ORIGINAL PAPER

Mineralogy, fluid inclusion, and oxygen isotope constraintson the genesis of the Lalla Zahra W-(Cu) deposit, Alouanadistrict, northeastern Morocco

Mohamed Sadequi & Mohammed Bouabdellah &

Abdellah Boushaba & Eric Marcoux & Alain Cheilletz

Received: 19 December 2011 /Accepted: 3 April 2012 /Published online: 28 April 2012# Saudi Society for Geosciences 2012

Abstract The Lalla Zahra W-(Cu) prospect of northeasternMorocco is hosted in a Devonian volcaniclastic and meta-sedimentary sequence composed of graywacke, siltstone,pelite, and shale interlayered with minor tuff and mudstone.Intrusion of the 284±7 Ma Alouana concentrically zoned,two micas, calc-alkaline, and post-collisional Alouana gran-itoid pluton has contact metamorphosed the host rocks,giving rise to a metamorphic assemblage of quartz, plagio-clase, biotite, muscovite, chlorite, and alusite, and cordierite.The mineralization occurs in and along subvertical, 20 to40 cm thick, and structurally controlled tensional veinscomposed of quartz accompanied by molybdenite, wolfram-ite, scheelite, base metal sulphides, carbonates, barite, andfluorite. Three main stages of mineralization (I, II, and III),each characterized by a specific mineral assemblage and/ortexture, are recognized. Quartz dominates in all the veinsand commonly displays multiple stages of vein filling and

brecciation, and a variety of textures. The early tungsten-bearing stage consists of quartz-1, tourmaline, muscovite,wolframite, scheelite, and molybdenite. With advancingparagenetic sequence, the mineralogy of the veins shiftedfrom stage I tungsten-bearing mineralization throughstage II, dominated by base metal sulphides, to stageIII with late barren carbonates and barite±fluorite miner-al assemblages. Pervasive hydrothermal alteration affect-ed, to varying degrees, the Alouana intrusion, resultingin microclinization, albitization, episyenitization, andgreisenization of all the granitic units. Fluid inclusiondata yield homogenization temperatures ranging from124°C to 447°C for calculated salinity estimates in therange of 0.4 to ~60 wt% NaCl equiv. Similarly, the δ18Ovalues for the three generations of quartz range from11.7‰ to 13.9‰ V-SMOW. Calculated δ18O values ofthe parent fluid in the range between −3‰ and +9‰ V-SMOW are consistent either with a mixture of water ofdifferent origins, including magmatic water, or an originfrom seawater or meteoric water that probably exchangedoxygen with rocks at elevated temperatures. The coexis-tence of CO2-rich and H2O-rich fluid inclusions reflectthe presence of a boiling fluid associated with the depo-sition of the early tungsten-bearing stage mineralizationat relatively high temperature. The general temperatureand salinity decrease with advancing paragenetic sequencesuggest that the early high temperature, magmatic, high-ly saline, and boiling fluid mixed with meteoric non-boiling fluid results in the precipitation of base metalsulphide and carbonate–barite stage mineral assemb-lages, respectively.

Keywords Northeastern Morocco . Alouana two micasintrusion . Tungsten vein-type mineralization . Fluidinclusions . Oxygen isotopes . Boiling

M. Sadequi :A. BoushabaUFR Pétrologie, Faculté des Sciences Dhar El Mahraz,B.P. 1796, Atlas-Fès, Fès, Morocco

M. Bouabdellah (*)Laboratoire des Gîtes Minéraux, Hydrogéologie &Environnement, Département de Géologie, Faculté des Sciences,Université Mohamed 1,B.P. 717, 60000, Oujda, Moroccoe-mail: mbouabdellah2002@yahoo.fr

E. MarcouxInstitut des Sciences de la Terre d’Orléans (ISTO),Université d’Orléans,B.P. 6749, Orléans cedex 2, France

A. CheilletzEcole nationale supérieure de géologie, ENSG-INPL,CRPG-CNRS, Université de Lorraine,BP 20, 54501, Vandoeuvre-les-Nancy,Nancy, France

Arab J Geosci (2013) 6:3067–3085DOI 10.1007/s12517-012-0571-0

Introduction

Vein-type tungsten-(± tin) ore deposits are closely related tothe crystallization and cooling history of highly fractionatedand greisenized two micas granitoid cupolas (Tischendorfand Förster 1990; Breiter et al. 1999; Cuney et al. 2002;Černý et al. 2005). Such spatial and/or temporal associationhas provided empirical guides to exploration and left nodoubt that granitic magmatism plays a prominent controlon the W–Sn enrichment, with the magmatic engine actingas a source of heat, magmatic volatiles, and ore-formingcomponents. The genetic relationships and specifically thecontributions of purely magmatic fluids, relative to fluidsfrom sources external to the granitoid intrusion, remain,however, controversial. Recent geochemical advances re-veal the involvement of both fluid types, and that interactionat the interface between a hydrospheric and a magmaticfluid regime appears to be the key process that determinesthe structure, location, size, and ore grade of W-(±Sn)deposits (Heinrich 1995).

In the Moroccan Massif Central, Paleozoic inliers, locallyintruded by Late-Hercynian (Variscan) granitoids, host mostof the economic W-(±Sn) deposits of the country. Theselatter are spatially and genetically related to the 290 Madifferentiated granites of Zaër, Ment, Oulmes, and JbelAouam (Fig. 1) (Termier et al. 1950; Giuliani and Sonet1982; Giuliani 1984; Cheilletz 1984; Sonnet and Verkaeren1989). The most productive deposits are those of El Karit(closed today with a total production of 725 t of concen-trates at 72 % to 74 % Sn; Heck 1946) and, most

importantly, Achmmach (Fig. 1), currently under evaluationwith probable geologic reserves of 9.73 Mt of 1.08 % Sn(Fettouhi et al. 2002). Conversely, Hercynian terrains ofnortheastern Morocco, as well as those of the rest of thecountry, have been judged as non-potential W-(±Sn)-bear-ing targets because of the small size of the intrusions andtheir primitive “non-specialized” geochemistry, as definedby Tischendorf (1977).

The Alouana district which hosts the newly discov-ered Lalla Zahra tungsten±copper prospect, describedherein for the first time, is located in the HercynianDebdou–Mekkam inlier of northeastern Morocco. Itwas discovered in the 1990s by the Moroccan Officeof Hydrocarbons and Mines (former BRPM), and at thattime, trenching, drilling, and soil geochemical prospec-ting delineated a mineralized zone of about 3×2 km(Fig. 2b). No follow-up work has ever been done inthis area, and the potential for undiscovered tungstenresources still exists. Up to now, only very limitedgeochemical studies have been carried out (Figuet1963; Lasri 1993), and there is no available informationon the precise mineralogy and geochemistry of fluidsresponsible for the emplacement of the W-(±Cu)-bearingmineralization. Accordingly, the main objectives of thecurrent study are (1) to describe the mode of occurrenceand mineralogy of the W-(±Cu)-bearing veins, (2) toprovide constraints of the temperature and compositionon the ore-forming fluids, and (3) to draw up a prelim-inary genetic model describing how the W-(±Cu) min-eralization formed.

ANTI ATLAS

Agadir

Marrakech

Oujda

AlouanaAchmmach

Hassian ed Diab

Ment

El Karit

Rif domain

Meso-Cenozoic Cover

Atlas system

Late-Hercynian granite

Hercynian inlier100 Km

RabatAtla

ntic O

cean

Mediterranean Sea

8° 2°

36°

32°

N

Oulmès

AouamZaer

Fig. 1 Geological mapshowing the spatial distributionof the main Paleozoic inliers ofnorthern Morocco inrelationship to late-Hercyniangranitoids and related tin–tung-sten deposits. Also indicated(inset) is the location of thestudied district

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Geologic setting

The Alouana district is located within the Hercynian Debdou–Mekkam inlier of the Oran–Moroccan Meseta of northeasternMorocco (Fig. 1). The regional stratigraphy consists of up to1,000 m of a Middle to Late Devonian (Givetian–Frasnian)(Marhoumi 1984) clastic metasedimentary and volcaniclasticsequence locally intruded by a Late Hercynian granitoid plu-ton known as the Alouana intrusion (Fig. 2a). The metasedi-mentary and volcaniclastic rocks are dominated by a flysch-like complex (Medioni 1980) made of olive-green to purpleschists and phyllites interlayered with immature sandstone-graywacke-siltstones and, to a lesser extent, tuffs and carbo-nates. Since their deposition, the Paleozoic rocks have under-gone the effects of ductile and brittle polyphase deformationrelated to the successive Caledonian and Hercynian orogenies.In this respect, three main Hercynian deformation episodes(D1, D2, and D3) have been recognized (Desteucq andHoepffner 1980), two of which (D1 and D2) are of pre-Late Visean age, and the third one (D3) is of post-LateVisean, resulting in the development of a network of struc-tures of various scales, including fracture cleavage or flowschistosity associated with faults and recumbent and isocli-nal folds. Available geochronological data indicate that themain phase of the Hercynian orogeny occurred at circa 368

to 372±4 Ma using the K/Ar method on metamorphicmineral separates (Huon 1985). No precise temperatureestimates have been published for the peak metamorphismthat reached greenschist facies grade, but due to the dom-inance of albite, muscovite, chlorite, and epidotes in pelitichost rocks, it is unlikely that peak temperatures were inexcess of 300°C. Conversely, thermal metamorphism pro-duced by the emplacement of the Alouana intrusion gaverise to a regionally extensive metamorphic aureole thatconsists of calc-silicate bands and spotted schist exhibitingporphyroblasts of cordierite, andalusite, chlorite, muscovite,and biotite (Fig. 3a, b) in a carbonaceous groundmass ofmuscovite, biotite, plagioclase, and detrital quartz. Thesemetamorphic equilibrium mineral assemblages indicatepeak thermal conditions ranging from 350°C in the chlo-rite–muscovite zone to 550–650°C at the granite contacts.Pressures are estimated to be between 0.5 to 1 kb,corresponding to batholith emplacement at depths rangingfrom 1.6 to 3.2 km (Targuisti 1983; Talha 1996; and thepresent study).

Unconformably overlying the Paleozoic succession is atabular transgressive cover, composed predominantly ofLiassic carbonates. These latter along with the Paleozoicpackage have been affected to varying degrees by theAtlasic brittle polyphase deformation whose effects are

Fig. 2 a Geological setting of the Alouana district showing the loca-tion of the Lalla Zahra W-(Cu) district (inset; modified after thegeological map of Debdou sheet at a scale of 1:100.000). b Geological

map of the Lalla Zahra prospect showing the distribution of “major”recently discovered W-(Cu)-bearing veins with the location of thestudied samples (filled circles)

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Fig. 3 Representative microphotographs (a–e) and hand-specimen sam-ples (f–h) illustrating aspects of mineralogy, texture, alteration, and min-eralization from the Alouana intrusion and Lalla Zahra W-(Cu) prospect.aCrossed-nicols-polarized light (XPL) microphotograph of contact meta-morphosed cordierite–biotite-bearing pelitic schist showing twinned por-phyroblast of cordierite, biotite lamellae, and muscovite resulting fromthe alteration “pinnitization” of cordierite. b XPL microphotograph ofcontact metamorphosed and alusite–cordierite-bearing pelitic schistshowing porphyroblasts of andalusite and cordierite post-dating the S2crenulated foliation but predating the S3 strain-slip cleavage. c XPLmicrophotograph of muscovite-bearing leucogranite showing majorrock-forming mineral dominated by K-feldspar and quartz. d XPL mi-crophotograph of biotite-bearing granite showing major rock-forming

mineral dominated by K-feldspar, plagioclase, and quartz. The mainmafic mineral phase consists of “kinked” biotite lamellae. e XPL micro-photograph highlighting the most widespread hydrothermal alterationassemblage dominated by the development of muscovite crystals overalbite (greisenization) in the hydrothermally altered leucogranite. f Min-eralized hand-specimen sample from the oxide-silicate stage showingmineral assemblage dominated by Qz-1 and wolframite. g Mineralizedhand-specimen sample from the sulphide stage consisting of Qz-2 andchalcopyrite, with this latter being oxidized to malachite (greenish color).h Late sheeted quartz tension vein composed of Qz-2 (base metal sul-phide stage) and Qz-3 (carbonate–barite±fluorite stage) crosscutting thepreviously brecciated Qz-1 and associated wolframite ore mineral assem-blage (tungsten oxide-silicate stage)

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materialized by the development of the ENE-trending faultthat has downthrown the western limit of the Alouanaintrusion (Fig. 2a).

The Alouana intrusion, dated at 284±7 Ma by Rb/Sr onmuscovite (Tisserant 1977), occurs as a post-collisional elon-gate NW–SE-trending 6×3.5 km oval-shaped, shallow-crustalbody of calc-alkaline composition (Fig. 2a). The intrusion iscompositionally complex and consists of three main petro-graphic units (Fig. 2a) corresponding to separate stages ofmagmatism. The following detailed mineralogical and geo-chemical description is based on the work by Targuisti (1983),Rosé (1987), Talha (1996), and El Hadi et al. (2003) andcompleted by field mapping and petrographic observationsof the authors. Accordingly, only a summary of the mainconclusions which are relevant to the present study is givenin the following discussion.

Indeed, the Alouana intrusion exhibits a distinct trendfrom marginal pink fine-grained muscovite–cordierite-bear-ing leucogranite “exogranite” (Fig. 3c), through intermedi-ate porphyritic- to medium-grained two-micas (biotite andmuscovite) granite, to extensively exposed central coarse-grained (>mm) equigranular biotite-bearing granite “endog-ranite” (Fig. 3d). These textural variations are characteristicof granites that have exsolved an aqueous phase (White etal. 1981). The contact between the marginal leucograniteand the adjacent porphyritic granite is sharp, whereas theboundary between the latter and the inner biotite-bearinggranite is rather gradational. Furthermore, a swarm of east-northeast-striking, steeply south-dipping intragranitic peg-matite and aplite along with microdiorite and dolerite-diabase dikes intrude all the granitic and supracrustal rocksdescribed previously. Miarolitic cavities are restricted to themarginal pink fine-grained leucogranite. No apparent expo-sures of granitoid intrusion crop out in the vicinity of LallaZahra prospect, but a buried granitic body, whose existencehas been initially suggested by geophysical data (i.e., ellip-tical gravity low), was intersected at 200 m depth duringexploration drilling. Macroscopic and microscopic exami-nations of a series of core samples reveal that the hiddengranitic intrusive corresponds to a microleucogranite whosetextural, mineralogical, and geochemical compositions aresimilar to those characterizing the Alouana marginal pinkfine-grained leucogranite.

Overall, the three granitic units show consistently homo-geneous mineral assemblages dominated by quartz (20–45 %), plagioclase (15–40 %), K-feldspar (15–35 %), biotite(3–11 %), and muscovite (<1–7 %), with minor apatite,zircon, allanite, topaz, tourmaline, and ilmenite rather thanmagnetite as reported by Talha (1996), though the proportionsof the mineral species vary from one specific unit to another.Quartz phenocrysts show embayments or other resorptiontextures especially near the top of the coupola. Altogether,the granitic rocks of Alouana intrusion are classified as

alkali-feldspar granite and granodiorite according to the Q-A-P diagram of Streckeisen (1974).

Geochemically, all granitic rocks are silica rich, withSiO2 contents ranging from 69 to 77 wt%, and exhibitvariable amounts of (Fe2O3)T, CaO, Na2O, K2O, andMgO, reflecting different modal compositions and varyingdegrees of alteration. The mineralogical and geochemicalevolutionary trends from marginal pink fine-grained leucog-ranite through porphyritic two-micas granite to coarse-grained biotite-bearing granite have been attributed to insitu fractional crystallization of a biotite granodioritic tomonzonitic magma parent (Targuisti 1983). The relativelyelevated initial 87Sr/86Sr ratios (0.7070±0.001) led Tisserant(1977) to suggest that the Alouana intrusion has been gen-erated by heating and partial melting of mantle and conti-nental crust protoliths.

In addition to their distinctive major element composi-tions, the Alouana granitic rocks are also enriched in F, Nb,Rb, Sr, Ba, Ga, Y, light REE (LREE), U, Th, W, and Pb butdepleted in TiO2, P2O5, Sn, Be, Li, and Eu. The REEpatterns exhibit LREE enrichment relative to heavy REE(HREE) with LaN up to 20 and LaN/LuN ratios ranging from3 to 13, and significant negative Eu anomalies (Eu/Eu*00.2)(Talha 1996).

Based on these geochemical data and values of A/CNK[i.e., Al2O3/(CaO+Na2O+K2O)]01.0 to 1.2, A/NK [i.e.,Al2O3/(Na2O+K2O)]01.1 to 1.5, and the aluminum satura-tion index (ASI) [i.e., Al2O3/(CaO+K2O+Na2O)]01.0 to1.2, the Alouana granitic rocks plot in the fields of W-related(Meinert 1993) peraluminous granitoid (Maniar and Piccoli1989).

Sampling and analytical procedures

Samples for this study were collected from the most miner-alized vein occurrences throughout the study area (Fig. 2b).A representative suite (11 samples) of each of the majorstages of mineralization was selected for mineral geochem-istry, fluid inclusions, and isotopic investigations.

Wolframite and scheelite major element compositions weredetermined on a CAMECA SX 50 wavelength dispersiveelectron microprobe at the BRGM-ISTO (University ofOrléans, France), at an accelerating voltage of 20 kV, a beamcurrent of 10 nA, and counting times of 10 s. Using pure metaland mineral standards for calibration, a detection limit of~0.01 wt% was achieved for most elements. A minimum ofseven analyses were obtained on traverses from each sampleand three for each grain. Results are listed in Table 1.

Petrography and microthermometric characterization offluid inclusions have been carried out on standard 100 μm-thick doubly polished quartz wafers using a Chaix-Mecaheating–freezing stage (Poty et al. 1976). More than 20 thin

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polished plates of representative quartz samples from theoxide-silicate (Qz-1), sulphide-bearing (Qz-2), and late, bar-ren carbonate (Qz-3) mineralization stages were investigat-ed under the microscope. Of these, ten were found suitablefor microthermometric measurements, and more than 200heating and freezing measurements were made. No suitablefluorite sample was encountered because of the scarcity offluorite and the fact that it is present only as submicroscopicgrains. Aqueous liquid (L), vapor (V), and daughter-solid(S) phases were identified in the studied fluid inclusions.The accuracy was ±0.2°C in the range of −60°C to +30°Cand ±0.5°C at higher temperatures. Total liquid–vaporhomogenization temperature (Th), homogenization tem-perature of any CO2 phases (Th(CO2)), solid CO2 melting(Tm(CO2)) and clathrate melting (Tm(clath)), last ice-melting(Tm(ice)), and the dissolution temperature of halite (Tm(NaCl))and sylvite (Tm(KCl)) daughter minerals were recorded. Involatile-bearing fluid inclusions, the presence of CO2 wasdetected by melting of the solid phase below −56.6°C andthe formation of clathrate compound. The volumetric frac-tion of the volatile phase has been estimated at room tem-perature by reference to the volumetric chart of Roedder(1984). The total salinity of the aqueous fluid inclusions,expressed as wt% NaCl equiv, was calculated in the H2O–NaCl system from Tm(ice), using the equation of Bodnar (1993).By contrast, salinities for halite±sylvite-bearing fluid inclu-sions were calculated using the SALTY software package(Bodnar et al. 1989) based on the equations of Hall et al.(1988) and Sterner et al. (1988).

For oxygen isotopic analysis, quartz separates from theoxide-silicate (Qz-1) and sulphide-bearing (Qz-2) stageswere handpicked from sieved and washed (deionized water)whole-rock chips. Each grain was carefully examined underbinocular in order to avoid inclusions. Oxygen isotopemeasurements were carried out at the stable isotope labora-tory of the Department of Earth and Planetary Sciences,University of New Mexico (USA), using the laser extractiontechnique with BrF5 as a fluorinating agent (Sharp 1990).The data are reported as per mil (‰) deviations relative toVienna standard mean ocean water (V-SMOW). Reproduc-ibility of δ18O measurements was 0.15‰ (1σ).

Hydrothermal alteration, vein mineralogy, and mineralparagenesis

Pervasive hydrothermal alteration affected, to varyingdegrees, all the granitic units of the Alouana intrusion andthe adjacent country rocks resulting in (1) replacement ofmagmatic biotite by muscovite and K-feldspar and plagio-clase by microcline (microclinization) and albite (albitiza-tion), (2) dissolution of quartz (i.e., episyenitization asdescribed in the Zaër two-micas granite; Cheilletz and Giuliani1982), and (3) greisenization of the marginal pink fine-grainedleucogranite (Fig. 3e). Of these, fracture-related greiseniza-tion is the most extensively developed. Greisenized areasrange in width from a few millimeters to several meters,with individual veins and isolated patches typically between0.1 and 10 m. The resulting greisen rock varies in compo-sition from a quartz-rich to a muscovite-rich assemblage(Fig. 3e). Typical greisen minerals include tourmaline, ilmen-ite, rutile, topaz, fluorite, and pyrite, in addition to quartz andmuscovite. Textural evidence for albitization includes thepresence of albite blebs in perthite and the development ofeuhedral grains replacing alkali feldspar. Albitization isthought to result fromNa-metasomatism during the final stageof magma crystallization (Taylor 1979). Furthermore,petrographic relationships indicate that greisenization occurredafter albitization.

Typologically, the Lalla Zahra W-(Cu) prospect consistsof a set of a dozen subparallel, predominantly NW–SE-trending, subvertical (>80 °), and tungsten–copper-bearingtensional quartz veins that cut the country rocks over an area>6 km2 (Fig. 2b). Less abundant, smaller stockwork-likeveins are recognized within the granite itself. Evidence of atensional environment includes vugs, branching of veins,variation in strike along individual veins, and the commonoverlapping of subparallel veins (Fig. 2b).

Individual quartz veins are up to 30 cm wide and haveexperienced post-ore movements causing deformation anddislocation. The veins are composed of quartz, K-feldspar,muscovite, tourmaline, carbonates (calcite, dolomite, and sid-erite) accompanied by W–Mo-bearing minerals (wolframite,scheelite, and molybdenite), sulphides (pyrite, arsenopyrite,

Table 1 Average microprobemajor element compositions(wt%) of wolframite andscheelite from the Lalla ZahraW-(Cu) prospect

Sample no. Mineral Number of analyses MnO CaO FeO TiO2 WO3 Total

Al-1 Wolframite 7 4.92 – 17.00 – 76.42 98.34

Al-2 Wolframite 8 6.73 – 15.71 – 76.09 98.53

Al-3 Wolframite 8 5.60 – 15.24 – 79.16 100.00

Al-4 Wolframite 8 5.76 – 15.34 – 79.27 100.00

Al-5 Wolframite 7 4.52 – 17.41 – 76.26 98.20

Al-6 Wolframite 8 5.32 – 16.52 – 76.22 98.07

Al-7 Scheelite 3 – 20.00 – 0.01 80.24 100.26

Al-8 Scheelite 3 20.04 – – 0.00 80.51 100.54

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chalcopyrite, sphalerite, and galena), and trace amounts of Biminerals (bismuthinite and native bismuth) and fluorite.

Quartz dominates in all the veins displaying multiplestages of vein filling and brecciation, and a variety oftextures, including saccharoïdal, cockade, and comb, dis-posed as crustiform bands. At least three distinct generationsof quartz are distinguished. The earliest quartz (Qz-1), rec-ognized in hand specimens by its white to milky appear-ance, is by far the most abundant and forms 40 % to 60 % ofthe veins by volume. Paragenetic studies show that Qz-1precipitated synchronously with wolframite during the earlyphase of W-mineralization (stage I; Fig. 3f). Qz-2 quartz,recognized in hand specimens by its translucent appearance,represents 20 % to 50 % of the veins by volume and isparagenetically related to the base metal sulphide stage(stage II; Fig. 3g, h). Conversely, Qz-3 quartz is restrictedin occurrence and volumetrically minor (<10 %) occurringas transparent euhedral crystals growing within vuggy porespace and as anastomosing microveinlets crosscutting Qz-1and Qz-2 (Fig. 3h). Paragenetically, Qz-3 is related to latecarbonate–barite±fluorite veins of stage III.

Based on internal vein mineralogy, macro- and microtex-tural relationships, and mineral assemblages, the depositionalsequence is subdivided into three distinct main stages (I, II,and III) (Fig. 4). Of these, the first one is a tungsten-oxide-silicate dominant stage, the second base metal sulphide, andthe final stage is a barren episode consisting of carbonates andbarite, with traces of fluorite. The established parageneticsequence is similar to major paragenetic stages characteriz-ing most tungsten–tin deposits worldwide (Kelly and Rye1979). The supergene post-ore “stage IV” consists of a richassemblage of covellite, malachite, azurite, Fe oxides, andhydroxides.

Stage I mineralization (Fig. 4) includes most of thewolframite-bearing veins visible on the surface at the LallaZahra prospect. Vein minerals are, in decreasing orderof abundance, Qz-1, K-feldspar, muscovite, wolframite,scheelite, molybdenite, tourmaline, and pyrite. No tin-bearing mineral has been observed. Wolframite and schee-lite comprise less than 1 % by volume of the bulk veinmineralogy. Geologic reserve and tungsten ore grade estimatesare not known.

Wolframite occurs as large (4 to 8 mm wide, up to 1.5 cmlong) commonly homogeneous black (Fig. 3f) to reddishbrown grains in the vein center, or as zoned dark flakycrystals exhibiting alternating bands of different colors.Cores of crystals are commonly dark mantled by reddishzones that are in turn surrounded by bright margins. Al-though wolframite tends to be concentrated towards thecenter of the veins, it may also be randomly distributedalong the veins, usually in association with muscovite andsulfides. Wolframite crystals are commonly partiallyreplaced by scheelite, with the replacement progressing

along fractures and cleavage to develop a shredded texture.The replacement by scheelite, a secondary process common-ly observed in hydrothermal veins (Ramdohr 1969), appearsto have occurred soon after deposition of wolframite andprior to the deposition of sulphides such as pyrite andchalcopyrite. Molybdenite was microscopically observedas small (150 μm) disseminated plates (<1 vol %) inter-grown with quartz. Microprobe data indicate that thecomposition of the wolframite crystals is relatively homo-geneous with WO3 ranging from 74.8 to 77.5 wt%, FeOfrom 14.6 to 17.8 wt%, and MnO from 4.2 to 8.4 wt%(Table 1), close to the theoretical composition (Fe, Mn)WO4. Nb, Ta, and Ti have not been detected. The wol-framite crystals are Fe rich (rather than Mn rich) and thuscorrespond to the ferberite member, with an average cal-culated structural formula of Fe0.74 Mn0.26WO4. In somecrystals, Fe contents tend to decrease from the core (15.4–16.3 wt%) to the edge (14.2–15 wt%), but no zoning hasbeen observed on backscattered electron images. Scheelitecontains 16.7 to 20.2 wt% CaO and 74.8 to 81.5 wt%WO3 (Table 1).

Stage II, the base metal sulphide-bearing stage (Fig. 4),occurs as NW–SE-trending, centimeter-thick quartz veinslacking both wolframite and greisen envelopes but contain-ing sulphides. Characteristic mineral assemblage includesQz-2, pyrite, chalcopyrite (Fig. 3g), sphalerite, galena,and arsenopyrite, with minor amounts of bismuth andbismuthinite. Pyrite and chalcopyrite, the most abundantsulphides, form coarse euhedral grains commonly inter-grown with arsenopyrite.

The stage III barren quartz–carbonate–barite±fluoritestage (Fig. 4) is mainly represented by carbonates (calcite,dolomite, siderite, and rhodocrosite) accompanied by thedeposition of centimeter-long euhedral quartz crystals (Qz-3; Fig. 3h), barite, and, to a lesser extent, fluorite in tension-al fractures which crosscut all earlier veins.

Fluid inclusion petrography and microthermometry

Petrography

Because of the paragenetic overlap of wolframite, chalco-pyrite, barite, and quartz, fluid inclusions from quartz coex-isting with wolframite (Qz-1), chalcopyrite (Qz-2), andbarite±fluorite (Qz-3) are assumed to be representative ofthe fluids which deposited the paragenetic sequence ofwolframite (stage I), sulphides (stage II), and barite±fluorite(stage III) as illustrated in Fig. 4. Classification of fluidinclusions follows the temporal classification scheme ofRoedder (1984) and Goldstein (2003), and the typologicnomenclature of Boiron et al. (1992) based on the type oftotal homogenization temperature Th (L-V to the vapor

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noted V, L-V to the liquid noted L), the presence of C–H–O-(N–S) compounds (subscript “c” when C–H–O–S speciesare dominant; “c-w” and “w-c-m,” when both water CO2

and CH4 species are present; “w” when C–H–O-(N–S)species are not detected by any methods), and solid daughterphases (“h” and “k” when identified solid minerals are haliteand sylvite, respectively).

Qz-1 and Qz-2, and, in a lesser extent, Qz-3 host abundant,randomly to densely distributed, small (rarely>15 μm), andisometric rounded to elliptical or negative crystal shape fluidinclusions, with variable liquid/vapor ratios. Even thoughmost of investigated fluid inclusions were pseudosecondaryand secondary, few of them have been identified as primary inorigin using the criteria of Roedder (1984) and Goldstein

(2003). However, we should keep in mind that in Sn–Wdeposits where the mineralizing system is dominated by re-peated phases of mineral deposition, brecciation and fractur-ing, and subsequent healing of the fractures, similarlyrandomly distributed pseudosecondary inclusions are inferredto be primary (Heinrich 1990; Wood and Samson 2000; Yanget al. 2012). Moreover, as secondary fluid inclusions do notdiffer in their modes of occurrence and compositions from thefew primary and pseudosecondary fluid inclusions, they arealso interpreted as being related to the same hydrothermalmineralizing event.

More than 90 % of the fluid inclusions are two or threephase at room temperature (liquid H2O, vapor CO2±liquidCO2), rarely multi-phase fluid inclusions exhibiting solid

Quartz

Muscovite

Molybdenite

Wolframite

Scheelite

Tourmaline

Topaz

Pyrite

Arsenopyrite

Chalcopyrite

Sphalerite

Galena

Bismuthinite

Stage

Bismuth

Fluorite

Barite

Calcite

Dolomite

Siderite

Rhodocrosite

Malachite

Azurite

Covellite

Fe-oxides and hydroxides

Qz-1 Qz-2 Qz-3

Tungsten oxide- Base metal Carbonate-

(Stage III) (Stage I) (Stage II) sulphide stage silicate stage barite stage

(Stage IV)

Superge post-

ore stage Mineral

Veining and fracturing

Veining and fracturing

Fig. 4 Mineral paragenesis andrelated quartz crystallizationsequence relative to thealteration and associatedW-(Cu) mineralization at LallaZahra prospect. Line weightindicates relative abundance ofmineral species. Dashed linesrepresent the uncertainty ofappearance of a given mineralin the corresponding stage

3074 Arab J Geosci (2013) 6:3067–3085

daughter phases. Since daughter minerals have grown fromthe fluid on cooling after trapping, their identification atLalla Zahra prospect would give important insights on thecomposition of the ore-forming fluids. Identification ofthese phases is based on criteria pointed out by Roedder(1971) and confirmed by Raman microscopy. Halite wasdistinguished by the following features: cubic form, isotro-pic, and colorless crystals. Sylvite was identified by itscommonly rounded form, isotropic, and colorless crystals.Halite has higher Tm(NaCl) on heating in comparison to Tm(KCl) of sylvite (Table 3). Additionally, halite recrystallizessuddenly on cooling, whereas crystallization of sylvite isslower.

From petrographic (i.e., modes of occurrence, shape, pro-portion, and number of the phases at room temperature) andmicrothermometric data, two main types (I and II), with somesubtypes (Lc-w, Vc-w, Vw, Lw, Vw(shk), Lw(sh)) of fluid inclu-sions, are recognized. Representative fluid inclusion assemb-lages are shown in Fig. 5. Type I is early aqueous-carbonicfluid inclusions (Lc-w, Vc-w, and Vw), whereas type II fluidinclusions (Lw) are aqueous. Solid inclusions (Vw(shk), Lw(sh)),consisting predominantly of trapped halite and sylvite cubeswithin type I and II fluid inclusions, are classified as a separatethird type (Type III). The volatile-bearing fluid inclusions arerestricted to the early oxide-silicate quartz veins, where-as the aqueous fluid inclusions which obviously over-printed the early volatile-bearing fluid inclusions arepresent in both oxide-silicate, sulphide-bearing and latebarren quartz–barite±fluorite veins. Locally, the two types offluid inclusions were found together along individual planararrays, indicating that they may have been trapped coevallyboth as primary and secondary inclusions and that they mightreflect immiscible fluid phases.

Type I fluid inclusions which occur as isolated, smallclusters, or trails are either three-phase (liquid H2O, vaporCO2, and liquid CO2) or two-phase (vapor CO2, liquid CO2)spherical or elongated fluid inclusions of variable size (mostcommonly between 10 and 20 μm), with a volatile/waterratio ranging from 0.3 to 1 (Lc-w) or exceeding 0.5 (Vc-w),and homogenization is to the vapor state. Many aqueous-carbonic fluid inclusions do not contain sufficient CO2 toform a separate liquid CO2 phase at room temperature butdo form clathrates upon freezing.

By contrast, type II fluid inclusions are by far the mostabundant and bi-phase (LV) and are dominated by aqueousH2O-rich fluids (Lw). They occur particularly in Qz-2 andQz-3 usually as isolated inclusions and more commonly astrails healing discrete microfractures. They are irregular inshape and exhibit larger sizes, from 10 to 50 μm, than theprevious type.

Type III solid-bearing fluid inclusions consist of small (upto 10 μm) halite-saturated±sylvite hypersaline fluid inclu-sions. Daughter minerals (i.e., halite±sylvite), recognized

optically and confirmed by Raman spectroscopy, occur withinQz-1 and Qz-2 (i.e., Vw(shk) and Lw(sh) fluid inclusion types) astrue daughter minerals that crystallized from the enclosed fluidafter trapping. Distinction between accidentally trapped andtrue daughter minerals can be made, as accidentally trappedsolids show highly variable liquid/solid volume ratios com-pared with halite±sylvite-bearing inclusions. The presence ofhalite and sylvite suggests that the principal salts dissolved inthe ore-forming fluids were dominated by NaCl, KCl, andCaCl2 ion complexes.

Microthermometry

One hundred and thirty heating and freezing temperaturemeasurements along with 72 halite and sylvite dissolutiontemperatures were recorded. Data are summarized in Table 2and plotted in Figs. 6 and 7.

Lc-w, Vc-w, and Vw volatile-rich vapor–liquid fluid inclu-sions from Qz-1 exhibit Tm(CO2) ranging from −59.9°C to−56.7°C (average0−56.9°C±0.6; 1σ, n025) and Th(CO2)values to the vapor phase from 29.9°C to 30.4°C(average030.1°C±0.1; 1σ, n025). Melting temperaturesof clathrate (Tm(clath)) range from 9.7°C to 12.5°C(average011°C±0.1, 1σ, n025). Decrepitation occurs priorto homogenization in the range of 250–300°C for mostinclusions. Homogenization temperature (Th) measurementsto the vapor phase (i.e., Vc-w and Vw fluid inclusions) rangefrom 323°C to 422°C, whereas those Th values, to the liquidphase, characterizing Lc-w fluid inclusions are much lower(129–158°C). Tm(clath) above +10°C coupled with low Tm(CO2) values (i.e., below −56.6°C which corresponds to theeutectic of pure CO2) indicates the presence of other gases,in addition to CO2, such as CH4 or higher hydrocarbons, N2,and H2S (Burruss 1981; Roedder 1984; Shepherd et al.1985). The presence of CH4 is indicated by Tm(clath) greaterthan 10°C and measured triple points for three-phase inclu-sions as low as −63.3°C (Burruss 1981). Accordingly, type1 fluid inclusions can be approximated by the H2O–CO2–NaCl–KCl system.

Lw fluid inclusions show Th values ranging from124°C to 394°C with modes of ~150°C, 250°C ,and350°C. Similarly, Tm(ice) varies over a wide range ofvalues from −20°C to −0.2°C, indicating salinity esti-mates in the range of 0.3 to 22.7 wt% NaCl equiv, withmodes of <5 and 20 wt% NaCl equiv. Moreover, most ofthe Lw fluid inclusions have eutectic melting temperaturesbetween −57°C and −34.7°C, indicating the presence, inaddition toNaCl, of other salts, likely divalent cation chlorides(Te(MgCl2)0−35°C, Dubois and Marignac 1997; Te(CaCl2)0−52°C, Davis et al. 1990).

Average vapor bubble disappearance in three-phasesolid-bearing fluid inclusions occurs at Th0305°C (±46,1σ, n047). Halite crystals dissolved at Tm(NaCl) between

Arab J Geosci (2013) 6:3067–3085 3075

256°C and 402°C (average0333°C±57; 1σ, n022), indicat-ing salinity estimates in the range of 35 to 48 wt% NaClequiv (average041±5 wt% NaCl equiv, 1σ, n022). Forfluid inclusions containing both halite and sylvite cubes, Tm(NaCl) and Tm(KCl) values range from 318.5°C to 376°C(average0339°C±15, 1σ, n025) and 155°C to 250°C

(average0203°C±35, 1σ, n025), respectively, correspondingto an average bulk salinity of 56.8 (±2.5, 1σ) wt% NaCl+KClequiv.

Overall, type I and II fluid inclusions are treated in theH2O–CO2–CH4–NaCl±KCl system, such that carbonicphases are assumed to be dominantly CO2–CH4.

Fig. 5 XPL representative photomicrographs displaying some of thetypical fluid inclusion (FI) types and textures hosted in quartz fromboth stage I and stage II of the Lalla Zahra W-(Cu) prospect. Arrowspoint to the constituents of some inclusions. a Cluster and trail ofdensely distributed coexisting Vc-w and Lw(sh) fluid inclusions, poten-tially produced by fluid immiscibility. b Solitary Lw(sh) fluid inclusioncontaining cube of halite closely associated with a trail of secondary

two-phase Lc-w fluid inclusions with variable volumetric proportionsfilling a fracture. c Slightly rounded solitary Vw(shk) fluid inclusioncontaining both cubes of halite and sylvite. d Cluster of closely spacedrandomly distributed Lc-w fluid inclusions. e Cluster of closely spaceelongated primary Lc-w fluid inclusions with ~45 vol.% vapor. f Detailof solitary slightly rounded primary Vc-w fluid inclusion

3076 Arab J Geosci (2013) 6:3067–3085

Table 2 Microthermometric data of fluid inclusions hosted in the three generations of quartz (Qz-1, Qz-2, and Qz-3) from the Lalla Zahra W-(Cu)prospect

Sample no. FI type Quartz generation Tm(CO2) Te Tm(ice) Tm(clath) Th(CO2) Th Tm(NaCl) Tm(KCl) Salinity

Al1-1 −56.8 −2.8 30.1 415.3 4.6

−56.8 −2.7 30.3 381.7 4.5

−56.9 −2.7 30.1 369.1 4.5

Al1-2 −56.8 −2.8 30 375 4.6

−56.9 −2.7 29.9 417.2 4.5

−57 −2.7 30.4 409.5 4.5

−57 −2.7 30.1 392.8 4.5

−56.7 −2.8 29.9 387.4 4.6

−56.7 −2.5 29.9 402.7 4.2

−56.8 −2.8 30.2 387 4.6

Al1-3 −56.8 −2.6 30.1 381.8 4.4

Vc-w Qz-1 −59.9 −2.7 30.3 397.3 4.5

−56.8 −2.8 30.1 404.6 4.6

−56.8 −2.7 29.9 421.8 4.5

−56.8 −2.7 30.1 348.9 4.5

Al1-4 −56.9 −5.8 30.1 386.7 8.9

−56.8 −5.3 30 389.4 8.3

−56.7 −2.9 29.9 345.8 4.8

−56.9 −2.8 30.3 353.7 4.6

−56.8 −2.8 30.1 363.8 4.6

−56.9 −2.7 30.2 390 4.5

Al2-1 −56.9 −2.7 30 409.5 4.5

−56.8 −2.8 30.1 405.7 4.6

−56.9 −2.7 30.2 411.2 4.5

−56.7 −2.8 30.3 397.8 4.6

−0.3 357.5 0.53

−0.1 347.4 0.18

−0.1 360.2 0.18

−0.2 323.4 0.35

Al2-2 −0.2 354.4 0.35

−0.1 329.2 0.18

−0.2 357.2 0.35

−0.1 349 0.18

Vw Qz-1 −0.1 358.5 0.18

−0.1 368.8 0.18

Al2-3 −0.1 361.9 0.18

−0.2 350.4 0.35

−0.2 369.8 0.35

−0.2 364 0.35

−0.2 366.8 0.35

−0.1 391.4 0.18

Al3-1 0 345.5 0

−0.1 358.5 0.18

−0.1 370.9 0.18

−3.1 12.3 147.8 5.1

−2.9 9.9 143.7 4.8

−0.6 11.4 144.4 1.1

Al3-2 −0.7 12.5 134 1.2

Arab J Geosci (2013) 6:3067–3085 3077

Table 2 (continued)

Sample no. FI type Quartz generation Tm(CO2) Te Tm(ice) Tm(clath) Th(CO2) Th Tm(NaCl) Tm(KCl) Salinity

−1.2 9.8 141.6 2.1

−1.3 10.4 145 2.2

−0.8 10.9 142.1 1.4

−1.3 9.7 136.1 2.2

Lc-w Qz-1 −0.6 10.6 144.1 1.1

Al3-3 Qz-2 −1.1 10.9 141.7 1.9

−1.2 12.3 150.3 2.1

−1.2 10.7 139.4 2.1

−0.6 11.5 158.5 1.1

−0.8 10.6 147 1.4

−0.7 11.6 145.1 1.2

Al4-1 −0.6 11 148.4 1.1

−1.4 10.9 143.5 2.4

−0.6 11.3 138.5 1.1

−0.6 10.9 129.4 1.1

−1.5 371.8 2.6

−1.5 354.3 2.6

−4.3 393.7 6.9

−1.4 365.5 2.4

Al4-2 −1.4 356.4 2.4

−4.5 390.7 7.2

−1.5 363.7 2.6

−1.6 375.5 2.7

−4.6 361.8 7.3

Al4-3 −1.6 372.9 2.7

−0.3 134.5 0.53

−0.5 138.9 0.87

−0.3 130.4 0.53

−0.2 124.9 0.35

Al5-1 Lw Qz-2 −0.3 128.2 0.53

Qz-3 −0.4 129.4 0.7

−0.3 146 0.53

−0.3 156.4 0.53

−0.5 133 0.87

−0.4 124.3 0.7

Al5-2 −0.3 148.6 0.53

−19.2 251.2 22.09

−18.8 248.5 21.79

−20 262.4 22.66

−20.1 252.6 22.73

Al6-1 −18.2 247 21.35

−20 256.4 22.66

−18.4 259.1 21.5

−45.6 363.5 391.1 46.5

369.7 393.4 46.7

Al6-2 −48 373.8 395 46.9

360.3 381.2 45.5

372.9 387.2 46.1

358.5 401.7 47.6

3078 Arab J Geosci (2013) 6:3067–3085

Oxygen isotope study

Oxygen isotope studies were carried out to investigate theorigin and history of the hydrothermal fluids. The results

and details of the analyzed vein samples are given in Table 3.Calculated δ18OH2O values of the ore-forming fluid for eachquartz generation using the quartz–H2O fractionation factorof Zheng (1993) and estimates of fluid temperatures derived

Table 2 (continued)

Sample no. FI type Quartz generation Tm(CO2) Te Tm(ice) Tm(clath) Th(CO2) Th Tm(NaCl) Tm(KCl) Salinity

370.3 392.5 46.6

Al7-1 −48.3 359.3 393.7 46.8

359.2 390.2 46.4

378.1 401.9 47.6

Lw(sh)a Qz-1 253.5 312.5 39.1

Qz-2 233.2 285.7 37.1

231.7 281.5 36.8

241.7 277.4 36.5

Al7-2 229.8 288.7 37.3

248.1 290.5 37.4

222.2 256.4 35.1

234.2 287.4 37.2

264.4 279.6 36.7

255.7 279.5 36.6

Al8-1 256.5 276.9 36.5

260.7 278.8 36.6

−56.5 311.5 337.2 249.9 59.9

295.5 320.9 229.7 57.5

Al8-2 318.3 341.8 240.4 59.5

288.9 331.6 235 58.5

301.7 323.4 227.2 57.5

345.9 367 232.3 60.6

329.9 351.7 222 58.9

328.2 348.1 243.8 60.1

Al9-1 310.4 322.3 228.6 57.5

323 359.8 230.1 59.9

313 351 237.6 59.9

Vw(shk)a Qz-1 −52.7 318.3 328.9 155.2 53.1

311.9 332 168.9 54.2

296.9 326.3 170.1 53.9

345.7 376.2 180.9 58

341.3 355.9 163 55.5

335.6 331.2 165.6 53.9

Al9-2 320.3 342.7 174.5 55.2

306.4 329.4 169.2 54

272.5 337.7 165.9 54.4

305.9 342.1 163 54.5

287.4 318.5 234.3 57.7

273.5 329.9 247.3 59.2

290.7 329.6 168 53.9

281.8 332.7 165 54

a In all daughter mineral-bearing inclusions, the salinity was calculated from the temperature at which the daughter mineral (i.e., halite and/orsylvite) disappeared

Arab J Geosci (2013) 6:3067–3085 3079

from the microthermometric measurements are also indicated(Table 3).

The measured δ18O values for the three generations ofquartz (Qz-1, Qz-2, and Qz-3) show variation from 11.7‰to 13.9‰ (Table 3). Qz-1 isotopic compositions (11.7–13.1‰, average012.2‰, n05) are somewhat lower thanδ18O values characterizing Qz-2 (12.5–13.9‰, average013‰, n06). These values are comparable to those character-izing specialized, weakly peraluminous granites (Sheppard1986) and fall within the range of δ18O measurementsreported for tungsten deposits worldwide (Kelly andRye 1979).

Discussion and conclusion

It is widely accepted that tungsten–tin ore-forming process-es are directly related to the crystallization and coolinghistory of highly fractionated and greisenized granitoidintrusions (Tischendorf and Förster 1990; Breiter et al.1999). Recent advances in isotope geochemistry and fluidinclusion studies show that the main processes involved inthe deposition of W–Sn mineralization are fractional crys-tallization, fluid-melt immiscibility, hydrofracturing andbreccia-pipe formation, and/or pervasive fluid-rock interac-tion (e.g., Davis and Williams-Jones 1985; Ramboz et al.1992; Breiter et al. 1999; Beuchat et al. 2004; Feng et al.2011; Liu et al. 2011; Naumov et al. 2011; Yang et al. 2012).

The Lalla Zahra fluid inclusion data display filling tem-peratures ranging from 124°C to 447°C with three distinctmodes at ~150°C, ~250°C, and ~380°C (Fig. 6). Similarly,salinity calculations span the whole range between 0.4 and~60 wt% NaCl+KCl equiv. These wide variations both intemperature and salinity estimates are comparable to thosereported for the contemporaneous westward late-HercynianW–Sn-bearing granitoids of the Moroccan Massif Central(Cheilletz 1984; Giuliani 1984; Boutaleb 1988; Boushaba1996; 2008; Aissa 1997; Nerci 2006) and fall within themicrothermometric and fluid composition ranges that charac-terize tungsten deposits worldwide (Naumov et al. 2011).

Calculated δ18O fluid composition for Qz-1, using selectedtemperature range of precipitation between 417°C and 323°C,indicates that Qz-1, and by inference wolframite mineraliza-tion, was precipitated from a fluid with δ18O values rangingbetween 5.4‰ and 8.9‰ V-SMOW. These values, eventhough not diagnostic in the absence of δD data, fall withinthe range expected for waters in isotopic equilibrium withmagmatic rocks at high temperature (6–10‰) as defined by

�Fig. 6 Histograms summarizing the distribution of quartz-hosted fluidinclusion homogenization temperatures (a) and calculated salinities (b)from the three generations of quartz (Qz-1, Qz-2, and Qz-3) of theLalla Zahra W-(Cu) prospect

3080 Arab J Geosci (2013) 6:3067–3085

Ohmoto (1986) and Sheppard (1986). By contrast, the δ18Ovalues of the parent fluid for Qz-2 and Qz-3, considering arange of temperature of precipitation between 262°C and125°C, yield, for Qz-2 and Qz-3, and by inference thesulphide-bearing and carbonate–barite mineral assemb-lages, much lower δ18OH2O values ranging from −3.3‰to +5‰ V-SMOW.

Furthermore, consideration of fluid inclusion data for thethree generations of quartz on a Th vs. salinity plot (Fig. 7)shows the existence of two populations of fluid inclusionscorresponding to two clearly defined trends. Trend I relatesto fluid inclusions formed during the Qz-1 and associatedW-mineralization (stage I), whereas trend II materializesfluid inclusions formed during the precipitation of Qz-2

and Qz-3 and their associated base metal sulphide andcarbonate–barite±fluorite mineralization related to stagesII and III; respectively.

The coexistence of type I CO2-rich fluid inclusions withtype II CO2-free, aqueous fluid inclusions either along sin-gle growth zone or even in close proximity constitutesstrong evidence that all the inclusions have been trappedcoevally both as primary and secondary inclusions and thatthe ore-forming fluid resulted from fluid immiscibility of aboiling H2O-rich fluid containing dissolved CO2, whichultimately exsolved a high salinity aqueous volatile phasefrom late-stage crystallization magma. This early separated,highly saline, and CO2-rich fluid was progressively evolvedand deposited, at relatively high temperature, W-bearing

Fig. 7 Plot of quartz-hostedfluid inclusion homogenizationtemperature vs. salinity fromthe three generations of quartz(Qz-1, Qz-2, and Qz-3) of theLalla Zahra W-(Cu) prospect

Table 3 Oxygen isotope compositions for the different generations of quartz separates (Qz-1, Qz-2, and Qz-3) from the major veins of the LallaZahra W-(Cu) prospect

Sample no. Vein type Quartzgeneration

δ18OV-SMOW

(‰)Temperaturerange (°C)a

Calculated δ18Ofluid range(‰)b

Al-1 Quartz+sulfides (chalcopyrite±pyrite) Qz-2 12.5 [156–262°C] [−2.4/+4.1]

Al-2 Quartz+sulfides (chalcopyrite±pyrite) Qz-2 12.8 [156–262°C] [−2.1/+3.4]

Al-3 Quartz+Fe-rich wolframite Qz-1 12.9 [417–323°C] [6.6–8.7]

Al-4 Quartz+Fe-rich wolframite Qz-1 13.1 [417–323°C] [6.8–8.9]

Al-5 Quartz+Fe-rich wolframite Qz-1 11.7 [417–323°C] [5.4−7.5]

Al-6 Quartz±Fe-rich wolframite±sulfides (chalcopyrite±pyrite) Qz-2 13.3 [156–262°C] [0.4–4.9]

Al-7 Quartz+sulfides (chalcopyrite±pyrite) Qz-2 12.7 [156–262°C] [−2.2/4.3]

Al-8 Quartz±Fe-rich wolframite±sulfides Qz-2 13.2 [156–262°C] [−1.7/3.4]

Al-9 Quartz + Fe-rich wolframite Qz-1 11.7 [417–323°C] [5.4–7.5]

Al-10 Quartz + Fe-rich wolframite Qz-1 11.8 [417–323°C] [5.5–7.6]

Al-11 Quartz + sulfides + carbonates Qz-3 13.9 [125-156°C] [−3.3/-1]

a Temperature range determined from fluid inclusion datab δ18 Ofluid values were calculated using the quartz-water oxygen isotope fractionation curve of Zheng (1993) and Th estimates as inferred fromfluid inclusion data

Arab J Geosci (2013) 6:3067–3085 3081

mineralization (i.e., silicate-oxide stage; Fig. 4). The existenceof miarolitic cavities which suggests fluid saturation at near-solidus conditions (Heinrich 1990) along with the occurrenceof resorption textures displayed by quartz phenocrysts con-stitutes additional evidences for boiling. Furthermore, loss ofH2 from reducedW-rich fluids by vapor separation and simul-taneous reaction with aluminosilicate rocks caused feldsparhydrolysis (i.e., greisenization) and, consequently, develop-ment of the widespread phyllic alteration halo that character-izes the marginal pink fine-grained leucogranite.

Heterogeneous entrapment after fluid phase separation ofa primary H2O–CO2–NaCl fluid as the mechanism respon-sible for the individualization of inclusions type I and,consequently, the formation of W mineralization are alsosupported by the near-vertical trend (Fig. 7) of increasingsalinity (~0.5–~60 wt% NaCl equiv) coupled with a gradualdecrease in Th (400–300°C) (trend I; Fig. 7). This suggeststhat no pressure correction is needed as pressures of entrap-ment could not have been greater than 600 bars as inferredfrom the boiling-point curve.

The halite–sylvite-bearing fluid inclusions exhibiting Thvalues (250–350°C) may represent the H2O rich end-member resulting from unmixing of the H2O–CO2–NaClparent fluid. The resulting vapor low-density phase wouldhave formed a buoyant plume which rose to the apicalregions of magma chambers and cooled to form a highsalinity condensate (Henley and McNab 1978; Candela

1991). Brittle failure triggered by post-late Visean (D3)episode would have favored rapid pressure changes condu-cive to fluid separation, thereby resulting in the decrease ofthe solubility of the wolframite and, consequently, its pre-cipitation. Thus, the high salinity estimates (up to 60 %wt%NaCl equiv) recorded at Lalla Zahra prospect suggest againthe involvement of a magmatic fluid component that expe-rienced phase separation at the time of entrapment (i.e., fluidimmiscibility; Bodnar 1995). Calculated δ18Ofluid composi-tions for Qz-1 which range from +5.4 to +8.9‰ V-SMOWare consistent with the range of δ18O values typical formagmatic fluids (5–10‰; Sheppard 1986). The two-phasefluid separation could influence mineral solubilities due tothe preferential fractionation of components between theliquid and vapor phases (Spycher and Reed 1986). Indeed,the presence of CO2 in a free fluid phase can induce unmix-ing of vapor and saline aqueous fluid at much greater depthsthan would normally occur for NaCl–H2O-only fluids(Lowenstern 1994; Hanley and Gladney 2011), and uponmixing from an initially single phase NaCl–H2O-CO2 fluid,it will cause the pH of the coexisting aqueous fluid toincrease, thereby inducing metal precipitation.

By contrast, trend II shows an overall continuum of de-creasing temperatures (300°C to 120°C) and decreasing salin-ities (60 to ~0 wt% NaCl equiv) from W- through base-metalsulphide to carbonate–fluorite stages (Fig. 7). These micro-thermometric values are distinct from those characterizing

Devonian schist with sandstone/quartzite

Porphyritic two-micas granite

Proximal-style W-(Cu) ore

Distal vein-style W-(Cu) oreMagmatic brine

Meteoric fluid

Jbel Idaj W-(Cu)Lalla Zahra W-(Cu) prospect Present surface Alouana intrusion

2 km100

mNE SW

Marginal pink fine-grained leucogranite

Mag

mat

ic b

rin

e +

vap

or

Magmatic vapor

Jbel Idaj W-(Cu)Lalla Zahra W-(Cu) prospect Present surface Alouana intrusion

2 km100

m

Mag

mat

ic b

rin

e +

vap

or

Fig. 8 Schematic reconstructed genetic model for the magmatic-hydrothermal system in the Lalla Zahra prospect. See text for explanations

3082 Arab J Geosci (2013) 6:3067–3085

trend I and suggest that the dominant process during whichQz-2 and Qz-3 precipitated was fluid mixing and coolingbetween a high salinity (~60 wt% NaCl+KCl equiv), high-temperature magmatic brine with a low salinity, and low-temperature H2O-rich fluid resulting in the precipitation ofbase metal sulfides of stage II and carbonate–fluorite ofstage III. Calculated δ18O signatures of the parent fluid forQz-2 and Qz-3 ranging from −3.3 to +4.3‰ V-SMOW mayindicate an origin from (1) seawater or meteoric water thatprobably exchanged oxygen with rocks at elevated temper-atures or (2) a mixture of water of different origins. The freshwater end-member (δ18O values around −3‰V-SMOW)mayrepresent the local meteoric water.

From the microthermometric and geochemical data dis-cussed previously, it becomes apparent that the overall min-eralogy of the Lalla Zahra hydrothermal system shiftedthrough time from CO2-rich to H2O-rich ore-forming fluidsresulting in the precipitation, with advancing parageneticsequence, of W-stage at relatively high temperature and basemetal sulphides and carbonate–barite±fluorite mineralassemblages at lower temperatures. Indeed, heating–freez-ing measurements show that during stage III, temperaturedecreased to 140°C or lower and salinities to values<to5 wt% NaCl equiv. This retrograde trend may indicate cool-ing and dilution fluid mixing processes, owing to the influxof progressively larger volumes of cooler fluids resulting inthe overprinting of the early magmatic-related mineralassemblages. Oxygen isotope data indicate that these exter-nally sourced fluids from the Alouana intrusion may corre-spond to meteoric or groundwater. These externally derivedfluids, capable of circulating deep into the upper part of theAlouana intrusion, through fracture networks, once solidi-fied, would have been in chemical equilibrium with graniticrocks (Fig. 8). Mixing of magmatic brine with cooler lowersalinity meteoric waters at the site of ore deposition trig-gered the deposition of sulphides, probably under fs2 con-ditions of less than 10-11 (Higgins 1985), and later oncarbonate–barite±fluorite.

In conclusion, we advocate that the Alouana tungsten vein-type mineralization resulted from three-stage fluid evolution(Fig. 8). During the early stage, the ore-forming system wasdominated by high-salinity magmatic fluid that exsolved fromthe crystallizing leucogranitic magma. Boiling resulted indeposition, at relatively high temperature, of the early wol-framite stage mineralization as evidenced by the coexistenceof CO2-rich and aqueous fluid inclusions, and the occurrenceof miarolitic cavities and resorption textures.

With decreasing temperatures, the orthomagmatic fluid thatmigrated away from the mucovite–leucogranite cupola alongthe regional faults (Fig. 8) was diluted by a cooler and lesssaline non-boiling meteoric fluid, resulting successively in theprecipitation of base metal sulphides and carbonate, barite±fluorite mineral assemblages related to stages II and III.

Acknowledgments We appreciate the constructive comments on ear-lier versions of the manuscripts by D.F. Sangster, G. Giuliani, and M.C.Boiron. Special thanks are due to R.J. Bodnar for his valuable help withsalinity calculations. E. Cardellach is also acknowledged for providing theδ18O data. The manuscript has been substantially improved by thethorough comments of the two anonymous reviewers and AbdullahM. Al-Amri for his editorial comments; their critical revisions are sin-cerely acknowledged. This research benefited from the financial supportprovided through grants from the Programme d'Appui à la RechercheScientifique of Morocco (PROTARS II/ P23/33) and the Moroccan-Spanish Scientific Research program (188/04/RE).

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