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Arabian Journal of Geosciences ISSN 1866-7511Volume 6Number 7 Arab J Geosci (2013) 6:2581-2597DOI 10.1007/s12517-012-0547-0

Rock magnetic study of basic intrusionsand massive sulphides in the Hercyniancentral Jebilets Massif (Occidental-Meseta),Morocco

Najib El Goumi, Mohammed Jaffal,Christian Rolf, Christoph Grissemann,Frank Melcher, Azzouz Kchikach,Mohammed Hibti, et al.

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ORIGINAL PAPER

Rock magnetic study of basic intrusions and massive sulphidesin the Hercynian central Jebilets Massif (Occidental-Meseta), Morocco

Najib El Goumi & Mohammed Jaffal & Christian Rolf &Christoph Grissemann & Frank Melcher &

Azzouz Kchikach & Mohammed Hibti &Torsten Graupner

Received: 23 October 2011 /Accepted: 10 February 2012 /Published online: 3 May 2012# Saudi Society for Geosciences 2012

Abstract This paper describes a multidisciplinary study ap-proach (petrography and rock magnetism) conducted on sam-ples collected from the study area to characterise the magneticmineralogy and to determine if the magnetisation of bothlithologies were induced or retain a remnant component.Petrophysical, mineralogical and geochemical analyses con-firm bimodal aspects, particularly in basic rocks; the two mag-netic modes depend essentially on the mineralogical andgeochemical characteristics of the samples. The ultramaficrocks comprise a highly altered primary mineralogy with chro-mite and magnetite as magnetic phases. The second type is ofmafic composition with a less altered primary mineralogy andessentially magnetite and/or (hemo-ilmenite) as a carrier ofmagnetic mineralisation. Sulphides are characterised by highconcentrations of Cu, Zn and Pb. The mineralogy is composedmainly of pyrrhotite (85% to 90%), sphalerite, galena, chalco-pyrite, arsenopyrite and, occasionally, stannite. Monocline pyr-rhotite seems to be the magnetic carrier of magnetisation inboth Draa Sfar and Koudiat Aïcha. However, we suspect adifferent amount of hexagonal pyrrhotite as the cause of differ-ent magnetic behaviour. Paleomagnetic and thermomagneticanalyses reveal different. The calculated characteristic direction

of natural remnant magnetisation for sulphides was used tomodel the magnetic anomaly of Draa Sfar. The proposedmodelmatch the geological features concluded from geological map-ping and boreholes. Results from this work can be very usefulfor any modelling processes of magnetic anomalies suspecteddue to a sulphide mineralisation in an area with poor outcropsand no presence of boreholes information or of any geologicalor geochemical data.

Keywords Magnetic behaviour . Magnetite . Pyrrhotite .

Basic rocks . Sulphides . Central Jebilets .Morocco

Introduction

The Hercynian central Jebilets Massif belongs to the west-ern Moroccan Meseta which is located in the middle-westportion of the Variscan belt (Morocco). It is mainly com-posed of deformed Hercynian rocks which outcrop north ofMarrakech (Fig. 1). This massif is one of the most importantmining districts in Morocco and was the target of numerousprospecting, mining and academic activities since the 1930s(Huvelin 1977; Bouloton and Le Corre 1985; Felenc et al.1985; Mellal and Maier 1988; Leblanc 1993). The potentialfor mineral resources in this massif is testified by the exis-tence of several massive sulphide deposits (Draa Sfar,Kettara, Koudiat Aïcha, etc.), which are associated with asuite of gabbroic and microgranitic intrusions forming linea-ments within the Visean shale of the Sarhlef series (Essaifiand Hibti 2008) (Figs. 1 and 2).

The discovery of a sulphide ore body at Hajar (34 km SWof Marrakech city) by an aeromagnetic survey shows thatmagnetic surveys are able to detect and to define massivesulphide deposits in the district of Jebilets-Guemassa(Bellot et al. 1991), due to the highly magnetic intensity

N. El Goumi (*) :M. Jaffal :A. Kchikach :M. HibtiFaculté des Sciences et Techniques,BP. 549 Marrakech, Moroccoe-mail: elgouminajib@gmail.com

C. RolfLeibniz Institute for Applied Geophysics (LIAG),Stilleweg 2,30655 Hannover, Germany

C. Grissemann : F. Melcher : T. GraupnerFederal Institute for Geosciences and Natural Resources (BGR),Stilleweg 2,30655 Hannover, Germany

Arab J Geosci (2013) 6:2581–2597DOI 10.1007/s12517-012-0547-0

Author's personal copy

character of the pyrrhotite-rich sulphides at the centralJebilets. Unfortunately, the basic intrusions associatedwith magnetic anomalies make it difficult to distinguishthe gabbroic intrusions from the sulphide deposits. In thepresent work, we investigated the magnetic properties ofboth massive sulphides and basic rocks of the centralJebilets area. First of all, we determined the physicalproperties (susceptibility and remnant magnetisation) forthe quantitative interpretation of the measured magneticanomalies in the area, and thereafter, we investigated theminerals carrying magnetisation.

We carried out a sampling campaign in the centralJebilets at different sites which correspond to the outcropsof basic rocks and massive sulphide (underground mines).

Geology and mineralisation

Geologic framework

The Palaeozoic Jebilets Massif is situated in the north ofMarrakech and forms a chain (about 170 km long and 40 kmwide) extending between the Haouz and the Bahira Basins(Fig. 1). This massif belongs to the western MoroccanMeseta, which constitutes a portion of the Hercynian foldbelt. It shows a well-exposed east–west section across theVariscan upper crust. In the Jebilets Massif, three structuraldomains have been recognised:

The western Jebilets unit, which corresponds to the stablecoastal block, has been described as formed essentially by

Fig. 1 Geological location. a Scheme of the geologic location of Jebilets Massif. b Geological sketch map of the Jebilets and Guemassa massifshowing massive sulphide deposits. WMD: Western Meseta Domain; C: central massif; R: Rehamna massif; J: Jebilets massif

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Cambro-Ordovician terranes (Huvelin 1977), Westphalo-Permian conglomerates and metapelites associated withPermo-Triassic basalts. This ensemble is unmetamorphosedbut slightly folded in submeridian direction.

The central Jebilets unit is formed by metasediment ter-ranes provenance “Sarhlef series”, whose age is attributed tothe Visean (Huvelin 1977). It is deformed and metamor-phosed in anchizone to epizone facies and intruded bybimodal plutonism magmatism. A large number of massivesulphide deposits are confined to this unit.

The eastern Jebilets unit, commonly termed as “Kharroubaflysch”, consists of a series of metasediments of Visean toNamurian age. In this unit, Huvelin (1977) has describedthe phenomenon of synsedimentary tectonics. Ordovicianto Devonian olistostromes were deposited by gravitate slipphenomena in a Visean basin (Bamoumen 1988).

Several magmatic intrusions (felsic and mafic) occur inthe Sarhlef schist; they are elongated (hundreds of metres) inthickness and kilometres in length, and they are localised atconstant distance from the massive sulphides deposits ofcentral Jebilets (∼1 to 1.5 km) (Essaifi and Hibti 2008)(Fig. 2). We distinguish two important mafic bodies, theKoudiat Kettara and Koudiat Ahril intrusions, which have

been sampled, with two other mafic intrusions (Fig. 2). On adistrict scale, together, the bimodal magmatism of centralJebilets match the general trend of the regional structures(folds, schistosity) (Essaifi and Hibti 2008).

Mineralisation

The sulphide deposits of the central Jebilets occur in the Sarhlefschist formation, outcropping as long and shifted lenses withsecondary iron caps (gossans) as a result of supergenic weath-ering effects. They are aligned parallel to the general directionof the regional structures (folds, schistosity) and, on a districtscale, distributed along three, generally N–S to NE–SW strik-ing subvertical lineaments (Bernard et al. 1988):

– The western ore lineament, which corresponds to the suc-cession of the Bouhane, Lachach, Jbel Hadid and KoudiatAïcha deposits. The Koudiat Aïcha deposit is the onlyoperational mine and contains about 3.6Mt of ore, grading3% Zn, 1% Pb and 0.6% Cu (Lotfi et al. 2006).

– The central ore lineament with the Kettara, Benslimaneand Kerkoz deposits. The Kettara ore deposit has beenmined between 1965 and 1981; the calculated reserve

Fig. 2 Geological mapshowing the bimodal magmaticplutonism of central Jebilets,the location of mined massivesulphide deposits and theapproximate location ofsamples. Gray rectangle areacovered by Fig.1b

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was approximately 25 Mt ore of grading 0.6% Cu(Huvelin 1977).

– The eastern ore lineament, including the Draa Sfar depositin the south, and the Nzalet el Harmel and Ben El Garngossans in the North. The operational (active) Draa Sfardeposit contains about 10Mt of ore, grading 5.3%Zn, 2%Pb and 0.3% Cu (Marcoux et al. 2008).

Thus, the massive sulphide deposits of central Jebilets areof great economic importance (Draa Sfar mine, KoudiatAïcha mine, etc.). They contain a large content of pyrrhotiteand are associated with magnetic anomalies.

Methods

Sampling

During May 2008, a sampling campaign was carried out in thecentral Jebilets area, in collaboration with Reminex Explorationand Sagax Maghreb. Fifty-eight core samples with 2.5-cmdiameters were collected from six sites (four outcrops of basicintrusions and cut from subsurface exposures of sulphides atseveral levels in two mines) (Fig. 2). Six to 11 cores per unitwere drilled with a gasoline-powered portable drill and thenoriented with magnetic and solar (where possible) compass. Inaddition, we collected six unoriented block samples from somesites for a preliminary petrographical analysis. Rock magneticanalyses were performed at the Paleomagnetic Laboratory

Grubenhagen of the Leibniz Institute for Applied Geophysics(LIAG).

Laboratory measurements

Petrophysical analysis

The cores were cut into 2.2-cm-long specimens in the lab-oratory; magnetic susceptibilities were measured using amagnetic susceptibility metre Minikappa KLF-3 (AGICO).A density was obtained, weighing the dry and water-saturated specimens in water and in air.

We corrected the magnetic susceptibility according to theinstrument characteristics. Thus, we used in the measure-ments a magnetic susceptibility metre with a nominal vol-ume of 10 cm³, while cores have different volumes.

The intensity of natural remnant magnetisation (NRM) ofthe samples was measured using a cryogenic magnetometre(760 SRM-RF-SQUID) from 2 G Enterprises (Rolf, 2000),and we used a spinner magnetometre (FA Magnon) fromMagnon GmbH for samples with highly intensive remnantmagnetisation (>10 A/m).

Petrographical and geochemical analysis

Four polished sections and 17 polished thin sections wereexamined in reflected and transmitted light with (×2.5, ×5,×10, ×20, and × 50) objectives using a Leica DMRP

Fig. 3 Histograms showing thedistribution of density,magnetic susceptibility,remnant magnetisation andKoenigsberger ratio forsampled basic rocks

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microscope to identify the magnetic minerals in both basicrocks and sulphides. In addition, three samples (KK01,DSG01 and MAR1) were examined using a scanning elec-tron microscope (SEM) (Quanta 600 FEG).

We prepared a mixture of samples, which were left overfrom preparation of polished and thin polished sections foreach site to carry out geochemical analysis. Samples wereprepared at the Federal Institute for Geosciences and NaturalResources (BGR), and powders were geochemically ana-lysed for the major and minor elements (X-ray fluorescence)and mineralogically by X-ray diffraction (XRD) (results notpresented in the paper).

Demagnetisation experiments

For isolating the characteristic remnant magnetisation byselective removal of secondary components, all sampleswere subjected to standard demagnetisation protocols (alter-nating field (AF); with AFD 1.2 Magnon Int.) or thermal(TH with Magnon International TD 700). During thermalexperiments after each step, magnetic susceptibility wasmonitored with a susceptibility bridge (mentioned alreadyabove) in order to detect mineralogical changes due toheating during thermal cleaning. For a detailed explicationof demagnetisation experiments, we refer to textbooks suchas Butler (1992) or Soffel (1991).

Thermomagnetic susceptibility measurements (kappa (T)curves)

High temperature variation measurements of magnetic sus-ceptibility were performed using a CS3 apparatus in combi-nation with a kappabridge KLY-3 in fields of 400 A/m(Hrouda 1994) from AGICO Inc. (Brno). Standard heat-ing–cooling runs were done in air or argon up to 700°C.

Petrophysical results

To improve the interpretations of magnetic surveys carriedout in the central Jebilets area and to constrain the effect ofthe magnetic remanence on the modelling of magnetic data,we adopted a direct approach: Sample measurement was asproposed by Morris et al. (2007). We measured the petro-physical properties of both basic rocks and sulphides.

Basic rocks of central Jebilets are characterised by twogroups of susceptibility- and NRM-intensity values, a behav-iour that is shown also by the basic rocks of the KoudiatKettara intrusion (Fig. 3). The unimodal density distributionsof basic rocks reflect their uniform total mineral composition(olivine–clinopyroxene–plagioclases) (Airo 1999). TheKoenigsberger ratio is an important parameter which indicatesthe relative importance of remnant and inducedmagnetisations(Clark 1997; Airo 1999) and gives also an indication ofamount, grain sizes and compositions of magnetic minerals(Airo 1999). The distribution for basic rocks shows a largescattering of values. Low values with Q<1 reflect the domi-nance of the induced magnetisation, average values with 1<Q<10 where the remnant magnetisation has to be included in theinterpretation and high Koenigsberger ratios with Q>10 andsometimes over 50 often indicate magnetite–ilmenite inter-growths formed during deuteric cooling (Airo 1999). Ahigh mineralisation of basic rocks can explain the anomalousvalue of Koenigsberger ratio observed in some basic rocks ofthe central Jebilets (Fig. 3).

It has been demonstrated that for Q<1, often, multido-main magnetite (Dunlop and Özdemir 1997) prevails. Onlysome samples from Koudiat Kettara site give Q values lessthan 1, indicating multi-domain magnetite (Fig. 4) as thedominant magnetic mineral.

Sulphides show a bimodal distribution of magnetic sus-ceptibility and NRM intensity, the density is again unimodal

Fig. 4 a Logarithmic scatter diagrams for basic rocks of central Jebilets. b Logarithmic scatter diagrams for sulphides of central Jebilets

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and the Koenigsberger ratio ranges between 2 and 11.Sulphides of the central Jebilets are characterised by similarmineralogy, but they typically show a predominance ofpyrrhotite (Essaifi and Hibti 2008; Bernard et al. 1988).Therefore, we assume that the variable nature of pyrrhotiteis responsible for the different magnetic behaviour of rocksfrom the two mines; these must be confirmed by furtheranalyses. We can distinguish between the sulphides of DraaSfar, which are strongly magnetic with susceptibility valuesup to 0.1 [SI] and those of Koudiat Aïcha characterised bymagnetic susceptibility values clearly less and ranging be-tween 0.002 and 0. 02 [SI]. This behaviour is confirmed bythe NRM intensities, too, with values ranging between 600and 2,000 mA/m for Koudiat Aïcha samples and valuesfrom 4,000 to 30,000 mA/m for samples from Draa Sfar.The Koenigsberger factor ranges from 2 to 11 for the twolocalities, demonstrating influence of remnant magnetisa-tion for observed magnetic anomalies (Fig. 4).

A bivariate scatter diagram density versus magnetic sus-ceptibility was plotted it showed three different groups(Fig. 5):

The first group corresponds to basic rocks with densityaround 2,900 kg/m3 and magnetic susceptibility rangingbetween 0.0003 to 0.3 SI. Others groups include massivesulphides with a density of about 4,500 kg/m3 and differentvalues of magnetic susceptibility (from 0.003 to 0.02 SI inKoudiat Aïcha and values around 0.1 SI)

These results permit us to discriminate between basicrocks and sulphides in the central Jebilets in term ofdensity. According to the magnetic susceptibility, thesulphides show, on average, higher values than the basicrocks; the high magnetic susceptibility could thereforereflect an additional magnetic mineral beside magnetite

in the basic rocks or high mineralisation as explainedbefore (Table 1).

Monocline pyrrhotite, which is marked to be typical forore deposits (Airo and Loukola-Ruskeeniemi 2004), seemsto be responsible for the increased magnetic susceptibilityand remanence, which is proven by our rock magneticinvestigations. Thus, the comparison between petrophysicalproperties of Draa Sfar ore and Koudiat Aïcha ore showsclear differences in the amount of monocline pyrrhotite,knowing that the two ore deposits are dominated bypyrrhotite.

It has been suggested (Henkel 1994; Airo 1997; Airo1999, Clark 1997) that the dominant magnetic mineral canbe concluded from the magnetic susceptibility versusKoenigsberger ratio scatter diagram. In our case, for basicrocks, magnetite dominates as showed by their high suscep-tibilities and Q ratio mainly below 10, some samples whichhave low susceptibilities and /or higher Q ratio will perhapspresent another magnetic mineral (Table 1). The followingpetrographical and rock magnetic analysis will give someindication of the nature of the second magnetic mineral.

Petrographical and geochemical results

General features

Petrographical investigations show that magnetite, ilmenite,chromite and some sulphides (pyrrhotite, pyrite and chalco-pyrite) form the major opaque minerals in basic rocks.Hematite was also observed in some samples. In massivesulphide samples, pyrrhotite, chalcopyrite, sphalerite, gale-na, pyrite and arsenopyrite were determined under reflected

Fig. 5 Scatter diagram ofdensity vs magneticsusceptibility for sampled rocksof the study area. Note that thediscrimination betweensulphides and basic rocks isevident, between basic rockswith high and those with lowmagnetic susceptibility.Sulphides are alsodiscriminated with highmagnetic susceptibility andvery high density

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Table 1 Main petrophysical da-ta for basic rocks and sulphidesfrom the central Jebilets area

NRM intensity of mean magneticremnant magnetisation, k volu-metric magnetic susceptibility, Ddensity, Q Koenigsberger ratiousing 0.032 mA/m

Sites Samples NRM (mA/m) k (SI) D (Kg/m3) Q

Gabbro Koudiat Ahril NJ01 3.88 0.000525 2.88 0.23

NJ02 17.12 0.000347 2.82 1.81

NJ58 470.80 0.007535 2.87 1.94

NJ59 1506.00 0.007175 2.95 6.49

NJ60 422.25 0.002075 2.86 6.38

NJ61 246.70 0.002545 2.87 3.00

Gabbro Koudiat Kettara NJ03 150.13 0.003400 2.87 1.32

NJ04 141.30 0.005415 2.84 0.81

NJ06 263.30 0.003175 2.76 2.57

NJ10 822.90 0.018025 2.77 1.37

NJ11 31.24 0.002570 2.82 0.37

NJ12 28.37 0.001007 2.88 0.95

NJ13 19.18 0.001493 2.86 0.40

NJ14 5725.00 0.036050 2.88 4.91

NJ15 1812.00 0.030250 2.96 1.86

NJ16 50.84 0.000613 2.85 2.57

Gabbro Draa Sfar NJ40 2470.00 0.002965 2.89 25.78

NJ41 1192.05 0.001474 2.88 22.75

NJ42 110.92 0.000905 2.89 3.81

NJ43 246.80 0.001300 2.90 5.87

NJ44 338.85 0.001293 2.92 8.07

NJ45 1847.67 0.003845 2.87 15.28

NJ46 494.80 0.002310 2.87 6.63

Gabbro Draa Lamnizeh NJ47 313.70 0.002790 2.91 3.48

NJ48 326.40 0.002740 2.90 3.69

NJ49 64.40 0.001278 2.87 1.56

NJ50 28.61 0.000733 2.88 1.21

NJ54 3507.50 0.005010 2.87 21.66

NJ56 5257.00 0.002503 2.83 63.77

NJ57 180.50 0.003418 2.83 1.63

Draa Sfar mine NJ18 14120.00 0.105700 4.40 4.13

NJ20 4316.00 0.042225 3.25 3.28

NJ22 5993.50 0.091250 4.42 2.03

NJ23 8295.00 0.105725 4.45 2.42

NJ24 9876.00 0.106750 4.54 2.87

NJ25 11279.00 0.114250 4.56 3.03

NJ26 14280.00 0.101600 4.56 4.35

NJ27 15095.00 0.092075 4.41 5.02

Koudiat Aicha mine NJ28 894.40 0.003958 4.45 7.20

NJ29 673.20 0.003108 4.43 6.62

NJ30 1081.20 0.006253 4.54 5.13

NJ31 1341.45 0.007938 4.46 5.23

NJ32 1272.15 0.004790 4.47 8.17

NJ33 978.80 0.005488 4.40 5.28

NJ34 1200.00 0.003730 4.46 9.96

NJ35 978.35 0.006243 4.23 5.01

NJ36 637.10 0.004035 4.41 5.02

NJ37 727.45 0.008025 4.50 2.88

NJ38 762.35 0.006065 4.44 3.85

NJ39 1398.30 0.013713 4.42 2.82

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light. Magnetite and pyrrhotite seem to be the carrier ofmagnetisation in the sampled rocks.

In summary, the observed samples encompass alteredbasic rocks and ore sulphides which are pyrrhotite-rich.The former consist of olivine–pyroxene–plagioclase as theprimary crystallisation sequence, supporting the petrophys-ical results which prove a unimodal density distribution ofbasic rocks. However, interstitial primary amphiboles withmicas, chlorite, talc and epidote have been determined asaccessory minerals, indicating possible metamorphic andhydrothermal events. Ore sulphides contain sphalerite, py-rite, chalcopyrite, arsenopyrite and galena besides pyrrho-tite. The mineralogical features were confirmed by XRD(results not presented here).

Mineralogically and geochemically, two types of basicrocks (mafic and ultramafic composition) have beendistinguished:

This first group which is constituted of magnetite-bearingultramafic rocks is present at Draa Lamnizeh and KoudiatKettara. The ultramafic rocks occur as banded lenses, cen-timetres to metres in size, at the base of a succession ofmafic rocks. The mineralogy of those rocks which showpreserved porphyritic to microcrystalline texture consistedof clinopyroxene, olivine and/or orthopyroxene, plagioclaseand subordinate opaque minerals. Olivine crystals wereobserved in non-altered rocks and are euhedral to subhedralrelicts (<500 μm). Locally, the olivine is altered to amphi-bole in fractures or along rims. Clinopyroxene crystals arepartly well preserved between olivine crystals and some-times are transformed to amphibole and chlorite. Relicts ofolivine/orthopyroxene, up to several millimetres in size, aregenerally converted into “bastite” in the pyroxenite of DraaLamnizeh. Plagioclase (anorthite) occurs interstitially as

annealed laths or crystals and is locally altered to carbonate,sericite and chlorite (Fig. 6a).

The altered minerals predominantly consist of chloriteand talc, as well as colourless clinoamphibole (tremolite/actinolite). Amphibole appears as automorphic crystals upto 500 μm long. An older amphibole generation consists ofcoarse fibrous crystals (several millimetres in length)(Fig. 6a).

The opaque phases are chromite rimmed by magnetite,as well as ilmenite and sulphides (pyrrhotite and chalco-pyrite. Chromite in all samples appears as idiomorphic tosubidiomorphic crystals, 20 μm to 1 mm in size, andsometimes rounded or fractured (Fig. 6b). Chromite crys-tals are locally rimmed by magnetite (10 μm thick) andsometimes completely transformed to magnetite (Fig. 6b).The relationship between chromite and magnetite hasbeen discussed by Barnes (2000), who concludes thatchromite rimmed by magnetite record metamorphic con-ditions and the rims of magnetite derived from serpenti-nisation of olivine in the host rock. In some samples,chromite grains are almost completely replaced by mag-netite with only locally small relicts of chromite. Theproportion of magnetite appears to increase with increas-ing metamorphic grade.

SEM analysis was done on typical zoned chromite todetermine the chemical nature of the zoning. Fe and Cr arerelatively enriched at the border whereas Al and Mg arehigher in the cores (Fig. 6j). This result agrees with anevolution from a ferriferous chromite in the centre to achromiferous magnetite at the border conformably with themetallographic observations. The same chemical featureswere revealed by Essaifi (1995) in some spinels in theultramafic rocks of Koudiat Kettara.

Fig. 6 Petrographical and mineralogical features for the basic rocksfrom the central Jebilets. a Microlitic texture of an altered pyroxenitefrom Draa Lamnizeh showing growth of mineral alteration on theprimary mineral association. b Chromite rimmed by magnetite. cIlmenite including lamellae of hematite. d Chalcopyrite and pyrrhotiteare intimately associated indicating mutual crystallisation. e SEMphotograph from Draa Sfar indicating ilmenite crystals and small grain

of baddeleyite. f Gabbro from Draa Sfar showing plagioclase andpyroxene laths; interstitial opaque minerals. g Myrmekitic texture ofilmenite from the Koudiat Ahril intrusion. h Ilmenite in fracture occursin small grains of sulphide. i Coated pyrite crystal rimmed withlimonite. j SEM photograph of chromite rimmed by magnetite. Ch:chromite; Mt, magnetite; Il, ilmenite; Ht, hematite; Po, pyrrhotite; Cp,chalcopyrite, Py, pyrite; Lm, limonite; Bd, baddeleyite

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Ultramafic rocks from the Bou Azzer ophiolite, Anti-Atlas, Morocco contain chrome spinel, which shows anhe-dral to rounded shape with intact core, and altered Cr-richmagnetite rim (Ahmed et al. 2005; El Ghorfi et al. 2008)have described the massive chromitites of the Bou Azzerophiolite which contain chromite grains. These show signsof alteration and replacement by “ferrit-chromite”, Cr-richmagnetite and/or magnetite. Similar features are comparablewith the mineralogical observation of ultramafic rocks fromthe Hercynian central Jebilets Massif. The processes gener-ating rimmed chromite by Cr-rich magnetite or magnetiteare serpentinisation in the Bou Azzer ophiolite while theyare both hydrothermally altered and green schist metamor-phosed in the case of the central Jebilets ultramafic rocks.

Magnetite appears as secondary mineral, e.g. as smallblebs rimming chromite crystals or infilling fractures inchromite (Fig. 6b), while ilmenites form an- and subhedral(several tens of millimetres) crystals. Some ilmenites con-tain separated hematite lamellae (Fig. 6c), which are partial-ly distorted.

Sulphides minerals presents in this rocks are: pyrrhotitewhich appears locally as tiny inclusions (20 to 200 μm),either isolated or associated with chalcopyrite (Fig. 6d), andchalcopyrite which is generally associated with chromiteand pyrrhotite in the ultramafic rocks. It appears as subidio-morphic crystals (50 to 200 μm), most often as small blebsin fractures within silicate minerals (Fig. 6d).

The second group formed of mafic rocks shows a por-phyritic texture with well-preserved interstitial plagioclasecrystals of 1–2 mm in length and poikilitic clinopyroxene(several millimetres large) that are partly present as oico-

crystals. Altered fine-grained minerals with slightly greenishcolour consist of muscovite/sericite, chlorite and talc; pos-sibly, they replace former orthopyroxene or olivine, butthere are no relicts present. Interstitial fine-grained epidoteattests to at least greenschist facies metamorphic overprint(Fig. 9a).

The dominant opaque phase is ilmenite, which appears asautomorphic or xenomorphic crystals (50 μm to 1 mm) andoccupies intergranular spaces (Fig. 6f). Small (<1 μm) sul-phides (pyrite) forming drops are rarely observed as relics insilicates minerals and ilmenite (Fig. 6h). Locally, ilmeniteappears as relicts in hematite and sometimes shows a myr-mekitic texture (Fig. 6g). We note that magnetite could notbe proven microscopically. Larger, usually idiomorphic,pyrite crystals are rimmed by limonite (Fig. 6i).

Massive sulphides

In the sampled drill core of the Draa Sfar deposit, the ore isstrongly distorted, sheared and locally laminated. It showsan association of pyrrhotite, sphalerite, galena, chalcopyrite,pyrite and arsenopyrite. Gangue minerals are represented bychlorite, quartz, talc, calcite and fragments of schist (Fig. 7).Pyrrhotite is the major sulphide of the ore body (80%to85%). It forms ribbons or massive lenses. Frequently,pyrrhotite forms sub- to anhedral crystals generally associ-ated with chalcopyrite in low-grade ore zones (Fig. 7a).

Sphalerite contains frequent teardrop like inclusions of ananisotropic mineral with green–grey reflection colour, whichwas identified as stannite (Fig. 11b, d). A standard energydispersive X-ray (EDX) analysis of the phase resulted in a

Fig. 7 Polished section photomicrographs (in reflected and plane-polarised light) representative of the massive sulphide deposit miner-alogy. a Growth of chalcopyrite and galena crystals along the borderbetween pyrrhotite and sphalerite. b Sphalerite, stannite, chalcopyriteand galena association in a matrix of pyrrhotite and gangue. c Idio-morphic crystals of arsenopyrite and galena in pyrrhotite with somesphalerite. d SEM photomicrograph showing the presence of stannite

in sphalerite; carbonate occurs as gangue. e Primary pyrrhotite–sphal-erite association with intergrowth of cubic pyrite crystals, interstitialsmall chalcopyrite grains. f Twinned crystal of pyrrhotite. g Sphaleriteand pyrrhotite matrix with chalcopyrite, galena and pyrite. h Growth ofarsenopyrite crystals in a matrix composed of pyrrhotite. Po, pyrrho-tite; Py, pyrite; Sp, sphalerite; Ga, galena; As, arsenopyrite; St, stannite;Gn, gangue; Cb, carbonate

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formula of (Cu1,2 Zn0,4) Fe1Sn0,9S4,5 (ideal formula of stan-nite: Cu2FeSnS4) (Table 2).

Galena is intimately associated with sphalerite and some-times remobilised along the contact sphalerite–pyrrhotite(Fig. 7). Chalcopyrite is less abundant than the other majorphases and forms thin inclusions and blebs in pyrrhotite, aswell as remobilisations and infillings of fractures in pyrrho-tite, pyrite and sometimes arsenopyrite (Fig. 7a, b).

Pyrite is relatively less abundant than other sulphides inthe ore deposit. They appear as euhedral to subhedral crys-tals (up to 300 μm), either scattered within different types ofore or as interfingered masses. Arsenopyrite appears asidiomorphic crystals in the association of sphalerite, galenaand pyrrhotite.

From geophysics, drilling and mining, it is known thatthe Koudiat Aïcha deposit forms two sub-vertical NNE–SSW elongate lenses, conformably with the host rocks.The ore lenses are 1 to 30 m thick, approximately 300 mlong and more than 250 m deep. The main sulphides arepyrrhotite, sphalerite, galena, chalcopyrite, pyrite and arse-nopyrite. At the base and the top of these lenses, the hostrocks are more altered and contain many veins of sulphidewhich resemble stringer zones.

The sulphide ore exhibits clear banding which is causedby different proportions of the main ore phases (pyrrhotiteand sphalerite) (Fig. 7). Pyrrhotite generally occurs withchalcopyrite in low-grade zones, whereas it shows mutualcrystallisation relationships with sphalerite and also appearsas inclusions in sphalerite in rich banded ore zones (Fig. 7).It contains idiomorphic pyrite, galena and chalcopyrite andis sometimes elongated conformably to the schistose fabricwith deformation twinning (Fig. 7f). It also contains inclu-sions of chalcopyrite (anhedral/subhedral), sphalerite andsilicates.

Chalcopyrite is the most remobilised sulphide in the ore.It is present as small inclusions in the other sulphides,sometimes in the boundaries of pyrrhotite and more ofteninfilling fractures in pyrite and arsenopyrite (Fig. 7g).Galena is rare and usually occurs at sphalerite boundaries.

Sphalerite contains inclusions of idiomorphic pyrrhotiteand pyrite grains, as well as small inclusions of chalcopyriteand galena (Fig. 7e, g).

Arsenopyrite appears as idiomorphic small grains whichare developed in pyrrhotite, sphalerite and chalcopyrite(Fig. 7h).

Geochemical analysis results

Geochemical analyses were carried out on the two types ofbasic rocks (mafic and ultramafic composition) and sul-phides of the central Jebilets area (Table 3).

The ultramafic rocks are poorer in SiO2 (40.73% to44.17%), Al2O3 (7.5% to 10.98%), CaO (4.75% to6.44%), TiO2 (0.30% to 0.59%) and P2O5 (0.03% to0.11%)) than the mafic ones which are characterised by arelatively high content in Si (46.89% to 47.85%), Al(17.08% to 18.09%), Ca (11.04% to 12.07%), TiO2

(0.85% to 1.14%) and P2O5 (0.09% to 0.15%). The lowcontent of Al and Ca in ultramafic rocks indicates lowercontents of plagioclase than in the mafic rocks, in accor-dance with microscopic observations.

The mafic rocks are highly enriched in MgO (23.82% to26.07%) compared with the mafic rocks (7.05% to 9.4%).This implies the abundance of olivine crystals which agreeswith microscopic observations. Ultramafic rocks are further-more distinguished by higher trace element concentrationslike Co (77 to 82 ppm), Ni (974 to 1,003 ppm) and Cr(2,473 to 3,541 ppm), with respect to the mafic ones.

The sulphides from the two mines show highly variableconcentrations of Cu (1,468 to 15,625 ppm), Pb (1,062 to54,419 ppm) and Zn (364 to 156,549 ppm).

Rock and paleomagnetic results

General features

We have carried out thermal and alternating field demagnet-isation experiments. This permits us to detect the magneticmineralogy and the characteristic remnant magnetisation ofboth basic rocks and sulphides. Consequently, we deducedinformation about the ancient magnetic field. From rock mag-netic experiments (measurements of the temperature depen-dence of magnetic susceptibility (Kappa T-curves)) therelationship between mineralogical changes and magneticbehaviour can be derived for basic rocks and sulphides.

Basic rocks and sulphides are characterised by big scatterof susceptibility, intensity and directions of NRM (Fig. 8 a,b; Table 1).

This very inhomogeneous behaviour is also demonstratedby demagnetisation experiments with alternating fields orthermal. For the basic rocks, hydrothermal alteration or highmineralisation are mostly the causes of such behaviour. Themain carrier of remanence of the basic rocks is magnetitenot only for samples with high intensity and susceptibility.

Table 2 EDX analysesof stannite

Wt weight, At atomicweight, S sulphur, Sntin, Fe iron, Cu copper,Zn zinc

Wt (%) At (%)

S 33.92 56.6

Sn 24.28 10.94

Fe 12.64 12.11

Cu 17.78 14.97

Zn 6.57 5.38

Total 95.19 100

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On the other side, the magnetic properties of the sulphidesare dominated by pyrrhotite (Fe7S8/Fe9S10). The main

magnetic minerals can be detected also by thermomagneticsusceptibility measurements.

Table 3 Geochemical analyses of basic rocks and sulphides

Location DLH DLH KK DSG DSG KAH KA KA KA KA DS DS

Samples MAR-1 TG-01-07 KK-01-08 MAR-2 DSG-01-03 KH-01-04 MAR-3 MAR-4 MAR-5 KA-01-04 DSE-01-03 MAR-6

SiO2 (Wt.%) 44.17 42.89 40.73 47.06 46.89 47.85 00.44 9.08 4.97 00.86 00.77 11.56

TiO2 00.384 00.59 00.305 1.138 00.851 1.001 00.005 00.058 00.124 00.001 00.006 00.005

Al2O3 7.5 9.63 10.98 18.09 17.08 17.79 <0.05 00.83 3.11 <0.05 00.1 1.15

Fe2O3 8.31 10.13 8.73 8.9 8.47 6.96 71.13 40.04 75.47 82.61 59.55 39.45

MnO 00.126 00.162 00.133 00.134 00.132 00.128 00.188 00.761 00.11 00.059 00.804 1.564

MgO 26.07 23.82 25.33 8.28 9.4 7.05 00.66 1.5 2.15 00.42 1.25 12.14

CaO 4.753 6.33 6.441 11.038 11.329 12.066 1.065 1.335 00.1 00.415 00.203 8.349

Na2O <0.01 00.07 00.35 2.57 2.23 2.19 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

K2O 00.017 00.025 00.356 00.225 00.147 00.744 00.01 00.063 00.217 00.011 00.028 00.018

P2O5 00.049 00.105 00.029 00.145 00.089 00.104 00.015 00.022 00.045 00.02 00.015 00.018

SO3 <0.01 00.09 00.13 00.06 00.16 00.63 00.76 00.55 00.1 00.13 00.34 9.68

Cl 00.004 00.011 00.044 00.018 00.017 00.018 00.002 00.028 00.002 00.01 00.017 00.004

F <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05

As (ppm) 3 4 <1 3 2 4 3133 3526 68 92 439 5963

Ba <4 <4 35 35 22 142 31 15 24 <6 10 14

Bi <4 <2 <2 <5 <3 <2 23 256 91 74 18 41

Ce <18 <18 <18 24 <19 <19 <27 <29 <27 <27 <29 <29

Co 80 77 82 41 41 26 93 9 156 151 <7 8

Cr 3541 2473 2886 337 444 753 <6 <6 <6 <6 38 <6

Cs <3 <3 <3 7 7 <3 <4 19 <4 <4 9 <4

Cu 75 55 105 55 45 114 15625 3633 1775 8119 1945 1468

Ga 8 9 8 15 14 14 <4 56 <4 <4 36 9

Hf <6 <7 <6 <7 <7 <6 <14 <18 <14 <15 <19 <14

La <14 <15 <15 <15 <15 <15 <21 <23 <22 <22 <23 <23

Mo <3 <3 <3 <3 <3 <3 <5 <6 <5 <6 <6 55

Nb 3 3 4 6 <3 3 7 12 6 6 <5 9

Nd <13 <13 <13 <14 <14 <14 <22 <22 <23 <24 <23 <22

Ni 1003 974 992 160 182 100 13 9 22 24 36 <5

Pb <3 <3 7 5 <3 4 1062 54419 2043 1736 39050 17103

Rb 4 7 22 16 13 16 14 9 18 13 17 12

Sb <7 <7 <7 <8 <8 <9 22 239 <9 10 76 30

Sc 19 22 14 28 33 37 <1 2 3 <1 <1 <2

Sm 17 <16 <15 <16 <16 <16 <35 <29 <36 <37 <33 <30

Sn <3 <4 <4 <4 <4 <4 90 145 7 <5 378 87

Sr 46 12 113 213 190 251 <4 10 <4 6 6 34

Ta <5 <5 <5 <5 <5 <5 <12 <13 <11 <13 <13 <10

Th 8 6 5 10 8 <4 25 25 26 16 35 26

U <4 <4 <4 <4 <4 <4 <7 <8 <7 <8 11 36

V 121 146 76 179 163 213 <8 <9 24 <8 <9 24

W <4 <4 <4 <4 <4 <4 82 53 <10 153 <18 <11

Y 6 13 9 24 24 20 <6 <10 <7 9 <9 <7

Zn 86 80 65 63 66 54 8234 156549 364 8051 133400 28396

Zr 23 40 21 86 60 44 10 22 32 10 <7 <6

KAH Koudiat Ahril, KK Koudiat Kettara, DSG Draa Sfar, DLH Draa Lamnizeh, DS Draa Sfar mine, KA Koudiat Aïcha mine

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Alternating field and thermal demagnetisation

AF demagnetisation was carried out using between six and11 steps up to a maximum field of 140 mT, and thermaldemagnetisation using between nine and 13 steps up to600°C on few specimens from basic intrusions and sulphides.

The program “PalMag” (Maier and Bachtadse 1994) wasused for analysis of the demagnetizing experiments. Thedemagnetisation data were displayed as orthogonal vectorplots (Zijderveld 1967) and intensity decay diagrams.Separation of stable primary and secondary magnetisationdirections was tried using principal component analyses(Kirschvink 1980) where the best-fitting line through asequence of data points (at least three) is calculated.

As shown in our experiments, basic rocks and sulphidescan be distinguished easily by their diverse intensity decaydiagrams for thermal demagnetisation (Fig. 9). Alternatingfield demagnetisation was also performed, but based on thefact that no mean characteristic primary direction for thedifferent outcrops could be calculated and that only limitedadditional information can be expected, these results are notpresented here in detail. Relevant for the present paper isonly the fact that the alternating field demagnetisationproved the varying domain structure of magnetite and pyr-rhotite (multi-, pseudo-single- and single-domain) shown bysmall or broad coercive spectra.

The gabbroic specimens are dominated by magnetite,which is proved by our rock magnetic experiments, too. Inaddition, few samples show very hard magnetic behaviourduring alternating field demagnetisation that hints to hema-tite as additional magnetic mineral but cannot be verified bythe rock magnetic investigations, maybe, due to the minormagnetic potency of hematite in comparison to magnetite(Fig. 9a–e).

All sulphide samples are characterised by a quasi-totalvanished magnetisation near to the Curie point of pyrrhotiteat 320°C. Further thermal demagnetisation above 500°Ctends toward irreversible transformation of pyrrhotite usual-ly to magnetite, which is demonstrated by increasing sus-ceptibility values. Increasing susceptibility at around 220°Cadverts to the well-known irreversible l-transition, typicalfor the breakdown of hexagonal Fe9S10 to monocline Fe7S8,proofing that natural pyrrhotite often contains a mixture ofFe9S10 and Fe7S8. After pyrrhotite is once heated above thel-transition, only the Fe7S8 phase is existent. These resultscould be impressive confirmed with our Kappa (T)-experi-ments. The sulphide samples show no stable primary mag-netisation that matches to the fact that pyrrhotite is wellknown as a common accessory mineral in igneous, meta-morphic and sedimentary rocks, although it seldom domi-nates the remanence (Dunlop and Özdemir, 1997).

After Thompson and Oldfield (1986), pyrrhotite has rare-ly been found to carry a useful paleomagnetic record of theancient geomagnetic field, but it is characterised by highsusceptibility that cause large local anomalies of the geo-magnetic field, as stated on the study area.

Thermomagnetic susceptibility measurements

To get additional information about the carrier of the mag-netisation, we performed thermomagnetic susceptibilitymeasurements. The thermomagnetic investigations showthat, for gabbroic rocks, magnetite minerals and, for sul-phides, pyrrhotite minerals (monocline as well as hexago-nal) are magnetically dominant.

For a first overview, we performed measurements in atemperature range from room temperature up to 700°C in

Fig. 8 Equal-area projection of NRM directions of a basic rocks and b sulphides of Koudiat Aicha mine; c sulphides of Draa Sfar mine

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one cycle. This method was useful to proving magnetitepresence (Fig. 10).

The kappa (T) experiments in Fig. 10 show only magne-tite as the main magnetic mineral responsible for the mag-netic properties and do not record for further magneticminerals. Due to the minor magnetic potency of hematitein comparison to magnetite, this mineral, if present togetherwith magnetite, is mostly not detectable in the kappa (T)curves.

For identifying iron sulphides (pyrrhotite), it was advisableto do several heating cycles with stepwise increase of maxi-mum temperature in the temperature range between 200°C to330°C (in 10–20°C steps). This was necessary to document, ifexisting, the so-called l-transition from hexagonal pyrrhotite(Fe9S10) to monocline pyrrhotite (Fe7S8) at temperaturesaround 220°C. In contrast to the ferrimagnetic Fe7S8(also called Weiss-type pyrrhotite), the hexagonal pyr-rhotite (Fe9S10) is antiferromagnetic, but, between thel-transition around 200°C and the Currie point of∼265°C, it is ferrimagnetic, too. The l-transition isdistinctive and diagnostic of hexagonal pyrrhotite(Dunlop and Özdemir 1997). Stepwise enhancement ofmaximum temperature up to 500°C showed that theirreversible oxidation of pyrrhotite to magnetite startednot until temperatures were above 500°C (Fig. 10a–d).

The stepwise heating of sample 39 (sulphide fromKoudiat Aïcha) is documented in Fig. 10i. Only the firstcycle up to 320°C shows the l-transition from hexagonal tomonocline pyrrhotite (Fig. 10e, f). After that run, onlymonocline pyrrhotite is detectable, due to the completeand irreversible transition of pyrrhotite. Stepwise heatingof the same material up to 500°C leads to thermomagneticcurves that are quite similar. Only the Hopkinson peak onthe verge of the Curie temperature of Fe7S8 is gettingsharper, meaning that more pyrrhotite minerals were shiftedto the single domain (SD) behaviour of pyrrhotite or thatmore and more SD pyrrhotite was built due to increasingtemperatures. Above 500°C, a sharp increase of susceptibil-ity started until the Hopkinson peak of magnetite, near tothe Curie point of magnetite (580°C), is achieved andsusceptibility decreases rapidly. In that diagram (Fig. 10g, h,i), Fe7S8 is only visible in a small kink at around 320°C.Creation of magnetite starts not until temperatures above500°C were reached.

Temperature-dependent susceptibility measurements in-dicate a dominance of monocline pyrrhotite (Weiss-type)(Kontny et al. 2000) with a small amount of l-type hexag-onal pyrrhotite. All sulphide ores show pyrrhotite as themain carrier of magnetic information but only few as diag-nostic for hexagonal pyrrhotite (Fig. 10e, f).

Fig. 9 Intensity decay diagrams for basic rocks, a hard magneticphase, b soft magnetic phase and c magnetite with narrow blockingtemperature spectrum near the Curie point of magnetite, indicative formagnetite as carrier of magnetic properties. d Normalised intensity

decay diagram and e Zijderveld diagram for thermal demagnetisationof specimens from Ore Koudiat Aïcha, representative for sulphiderocks

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Fig. 10 a–d Kappa (T) curvescharacteristic of basic rocks in thestudy area. Kappa (T) curves fromsample 39 typical for λ-transitionat 220°C (e) and transformationof monocline pyrrhotite tomagnetite above 500°C (f)(explanation see text). g–iExamples for pyrrhotite bearingrocks from the sulphide ore

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Forward magnetic modelling of magnetic anomalies

The magnetic map of Draa Sfar north corresponds to anormally polarised anomaly oriented NE–SW and centredover the mineralised area (Fig. 11a). The anomaly has ahigh amplitude of about 400 nT. This anomaly wassampled along the profile A-A’ (Fig. 11a) for the mod-elling profile that fits to the measured line 100 N.Forward modelling was performed using a semiautomatic2.5D gravity and magnetic modelling program SAKI,written by Webring (1986) for the USGS based onTalwani et al. (1959) and Talwani (1965). Magneticmodelling was conducted assuming the measured mag-netic, paleomagnetic properties (obtained after eliminat-ing the highly remnant magnetic samples) and geometricalproperties of Draa Sfar taken from the borehole (DS 125)and geological sections to constrain the interpretation.We analysed several polygonal models with variouscombinations of magnetic characteristics, and the bestfit suggests two important, dipping, remnant ore-bodieswith a small body that fit the south-western part of theprofile (Fig. 11b).

The acceptable model required a magnetic susceptibility inorder of 0.095 SI, remnant intensity of about 10.41 A/m anddirection of remanence of D0344.4°, I029.2° (data takenfrom the measured sulphides samples in Draa Sfar), for allbodies. The ambiguity of the modelling process was con-strained by adding a small body which have the same proper-ties as the twomain lenses and decreased the uncertainty in thesize and the geometrical properties of the proposed bodies.

The fitted curve shows small deviation from the observedone. This can be considered as small outcrops (iron caps).Therefore, we conclude our model is representative for themain mineralisation source bodies that produce the anomalyof Draa Sfar north.

Discussions and conclusions

Petrophysical studies carried out in the central Jebilets areareveal that the remanence of gabbroic rocks is different fromsite to site. Intensity of remanence is about 34.53 A/m inKoudiat Kettara, 8.55 A/m in Draa Lamnizeh, 0.36 A/m inKoudiat Ahril and 0.88 A/m in Draa Sfar.

Mineralogical, geochemical and paleomagnetic investi-gations show that the remanence variability intimatelydepends on the mineralogical composition. However, itappears clear that, in all samples, the magnetic mineralsare magnetite, hematite (not directly proven) and pyrrhotite.The mineralogical and geochemical characteristics of mag-matic rocks permit to distinguish two types: ultramafic onewhich is chromite, magnetite and occasionally pyrrhotiterich and mafic one which contains principally ilmenite,

hematite and pyrite. This mineralogical difference certainlyexplains the difference of the revealed remanence.

The basic rocks remanence appears to be dependent onthe abundance of magnetic mineral (magnetite) which rep-resents the transformation of primary Chromite in ultramaficrocks in connection with hydrothermal and metamorphicoverprint. Effectively, only the ultramafic rocks containpyrrhotite and primary chromite which was, in most cases,transformed to magnetite.

The ore deposits remanence is clearly explained by thetypical abundance of monocline pyrrhotite which representsabout 85% to 90% in the Draa Sfar and Koudiat Aïchadeposits. The difference of remanence observed in the two

Fig. 11 a Map view of Draa Sfar north orebody and location ofborehole DS125 and profile A-A’ (Moreno et al. 2008, modified).Total magnetic field intensity map of Draa Sfar area. Notice thepresence of a bipolar anomaly over mineralised area. Values are givenin nanotesla (nT). AA’ is location of the modelled profile. b Profile ofthe magnetic anomaly measured over Draa Sfar orebody and hiscorresponding quantitative model. Local geomagnetic field parametersare: declination D0−4.2°, inclination I042°, and total magnetic fieldintensity F040,448 nT. We used 2D bodies that extend 150 m along-strike perpendicular to this profile. Magnetic data for the three bodiesare: NRM declination0344.4°, NRM inclination029.2°, NRMintensity010.41 A/m; and magnetic susceptibility00.095 SI

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deposits is particularly explained by the higher abundanceof hexagonal pyrrhotite in the Koudiat Aïcha compared withthe Draa Sfar deposit.

Paleomagnetic and thermomagnetic analysis reveal thatthe basic rocks were affected by hydrothermal alteration andmetamorphism at about 330°C, in accordance with the min-eralogical observations and in agreement with the studiescarried out by Barnes (2000). During hydrothermal alter-ation and metamorphism, chromite in ultramafic rocks isreplaced by magnetite; this overprint occurred subsequent tothe emplacement of bimodal intrusions at about 330 Ma.The massive sulphide deposits seem to be older than313 Ma, according to an important post-tectonic thermalevent detected in the Hajar mine within the massif ofGuemassa (40Ar/39Ar dating) (Watanabe 2002).

According to Henkel (1994) “Pyrrhotite lithologies occurin structures generating almost symmetric magnetic anomalieswhich are insensitive to dip variations. Therefore, total fieldmagnetic anomalies due to pyrrhotite tend to be more centeredabout the magnetic source than those caused by magnetite”and “Lithology containing ferrimagnetic pyrrhotite can havetotal magnetisations up to 10 times larger than those contain-ing magnetite”. Thus, taking into consideration our results,basic intrusions and sulphides deposits having the same ge-ometries and shapes can be distinguished by the symmetryand the intensity of the generated magnetic anomalies.

The high intensity of the remanence is significant and deci-sive for the quantitative modelling interpretation. Rock magne-tism combined with field checks of source mineralisation canprovide helpful information on massive sulphide explorationwith the magnetic method in the central Jebilets. We stronglyrecommend such interpretation of the ground and aeromagneticdata to be performed in the study and similar areas as part of theexploration process. We also suggest the use of new parametersresulting from this study to model other magnetic anomalies inthe area to evaluate their economic potential.

Acknowledgements The present paper is the second part of thedoctoral thesis of the first author at the University CADI AYYAD ofMarrakech (UCAM). The first author would like to thank the CentreNationale de la Recherche Scientifique (CNRST, Morocco) for grant-ing his PhD Thesis (Ref. C03/008). We wish to thank also the DeutscheAkademische Austauchdienst (DAAD) for financial support duringtraining in Germany. Laboratory measurements were performed atthe Geozentrum of Hannover, within the institutes of the Bundesanstaltfür Geowissenschaften und Rohstoffe (BGR) and the LIAG. Directorsof laboratories and technicians are gratefully thanked for help duringmeasurements and the use of their facilities.

We also thank the directors of Sagax Maghreb Company, JöelSimard, and Reminex Exploration Company, Lhou Maacha and Baj-jedi Amine and Haj Saoudi for giving us access to their field area (DraaSfar and Koudiat Aïcha mines), data and logistic help during thesampling campaign. The authors are grateful to the reviewers for theirvaluable remarks and suggestions.

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