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IntroductionMORE than 215 minerals have been identified from the BouAzzer district, of which cobalt and nickel minerals are themost important and common. Cobalt, Ni, and Fe behave geo-chemically similarly and Co is widely dispersed in the Earth’scrust, with an average abundance of 25 ppm; it is most abun-dant in ultramafic rocks where its average concentration is110 ppm (Smith, 2001). Processes such as magmatic differ-entiation, chemical weathering, and hydrothermal activity canconcentrate the metals into potentially exploitable deposits.Cobalt deposits worldwide fall into a number of categories:(1) Cu-Co deposits hosted by sedimentary rocks representedby the Copperbelt of Zambia and the Democratic Republic ofCongo (Lhoest, 1995; Smith, 2001); (2) lateritic Ni-Co de-posits leached from ultramafic rocks, most notably in NewCaledonia, Cuba, and Australia (Smith, 2001; Elias, 2002;Jaques et al., 2002); (3) primary magmatic Ni-Cu-(Co-PGE)sulfide deposits such as in the Bushveld, Noril’sk, Sudbury,Voisey’s Bay (Lightfoot et al., 2001; Gavrilenko et al., 2002;

Kravtsova et al., 2002; Marakushev et al., 2002; Ripley et al.,2002); (4) hydrothermal and volcanogenic polymetallic de-posits, including the ophiolite-hosted Co-Ni arsenide depositsat Bou Azzer, Morocco (Leblanc and Fischer, 1990; Wilde etal., 2002; Dolansky, 2007), the Ni-Co-Ag-As-Bi vein-type de-posits of Ontario (Marshall and Watkinson, 2000; Bendingand Scales, 2001), and the epigenetic Cu-Au-Co deposits ofthe Idaho cobalt belt (Kolkovski et al., 2003).

The Bou Azzer mining district in southern Morocco of thecentral Anti-Atlas mountain belt is one of the main producersof Co in the world today. Co production started in 1928 andcurrently represents about 8 percent of the annual world Coproduction (Hawkins, 2006). The deposit comprises morethan 60 orebodies, all of which are spatially associated withserpentinites of a Neoproterozoic ophiolite sequence in con-tact with propylitized quartz diorite and sedimentary and vol-caniclastic rocks. Despite its economic significance, the BouAzzer Co-Ni arsenide mineralization has received little scien-tific attention and very little is known about its mineralogyand mineral paragenesis. Previous studies by Leblanc and hiscoworkers (Leblanc, 1975, 1976, 1981, 1986; Leblanc andBillaud, 1982; Leblanc and Lbouabi, 1988; Leblanc and Fis-cher, 1990) concentrated mainly on the mode of occurrence,

Mineralogy and Paragenesis of the Co-Ni Arsenide Ores of Bou Azzer, Anti-Atlas, Morocco

AHMED HASSAN AHMED,†,* Department of Geology, Helwan University, Cairo, Egypt

SHOJI ARAI, Department of Earth Sciences, Kanazawa University, Kanazawa, Japan

AND MOHA IKENNE

Department of Geology, Ibn Zohr University, Agadir, Morocco

AbstractThe Co-Ni arsenide ore deposits at Bou Azzer, Morocco, represent an uncommon type of Co mineralization.

Several Co-Ni-As-Au-Ag mines are active in Bou Azzer and adjacent areas. The Co-Ni arsenide ores are struc-turally controlled and concentrated mainly within quartz-carbonate veins along the boundaries of Neoprotero-zoic serpentinized mantle peridotites, quartz diorite, and Precambrian volcanic rocks. They have varyingshapes and sizes; however, all deposits have the same general mineralogy in decreasing order of abundance:arsenides, sulfarsenides, and sulfides. Arsenide minerals include skutterudite (CoAs3), safflorite (CoAs2),loellingite (FeAs2), nickeline (NiAs), rammelsbergite (NiAs2), and S-rich Ni-Co diarsenide. Co-rich ores arepredominant over Ni-rich ones in almost all deposits. Arsenopyrite and, to a lesser extent, gersdorffite are themain sulfarsenide minerals in almost all samples studied. Bornite, an unidentified Cu-Zn sulfide, chalcocite,chalcopyrite, pyrite, pyrrhotite, sphalerite, and tetrahedrite, in decreasing order of abundance, are the mainsulfide minerals present in Bou Azzer ores. Gold-silver alloys, with about 15 wt percent Ag on average, arecommonly associated with Ni-Co arsenides and/or disseminated in quartz veins. In the Ni-rich assemblagepure nickeline with high Ni content (~48 wt %) and very low Co content (≤ 0.1 wt %) grades outward to com-positions with less Ni (~20 wt %) and very high Co (up to 18 wt %). This suggests a depositional sequence fromNi monoarsenide (nickeline) to Ni-Co diarsenide to cobalt triarsenides (i.e., a progressive increase in Co andAs). The ore-forming elements (Co, Ni, Fe, and As) were originally leached from serpentinites by acidic mag-matic fluids under moderately reducing conditions at high fluid/rock ratios. Deposition of ore mineralsoccurred in response to increasing pH and decreasing oxygen fugacity of the mineralizing fluids caused by mix-ing between the magmatic brines and meteoric water. The predominance of Co over Ni arsenide minerals inthe Bou Azzer mineralization despite the high Ni/Co ratio of the serpentinites may be attributable to the dif-ferent solubilities of Ni and Co in the hydrothermal system.

† Corresponding author: e-mail, ahmh2@yahoo.com*Present address: Faculty of Earth Sciences, King Abdulaziz University,

Jeddah, Saudi Arabia.

©2009 Society of Economic Geologists, Inc.Economic Geology, v. 104, pp. 249–266

precious metals content and provinence of the Bou Azzer Co-Ni arsenide mineralization, however there is no detailed min-eralogical study of these important deposits. There is goodagreement in the sequence in the Bou Azzer mineralizationwith Leblanc (1975), En-Naciri (1995) and En-Naciri et al.(1997), who proposed a generalized sequence of depositionstarting with the Ni-rich phases, followed by Co-rich phases,then by Fe-rich phases, and finally by sulfarsenides and sul-fides. In this paper, we present detailed mineralogical studiesof the ore minerals and highlight the possible source(s) of Coand Ni and their behavior in the hydrothermal system. Thecomposition of ore-forming fluids and the possible source(s)of arsenic are also discussed.

Geologic BackgroundThe Anti-Atlas belt of the southwestern Moroccan desert

(Fig. 1) is well exposed. It is located at the northern edge ofthe Eburnian Western African craton, the oldest rocks in theAnti-Atlas belt (2000 Ma: Thomas et al., 2002; Walsh et al.,2002), which are separated from the High Atlas Mountains(Fig. 1) to the north by the South Atlas fault (Gasquet et al.,2005). The final assembly of most of the African continentalcrust took place during the 700 to 600 Ma Pan-Africanorogeny (Hefferan et al., 2000). The Precambrian basement

in the Anti-Atlas was uplifted by compressional tectonics re-lated to the Carboniferous Hercynian (Variscan) orogeny,which resulted in the development of a regional-scale domeand basin topography (Caritg et al., 2004). Precambrian base-ment of the Anti-Atlas belt comprises several units of Paleo-proterozoic to Neoproterozoic age (Gasquest et al., 2005).Several Precambrian inliers, including Bou Azzer (e.g., Ifni,Kerdous, Akka, Igherm, Sirwa, Zenaga, Saghro, and Ougnat)are distributed along two major fault zones: the South Atlasfault and the Central Anti-Atlas fault (Gasquest et al., 2005).Remnants of a Pan-African suture zone represented by a dis-membered ophiolite series, within the Bou Azzer inlier (Fig.2) enhance our understanding of the Pan-African events(Leblanc, 1976, 1981; Leblanc and Billaud, 1982; Bodinier etal., 1984; Saquaque et al., 1989; Hefferan et al., 2000; Gas-quet et al., 2005; D’Lemos et al., 2006).

The Bou Azzer district lies in the central part of the Anti-Atlas Mountains of southern Morocco within the saddle-shaped ridge of Bou Azzer-El Graara. The Bou Azzer ophiolitecomplex forms a 60-km-long belt that trends west-north-west–east-southeast along the Anti-Atlas major fault (Fig. 2).Ophiolitic rocks in the central portion of the inlier form tec-tonic slices of approximately 5-km thickness that are bestviewed in Wadi (= valley) Ait Ahmane and comprise the

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FIG. 1. Geologic map of the Anti-Atlas belt, southern Morocco (modified from Gasquet et al., 2005), showing the loca-tion of main ore deposits and the Bou Azzer study area (rectangular area).

following rock units: serpentinized upper mantle peridotites(~2,000 m), mafic-ultramafic cumulates (~500 m, locally lay-ered gabbro), dike swarms, submarine basaltic to intermedi-ate pillow lavas (~500 m, spilites and keratophyres), and anassociated volcano-sedimentary sequence (Leblanc, 1976;Bodinier et al., 1984; Thomas et al., 2002). Quartz dioriticbodies of 650 to 640 Ma age are believed to have been em-placed syntectonically during the main period of the Pan-African orogenesis (Inglis et al., 2005) and intruded manyunits of the central and northern parts of the Bou Azzer inlier.These ophiolitic rocks are unconformably overlain succes-sively (Fig. 2) by the following: (1) a volcano-sedimentary se-ries, (2) a weakly metamorphosed molassic sedimentary suc-cession (Tiddeline Formation), (3) a thick succession ofsubhorizontal ignimbrites and conglomerate (OuarzazateFormation), and (4) a thick, shallow marine carbonate se-quence (Adoudounian Formation) of late Proterozoic to earlyCambrian age (Gasquet et al., 2005; D’Lemos et al., 2006).

The Precambrian rocks experienced multiple episodes ofdeformation, resulting in overthrusts, horizontal and verticalfracturing, brecciation, and the creation of cavity-rich perme-able zones in which hydrothermal solutions circulated (En-Naciri, 1995). Within the Bou Azzer inlier, two Hercynian de-formation events are evident along the borders of the uplifted

basement rocks and are found as vertical slices with injectionsof serpentinites into the overlying strata (Leblanc, 1975). Theage of the main episode of deformation is generally consid-ered as Late Devonian to Late Carboniferous (Piqué andMichard, 1989; Soulaimani et al., 1997; Caritg et al., 2004).

Field Observations and Description of OrebodiesAmong more than 60 Co-Ni arsenide deposits, only five

mines currently remain in operation. Sampling for the presentstudy was from, west to east, the Bou Azzer East, Aghbar,Bouismas, Tamdrost, Agoudal, and Ait Ahmane deposits (Fig.2). All are underground mines. More than 75 samples of orewere collected from various levels in the mines, either from al-ready worked out (Ait Ahmane) or currently active mines (BouAzzer East, Aghbar, Bouismas, Tamdrost, and Agoudal). Ageneral feature for the Bou Azzer mineralization is that all Co-Ni arsenide ores occur at the contacts between the serpenti-nite and the intruded quartz diorite (Fig. 3A). Most of the oredeposits are bound either by Precambrian structures or theyounger fault zones of the Variscan orogeny (late Paleozoic).The richest orebodies are found where deformation was mostintense, particularly in the west-northwest– and/or east-southeast–striking zone of contact between quartz dioritesand serpentinites (Leblanc and Billaud, 1982; Leblanc, 1986).

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FIG. 2. Simplified geologic map of the Bou Azzer inlier (rectangular area of Fig. 1), showing the distribution of the mainCo-Ni arsenide ore deposits in relationship to the serpentinite massifs (modified from Leblanc, 1975).

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FIG. 3. Photographs showing the field observations of the Bou Azzer Co-Ni ore deposits. (A). Mining activity at old AitAhmane mine located at the contact between serpentinite and quartz (Qz) diorite intrusion. (B). Co arsenide veinlets asso-ciated with quartz-carbonate gangues (white), crosscutting the serpentinite host, Bouismas mine. (C). Hand specimen ofmassive Co arsenide ore associated with quartz-carbonate gangues (buff color patches), from Bou Azzer East mine. (D).Chalcopyrite concentrations (Cp) within cavities of massive Co arsenide ore, Bou Azzer East mine. (E). Erythrite (pink) inthe oxidation zone around the Co arsenide mineralization, old Ait Ahmane mine. (F). Coarse magnetite aggregates on thesurface of serpentinite massifs, Ait Ahmane area.

A B

C D

E F

Different shapes and sizes of orebodies are recognized; theforms and sizes of the orebodies are highly variable fromveins, seams, and lenses to stockworks (Leblanc and Billaud,1982; Leblanc and Fischer, 1990). The most economically im-portant are the thick subvertical lenses and lodes (amas type),5 to 20 m thick and 50 to 600 m long, which are concentratedalong irregular contacts between serpentinites and Precam-brian volcanic rocks (Tamdrost) or quartz diorites (Bou AzzerEast and Bouismas). The most common orebodies consist ofsmall subhorizontal lenses (flat lenses), veins (Fig. 3B), andcomplex concentrations that occur at the top of the serpenti-nite masses (Aghbar), in contact with quartz diorite or Pre-cambrian volcanic rocks. Mineralized veins have been ex-ploited to different depths (90 m at Agoudal, 300 m at Aghbarand Bouismas, and up to 540 m at Bou Azzer East). Thecobalt ore minerals form massive lenses and occur as dissem-inated mineralization in gangue within veins (Fig. 3C), lesscommonly in the wall rocks, and locally associated with Cu-rich mineralization (Bou Azzer East; Fig. 3D). Hydrothermalalteration associated with mineralized veins is observed inboth types of country rock (serpentinite and quartz diorite),with alteration zones ranging from several centimeters to ameter or more in width. Proximal to veins, quartz diorite is in-tensely chloritized, silicified, or less commonly, carbonatized.Alteration in serpentinites consists of intensely carbonatized,silicified, or talc-rich zones (Ahmed et al., 2009). Detailed de-scriptions of different types of orebodies can be found inLeblanc and Billaud (1982).

A positive indicator for mineralization is erythrite (hydratedcobalt arsenate), which is locally found on surface near min-eralization (Fig. 3E); mainly in the oxidation zones that ex-tend up to 200 m away from mineralization. Erythrite is com-mon in the Bou Azzer East and old Ait Ahmane mines whereit occurs as acicular crystals forming globular aggregates (pinkto deep purple in color). Magnetite veins (Gahlan et al., 2006)and the presence of magnetite aggregates in the serpen-tinized peridotites (Fig. 3F) are notable features in the min-eralized areas of the Bou Azzer district.

Characteristics of the Host SerpentinitesThe mantle section of the Neoproterozoic Bou Azzer ophi-

olite is dominated by serpentinized harzburgites with lessabundant dunites and small-scale chromitite pods. Almost allprimary silicate minerals in harzburgites and dunites havebeen altered to secondary serpentine, talc, carbonate, andchlorite. Chromian spinel is the only primary mineral pre-served (Ahmed et al., 2005, 2009). Almost all the upper man-tle rocks of the Bou Azzer ophiolite were metamorphosed togreenschist facies during the Pan-African orogeny; the min-eral assemblage antigorite + chlorite + carbonate + ferrit-chromite and/or magnetite is very common in all rock units(Evans, 1977; Leblanc, 1981; Bodinier et al., 1984). The chro-mian spinel cores in the harzburgites and dunites of the BouAzzer ophiolite, which represent the relics of primary spinel,are of the high Cr varieties (Ahmed et al., 2009).

Serpentinized dunites and harzburgites of the Bou Azzerophiolitic upper mantle have high Co, Ni, and Cu contentscompared with other ophiolitic rocks (Leblanc and Billaud,1982; Ahmed et al., 2009). The Ni content averages approxi-mately 2,500 ppm, and the Cu content averages approximately

52 ppm. The Co content of serpentinites ranges from 40 upto 160 ppm (Leblanc and Billaud, 1982; Gahlan et al., 2006).Fibrous magnetite veins of Bou Azzer are richest in Ni andCo; containing up to 1.0 wt percent and 3,000 ppm, respec-tively (Leblanc and Billaud, 1982; Gahlan et al., 2006; Ahmedet al., 2009). The serpentinized harzburgites and dunites ofBou Azzer ophiolite have intermediate to low PGE contentssimilar to the Co-Ni arsenide ores (see below). Total PGEcontents vary from 22 up to 53 ppb in dunites, with an aver-age of 38 ppb, and from 40 up to 68 ppb in harzburgites, withan average of 56 ppb (Ahmed et al., 2009). All of the peri-dotite samples also display positive Ru and Rh anomalies(Fig. 4).

Mineralogy and Petrography of the OresSeventy-five samples of Co-Ni arsenide mineralization

were prepared as polished thin sections to be studied by re-flected-light microscopy and electron probe microanalysis.The Bou Azzer Co-Ni arsenide ores have complex texturesand mineral assemblages, reflecting several stages of mineral-ization. The most common and distinctive textural and min-eralogical features are replacement and overgrowth, with ageneral paragenesis, in decreasing order of abundance of ar-senides, sulfarsenides, and accessory sulfides. The total PGEcontent of the Co-Ni arsenide ores is low to intermediate,ranging from 11 up to 74 ppb. The bulk PGE analyses displayunfractionated patterns with Pd/Ir ratios between 1.0 and2.25 (Fig. 4). The PGE spidergrams for Co-Ni arsenides showminor positive Ru and Rh anomalies, which resemble that ofthe host serpentinites (Fig. 4). There is a wide range of Ni andCu contents in the Co-Ni mineralization samples; up to 1 wtpercent Ni and 0.8 wt percent Cu. Gold contents range from

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FIG. 4. Chondrite-normalized PGE distribution patterns of the Co-Ni ar-senide ore deposits of the Bou Azzer, Morocco. Ten samples of Co-Ni ar-senide ores and host rocks were analyzed for all PGE (Os, Ir, Ru, Rh, Pt, andPd), Au, Ni, and Cu. The analysis was conducted using ICP-MS, after the Nisulfide fire assay collection at the Genalysis Laboratory Services, Australia.Detection limits are 2 ppb for Os, Ir, Ru, Pt, Pd, and Ni, 1 ppb for Rh andCu, and 5 ppb for Au. Normalization values are taken from Naldrett andDuke (1980). The compositional field of the Bou Azzer serpentinites PGEvalues is shown for comparison.

8 up to 2,000 ppb. The Co arsenide ores display a positive Auanomaly relative to other PGE in almost all the analyzedsamples (Fig. 4).

Arsenides

Generally, Co-Ni-Fe–bearing arsenide phases representthe main arsenide minerals of the Bou Azzer mineralization.Co-bearing arsenide minerals are predominant over the Ni-and Fe-bearing ore minerals; the Ni-bearing arsenide miner-als appear to have been paragenetically early and are replacedby Co- and Fe-bearing arsenides. Most Ni-bearing arsenidesare zoned. The main arsenide minerals are skutterudite, saf-florite, loellingite, nickeline, rammelsbergite, and S-rich Ni-Co diarsenides. Identification of these mineral phases isbased on reflected light microscopy and microprobe analysis.The presence of polymorphic phases such as clinosaffloriteand pararammelsbergite, which has been previously docu-mented from Bou Azzer ores (En-Naciri, 1995), cannot beconfirmed here.

Skutterudite (Co triarsenide) is one of the most commonarsenide minerals in the Bou Azzer ores. It forms homoge-neous euhedral to subhedral crystals and lacks zoning (Fig.5A). It occurs either as clusters of small euhedral crystals oras isolated, stout euhedral to subhedral crystals; all are em-bedded in quartz-carbonate gangue (Fig. 5A, B). Safflorite(Co diarsenide) and loellingite (Fe diarsenide) are wide-spread. Safflorite occurs as small to medium, euhedral to sub-hedral, crystals locally associated with skutterudite (Fig. 5C).Loellingite is widely distributed as lustrous granular massesand scaly aggregates. It has a brilliant silver-white color andmetallic luster and is opaque and brittle. It is easily confusedwith arsenopyrite, but arsenopyrite is commonly prismatic.Loellingite occurs as disseminations in and intergrown withbornite, all of which are imbedded within a quartz-carbonategroundmass (Fig. 5D). Oscillatory zoning between Co- andFe-rich diarsenides has been observed and, in places, theycomprise aggregates with massive or dendritic forms (Dolan-sky, 2007).

Nickeline is one of the most common Ni-bearing arsenideminerals in the Bou Azzer district, especially at the Ait Ah-mane mine. It occurs as bronze-colored masses and/or as re-sorption remnants within Ni-Co diarsenides (Fig. 5E) and asdendrites in dolomitic carbonate veins, with gray, cubic crys-tals of Ni skutterudite. Nickeline is locally associated withand/or replaced by Ni-Co and Ni diarsenide (rammelsber-gite) minerals. Rammelsbergite is comparatively rare; it isfound as metallic gray masses (silver-white to silver-gray incolor) with perfect cleavage at the Ait Ahmane mine. Its sur-face is commonly dull, perhaps due to alteration of associatedgersdorffite. It is observed as a replacement of earlier nicke-line and contains small remnants of nickeline, and it is in-truded and/or replaced by S-rich Co-Ni minerals (Fig. 5F).

Sulfarsenides

Arsenopyrite and, to a lesser extent, gersdorffite andcobaltite are the main Co-Ni-Fe sulfarsenide minerals at BouAzzer ores, especially in the old Ait Ahmane mine. The sul-farsenides mainly occur as euhedral homogeneous crystalsand rarely exhibit oscillatory zoning. Arsenopyrite is espe-cially common as discrete shiny prismatic crystals, up to 1 mm

across that are entirely imbedded within bornite (Fig. 6A).Ni-Co sulfarsenides with wide compositional ranges due toNi-Co and As-S substitutions locally replaced rammelsbergite(Fig. 6B). Gersdorffite is rare but forms prismatic crystals thatappear to replace rammelsbergite and S-rich Co arsenides(Fig. 6C). In the serpentinized peridotite country rocks, gers-dorffite is characteristically intergrown with and/or replacedby chalcopyrite (Fig. 6D) within carbonatized serpentinitematrix. Although cobaltite has been previously documentedin the Bou Azzer area (Leblanc and Billaud, 1982; Dolansky,2007), it was not observed in the samples studied.

Sulfides

Sulfides are accessory minerals in the Bou Azzer Co-Nimine and postdate the arsenide and sulfarsenide ore miner-als, filling fractures and cavities in the ore and altered wallrocks. Bornite, an unidentified Cu-Zn sulfide, chalcocite,chalcopyrite, pyrite, pyrrhotite, sphalerite, and tetrahedriteare the main sulfide phases observed. Bornite is the mostcommon and occurs as fracture filling (Fig. 7A) and as thegroundmass to the small arsenopyrite crystals and other sul-fides (see Fig. 6A). Bornite locally shows replacement alongthe peripheries and cracks by Cu-Zn sulfide (Fig. 7B). Thelatter also locally forms dendritic and irregular islands withinbornite (Fig. 7B). Chalcocite is found as disseminated com-posite grains associated with bornite and sphalerite (Fig. 7C).Chalcopyrite fills cavities and fractures in the Co arsenide oreor replaces earlier pyrite crystals along cracks (Fig. 7D) in thehost serpentinized mafic-ultramafic rocks. In quartz-carbon-ate gangue (see Fig. 3D), chalcopyrite is concentrated in cav-ities and contains large brecciated euhedral pyrite crystals(Fig. 7E). Pyrite, on the other hand, locally cements large eu-hedral crystals of pyrrhotite (Fig. 7F). Sphalerite and tetra-hedrite occur as small rounded crystals associated with ar-senopyrite and/or included within bornite in the Co-Fearsenide ore (Fig. 8A). Sphalerite also occurs as large an-hedral crystals associated with loellingite within a quartz-car-bonate matrix (Fig. 8B).

Gold

Gold is an important byproduct of the Bou Azzer districtand has been the object of numerous investigations (Leblancand Fischer, 1990; En-Naciri, 1995; Ghorfi et al., 2005). Goldoccurs in two specific modes: (1) as fine rounded discrete mi-croscopic grains (up to 50 µm) within early formed Ni-rich(i.e., nickeline) mineral phases (Fig. 8C), or in the laterformed Co-rich minerals (Fig. 8D), and (2) as comparativelylarge crystals (up to 100 µm) within gray-white quartz veinscrosscutting the serpentinized peridotites (Fig. 8E). Gold inquartz veins is locally aligned along fractures and cracks thatare partly filled by Fe-Mn–rich carbonates (Fig. 8E, F). Al-though silver content ranges up to 18 wt percent in the gold,native silver was not observed.

Gangue minerals

Quartz and carbonate minerals mainly fill spaces and frac-tures between the ore minerals (Leblanc and Billaud, 1982)and likely postdate Co-Ni arsenide mineralization since theyare infill fractures and cavities in the ore and altered wallrocks. Dolomite, ankerite, and calcite are the main carbonate

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FIG. 5. Photomicrographs of the Co-Ni arsenide ore minerals. (A). Idiomorphic crystals and masses of skutterudite (Skut.)imbedded within quartz-carbonate minerals, plane-polarized reflected light. (B). Euhedral skutterudite crystals associatedwith quartz (Qz) carbonate (Carb.) gangues, crossed-nicoles transmitted light. (C). Subhedral to anhedral safflorite crystalsintergrown with skutterudite (Skut.), all are imbedded within carbonatized serpentinite matrix, plane-polarized reflectedlight. (D). Subhedral to anhedral loellingite intergrown with bornite, within carbonate-rich serpentinite matrix, plane-polar-ized reflected light. (E). Disseminated irregular masses and resorbed grains of nickeline replaced by Co-Ni diarsenideswithin serpentinite matrix, plane-polarized reflected light. (F). Massive nickeline replaced by rammelsbergite with replace-ment texture, rammelsbergite is, in turn, replaced by S-rich Ni-Co diarsenides, plane-polarized reflected light. The whitedotted line delineates the boundary between S-rich diarsenide and rammelsbergite.

BA

C D

E F

minerals observed (Leblanc and Billaud, 1982; Dolansky,2007). Talc, chlorite, and serpentine are more abundant inthe host serpentinized peridotites. Chromite grains, stichtite,and magnetite with the same characteristics as those in ser-pentinites are also locally present in the mineralization.

Paragenetic SequenceTextural relationships suggest a depositional sequence pro-

ceeding from Ni monoarsenide (nickeline) to Ni-Co-Fe di-arsenides (rammelsbergite, Ni-Co diarsenides, Co-Fe di-arsenides) to cobalt triarsenide (skutterudite) reflectingprogressive increase in Co and As concentrations (see Fig.5A-F). Five stages of mineral formation can be distinguishedbased on the mineral textures and associations (Fig. 9): stage1 consists mainly of nickeline; stage 2, rammelsbergite, (Ni-Co)As2, and to a lesser extent skutterudite; stage 3, mainly

Co- and Fe-rich arsenides (skutterudite, safflorite, andloellingite) and (Co-Ni)As2-xSx; stage 4, sulfarsenides (gers-dorffite and arsenopyrite) and some sulfides (sphalerite andtetrahedrite), and pyrrhotite; and stage 5, mainly character-ized by the formation of Cu sulfides such as bornite, chal-cocite, and chalcopyrite.

In stage 1 nickeline forms euhedral masses and crystals,which are replaced by other arsenide minerals such as (Co-Ni)As2 (Fig. 5E). Stage 2 is characterized by an increase in Asand Co, which resulted in the replacement of nickeline byrammelsbergite (Fig. 5F). During stage 3, the Co, Fe, and Asconcentrations are even higher, as indicated by the crystal-lization of Co triarsenides (skutterudite) and Co-Fe di-arsenides (safflorite and loellingite), including the completesolid solution between safflorite and loellingite (see Fig.10B). The S-rich Ni-Co diarsenide, (Co-Ni)As2-xSx, replacing

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FIG. 6. Photomicrographs (A-C) and backscattered electron image (D) of the Co-Ni arsenide and sulfarsenide minerals.(A). Small euhedral rhombs of arsenopyrite (Asp) entirely imbedded within bornite (Bo) and itergrown with skutterudite(Skut), plane-polarized reflected light. (B). Sulfur-rich Ni-Co diarsenides replacing rammelsbergite (Ramm.); the latter isalso selectively replacing nickeline, plane-polarized reflected light. The white dotted line delineates the boundary betweenS-rich diarsenides and rammelsbergite. (C). Well-defined euhedral gersdorffite crystals (Gr) intergrown with rammelsber-gite and S-rich Ni-Co diarsenides, plane-polarized reflected light. (D). Backscattered electron image of gersdorffite (Gr) in-tergrown with chalcopyrite (Cp) within carbonatized serpentinite.

BA

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FIG. 7. Photomicrographs (A-C, E, F) and backscattered electron image (D) of the Co-Ni arsenide and sulfide minerals.(A). Bornite (Bo) infilling cracks and fractures of skutterudite (Skut), plane-polarized reflected light. (B). Unidentified Zn-rich Cu sulfide mineral rimmed and selectively replacing bornite (Bo), plane-polarized reflected light. (C). Composite dis-seminated sulfide grains in serpentinite comprising bornite (Bo), chalcocite (Cc), and sphalerite (Sphl) mineral assemblage,plane-polarized reflected light. (D). Backscattered electron image of euhedral pyrite crystal (Py) selectively replaced alongcracks by chalcopyrite (Cp) within serpentinite host rock. (E). Fractured and brecciated euhedral pyrite crystals (Py) ce-mented by chalcopyrite (Cp) within cavities of the Co-Ni arsenide mineralization, plane-polarized reflected light. (F). Brec-ciated euhedral to subhedral pyrrhotite (Po) cemented by pyrite (Py) in the sulfide-rich zone of the Co-Ni arsenide miner-alization, plane-polarized reflected light.

A B

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E F

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FIG. 8. Photomicrographs (A-F) of the Co-Ni arsenides, sulfarsenides, sulfides, and gold of Bou Azzer mineralization, Mo-rocco. (A). Small euhedral arsenopyrite (Asp) rhombs intergrown with rounded to subhedral crystals of sphalerite (Sphl) andtetrahedrite (Tet), all are imbedded within bornite (Bo) matrix, plane-polarized reflected light. (B). Sphalerite (Sphl) inter-grown with loellingite (Loel) and bornite (Bo), plane-polarized reflected light. (C). Discrete rounded gold inclusion withinnickeline replaced by rammelsbergite (Ramm) and S-rich Ni-Co diarsenides, plane-polarized reflected light. (D). Roundedgold grain within the late-stage S-rich Ni-Co diarsenides, plane-polarized reflected light. (E). Gold grains in quartz veinswithin the Bou Azzer serpentinites, reflected light. (F). Different sizes of gold grains concentrated with the Fe-Mn–rich car-bonates along cracks of quartz veins within the Bou Azzer serpentinites, plane-polarized reflected light.

A B

C D

E F

rammelsbergite, and sulfarsenides, gersdorffite, and, to alesser extent, arsenopyrite also started to appear in this stageand in stage 4. Sphalerite was also part of stage 3, as inferredfrom its intergrowth with loellingite and safflorite (see Fig.8B). Stage 4 was apparantly marked by an increase of sulfurfugacity, which would explain sulfarsenides (gersdorffite andarsenopyrite) and some sulfides (sphalerite and tetrahedrite)becoming prevalent during this stage (see Fig. 6A). Euhedralpyrite crystals also formed during this stage (Fig. 7E, F). Fi-nally, during stage 5 the widespread occurrence of Cu sulfides(bornite, chalcocite, and chalcopyrite) replacing or rimmingall the previously formed mineral phases (see Figs. 6A, 7A,E). Bornite fills cavities and cracks within Co-Ni-Fe arsenideminerals (Fig. 7A) and forms the matrix to sulfarsenides andsulfides (Fig. 8A). Chalcopyrite also fills cavities and crackswithin Co-Ni-Fe arsenides as well as in the quartz-carbonategangue (see Fig. 3D). Gold is found in almost all of the above-mentioned mineralization stages, as inferred from its pres-ence as discrete grains with various arsenide minerals and inlater Fe-rich quartz-carbonate veins (Fig. 8C-F).

Mineral Chemistry

Analytical techniques

The chemical compositions of minerals were determinedusing a JEOL electron-probe microanalyzer JXA-8800 at theCenter for Cooperative Research of Kanazawa University,Japan. The ore minerals were analyzed for Co, Ni, Fe, Cu,Au, Ag, Zn, Sb, Bi, As, and S. Analytical conditions were 25-kV accelerating voltage, 20-nA probe current, and the count-ing time was 20 s for each element. The standards usedwere pure metals for the elements Cu, Au, Ag, Zn, Sb, and Bi,

gallium arsenide for As, cobaltite for Co and pentlandite forS, Fe and Ni. Representative analyses of the investigated min-erals are given in the Appendix.

Although mineralogical and chemical characteristics of theBou Azzer arsenides are extremely variable, the investigatedminerals can be simply divided into two main groups: group Iconsists of Co-Fe–rich, Ni-poor arsenides, which include thetriarsenide skutterudite and diarsenides (safflorite andloellingite); group II consists of Ni- and Ni-Co–rich ar-senides, which include Ni monoarsenide (nickeline), Ni di-arsenide (rammelsbergite), and S-rich Ni-Co diarsenide.There is general enrichment in Co and Fe, and depletion inNi in group I, and general enrichment in Ni, and to a lesserextent Co, and substantial depletion in Fe in group II.

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FIG. 9. Paragenetic sequence of mineralization (e.g., arsenides, sul-farsenides, sulfides, and gold) in the Bou Azzer Co-Ni arsenide ore deposits.The dashed lines represent the uncertainty of appearance of a mineral in thecorresponding stage.

FIG. 10. Compositional variations (at. %) of the Bou Azzer Co-Ni arsenidemineralization. (A). Triarsenide (i.e., skutterudite) composition in the CoAs3-NiAs3-FeAs3 ternary diagram. The full shaded area shows the range of skut-terudite composition previously analyzed from Bou Azzer (En-Naciri, 1995;Dolansky, 2007), and the dashed area outlines the solid-solution field of skut-terudite experimentally determined by Roseboom (1962). (B). Diarsenidecompositions in the system CoAs2-NiAs2-FeAs2. The compositional limits(dashed area) of natural (Co, Ni, Fe) diarsenides (full dashed area) are alsoshown for comparison.

Triarsenides

Skutterudite is the only Co-Ni-Fe triarsenide in the studiedsamples and its composition varies from sample to sample oreven within individual samples. The Co end member ofskuterrudite was not found. The Ni content varies from 0.05to 7.1 at. percent (App.), and Co varies from 14.7 to 25.3 at.percent. Iron content increases with Ni and somewhat in-versely with Co; it varies from 0.88 to 7.3 at. percent. Skut-terudite compositions (Fig. 10A), lie entirely within the com-positional range of previously analyzed skutterudite from BouAzzer mineralization (En-Naciri, 1995; Dolansky, 2007) andnatural skutterudite solid solutions reported by Roseboom(1962). Sulfur content also varies widely from nil up to 6.9 at.percent and varies inversely with As (Fig. 11A) and somewhatpositively with Ni and Co (App.). The chemical formula ofskutterudite analyzed in this study ranges from (Co0.94

Fe0.06)As3.00 to (Co0.64Ni0.18Fe0.18)As2.74S0.26, with substantialsubstitution of Ni and Fe for Co (average formula of(Co0.82Ni0.09Fe0.09)As2.90S0.10).

Diarsenides

Safflorite (CoAs2), loellingite (FeAs2), and rammelsbergite(NiAs2) are the main diarsenide minerals investigated. Thereis a wide compositional variation along the three joins of thetriangle CoAs2-FeAs2-NiAs2 with a few analyses located at thepure Ni and Fe apices (Fig. 10B).

Safflorite has a wide range of Fe content from 5.3 to 21.5at. percent (App.); Co contents range from 14.7 to 30.9 at.percent. There is a strong negative correlation between Asand S in safflorite (Fig. 11A), with S contents from nil to 2.3at. percent. There is no correlation between Ni and Co in saf-florite (Fig. 11B), although all the safflorite analyses showvery low Ni contents (App.). The chemical formula of saf-florite, therefore, ranges from (Co0.73Fe0.27)As2.00 to (Co0.59

Fe0.41)As1.93S0.07, with an average of (Co0.62Fe0.38)As1.96S0.04.Loellingite in the studied samples has a wide range of Co

and Ni contents. The Ni content varies from nil to 19.7 at.percent (App.). The Ni-rich loellingite also locally contains upto 11.2 at. percent Co (Fig. 10B) and up to 3.2 at. percent Zn.There is a nearly complete solid solution between the Co-richloellingite and Fe-rich safflorite (Fig. 10B) from 0.06 up to23.4 at. percent Co. The S content of loellingite also varieswidely from nil to 6.5 at. percent. There is a strong negativecorrelation between S and As (Fig. 11A) as expected, butthere is no correlation between S, Co, and Ni. The chemicalformula of loellingite in this study varies from pure Fe endmember FeAs2, to (Fe0.46Co0.29Ni0.25)As1.81S0.19, with an aver-age formula of (Fe0.68Co0.21Ni0.11)As1.93S0.07.

Rammelsbergite in the studied samples varies from thepure Ni end member to Co-bearing varieties (Fig. 10B). TheCo content of rammelsbergite varies from 0.09 up to 20.2 at.percent (App.). The Fe substitution for Co is very limited; Fevaries from nil to 1.1 at. percent. There is a very good nega-tive correlation between S and As (Fig. 11A) and Ni and Co(Fig. 11B), with S contents varying from nil in the Co-poorend member to 8.7 at. percent in the Co-rich varieties (App.).The chemical formula of rammelsbergite varies from the pureNi end member (Ni0.99Co0.01)As2.00 to (Ni0.64Co0.34Fe0.02)As1.76S0.24, with an average formula of (Ni0.93Co0.07)As1.94S0.06.

An S-rich diarsenide mineral with a general formula (Ni-Co)As2-xSx typically is associated with and/or replaces rammels-bergite in the Ni-rich arsenide ore. The Co content varies from2.3 up to 21.1 at. percent, whereas the Ni content varies from13.3 to 32.9 at. percent (Fig. 10B, App.). The S content is highin most of the analyses of this mineral, varying from 3.4 to 26.3at. percent, and there is a strong negative correlation both be-tween S and As (Fig. 11A) and between Ni and Co (Fig. 11B),with continuous substitution of S for As and Co for Ni. Analy-ses with the lowest As and Ni contents have the highest S andCo contents. The chemical formula of this S-rich diarsenidevaries from (Ni0.85Co0.15)As1.83S0.17 to (Ni0.59Co0.38Fe0.03)As1.39

S0.61, with an average formula (Ni0.66Co0.33Fe0.01)As1.66S0.34.

Monoarsenides

Nickeline in the studied samples is very homogeneous inchemistry and has a very simple structural formula (NiAs)

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FIG. 11. Binary plot of compositional variations of the Co-Ni arsenidemineralization of Bou Azzer. (A). Variations of As vs. S (at. %) of various Co-Ni-Fe arsenide minerals showing a good negative correlation between As andS in all arsenide minerals. (B). Compositional variations of Ni vs. Co (at. %)of the Co-Ni-Fe arsenide minerals, showing a negative correlation betweenCo and Ni in all arsenide minerals except for nickeline and safflorite wherethe Co and Ni contents are negligible.

with very limited Co and Fe substitution for Ni. The Ni con-tents range from 52.5 to 55.1 at. percent (App.). Cobalt andFe contents are very low; from 0.02 to 1.0 at. percent for Co,and from nil to 0.2 at. percent for Fe (App.). The S contentsvary within a limited range from nil to 3.1 at. percent. Thechemical formula of nickeline is approximately around thepure Ni end member, which has the average formula ofNiAs0.98S0.02.

Sulfarsenides

The arsenopyrite (FeAsS) and gersdorffite (NiAsS) are themain sulfarsenide minerals identified in the studied samples.Arsenopyrite has a homogeneous composition; the Fe con-tents vary within a limited compositional range from 33.8 to36.9 at. percent (App.). The S contents range from 31.5 to34.9 at. percent, and Co from 0.03 to 1.2 at. percent. The av-erage chemical formula of arsenopyrite is (Fe0.99Co0.01)As0.96S1.04.

Gersdorffite of the studied samples is more or less homo-geneous in composition; limited amounts of Co and Fe sub-stitute for Ni. The Co contents range from 2.1 to 3.3 at. per-cent (App.). The Fe content has a limited range from 1.7 to2.6 at. percent, and S contents range from 22.8 to 31.0 at. per-cent. The average chemical formula of the analyzed gersdorf-fite is (Ni0.86Co0.08Fe0.06)As1.13S0.87.

Sulfides

There is very good inverse correlation between S and metalcontents (Me = Cu + Fe + Zn + Sb + Bi + Ag + Au in at. %)in the sulfide minerals of the studied samples (Fig. 12). TheCu sulfides (i.e., bornite and chalcocite) have the highest Mecontents and lowest S contents, whereas the Fe sulfides(pyrite and pyrrhotite) have the lowest Me contents and high-est S contents (Fig. 12). The Cu-Fe sulfide (chalcopyrite), theCu-Sb sulfide (tetrahedrite), and the Zn sulfide (sphalerite)have intermediate Me and S contents (Fig. 12).

Bornite (Cu5FeS4) shows a limited compositional range,where the Cu contents range from 47.4 to 51.5 at. percent, Fe

from 8.4 to 11.1 at. percent, and S from 37.9 to 43.3 at. per-cent (App.), showing an average chemical formula ofCu4.81Fe1.19S3.92. Minor amounts of Au, Ag, and Co are alsopresent in bornite.

Bornite is usually rimmed and/or replaced by an unidenti-fied Zn-rich Cu sulfide that contains 53.7 to 57.8 at. percentCu, 0.5 to 2.2 at. percent Fe, 3.2 to 6.3 at. percent Zn, and37.9 to 38.6 at. percent S (App.), with an average chemicalformula of Cu11.59(Fe0.2Zn0.8)S7.86. There is no similarity incomposition with any recorded Cu minerals, which suggeststhat may be a new mineral species, but further investigationsare required.

Chalcopyrite (CuFeS2) of the studied samples has a limitedcompositional range, containing 20.4 to 23.8 at. percent Cu,22.4 to 25.6 at. percent Fe, and 50.6 to 56.6 at. percent S(App.). Chalcopyrite is almost free of other elements, con-taining just 0.05 at. percent Co on average . It is very close tothe stoichiometric chalcopyrite, which has an average chemi-cal formula of Cu0.93Fe1.07S2.10.

Chalcocite (Cu2S) is reported only from the disseminatedores of the Bou Azzer Co-Ni mineralization. Its chemicalcomposition has a restricted variation, where the Cu contentsrange from 62.7 to 67.1 at. percent and S contents from 32.3to 34.9 at. percent (App.). It contains a detectable amount ofFe, from 0.10 to 2.3 at. percent, and very low contents of Agand Au (around 0.02 at. % for each). Chalcocite of the stud-ied samples has an average chemical formula of Cu1.92Fe0.02

S1.06. Pyrite and pyrrhotite are the main Fe sulfides in the stud-

ied samples. Iron in pyrite ranges from 33.1 to 35.3 at. per-cent (App.) with low Co, 0.12 at. percent on average. Thepyrite is stoichiometric, showing an average chemical formulaof Fe1.07S1.93. Pyrrhotite also has a restricted chemical compo-sition with Fe contents ranging from 44.3 to 46.7 at. percent,with an average chemical formula Fe0.80S1.20.

Sphalerite (ZnS) in the studied samples consists of 43.7 to46.5 at. percent Zn (App.). There are detectable amounts ofFe, Co, and Ni with an average of 0.5, 0.2 and 0.4 at. percent,respectively. Sphalerite has a substantial content of Au, up to1.88 wt percent (0.44 at. %) on average. The average chemi-cal formula of sphalerite is Zn0.97(Fe0.01Ni0.01Au0.01)S1.19.

Tetrahedrite is the only Sb-rich mineral in the studied sam-ples. Its chemical composition is very homogeneous, contain-ing about 34 at. percent Cu and 12.3 at. percent Sb (App.).There are detectable amounts of Zn, Fe, Bi, As, Ag, and Au,with average contents of 4.3, 2.1, 0.5, 0.4, 0.1, and 0.1 at. per-cent, respectively. The average chemical formula of tetra-hedrite is slightly different from the stoichiometric form; ithas a chemical formula of (Cu0.94Fe0.06)9.10(Sb0.7Zn0.26Bi0.03

As0.02Ag0.01)4.41S11.49, with some deficiency in Cu-Fe and Ssites.

Gold

As mentioned previously, two modes of occurrence exist forgold in the studied samples; microscopic inclusions within Ni-and Co-rich arsenides and visible grains along microfracturesand carbonate-rich quartz veins in surrounding serpentinites.The composition of the gold grains is quite different in thetwo types, being higher in Ag in the former than in the lattertype (Fig. 13). There is a general inverse correlation between

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FIG. 12. Binary plot of S vs. metal contents (Me = Cu + Fe + Sb + Bi +Au + Ag, at. %) of various sulfide minerals in the Bou Azzer district. Note theperfect inverse correlation between the metal contents and S of various sul-fide minerals.

Au and Ag contents of the gold analyses (Fig. 13). The Aucontents of the first type of gold range from 58.2 to 70.6 at.percent (App.), with Ag contents varying from 23.7 to 25.5 at.percent (Fig. 13; App.). Detectable amounts of Ni and S arepresent, which may be a result of contamination from the sur-rounding minerals due to the small size of the gold grains.The gold grains in the carbonate- and Fe-rich quartz veinshave a wide range of Au contents, from 78.1 to 95.5 at. per-cent (Fig. 13, App.). The Ag contents of this type of goldgrains range from 3.2 to 13.2 at. percent. There is also a de-tectable amount of Bi and S in the gold analyses of this type.

Discussion

Source(s) of Co, Ni, and Fe, and fractionation of Co and Ni

The Bou Azzer mineralization is dominated by Co tri-arsenide (skutterudite), diarsenides (safflorite, loellingite),and Ni arsenides (nickeline and rammelsbergite). Hence, Cois the main metal constituent, followed by Ni and Fe, whereasarsenic is the element common to all the ore minerals. Logi-cally, if serpentinite is the ultimate source of Co and Ni in theBou Azzer mineralization (Leblanc, 1975; Leblanc and Bil-laud, 1982; Dolansky, 2007), and if the Ni and Co wereequally leached from serpentinites, Ni should be the domi-nant metal in the ore minerals, since its concentration ismuch higher than Co in serpentinites (Leblanc and Billaud,1982, Shiga, 1987). Serpentinites are enriched in Ni and Corelative to other ophiolitic rock types in the Bou Azzer inlier,containing about 2,800 ppm Ni and about 76 ppm Co (aver-age of six serpentinite analyses from Leblanc and Billaud,1982; Gahlan et al., 2006), equivalent to an atomic proportionof Ni to Co of 37/1. The maximum concentration of 1 wt per-cent Ni and 3,000 ppm Co has been recorded in magnetiteveins (Leblanc and Billaud, 1982; Gahlan et al., 2006; Ahmedet al., 2009). The serpentinites are the dominant lithology ofthe Bou Azzer ophiolite suite, extending up to 2 km in plan(Leblanc, 1975), and thus, volumetrically, they comprise aplausible source rock for the adjacent ore deposits.

The source of Fe of the Fe-rich minerals in the Bou Azzerores and magnetite veins likely derived by the breakdown ofolivine in the ophiolitic peridotites (Frost, 1985; Shiga, 1987;Frost and Beard, 2007). During serpentinization, Fe was mo-bilized by CO2-rich hydrothermal fluids; meteoric and/ormetamorphic waters activated by regional metamorphismduring emplacement of the Neoproterozoic ophiolite. Mag-netite in highly serpentinized peridotites commences crystal-lizing only after a significant amount of hydration has takenplace (Bach et al., 2006; Frost and Beard, 2007). Thefluid/rock ratio during the serpentinization process of theBou Azzer upper mantle, therefore, was possibly very high,as inferred from the common abundance of magnetite veinsand frequently distributed magnetite grains throughout theserpentinite massifs (Oufi et al., 2002; Bach et al., 2006;Gahlan et al., 2006). The common abundance of Co-, Zn-,and Mn-rich spinel in the Bou Azzer mineralization district(Gahlan and Arai, 2007; Ahmed et al., 2009) is further evi-dence for the high fluid/rock ratio during serpentinization,since these elements presumably were incorporated intospinel by intensive leaching of surrounding rocks via hy-drothermal fluids (Gahlan and Arai, 2007). The highfluid/rock ratios were locally accompanied by high silica con-tents, either through serpentinization of orthopyroxene orthrough external supply of silica (Frost and Beard, 2007).The presence of free quartz- and carbonate-rich veins adja-cent to the magnetite veins in the Bou Azzer serpentinite(Ahmed et al., 2009; Fig. 2D) is additional evidence for ahigh fluid/rock ratio during serpentinization.

If we assume that serpentinite was the source for Co and Nifor the ores, as suggested by Leblanc (1975), Leblanc and Bil-laud (1982), Dolansky, (2007), then Ni minerals would be ex-pected to predominate over Co minerals. However, this is notthe case in the Bou Azzer arsenide mineralization, where Coarsenides are more abundant. This suggests that there is a sig-nificant difference in mobility between Ni and Co during thehydration process of peridotites (Hem et al., 2001; Ertsevaand Tsymbulov, 2002; Fanlo et al., 2004). Studies by Hem andMakovicky (2004) and Fanlo et al. (2004) suggest that Co ismore soluble than Ni in hydrothermal fluids at temperatureslikely to pertain at Bou Azzer mineralization. This is consis-tent with the deficiency of Co relative to Ni in the Bou Azzerserpentinites (Gahlan and Arai, 2007). Ni and Co fractiona-tion could also result in sequential precipitation of ore miner-als: Ni was precipitated as Ni monoarsenides and diarsenidesfrom high-temperature fluids before the formation of Co-richarsenides. The change in concentration and behavior of met-als (Ni, Co, and Fe) in the mineralizing fluids is, therefore,the main factor controlling the trend of crystallization se-quence of the Bou Azzer mineralization. It is noteworthy thatnickeline is the arsenide richest in metals and most deficientin As (Fig. 14). The sequential formation, after nickeline, ofdiarsenides (rammelsbergite, Co-NiAs2-xSx, safflorite, loellin-gite), and then triarsenides (skutterudite) shows a crystalliza-tion trend characterized by a progressive increase in the ac-tivity of Co and As (Hem et al., 2001; Fanlo et al., 2004). Theleaching of ore elements (i.e., Co, Ni, and Fe) from serpen-tinites in the Bou Azzer mineralization can also be supportedby the similar patterns of PGE spidergrams from both ser-pentinites and the arsenide ore deposits (see Fig. 4).

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FIG. 13. Compositional variations of Au and Ag (at. %) of the gold grainsfrom the Co-Ni arsenide mineralization and quartz veins. Note that there isa general increase in Ag contents from the gold in quartz veins to the gold in-clusions in Co-Ni arsenide minerals.

Source(s) of arsenic

Arsenic is a chalcophile element with a strong affinity for S,and sulfide minerals are considered as one of the main hostsfor As in a variety of crustal rocks (Ryan et al., 1995; Leeman,1996; Noll et al., 1996). It is one of the most soluble chal-cophile elements and is highly enriched in arc magmas fromthe mantle wedge (Ryan et al., 1995; Leeman, 1996; Noll etal., 1996). Arsenic is also soluble in aqueous fluids even at lowtemperatures (Smedley and Kinniburgh, 2002). Serpen-tinized ultramafic rocks contain high As contents rangingfrom 10 up to 450 ppm As (Hattori et al., 2005; Sullivan,2007).

The source of As in the Bou Azzer arsenide mineralizationis still controversial. Several sources of As have been previ-ously suggested including: (1) serpentinites (Leblanc, 1975;Leblanc and Billaud, 1982; Leblanc and Fischer, 1990;Dolansky, 2007); (2) late Neoproterozoic felsic volcanic coverof the Ouarzazate Supergroup, which contains As ≈ 100 to300 ppm (Leblanc, 1975; Leblanc and Billaud, 1982); (3) dis-crete sulfide-rich horizons in the serpentinites which containbetween 50 and 300 ppm As (Leblanc and Billaud, 1982); and(4) Silurian organic-rich shales (Dolansky, 2007). Althoughthere is no definitive evidence to constrain the source(s) of Asfor the Bou Azzer arsenide mineralization, the serpentinitemass would be a sufficient As source.

Timing and conditions of ore formation

The timing of the Bou Azzer arsenide mineralization re-mains controversial. The present consensus is that the BouAzzer arsenide mineralization postdates the emplacement ofthe Neoproterozoic ophiolite fragments (Leblanc, 1975;Leblanc and Billaud, 1982; Dolansky, 2007), but the exact ageof ore formation is uncertain. Recently, Dolansky (2007) con-strained the timing of the Bou Azzer Co-Ni mineralizationbased on in situ dating of brannerite, suggesting a range from383 ± 7 to 257 ± 8 Ma.

The resultant fluids from the obduction of the Bou AzzerNeoproterozoic ophiolite were likely enriched in metals (Co,Ni, Fe), CO2, Si, Ca, and S due to interaction with serpen-tinites (En-Naciri et al., 1997). Based on recent fluid inclu-sion studies (En-Naciri et al., 1997; Dolansky, 2007), the BouAzzer mineralizing fluids were highly saline brines probablyderived from a felsic magmatic source related to the onset ofthe Hercynian (Variscan) orogeny (En-Naciri et al., 1997).

ConclusionsThe present study discusses several important aspects of

the genesis of the Co mineralization at Bou Azzer district thathave been only poorly constrained by previous investigations.It thus contributes to a better understanding not only of theformation of this particular deposit but also of general aspectsof hydrothermal Co ores. The Bou Azzer mineralization isdominated by Co triarsenides (skutterudite), diarsenides (saf-florite, loellingite), and Ni arsenides (nickeline and rammels-bergite). Sulfarsenides (gersdorffite and arsenopyrite) andsulfides (bornite, chalcocite, chalcopyrite, pyrite, pyrrhotite,sphalerite, and titrahedrite) are accessory phases of a subse-quent later stage after the formation of the main arsenide oreminerals. Hence, Co is the main metal constituent, followedby Ni, whereas arsenic is minor but common to all the oreminerals at Bou Azzer ores. Detailed chemical analysis of(Co, Ni, Fe) arsenides, sulfarsenides, and sulfides show com-positional ranges wider than previously reported for thesephases (App.).

The mineral paragenetic sequence in the Bou Azzer ar-senide mineralization was started by the high-temperaturephases of Ni monoarsenides (nickeline) and followed by Nidiarsenides. With decreasing temperature and increasing sul-fur fugacity, Co and Fe arsenides became more stable to formthe main Co arsenide ore minerals. Sulfarsenides and sulfideswere deposited at relatively low temperature during the latestage of the serpentinization process. The source of metals(Co, Ni, and Fe) and As is most likely the serpentinites,leached by acidic fluids under a high fluid/rock ratio andmoderately reducing conditions.

The mineralizing fluids at the Bou Azzer district were pos-sibly a mixture of two types: a relatively high temperature, ox-idizing, NaCl-dominated saline brines derived from a felsicmagmatic source related to the Hercynian orogeny, and rela-tively low temperature, oxidizing, and CO2-bearing meteoricwaters. The mixing between these fluids led to an increase inpH and sulfur activity, as well as decrease in oxygen fugacity,and resulted in the formation of quartz-carbonate ganguesand associated late-stage hydrothermal sulfides.

AcknowledgmentsWe are thankful to T. Morishita, A. Tamura, and K. Tazaki

of Kanazawa University for their assistance with microprobeanalysis. We also express our gratitude to the staff membersof the Bou Azzer Co. mine (C.T.T.) for their hospitality andguidance during the field work. This study was partly sup-ported by the Japan Society for the Promotion of Science(JSPS) Grant-in-Aid for Creative Research (19GS0211).Helpful comments by D. Marshall, G. Olivo, and S.J. Pierceyimproved the clarity of the first version of the manuscript.Mark Hannington, coeditor of Economic Geology, is highly

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FIG. 14. Compositional variations (at. %) of As vs. metal contents (Me =Co + Ni + Fe) of the Co-Ni-Fe arsenide ore minerals. Note the general neg-ative correlation between the metal and As concentrations. In some instancesmetal contents are stable with decreasing As content.

thanked for his useful comments and advice to improve theearlier version of the manuscript. Finally, the comments ofLarry Meinert, the editor of Economic Geology, are highlyappreciated to put the manuscript in its final form.

March 25, 2008, February 15, 2009

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APPENDIXRepresentative Microprobe Analyses of All the Analyzed Minerals (arsenides, sulfarsenides, and sulfides)

from the Bou Azzer Arsenide Mineralization, Morocco

Wt % Skutterudite Safflorite Loellingite

Ni 2.33 1.73 3.61 2.17 0.02 1.61 2 2.95 0.04 nd 0.04 0.03 0.01 nd 0.02 7.2Co 19.26 19.97 17.1 19.91 26.35 20.53 20.34 19.06 22.08 18.28 21.27 23.39 0.1 0.46 11.95 1.09Cu nd 0.31 nd nd nd nd nd nd nd 0.02 nd nd nd 0.21 nd 0.04Fe 1.44 1.5 2.67 1.18 4.29 0.83 0.83 1.04 8.62 12.48 9.18 7.45 29.86 29.45 18.52 23.07Au 0.2 0.16 0.12 0.09 0.1 0.16 0.08 0.05 0.14 0.05 0.14 0.17 0.04 0.1 0.11 0.1Ag nd 0.1 0.06 nd 0.04 nd nd nd nd nd nd 0.04 nd 0.01 nd ndZn 0.03 nd 0.03 nd 0.03 nd 0.02 nd 0.03 nd nd nd 0.04 0.01 0.01 ndSb nd 0.01 nd nd nd nd nd nd nd nd nd nd nd nd nd ndBi nd nd nd nd nd nd 0.02 0.06 nd 0.07 0.05 nd 0.04 0.09 nd ndAs 75.61 74.92 75.25 74.91 66.89 74.63 74.68 75.7 66.99 67.18 67.74 67.54 67.27 66.81 67.75 65.37S 1.29 1.17 1.21 1.26 0.94 1.79 0.7 0.79 1.1 0.68 0.09 0.97 1.68 1.81 0.92 2.21Total 100.16 99.87 100.06 99.52 98.67 99.56 98.67 99.65 99 98.77 98.5 99.6 99.02 98.95 99.28 99.08

Wt % Rammelsbergite S-rich Ni-Co diarsenides Nickeline

Ni 26.99 31.73 31.72 31.65 18.84 22.87 28.24 18.11 47.66 47.73 47.29 48.17 48.39 48.09 48.07 48.44Co 4.87 0.15 0.17 0.14 14.36 10.45 4.25 15.46 0.06 0.07 0.08 0.09 0.09 0.06 0.08 0.07Cu 0.04 0.01 0.02 0.04 0.01 0.02 0.02 0.02 0.06 0.07 0.1 0.06 0.05 0.05 0.07 0.05Fe 0.11 0.02 nd nd 0.24 0.14 0.13 0.24 nd nd nd nd nd nd 0.02 0.01Au 0.15 0.13 0.09 0.15 0.19 0.18 0.07 0.05 0.18 0.14 0.14 0.05 0.18 0.1 0.12 0.15Ag nd nd nd nd 0.04 nd 0.01 nd nd nd nd 0.01 nd nd nd ndZn nd nd nd 0.01 nd nd nd nd nd nd 0.01 nd nd nd nd ndSb nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd ndBi nd nd nd nd 0.06 nd nd nd nd nd nd nd nd nd nd ndAs 65.72 67.76 67.44 68.06 59.85 60.56 63.67 58.44 51.21 51.34 50.35 50.42 50.39 50.98 51.22 50.13S 2.21 nd 0.9 nd 6.24 5.91 2.73 7.72 1.27 0.48 1.26 0.81 1.44 0.68 0.55 0.22Total 100.08 99.79 100.35 100.04 99.81 100.13 99.122 100.04 100.43 99.83 99.24 99.6 100.53 99.96 100.12 99.06

Zn-rich Wt % Gersdorffite Arsenopyrite Bornite Cu sulfide Chalcocite Chalcopyrite Pyrite Pyrrhotite

Ni 31.52 31.74 nd 0.03 nd nd nd nd nd nd 0.12 nd 0.02 nd nd ndCo 3.5 3.15 0.07 0.11 0.02 0.06 0.02 0.01 0.01 nd 0.31 0.07 0.05 0.13 0.09 0.15Cu 0.02 0.03 0.02 0.01 61.54 61.59 63.14 69.94 76.96 75.85 29.74 33.12 nd nd 0.02 ndFe 2.2 2.23 36.54 36.37 11.65 11.5 2.31 0.5 0.1 0.3 28.31 31.26 46.74 46.47 60.02 60.12Au 0.04 0.11 0.05 0.12 0.14 0.02 0.29 0.15 0.01 0.05 0.11 0.04 0.05 0.09 0.12 0.05Ag nd nd 0.02 nd 0.18 0.17 0.07 0.11 nd 0.14 nd nd nd nd nd ndZn 0.04 0.01 nd nd nd nd 7.26 3.74 nd nd nd nd nd 0.01 0.03 ndSb nd nd 0.01 0.01 nd nd nd 0.01 nd nd 0.03 nd nd nd nd ndBi nd nd nd nd 0.1 0.12 nd 0.01 nd 0.08 nd nd nd 0.1 0.08 ndAs 44.54 44.99 42.31 42.67 nd nd nd nd nd nd nd nd nd nd nd ndS 17.78 17.81 19.39 20.34 26.15 26.75 22.49 23.55 19.97 20.37 41.17 35.58 53.27 52.51 39.39 41.27Total 99.64 100.07 98.42 99.65 99.79 100.2 95.62 98.04 97.05 96.81 99.79 100.09 100.13 99.296 99.77 101.59

Gold in Wt % Sphalerite Tetrahedrite Gold inclusions Qz veins

Ni 0.97 0.18 nd nd 2.48 0.82 nd 0.01Co 0.32 0.35 nd nd 0.03 nd nd 0.01Cu nd nd 38.04 37.96 0.02 0.07 nd ndFe 0.87 0.52 1.67 2.12 0.01 nd nd ndAu 1.84 1.75 0.3 0.21 77.72 80.44 96.24 96.74Ag nd nd 0.11 0.24 16.18 15.97 2.14 2.19Zn 60.83 63.16 5.1 5.04 0.07 nd nd ndSb 0.03 nd 26.12 26.3 nd nd nd 0.06Bi 0.05 nd 1.61 1.81 0.61 0.52 0.87 0.67As 0.01 nd 0.4 0.28 1.1 nd nd ndS 35.81 34.78 25.67 26.07 0.07 1.08 0.14 0.35Total 100.73 100.77 99.02 100.02 98.28 98.88 99.4 100.03