include Znrich hercynite, biotite, zircon and baddeleyite; alteration minerals include chlorite, serpentine and actinote.

The Babbitt area, located in the north of the complex (about 60 km N of the Boulder Lake area) contains a vein-like massive sulphide mineralization (with pyrrhotite, cubanite and chalcopyrite) known as the Babbitt deposit. It is also characterized by the presence of apatite as up to 10 volume % of euhedral crystals. The nature of the apatite, included in massive sulphide, is identical to that in Fe-Ti oxides from the Boulder Lake area. Moreover the minerals found in the massive sulphide mineralization at Babbitt are identical to those found in the nelsonite occurrences in the Boulder Lake area, but in different proportions: ilmenite, magnetite, Zn-hercynite, biotite, zircon and baddeleyite occurring as accessory minerals.

For [START_REF] Ripley | Evidence for sulfide and Fe-Ti-P-rich liquid immiscibility in the Duluth Complex, Minnesota[END_REF], the presence of rounded sulphide droplets in apatite and oxide as well as interstitial grains of sulphide in a framework of oxide and apatite crystals suggest that an immiscible sulphide liquid accompanied the apatite-oxide liquid, which itself is most consistent with an origin through Fe-Ti-P liquid immiscibility. The fact that both the nelsonite and sulphide occurrences contain accessories such as biotite, Zn-rich spinel, zircon and baddeleyite suggests that small amounts of incompatible elements may be found in both immiscible sulphide and Fe-Ti-P liquids [START_REF] Ripley | Evidence for sulfide and Fe-Ti-P-rich liquid immiscibility in the Duluth Complex, Minnesota[END_REF] concluded that the apatite-bearing sulphide mineralization at Babbitt and the sulphide-bearing apatite-Fe-Ti oxide rock occurrences at Boulder Lake represent end members of immiscible liquids formed during crystallization of tholeiitic magmas, most likely in the presence of a volatile phase. Nelsonite at Boulder Lake and massive sulphide at Babbitt are associated with a ferrogabbro host rock and derivation of each type of mineralization from such a parental melt is a strong possibility, but the nature of the parental magma from which the immiscible liquids were derived remains poorly constrained.

The Lanjanina mineralization probably represents the same processes but with sulphide immiscibility occurring within the oxide-apatite rich liquid. The sulphide composition, dominated by Fe-sulphide, confirms that S immiscibility occurred in an evolved liquid, having a composition close to the Fe-Ti-P gabbro, strongly impoverished in Ni and Cu.

The geology of the Itsindro gabbro massif is poorly constrained, and we have no field observations showing the relationships between the "Lanjanina" facies and the other gabbro facies. The geochemical and geophysical exploration campaign conducted in the Itsindro massif enabled evaluation of the extensions of two anomalies caused by the sulphide and Fe-Ti-P mineralization, covering an area of a few ha. Only the southern one (Lanjanina) was drilled.

We speculate that these two anomalies trace the outline of the Lanjanina facies, corresponding to a quite limited volume at the scale of the Massif. Thus, by analogy with the Duluth Complex's Boulder Lake area, it is suggested that the Lanjanina type ultramafic mineralized facies form intrusive bodies within the gabbroic rocks corresponding to a specific magmatic evolution.

Contamination and S immiscibility

A common feature for a large number of sulphide deposits is the presence of graphite associated with ore, with graphite-sulphide-bearing schist in the vicinity of intrusions [START_REF] Boyd | The nickel mineralization of the Råna mafic intrusion, Nordland, Norway[END_REF][START_REF] Mainwaring | Country-rock assimilation and the genesis of Cu-Ni sulphides in the water Hen intrusion, Duluth complex, Minnesota[END_REF][START_REF] Peltonen | Magma-country rock interaction and the genesis of Ni-Cu deposits in the Vammala Nickel Belt, SW Finland[END_REF].

Assimilation/contamination processes, and in particular assimilation of graphite-sulphurbearing rocks, appear to be important in the formation of sulphide ore deposits [START_REF] Naldrett | World-class Ni-Cu-PGE deposits: key factors in their genesis[END_REF], because of the modifications of magmatic processes they imply. Note however that graphite can also be a primary magmatic phase precipitated from the partially outgassed residual liquid trapped between silicate crystals, when saturation of the liquid in carbon is reached [START_REF] Pineau | Primary igneous graphite in ultramafic xenoliths : II. Isotopic composition of the carbonaceous phases present in xenoliths and host lava at Tissemt (Eggéré, Algerian Sahara)[END_REF]. This process of crystallisation of graphite under magmatic conditions from a carbon outgassed melt is also applicable to mineralised samples. The role of the host-rock is important because the carbon contamination of magma enhances the crystallisation of graphite from the residual [START_REF] Pineau | Primary igneous graphite in ultramafic xenoliths : II. Isotopic composition of the carbonaceous phases present in xenoliths and host lava at Tissemt (Eggéré, Algerian Sahara)[END_REF]. Study of carbon isotopes allows the contribution of contamination to be established [START_REF] Lerouge | A stable isotopic study of the Bruvann Ni deposit, Norway[END_REF].

The strong sulphide enrichment in the Lanjanina area associated with highly differentiated facies must arise from the incorporation of S from the country rock. This is clearly not the case, for example, for the PGE-poor Fe-Ni-Cu sulphides described as tiny blebs (1 to 3 vol. %) in Fe-Ti oxide-bearing layered intrusions in the Tarim igneous province, NW China, for which a mantle origin has been demonstrated, with sulphide saturation occurring at a late stage of the fractional crystallization process [START_REF] Cao | Triggers on sulfide saturation in Fe-Ti oxide-bearing, mafic-ultramafic layered intrusions in the Tarim large igneous province, NW China[END_REF].

The host "schisto-quartzo-calcaire" series is the probable source of the S contamination. We have no evidence however that the molybdenite, associated with the sulphides, reflects this contamination episode. Moreover Re, almost systematically associated with molybdenite in the form of rheniite, is generally interpreted as evidence of a magmatic hydrothermal process.

A tentative model

Several papers have recently proposed models to account for the formation of Fe-Ti oxide deposits. Some are focused on the large Permian Emeishan igneous province (ELIP, SW China, [START_REF] Liu | Using multiphase solid inclusions to constrain the origin of the Baima Fe-Ti-(V) oxide deposit, China[END_REF][START_REF] She | The formation of Prich Fe-Ti oxide ore layers in the Taihe layered intrusion, SW China: Implications for magmaplumbing system process[END_REF][START_REF] Xing | Trace element compositions of apatite from the middle zone of the Panzhihua layered intrusion, SW China: Insights into the differentiation of a Pand Si-rich melt[END_REF][START_REF] Zhou | Geochemistry, petrogenesis and metallogenesis of the Panzhihua gabbroic layered intrusion[END_REF]2008;2013).

They first consider a deep-seated magma chamber into which is injected a high-Ti picritic mantle-derived magma that will accumulate ultramafic minerals. The residual liquid will be Fe-Ti enriched. After immiscible separation, the two magma compositions will evolve separately.

By analogy with the ELIP, the model proposed (Fig. 12) considers a mantle-derived primitive magma. This magma, stored in a deep-seated magma chamber, will crystallize ultramafic cumulates. Analogues of such ultramafic facies are known in the area, such as the Ambodilafa complex [START_REF] Tucker | Notice explicative des cartes géologique et métallogéniques de la République de Madagascar à 1/1 000 000[END_REF]. The residual liquid will be an evolved Fe-Ti-rich magma that will continue its fractional crystallization process and generate the various pyroxenite and gabbro units that form the Itsindro complex. Part of this magma (or a different pulse) will have a different evolution and follow two compositional trends after immiscible separation: an Si-rich magma and an Fe-Ti-P-rich, Si-poor magma. The Si-rich magma will be expelled from the Si-poor melt and form syenitic and granitic dykes and lenses in the complex. Owing to crustal contamination, the Fe-Ti oxide-rich magma will become enriched in S, and at a certain stage of its evolution (cooling, changes in physical and chemical parameters), S saturation will result in the formation of an immiscible sulphide liquid. The massive crystallization of Fe-Ti oxides will drag down droplets of sulphide liquid, resulting in various mineralized Fe-Ti oxide and Fe-(Cu-Ni) sulphide-rich ore types. The Fe-rich nature of this evolved magma explains the composition of the Fe-Cu-Ni sulphides, largely dominated by pyrrhotite.

Conclusion

The Lanjanina series drilled in the Itsindro Gabbro Complex corresponds to a mineralized part of the complex. As such, it is not possible to extrapolate the result obtained here to the whole complex. The Lanjanina series provides an illustration of complex immiscibility and contamination processes. The first immiscibility affecting an evolved mafic magma results in the segregation of two liquids, a "light" Si-rich magma that will tend to coalesce and be evacuated through dykes (conduits) to form an upper granitic layer, and a dense Fe-Ti-rich, Si-poor magma. At a certain stage, this magma will be contaminated by S. The dense magma will start to crystallize according to a normal fractional crystallization process, with cumulus olivine, apatite, pyroxene, and cumulus to intercumulus plagioclase. Early in its evolution, S saturation will result in sulphide immiscibility, with formation of sulphide droplets.

Fe-Ti oxides will tend to grow on the S droplets and both heavy phases will accumulate locally to form layers. The Fe-Ti-rich composition of the magma explains both the abundance of Fe-Ti oxides, and the composition of the sulphide (the magma being totally impoverished in Cu, Ni and PGE).

The mineral composition exhibits two trends in the ultramafic units. The first, observable from the bottom to 163 m, reflects a normal fractionation trend, whereas the second marks an abrupt change reflecting perturbations in the system. The main mineralized zone is found in the second part, but there is no evidence of a link between the two.

By definition, the Lanjanina area is an "anomaly" in the complex (it corresponds to a geochemical anomaly). The relationship between this series and the whole complex has not been established. The complex has been interpreted as a series of gabbro-forming sills or laccoliths in a metamorphosed sedimentary series, with at least four gabbro units. It is further envisaged that the evolved mafic magma from which the Lanjanina series has been derived could be related to one of these magma pulses.

and associated Fe-Ti-V oxide deposits, Sichuan Province, SW China. Journal of Petrology 929 46, 2253-2280. F, Detail of the complex contact between titanomagnetite (with ilmenite exsolution) and ilmenite (dark grey, sample LJ12). In black, Al-Zn spinel surrounding the interfingering of titanomagnetite in ilmenite. G, Detail of the complex ilmenite exsolution system in titanomagnetite (sample LJ4). H, I, molybdenite (Mol) and rheniite (Re) included in pyrrhotite (samples LJ10 and LJ12 respectively). Experimental values from Perkins and Viezeuf (1992). Selected electron microprobe analyses of ilmenite.

List of Figures

  Cu-Ni sulphides and Fe-Ti oxides occur throughout the LJ1 borehole and there are no occurrences of oxide without sulphide or sulphide without oxide. The collected samples exhibit various proportions of Fe-Cu-Ni sulphides and Fe-Ti oxides (as reflected by whole-rock composition) and in spite of local variations the average proportion may be estimated at 50/50. At centimetre scale, nearly massive Fe-Cu-Ni sulphides with inclusions of Fe-Ti oxides alternate with sub-massive Fe-Ti oxides containing Fe-Cu-Ni sulphide inclusions.

  Major elements were analysed by ICP-AES after lithium borate fusion; Ag, Cu, Cd, Co, Mo, Ni, Pb, Sc, and Zn by ICP-AES after four-acid dissolution; other trace elements and rare elements by ICP-MS after lithium borate fusion; C and S by combustion furnace; and As, Bi, Hg, Sb, Se, and Te by ICP-MS after aqua regia dissolution. Silicates and oxides were analysed by electron probe microanalyser (EPMA) with the CAMECA SXFive operated by the "Plateforme analytique régionale" (BRGM -Université d'Orléans -CNRS Orléans) equipped with 5 WDS spectrometers under the following conditions: acceleration voltage of 15 keV, beam current of 12 nA, counting time of 10 sec.

Fig. 1 .

 1 Fig. 1. Geological map of the Itremo region, showing the Itsindro gabbro and the position of the LJ1 drilling (star). After Handke et al. (1999).

Fig. 2 .

 2 Fig. 2. Simplified stratigraphic section of the Itsindro gabbro in the Lanjanina area showing the location of the analysed samples collected in borehole LJ1 (after Fournié 1970).

Fig. 3 .

 3 Fig. 3. Photomicrographs illustrating the rock types and ore textures of the Lanjanina borehole. Vertically, same image in plane-polarized light (top) and cross-polarized light (bottom). A, D Pyroxenite (sample LJ14); B, E Olivine gabbro showing a magmatic layering (sample LJ30). C, F Typical net texture in an olivine pyroxenite. Opaque minerals are Fe-Ti oxides (dominant) and Fe-Cu-Ni sulphides (sample LJ31); G, J Euhedral apatite enclosed in ore minerals. Olivine gabbro LJ28. H, K Detail of the net-texture, with interstitial ore minerals, euhedral apatite, pyroxene and olivine (sample LJ19). I, L Detail of the net texture, with interstitial ore minerals, euhedral pyroxene and olivine forming a thin rim between clinopyroxene and the ore minerals (sample LJ19). Scale bar is 2 mm, except for H, I, K, L where it is 0.5 mm. Abbreviations: Opa undifferentiated opaque minerals, Cpxclinopyroxene, Ol -olivine, Ap -apatite.

Fig. 4 .

 4 Fig. 4. Scanning electron microscope images (backscattered electron mode). A, Typical nettexture showing the interstitial ore minerals (euhedral Fe-Ti oxides -grey-in a Fe-Cu-Ni sulphides matrix -white) surrounding cumulus silicates -black (sample LJ4); B, detail of A showing the relationships between the ore minerals and the cumulus silicates; C, euhedral Fe-Ti oxides (grey) enclosed in Fe-Cu-Ni sulphides (white), adjacent silicates in black (sample LJ6); D. euhedral (elongated) apatite (dark grey) and euhedral Fe-Ti oxide (grey) enclosed in Fe sulphide (sample LJ10); E, titanomagnetite (Mag) with ilmenite exsolutions adjacent to ilmenite (Ilm). Silicate in black, sulphide droplets (BMS) in white (sample LJ12).

Fig. 5 .

 5 Fig. 5. Binary diagrams showing the correlation between major elements in pyroxenite (open diamond) and gabbro (grey square).

Fig. 6 .

 6 Fig. 6. Binary diagrams showing the correlation between minor and trace elements in pyroxenite (open diamond) and gabbro (grey square).

Fig. 7 .

 7 Fig. 7. Rare earth element content in selected samples of the Lanjanina bore hole. A correlation of REE vs P 2 O 5 , B, C and D chondrite-normalized REE patterns for gabbro (B), apatite-poor pyroxenite (C) and apatite-rich pyroxenite (D). Tm is below the detection limit.Normalization according to[START_REF] Mcdonough | Composition of the Earth[END_REF].

Fig. 8 .

 8 Fig. 8. Plots of (A) TiO 2 in wt percent and (B) Al 2 O 3 in wt percent vs. Mg/(Mg+Fe) in molar percent from clinopyroxenes in pyroxenite and gabbro from the Itsindro Complex.

Fig. 9 .

 9 Fig. 9. Clinopyroxene, olivine and plagioclase compositions along stratigraphic height. MMZ, main mineralized zone, ML, marker level.

Fig. 10 .

 10 Fig. 10. Plot of MnO in wt. percent vs Mg/(Mg+Fe) in molar percent in olivine in pyroxenite and gabbro from the Itsindro Complex.

Fig. 11 .

 11 Fig. 11. Distribution of Mg/(Mg+Fe) in molar percent between olivine and clinopyroxene.

Fig. 12 .

 12 Fig. 12. Schematic model showing the various liquid immiscibility processes and crustal contamination leading to the formation of the complex Fe-Ti oxide Fe-(Cu-Ni) sulphide mineralization in the Itsindro Complex, Madagascar.

Table 9

 9 Selected electron microprobe analyses of Al-Zn spinel. Fe 2 O 3 calculated assuming stoichiometry.

	Table 10
	Calculated proportions (percent) of chalcopyrite (Ccp), pentlandite (Pn), pyrrhotite (Po),
	ilmenite (Ilm), magnetite (Mag), apatite (Ap) and silicates in pyroxenite and gabbro (see text
	for explanation).

Acknowledgements

We thank Guillaume Wille for the EPMA analyses obtained with the Cameca SXFive at the "Plateforme analytique régionale" (BRGM -University -CNRS, Orléans), O. Rouer and D.

Ohnenstetter for the EPMA analyses of apatite and Prof. J.C. Duchesne, Prof. H.G. Dill and an anonymous reviewer for their critical comments. The English text was edited by J.V. Guy-Bray.

Code 2 Fe 0.020 0.011 0.025 0.016 0.000 0.017 0.014 0.000 Mn 0.000 0.000 0.000 0.000 0.003 0.000 0.011 0.000

List of Tables

 Table 1Characteristics of the samples analysed in the Itsindro intrusion. For Fe-Cu-Ni sulphides (undifferentiated sulphides) and Fe-Ti oxides (titanomagnetite, ilmenite and spinel), the number and size of "X" reflect the relative proportions of each. MMZ, main mineralized zone, from 72 to 90 m in the pyroxenite unit. Stars at 162.8 correspond to the marker level. Code: 1 pyroxenite, 2, mineralized pyroxenite, 3 gabbro, 4, mineralized gabbro.

Table 2 Major and trace element compositions of selected samples from the Lanjanina borehole.

Table 3 Selected electron microprobe analyses of clinopyroxene.

Table 4

Selected electron microprobe analyses of olivine.

Table 5 Selected electron microprobe analyses of plagioclase.

Table 6

Selected electron microprobe analyses of apatite. OH calculated assuming full site occupancy.