Lead and sulfur isotope constraints on the genesis of the polymetallic mineralization at Oued Maden, Jebel Hallouf and Fedj Hassene carbonate-hosted Pb–Zn (As–Cu–Hg–Sb) deposits,

Journal of Geochemical Exploration 132 (2013) 6–14

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Lead and sulfur isotope constraints on the genesis of the polymetallic mineralizationat Oued Maden, Jebel Hallouf and Fedj Hassene carbonate-hostedPb–Zn (As–Cu–Hg–Sb) deposits, Northern Tunisia

Nejib Jemmali a,⁎, Fouad Souissi a,b, Emmanuel John M. Carranza c, Mohammed Bouabdellah d

a Laboratoire des Matériaux Utiles, Institut National de Recherche et d'Analyse Physico-chimiques, 2026 Technopole de Sidi Thabet, Tunisiab Université de Tunis El Manar, Faculté des Sciences, Département de Géologie, 2092 El Manar, Tunis, Tunisiac Economic Geology Research Unit, School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland, Australiad Laboratory of Mineral Deposits, Hydrogeology and Environment, Department of Geology, Faculty of Sciences, B.P. 717, 60000 Oujda, Morocco

⁎ Corresponding author at: Department of Geology, FaAhmed Zarrouk, 2112 Tunisia.

E-mail address: [email protected] (N. Jemmali

0375-6742/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.gexplo.2013.03.004

a b s t r a c t

Article history:Received 14 December 2012Accepted 22 March 2013Available online 6 April 2013

Keywords:Oued MadenJebel HalloufFedj HasseneNappe zoneSulfur and lead isotopesNorthern Tunisia

The Oued Maden, Jebel Hallouf and Fedj Hassene Pb–Zn (Ba–Sr–F–Fe–Hg) hydrothermal ore deposits arelocated in the Nappe zone of Northern Tunisia. These ore deposits occur as epigenetic veins, karst andstockwork fillings in Upper Cretaceous limestones. The ore mineralogy consists mainly of galena and sphalerite,accompanied by minor amounts of jordanite, pyrite, chalcopyrite, orpiment, realgar, and other sulfosalts. Sulfurisotope data from sphalerite and galena indicate that the reduced sulfur was derived through thermochemicaland/or bacterial reduction of dissolved sulfate resulting in metal precipitation. Lead isotope ratios and corre-sponding calculated age models indicate that the Pb in galenas has been derived from a homogenous crustalsource during Upper Miocene time.

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1. Introduction

Northern Tunisia hosts several Pb–Zn (Ba–Sr–F–Fe–Hg) hydrother-mal deposits that occur throughout the main structural domains ofthe country (i.e., Nappe zone, Diapir zone, Jurassic Mountain zone,and Graben zone), preferentially along the contact zone between theTriassic and the overlying package of Mesozoic–Cenozoic sedimentarycover (Fig. 1). Previous studies have focused on the mineralogical andgeochemical characteristics of the most productive deposits such asthose of Bou Grine (Bechtel et al., 1996, 1998, 1999; Orgeval, 1994),Fedj-el-Adoum (Charef, 1986; Charef and Sheppard, 1987; Sheppardet al., 1996), Sidi Driss (Decrée et al., 2008), Jebel Ressas (Jemmali etal., 2011a), Jalta, Jebel Ghozlane, and Guern Halfaya (Jemmali et al.,2011b, 2012) (Fig. 1). The intimate spatial and temporal relationshipsbetween some of these deposits and Neogene magmatism have ledmany authors to argue for a genetic relationship (Decrée et al., 2008;Jemmali et al., 2011b; Talbi et al., 1999). However, for those depositsnot showing obvious connection to igneous activity like the depositsin the Diapir Zone (Fig. 1), their genesis has been related to a complexinterplay between basin evolution, thermal maturation and biogeo-chemical alteration of hydrocarbons (Bechtel et al., 1996, 1998, 1999;

culty of Sciences of Gafsa, Sidi

).

rights reserved.

Montacer et al., 1988; Orgeval, 1994, 1995; Orgeval et al., 1986), hydro-logic and tectonic processes and halokinesis (Rouvier et al., 1985).

Hybrid models calling upon the involvement of a mixture of mag-matic hydrothermal solutions and meteoric water have also beenproposed (Charef and Sheppard, 1987, 1991; Decrée et al., 2008;Sheppard and Charef, 1990; Sheppard et al., 1996). Uncertainty inthe absolute ages of mineralization, coupled with opposing interpre-tations of relationships among mineralizing events (syngenetic vs.epigenetic), has hampered the development of reliable geneticmodels, constraining thereby exploration efforts to areas adjacent toknown deposits.

The Upper Cretaceous Oued Maden, Jebel Hallouf and FedjHassene carbonate-hosted Pb–Zn (As–Cu–Hg–Sb) deposits, describedherein, occur in small Neogene post-nappe basins developed withinthe fold–thrust “Nappe Zone” (Fig. 1), and belong to a large group ofknown potentially economic Pb–Zn deposits in Tunisia. Altogether,these deposits, which were mined from 1900 to 1950 by open-pit andunderground operations, have produced about 400,000 tons Pb,70,000 tons Zn, and 89 tons Cu with 350 g/t Ag (Cherif Ben Hassene,2006; Gharbi, 1977; Sainfeld, 1952).

Despite their economic importance, knowledge of the origin ofthese deposits ore remains poorly constrained owing to the lack ofdetailed mineralogical, isotopic, and geochemical data. Earlier studieshave focused on the geologic setting and mineralogical characteristicsof the deposits (Bejaoui et al., 2011; Charef, 1986; Cherif Ben Hassene,

Fig. 1. Geological zones, Triassic exposures and Pb–Zn deposits in Northern Tunisia.Adapted from Sainfeld (1952), Burrolet (1991) and Perthuisot (1978).

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2006; Gharbi, 1977; Mansouri, 1980; Rouvier, 1971; Sainfeld, 1952;Slim-Shimi, 1992) without attempting to correlate data from thesePb–Zn occurrences to the regionally surrounding deposits (e.g., Nefzadistrict Decrée et al., 2008, Bir el Afou, Bou Dina, Saf-Saf Sainfeld,1952). Less attention has also been paid to the interrelationshipsbetween the nature and intensity of hydrothermal alteration and the re-gionally spatially associated Cenozoic magmatism.

The present manuscript intends to document for the first time theS and Pb isotopic signatures of the Oued Maden, Jebel Hallouf andFedj Hassene carbonate-hosted Pb–Zn deposits, in conjunction withrelevant data and interpretations in the literature, to clarify (a) thesources of sulfur and metals, (b) the processes of fluid–rock interac-tion and ore deposition, and (c) the timing of mineralization.

2. Regional geology

The alpine “Nappe zone” structural domain of northern Tunisia con-sists of a series of allochtonous stacked nappes (Rouvier, 1977) thatwere displaced tectonically southeastward upon the autochtonoussubstratum (Khomsi et al., 2009). Numerous post-nappe Neogene(Serravallian to Quaternary) basins, some of which are of economicinterest as those described in the present paper, formed within NE–SWsinistral strike–slip fault-controlled rift systems (Melki et al., 2011) in re-sponse to the Lutetian and Priabonian compressional phases (e.g., Frizonde Lamotte et al., 2006) and theOligocene extensional stage (Melki et al.,2011). The Late Miocene compressional phases resulted in folding ofboth the allochtonous andautochtonous terranes alongNNE–SSWdirec-tions, triggering the emplacement of the nappes. This main compressive

event was followed by development of extensional longitudinal faultsthat acted as major pathways for bimodal volcanism and associated hy-drothermal fluids (Halloul and Gourgaud, 2012; Laridhi-Ouazaa, 1994;Mauduit, 1978; Tlig et al., 1991).

Overall, the lithostratigraphic column consists of a pre-Miocenebasement made of Triassic evaporites, Cretaceous and Eocene shalesandmarls, unconformably overlain by a thick (up to 1800 m) continen-tal sequencemade of syn- to post-orogenicmolasses ofMiddle to UpperMiocene age, followed by Pliocenemarine argillaceous and sandy lithol-ogies (Melki et al., 2011). TheQuaternary ismade of a succession of con-tinental and marine rocks, which cap the lithostratigraphic column.

Dominant tectonic structures include sheets thrust over the Triassicsalt (i.e., Numidian allochtonous flysch unit, Perthuisot and Rouvier,1992; Rouvier, 1973, 1977) and inherited fault systems and titled blocksthat affected the autochtonous terranes (Khomsi et al., 2009). Theinherited faults are connected at depth to a major “décollement” levelrepresented by the interface Triassic salt/Mesozoic–Cenozoic cover(Khomsi et al., 2009) whose extension is punctuated by numerousPb–Zn occurrences including those related to the Jalta deposit(e.g., Jemmali et al., 2011b). Halokinesis resulting in development ofTriassic diapirs and associated dome structures played a key role inbasin evolution (Melki et al., 2011) and subsequent associated mineral-ization (Jemmali et al., 2011a, 2011b; Rouvier et al., 1985).

The Neogene sedimentary country rocks are intruded locally by aseries of extensional post-collisional bimodal intrusions (Halloul andGourgaud, 2012), pyroclastics, and lava flows whose compositionsrange from basaltic (8.4–5 Ma; Bellon, 1976) to rhyolitic (12.9–8.2 Ma;Bellon, 1976; Faul and Foland, 1980; Rouvier, 1977) end-members.

8 N. Jemmali et al. / Journal of Geochemical Exploration 132 (2013) 6–14

Some of the mafic and/or lava flows are interbedded within Neogenesediments in the Nefza area (Fig. 1). The magmatism was initiallycalc-alkaline exhibiting a subduction signature (Belayouni et al., 2010)and became transitional with time, with a double signature of bothintra-plate and orogenic magmatism (Halloul and Gourgaud, 2012).Another outcrop of magmatic rocks is La Galite Island, located betweenSardinia and Tunisia, 45 km off the Cap Serrat, NE to the study deposits(Fig. 1). Magmatic rocks at La Galite are represented by massivemicrogranular granodioritic sub-intrusives with microgranular granitesand aplites (Belayouni et al., 2010).

A wide variety of hydrothermal ore deposits spatially associatedwith Tertiary carbonate-dominated host rocks are located withinthe Nappe zone (e.g., Oued Maden, Jebel Hallouf, and Fedj Hassenedeposits described herein; Fig. 1). Based on mineralogy and petrogra-phy of the host rocks, two groups of carbonate-hosted Pb–Zn depositsare recognized (Rouvier et al., 1985). Group 1 consists of arsenic- andantimony-bearing Pb–Zn ore bodies within the Neogene continentalstrata (i.e., Bou Aouane and Jebel Hallouf deposits); whereas the secondgroup of ore deposits consists of arsenic- and mercury-bearing Pb–Znmineralization emplaced within extensional fractures and are spatiallyand/or genetically related to Neogene magmatism (i.e., Oued Maden,Jebel Hallouf and Fedj Hassene deposits; Rouvier et al., 1985).

3. District-scale geology

TheOuedMaden, JebelHallouf, and FedjHassenedeposits constitutethree of numerous Pb–Zn deposits that occur within the Mesozoic–Cenozoic cover rocks of the Nappe Zone, and are classified as carbonate-hosted Pb–Zn in the descriptive sense (Leach and Sangster, 1993). Theregional geology and tectonic evolution, along with ore mineralogyand alteration of these deposits have been thoroughly described in liter-ature (e.g., Gharbi, 1977; Rouvier, 1977; Sainfeld, 1952; Slim-Shimi,1992) and summarized herein.

3.1. Oued Maden Pb–Zn (Cu–As–Hg–Sb) deposit

Oued Maden mine is located 35 km to the NW of Jendouba city,close to the Algerian–Tunisian border (Fig. 1). The regional geologyconsists of a series of stacked nappe structures produced by regionalwestward intra-Miocene tangential over-thrusting and Upper Mioceneto Pliocene–Quaternary post-nappe tectonic phases (Gharbi, 1977;Rouvier, 1977). Two main nappes are recognized (Rouvier, 1977),namely (i) the Numidian nappe consisting of Oligocene clay-richsandstones and (ii) the Ed-Diss nappe comprising Senonian (Upper

Fig. 2. Geological map of Oued Maden showing thAdapted from Slim-Shimi, 1992.

Campanian–LowerMaastrichtian) and Eocene sediments. The autoch-thon beneath these nappes displays a normal stratigraphic sequenceconsisting of Santonian–Maastrichian (Late Cretaceous) rocks (Fig. 2).The Upper Santonian–Lower Campanian sedimentary package in fault-contact with the underlying Triassic rocks consists of thinly beddedgray marls with rare intercalations of marly limestones, overlain by a5-m-thick succession of marls and limestones of Upper Campanian–Lower Maastrichtian age, followed by up to 200-m-thick gray sulfide-rich limestones of Middle Maastrichtian age. Brittle structuresdeveloped within the gray limestones contain most of the documentedmercury mineralization (Gharbi, 1977).

From 1900 to 1955, about 11,500 tons of Pb and 89 tons of Cuconcentrates (with ca. 350 g/t Ag) were produced from open pit andunderground workings (Gharbi, 1977; Sainfeld, 1952). Pb–Zn–(Cu–As–Hg–Sb) mineralization occurs as open-space fillings of veinlets andstockwork structures superimposed on the NE–SW-trending Grouraand Ferza faults cutting across the Triassic dolostone and the Campa-nian–Maastrichtian limestone country rocks. An iron oxide cap thatcan be followed for 2.5 km along strike mantles the surface of theGroura fault. The Groura and Ferza faults (Fig. 2) are part of a regionalmajor structure referred to as the Ghardimaou–Cap Serrat Fault(Fig. 1). The ore mineralogy is dominated by galena, sphalerite, pyriteand sulfosalts (mainly tetrahedrite). The sequence of mineral deposi-tion shows the existence of three successive and overlapping stages ofmineralization (Slim-Shimi, 1992). These three stages are distinguishedby megascopic and microscopic textural and cross-cutting relation-ships, and mineral assemblages. Stage I is the earliest and consists offramboidal pyrite and schalenblende accompanied by calcite, celestiteand barite. Stage II, referred to as “main-stage ore”, is economicallythe most important accounting for more than 80% of the extractedPb–Zn resources. The mineral paragenesis consists mainly of fine-grained galena and sphalerite occurring within the Upper Cretaceousorganic-rich limestones as open-spacefillings of veinlets and stockworkstructures. Microprobe data show that sphalerite contains 0.1–0.8 wt.%Fe and up to 2.7 wt.% Cd and 8.2 wt.% Hg (Slim-Shimi and Tlig, 1993)whereas In, Ge and Ga are below the detection limit. Similarly galenashows abnormally high contents of As and Sb (up to 1.6 and 3.2 wt.%,respectively). The latest, stage III mineralization consists of cinnabarand As–Sb-rich sulfosalts.

3.2. Jebel Hallouf Pb–Zn (As–Sb) deposit

The Jebel Hallouf and its satellite Sidi Bou Aouane ore bodies,referred to hereafter as the Jebel Hallouf deposit, are located in the

e locations of the major Pb and Hg orebodies.

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external zone of the Nappe Zone, 17 km west of Beja city (Fig. 1).Regional stratigraphy consists of a Triassic–Miocene series overlain infault-contact by the Kasseb Paleocene–Oligocene allochthonous unit,which in turn is overlain by the Neogene post-nappe continental series(Mansouri, 1980; Rouvier, 1977) (Fig. 3). The Triassic package consistsof a succession of interbedded dolomitic breccia and evaporites, uncon-formably overlain by Lower–Upper Cretaceous clay-rich limestones(Fig. 3). The Paleocene–Upper Cretaceous series, regionally referred toas transition zone, consists mainly of marls, which in turn are overlainby Eocene limestones. The tectonically overlying Oligocene–Miocenestrata are made of sandstones.

Strata exposed in the Jebel Hallouf area have undergoneupper crust-al polyphase brittle to ductile deformation that included at least fivesuccessive extensional and compressional episodes (Charef, 1986;Mansouri, 1980). The resulting structures, developed at different scalesduring the deformation of the Triassic evaporites (gypsum and salt),include NE–SW-trending upright isoclinal open to closed folds, and asuccession of closed-to-tight N30°–40°E oriented evaporite-cored anti-clines and broad E–W to N–S-trending open-to-gentle synclines, trun-cated and/or accompanied by a series of ENE–WSW, NW–SE, E–Wand N–S trending faults.

Available resource data indicate that the Jebel Hallouf deposit,which was mined by the SOTEMI mining company between 1965and 1986, has produced since its exploitation in 1910 the equivalentof 326,541 tons of Pb and 14,207 tons of Zn (Cherif Ben Hassene,2006). At Jebel Hallouf, mineralized veins or karstic mineralizationare hosted by Campanian–Maastrichtian limestones. The depositexhibits two styles of mineralization (Charef, 1986; Mansouri, 1980;Rouvier, 1971). One style of mineralization consists of cavity-fillingof karstic features (Fig. 4) on the north side of the Jebel Halloufanticline. Vertical cavities developed from WSW–ENE and NNW–SSEjoints were partially filled up by stratified mineralized calcite(Charef, 1986). The mineral association consists of calcite, sphalerite,galena, jordanite and pyrite as microspheres. These minerals arefound as incrustations on cavity floors, walls and ceilings; in certaincases, they alternate with micro-crystalline calcite precipitates. Theyare also found within layered sediments issued from reworking ofthe former deposits, as well as in concentric rings of stalactites. Theother style of mineralization is vein-filling in fractures orientedNW–SE in the Cretaceous sediments. The mineralogy of the veinsconsists of galena, sphalerite, jordanite and cerussite, smithsonite, asalteration products.

Fig. 3. Geological map of Jebel Hallouf shoAdapted from Mansouri, 1980.

3.3. Fedj Hassene Zn–Pb (Cu–As–Hg) deposit

The Fedj Hassene deposit is located in the front of the Numidiannappe, 10 km south of the Ghardimaou city (Fig. 1). The geology ofthe deposit consists of (Fig. 5; Sainfeld, 1952): (i) Triassic series of asuccession of carbonates (i.e., mainly limestone and dolostone), andgypsum-bearing red argillites breccia, locally truncated by regionalE–W striking faults; (ii) Aptian gypsum-bearing dolomitic breccias;(iii) Albian marls and limestones; (iv) the overlying Cenomanianlimestones and marls capped by a Turonian succession of marls andlimestones; (v) Eocene nummilitic limestones; (vi) Oligocene marlsand sandstones; and (vii) continental Neogene marls, sandstonesand conglomerates.

The thickness of the Cretaceous package, includingAptian, Albian, andCenomanian–Turonian, varies from 150 m to 1250 m. The Cretaceousends with the 500-m-thick Coniacian–Santonian marls followed by asuccession of Campanian–Maastrichtian marls and limestones (up to200 m thick). The major structure in the Fedj Hassene deposit is theAin el Kohla ESE–WNW-trending fault.

During the lifetime of the mine that exploited the Fedj Hassenedeposit until 1992, about 55,600 tons of Zn and about 300 tons of Pbhave been produced (Cherif BenHassene, 2006). The Pb–Znmineraliza-tion mainly occurs as ESE–WNW-trending veins and stockworksenclosed in Upper Cretaceous limestones (Figs. 4, 5). In addition to Pband Zn, minor amounts of Cu–As–Hg expressed mineralogically assulfides and sulfosalts were also recovered (Slim-Shimi, 1992). The sul-fide mineralogy consists mainly of sphalerite and galena with minoramounts of pyrite and chalcopyrite. Orpiment and realgar are locallypresent (Bejaoui et al., 2011). According to the latter authors, two para-genetic stages are distinguished: an earlier stage I with galena, pyrite,calcite, sphalerite and barite; and an older second stage II with realgar,and orpiment. The alterationminerals are cerussite, goethite and smith-sonite. Trace element compositions of sphalerite show 0.8–5 wt.% Fe,0.2–0.4 wt.% Cd and b0.7 wt.% Hg (Shimi and Tlig, 1993).

4. Sampling and analytical methods

A suite of 23 representative sulfide separates was collected frommine workings and outcrops. From this suite, four sphalerite grainsand 12 galena separates were extracted from crushed and washedfraction of samples. Each grain was carefully examined under binocu-lar in order to avoid inclusions. Only pure separates of each sulfide

wing the locations of Pb–Zn deposits.

Fig. 4. Selectedmineralized samples from the studied ore deposits showing typical: (a) karsticmineralization of Jebel Hallouf with galena and jordanite (Gn + jordanite) and calcite (Ca);(b) karstic mineralization of Jebel Hallouf with galena, jordanite (Gn + jordanite), and calcite (Ca); (c) galena (Gn) hosted by Upper Cretaceous micritic limestone (L) at Fedj Hassene;(d) banded sphalerite (Sp) with calcite (Ca) hosted by Upper Cretaceous limestone (L) at Fedj Hassene; and (e) stockwork sphalerite (Sp) with calcite (Ca) hosted by Upper Cretaceouslimestone (L) at Fedj Hassene.

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mineral were analyzed at the laboratory of the Institute of Mineralogyand Geochemistry of the University of Lausanne (Switzerland). Thesulfur isotope ratios were determined using a Carlo Erba 1100 ele-mental analyzer (EA) connected to a Thermo Fisher Delta S isotoperatio mass spectrometer (IRMS) that was operated in the continuousHe flowmode via a Conflo III split interface (EA-IRMS). The sulfur iso-tope compositions are reported as per mil (‰) deviations relative tothe Canyon Diabolo troilite (V-CDT) standard. The reference SO2 gaswas calibrated against the IAEA-S-1 sulfur isotope reference standard

(Ag2S) with δ34S value of −0.3‰. Reproducibility of measurementswas ±0.2‰ (1σ) as monitored by replicate analysis of standardsNBS-123, IAEA-S-1 and IAEA-S-2.

For Pb isotope measurements, 2–3 mg of galena sample wasdissolved using ultrapure (double distilled) HCl. The Pb isotope com-positions were analyzed using a multi-collector inductively coupledplasma mass spectrometer instrument within the Radiogenic Isotopefacility at the University of Bern (Switzerland). Sample aliquots weresubsequently mixed with ~1.5 ml of a 2% HNO3 solution spiked with

Fig. 5. Geological map of Fedj Hassene showing the locations of the main Pb–Zn orebodies.Adapted from Sainfeld, 1952.

Table 1Sulfur isotope data of sulfides from the Oued Maden, Fedj Hassene and Jebel Halloufdeposits (values in brackets are δ34SH2S calculated using a temperature of 130 °C, seethe text).

Sample ID Deposit Mineral Mineralization style δ34S (‰, VCDT)

OM-1 Oued Maden Galena Vein −3.2 (0.74)OM-2 Oued Maden Galena Vein −2.9 (1.04)OM-3 Oued Maden Galena Vein −3.5 (0.44)OM-4 Oued Maden Galena Vein −3.2 (0.74)FH-1 Fedj Hassene Galena Vein 16.2 (20.14)FH-2 Fedj Hassene Galena Vein 15.3 (19.24)FH-3 Fedj Hassene Galena Vein 15.2 (19.14)FH-4 Fedj Hassene Galena Vein 16.0 (19.94)FH-5 Fedj Hassene Sphalerite Vein 5.0 (4.38)FH-6 Fedj Hassene Sphalerite Vein 5.3 (4.68)FH-7 Fedj Hassene Sphalerite Vein 4.8 (4.18)FH-8 Fedj Hassene Sphalerite Vein 5.1 (4.48)JH-1 Jebel Hallouf Galena Karstic 16.9 (20.84)JH-2 Jebel Hallouf Galena Karstic 17.4 (21.34)JH-3 Jebel Hallouf Galena Karstic 17.3 (21.24)JH-4 Jebel Hallouf Galena Karstic 16.6 (20.54)

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the NIST SRM 997 Thallium standard (2.5 ppb), and aspirated(~100 μl/min) into the ICP source using an Apex™ desolvating nebuliz-er (Nu Instruments Ltd). Simultaneous measurements of Pb and Tl iso-topes, and 202Hg ion signal were achieved by using seven Faradaycollectors. The 205Tl/203Tl ratio was measured to correct for instrumen-tal mass bias (exponential law; 205Tl/203Tl = 2.4262). Upon sampleintroduction, data acquisition consisted of 2 half-mass unit baselinemeasurements prior to each integration block, and 3 blocks of 20scans (10 s integration each) for isotope ratio analysis. 204Hg interfer-ence (on 204Pb)wasmonitored and corrected using 202Hg. At the begin-ning of the analytical session, a 25 ppb solution of the NIST SRM 981 Pbstandard, which was also spiked with the NIST SRM 997 Tl standard(1.25 ppb), was analyzed. The two-year long-term reproducibility ofthe NIST SRM 981 defines a variance of ca. 2 × 10−4, with no adjust-ment relative to common literature values (Cattin et al., 2011). The ex-ternal reproducibility of individual analytical sessionswas ca. 1 × 10−4.

5. Results

The analyzed sulfides yielded δ34S ratios ranging from+17 to−3.5‰(Table 1), with highest values corresponding to galenas from the JebelHallouf deposit and lowest values to galenas from the Oued Madendeposit. The present data from the Jebel Hallouf deposit are consistentwith δ34S values 13–23‰ reported by Mansouri (1980) from the samedeposit. In the Fedj Hassene deposit where both sphalerite and galenacoexist, the δ34S values obtained from sphalerites are systematicallylower than those obtained from galenas (Table 1), indicatingdisequilibrium.

The Pb isotope compositions of galenas from the three depositsare homogeneous (Table 2), with 206Pb/204Pb, 207Pb/204Pb, and208Pb/204Pb ratios ranging from 18.788 to 19.002, 15.663 to 15.684,and 38.838 to 38.917, respectively. On the 207Pb/204Pb vs. 206Pb/204Pband 208Pb/204Pb vs. 206Pb/204Pb diagrams (Fig. 6), galenas from thethree deposits plot close to the Upper Crust curve of Zartman and Doe(1981).

6. Discussion

6.1. Sources of sulfur

The sulfur isotope compositions of hydrothermal sulfides are classi-cally interpreted to be indicative of sulfur sources and ore depositionalmechanism (Ohmoto and Rye, 1979). In this respect, the large varia-tions in δ34S values recorded from the three studied deposits suggest ei-ther different sulfur sources or that the analyzed sulfides precipitatedunder different physico-chemical conditions (i.e., changes in intensiveparameters such as temperature, pH, and oxygen fugacity) (Ohmoto,

1972). Fluctuations in temperature could not have been a major fac-tor of large variations in δ34Ssulfide ratios because the effect of tem-perature on isotopic fractionation between coeval precipitatingsulfides (i.e., sphalerite and galena) and H2S is small and virtually inde-pendent of temperature (≤±2‰) (Ohmoto and Rye, 1979). Similarly,variations in pH and fO2 would produce negligible δ34S variations(Cole and Ohmoto, 1986; Ohmoto, 1972; Rye and Ohmoto, 1974). Fur-thermore, fluid inclusion microthermometric data from the OuedMaden and Fedj Hassene deposits (Bejaoui et al., 2011; Slim-Shimi,1992) suggest mineralizations under roughly similar temperatures of130 ± 10 °C whereas Jebel Hallouf mineralization was under lowertemperatures of ca. 50 °C (Charef, 1986).

By using a mean temperature of 130 °C inferred from fluid inclusionmicrothermometric data (Bejaoui et al., 2011; Slim-Shimi, 1992) andthe fractionation equation of Li and Liu (2006), calculated δ34SH2S valuesfor reduced sulfur in galena and sphalerite from the three studied de-posits range from −3.5 to +17.4‰. These values are, for most of thestudied samples, much higher than those derived from an igneoussource (δ34S = 0 ± 5‰; Ohmoto and Rye, 1979), except for OuedMaden galenas and Fedj Hassene sphalerites in which a possible in-volvement of magmatic sulfur could be hypothesized, although lessprobable as suggested by the absence, in the studied area, of contempo-raneous magmatic activity during ore deposition.

Therefore, a better interpretation of the large variations inδ34Ssphalerite-galena ratios is fractionation of sulfur isotopes during inor-ganic (TSR) to bacteriogenic (BSR) reduction of seawater sulfate or

Table 2Lead isotope data of galenas from the Oued Maden, Fedj Hassene and Jebel Hallouf deposits.

Sample ID Deposit Mineral 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb Calculated age (Ma) of mineralization

OM-1 Oued Maden Galena 18.890 15.680 38.907 −28.48OM-2 Oued Maden Galena 18.894 15.684 38.917 −23.13OM-3 Oued Maden Galena 18.884 15.675 38.888 −34.46FH-1 Fedj Hassene Galena 18.792 15.666 38.849 12.90FH-2 Fedj Hassene Galena 18.788 15.663 38.862 9.57FH-3 Fedj Hassene Galena 18.788 15.663 38.838 9.57JH-1 Jebel Hallouf Galena 18.820 15.678 38.914 17.49JH-2 Jebel Hallouf Galena 18.820 15.680 38.913 21.62JH-3 Jebel Hallouf Galena 18.820 15.676 38.869 13.35

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pore seawater at different rates of sulfate availability (i.e., open versusclosed reservoirs with respect to sulfate) (Amrani et al., 2006; Butleret al., 2004; Machel, 2001; Vikre et al., 2011). Both BSR and TSR takeplace in twomutually exclusive thermal regimes. The upper limit for ef-fective BSR is approximately 60–80 °C whereas TSR requires tempera-tures of at least 150–200 °C to be significant (Machel et al., 1995; Orr,1974). Although attempts have been made to distinguish BSR fromTSR processes by means of sulfur isotope fractionation (Machel et al.,1995), there is so much overlap in fractionation ranges such thatLeach et al. (2005, p. 576) concluded that “…proof of the reduction pro-cess is equivocal”. Thus, although the ultimate source of sulfur in thestudied ore deposits is very likely marine sulfate, the specific sulfate

206Pb/204Pb18.4 18.6 18.8 19.0 19.2 19.4 19.6

208 P

b/20

4 Pb

38.0

38.4

38.8

39.2

Jebel Ghozlane (Jemmali et al., 2011b)Jalta (Jemmali et al., 2011b)Bou Grine (Jemmali et al., 2012)Guern Halfaya (Jemmali et al., 2012)Oued Maden (this study)Fedj Hassene (this study)Jebel Hallouf (this study)Granodiorite La Galite (Juteau et al., 1986)

Orogene

Upper crust

206Pb/204Pb

18.4 18.6 18.8 19.0 19.2 19.4 19.6

207 P

b/20

4 Pb

15.56

15.60

15.64

15.68

15.72

Jebel Ghozlane (Jemmali et al., 2011b)Jalta (Jemmali et al., 2011b)Bou Grine (Jemmali et al., 2012)Guern Halfaya (Jemmali et al., 2012.)Oued Maden (this study)Fedj Hassene (this study)Jebel Hallouf (this study)Granodiorite La Galite (Juteau et al., 1986)

Upper crust

Orogene

Fig. 6. Plots of 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb for the Pb–Zndeposits in Oued Maden, Jebel Hallouf and Fedj Hassene (this study), Jebel Ghozlaneand Jalta (Jemmali et al., 2011b), and Guern Halfaya and Bou Grine (Jemmali et al.,2012). Curves of growth trends for Pb isotope ratios are from the plumbotectonicmodel of Zartman and Doe (1981). The granodioritic intrusion of the La Galite hasbeen emplaced at 15 Ma (Juteau et al., 1986).

source remains unknown as does the process(es) by which sulfatewas reduced. The positive δ34S values are thus interpreted as resultingfrom rapid and complete thermochemical reduction of pore-water sul-fate. In this respect, TSR has been proposed as themain ore depositionalprocess that operated during the deposition of most Pb–Zn deposits innorthern Tunisia, with the reduced sulfur probably derived from sul-fates in Mesozoic–Cenozoic marine sediments (Amouri, 1989; Charefand Sheppard, 1991; Decrée et al., 2008; Jemmali et al., 2011a, 2011b,2012; Orgeval, 1994; Salmi-Laouar et al., 2004). Conversely, the nega-tive δ34S values are suggestive of a system opened to sulfate.

Although primary gypsum innearby Triassic argillite (with seawatersulfate of ~17‰; Claypool et al., 1980) would appear to be the mostobvious source, because it occurs in rock outcrops in the vicinity ofthe OuedMaden and Fedj Hassene deposits, other sources are possible.For example, because ore deposition has been shown to be of LateTertiary–Quaternary age (see Section 6.2 below) and, as stated byBouabdellah et al. (2012), any sulfate-bearing strata (i.e., Cretaceousand Neogene sedimentary sequences) or even sulfate dissolved inconnate water must also be considered as potential sources forsulfide-sulfur at the Oued Maden, Jebel Hallouf and Fedj Hassenedeposits.

6.2. Sources of metals and age of mineralization

The homogeneity in Pb isotope compositions likely indicates theinvolvement of either a well-mixed multi-source in large hydrother-mal cells (e.g., Thorpe, 1999) or a single homogeneous source. Inthe 206Pb/204Pb vs. 207Pb/204Pb diagram (Fig. 6), galena Pb isotope ra-tios plot between the Orogene and Upper crustal curves of Zartmanand Doe (1981), consistent with a dominantly upper crustal reservoirwith a negligible contribution of juvenile mantle lead. Compared toadjacent and typologically similar classes of Pb–Zn deposits, the Pbisotope compositions of the studied deposits are somewhat similarto those of Jalta, Jebel Ghozlane, Guern Halfaya, Bou Grine andFedj-el-Adoum deposits (Charef, 1986; Jemmali et al., 2011b, 2012;Orgeval, 1994). Because Pb isotope compositions of possible sourcerocks in the region are lacking, the ultimate source(s) of Pb remainsunclear. However, the similarity between the Pb isotope signatureof the analyzed galenas and the unique available whole-rock Pb isoto-pic composition of the 15 Ma dated microgranite and granodiorite ofLa Galite Island (Juteau et al., 1986) (Fig. 6) supports the concept thatbasement microgranite and granodiorite as likely main sources of Pbin the studied deposits.

The plausibility of basement rocks as the deep-seated source of Pb inthe Oued Maden, Jebel Hallouf and Fedj Hassene deposits is supportedby the presence of jordanite, orpiment and realgar minerals. Jordaniteis also present in the Lengenbach Pb–Zn–As–Tl–Ba deposits, which arehosted in Triassic dolostones in the Swiss Alps, for which it was proventhat Pb and other metals were leached from basement rocks (Hofmannand Knill, 1996; Köppel and Schroll, 1988). Another support for theplausibility of basement rocks as the deep-seated source of Pb in thestudied deposits is the presence of inherited faults connected todeep-seated faults cutting theMesozoic–Cenozoic cover and the Triassic

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salts (Fig. 3) and possibly extending into the basement (Khomsi et al.,2009), like the NE–SWGhardimaou–Cap Serrat basement fault. Howev-er, the occurrence of the studied deposits mainly in Upper Cretaceouslimestones also suggests plausible late remobilization of metals, as pro-posed by Moisseeff (1959) and Slim (1981) for other Pb–Zn deposits inTunisia.

Because the Pb–Zn mineralization in the studied deposits isfault-controlled and spatially associated with the post-nappe Mioceneseries, one is led to hypothesize that themineralization occurred duringthe last paroxysmal phase of the Alpine folding (i.e., Miocene age). AMiocene age has also been proposed for similar adjacent Pb–Zndeposits(Decrée et al., 2008; Jemmali et al., 2011a, 2011b; Slim-Shimi and Tlig,1993). Calculated model ages using the Pb isotope model of Staceyand Kramers (1975) range from 2.7 to 21.6 Ma with a median of12.5 Ma (excluding negative values), indicating an Upper Tertiary–Quaternary age (Table 2). This calculated age range, which is similarto that recently proposed for the world-class Touissit-Bou Beker districtof northeastern Morocco (Bouabdellah et al., 2012), coincides with theSerravallian–Tortonian magmatism event (Jallouli et al., 1996, 2003;Zouiten, 1999), and with the mid-Miocene Alpine compressional tec-tonics in northern Tunisia (Bouaziz et al., 2002; Frizon de Lamotte etal., 2006; Rouvier, 1977; Tricart et al., 1994). Similarly, a genetic link be-tween the Messinian mafic magmatism and Sidi Driss Pb–Zn deposit(Fig. 1) has been proposed by Decrée et al. (2008). In the Oued Madendeposit, the Pb-isotopic data give negative model ages. This impliesthat an anomalous Pb was introduced to characterize ore Pb in theOued Maden deposit, which gave negative or excess model agesuggesting significant radiogenic contamination. This kind of highlyradiogenic Pb known as J-type Pb (Evans, 1997) could be derivedfrom high U and Th crustal source reservoirs. Accordingly, resultingradiogenic Pb was likely stored in Paleozoic rocks before beingremobilized and re-deposited in Cretaceous rocks in the studiedarea.

6.3. Heat-source and structural controls

Our interpretations about the source of sulfur and dominantmechanism of sulfate reduction imply heat-source control on sulfidemineralizations in the Oued Maden and Fedj Hassene deposits.According to Machel (2001), the efficiency of TSR requires tempera-tures of ≫100 °C. Due to the lack of relevant data, we turned to theliterature to support our interpretations. Measured homogenizationtemperatures at Oued Maden and Fedj Hassene deposits are ca.130 °C (Bejaoui et al., 2011; Slim-Shimi, 1992). At the nearby SidiDriss Pb–Zn deposit (Fig. 1), where a genetic link between the em-placement of the Messinian mafic magmatism and ore genesis hasbeen proposed (Decrée et al., 2008), highest homogenization temper-atures are in the range of 160–190 °C. Contemporaneous Serravalian–Tortonian igneous activity in the Nefza area, which is close to thestudied deposits, is associated with high thermal gradient in excessof 10 °C/100 m and related thermal anomalies (Jallouli et al., 1996,2003; Zouiten, 1999). Accordingly, the Serravalian–Tortonian igneousactivity is proposed here as the heat-source control involved in theTSR of Triassic sulfates that triggered sulfide deposition at the OuedMaden and Fedj Hassene deposits. At Jebel Hallouf, the cave-fillingbase metals are likely due to “meteoritisation” (remobilization by sur-face processes) of the Pb–Zn “post-nappe” Miocene strata-bound orebodies (Rouvier, 1971) or to mixing between hydrothermal solutionsand meteoric water (Ford and Kings, 1965).

The predominant occurrence of the studied Pb–Zn deposits as veinsin Cretaceous dolostones indicates that they were formed by epigeneticprocesses including structural control. We emphasize, however, thatalthough tectonic movements have certainly favored diapiric ascent ofTriassic salts. The high salinity of fluids could relate the emplacementof mineralization to salt-diapir movement in Northern Tunisia. Rather,the tectonic movements that have affected the Triassic rocks have

provided the plumbing system for the mineralizing fluids originatingat depth and have enhanced the structural permeability of the carbon-ate host rocks. Nevertheless, the two documented phases of halokinesisthat occurred during the Cretaceous and Miocene times (Slim-Shimiand Tlig, 1993) help to constrain the age of Pb–Zn mineralizations inthe studied ore deposits.

7. Conclusions

Sulfur isotope data support the hypothesis that sulfur in sulfides at theOued Maden and Fedj Hassene Pb–Zn deposits of northern Tunisia hasbeen derived from Triassic evaporites or seawater through thermochem-ical sulfate reductionwhereas sulfur at the Jebel Hallouf deposit has beenderived by bacterial sulfate reduction. Additionally, Pb isotope ratios sup-port the hypothesis that Pb has been derived from a homogenous crustalsource during theUpperMiocene time. Thus, thermal anomalies associat-edwith the Neogenemagmatism (heat-source) are proposed as the driv-ingmechanism for hydrothermal fluids responsible for the genesis of theOued Maden and Fedj Hassene Pb–Zn deposits. Tectonic activity andresulting brittle structures, which likely triggered diapiric ascent of Trias-sic salts, may have provided escape pathways that allowed fluids residingin the basement to ascend into the cover rocks.

Acknowledgments

This work has been performed at the Departments of Geology at theuniversities of Bern and Lausanne, Switzerlandwithin the framework ofa trainingship granted by the Tunisian Ministry of High Education andScientific Research. Special thanks to Editor-in-Chief Dr. Robert Ayusoand two anonymous reviewers for their comments that help us improveour manuscript.

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