Lithology, mineralogy and geochemical

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Journal of African Earth Sciences 92 (2014) 76–96

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Lithology, mineralogy and geochemical characterizations of sediment-hosted Sr–F deposits in the eastern Neo-Tethyan region – With special reference to evaporation and halokinesis in Tunisia H.G. Dill a,⇑, N. Nolte b, B.T. Hansen b a b

Federal Institute for Geosciences and Natural Resources, P.O. Box 510163, D-30631 Hannover, Germany Geoscience Centre of the Georg-August University, Goldschmidtstrasse 3, D-37077 Göttingen, Germany

a r t i c l e

i n f o

Article history: Received 24 September 2013 Received in revised form 23 December 2013 Accepted 10 January 2014 Available online 31 January 2014 Keywords: Fluorite Rare-earth-elements (REE) Sr and Nd isotopes Evaporation Neogene Tunisia

a b s t r a c t The Neo-Tethyan basin is known for its sediment-hosted Sr deposits in Spain, Turkey, Cyprus, and the Gulf Region. Sediment-hosted Sr–F deposits with base metals formed in the rim sinks and on top of salt domes resulting from halokinesis of Triassic evaporites near the southern edge of the Mediterranean Sea in Tunisia. These evaporites delivered part of the elements, created a basin-and-swell topography and provided the local and regional unconformities to which many of the mineral deposits are related. Five mineralizing processes, each with characteristic sedimentary ore textures, are related to this subsurface salt movement: (1 + 2) Early- and late-stage replacement (‘‘zebra rocks’’), (3) hydraulic fracturing (‘‘fitting breccia’’ sensu Dill and Weber, 2010b), (4) remobilization (‘‘spinifex structures’’), and (5) open-space filling (‘‘caves and vein-like deposits’’). Basinal brines from Mesozoic aquifers delivered Pb, Zn, Cd, REE, Y, Hg, and Se, while Sr, Cs, Be, Li, Cu and Co have been derived from Cenozoic salinas of the Neo-Tethyan basin. Mixing of Mesozoic and Cenozoic brines between 28 and 19 Ma provoked the emplacement of Sr–F mineralization at temperatures below 200 °C under strong alkaline conditions. Epigenetic polyphase Sr–F deposits bearing base-metals which are closely related to salt domes (Tunisian-Type) may be traced into epigenetic monophase Sr deposits within bioherms (Cyprus-Type) devoid of Pb, Zn and F. Moving eastward, syndiagenetic monophase Sr deposits in biostromes (Gulf-Type) herald the beginning of Sr concentration in Miocene sabkhas of the Neo-Tethys. The current results are based upon field-related sediment petrography and on mineralogical studies, which were supplemented by chemical studies. The present studies bridge the gap between epigenetic carbonate-hosted MVT and syndiagenetic evaporite deposits, both of which developed during the same time span (Neogene) and were hosted by the same environment (near-shore marine marginal facies of the Neo-Tethys basin). Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Fluorine concentrations associated with strontium mineralizations is of very widespread occurrence in sedimentary rocks, especially in calcareous host rocks (Demark, 2003; Fisher, 2004; Tritla et al., 2004; Dill, 2010b). Numerous so-called Mississippi-ValleyType (MVT) fluorite-bearing deposits are located in the USA, Great Britain, Mexico and in Central Europe (Dill et al., 2008). In the Mediterranean Region several sediment-hosted F–Sr deposits are situated in northern Tunisia that have been mined out already in the past (Fig. 1) (Gottis and Sainfield, 1952; Nicolini et al., 1966; Office National des Mines, 1985; Bouhlel et al., 1988; Tlig, 1991; Souissi ⇑ Corresponding author. Tel.: +49 0511 643 2361. E-mail address: [email protected] (H.G. Dill). URL: http://www.hgeodill.de (H.G. Dill). http://dx.doi.org/10.1016/j.jafrearsci.2014.01.009 1464-343X/Ó 2014 Elsevier Ltd. All rights reserved.

et al., 1998, 2010). There were some attempts made to account for the origin of these Tunisian sediment-hosted mineral deposits (Souissi et al., 1998, 2010). The quoted papers focused either on one fluorite deposit or they confined their studies on one F–Ba– Sr mineralizing district in Tunisia only, casting aside the exceptional evolution of the Mediterranean Region during the Cenozoic when vast periods of strong evaporation occurred in what is called the Messinian Salinity Crisis (late Miocene) leading to sulfate deposition as far east as Cyprus and the Persian Gulf (Hsü, 1972; Rouchy, 1980; Cita and McKenzie, 1986; Dill et al., 2005, 2009; Lozar et al., 2008; Fischer and Garrison, 2009). In the current study various sediment-hosted Sr-bearing fluorite deposits in Tunisia have been investigated for their mineralogical and chemical composition. Particularly the structural and textural inventory has been addressed during the current study. The Tunisian mineral deposits were compared with sulfate-bearing


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H.G. Dill et al. / Journal of African Earth Sciences 92 (2014) 76–96

A B C

b

a I

c

5km

Qued M´tek

N Mediterranean Sea TUNIS II

J.Mokta Quarternary Pliocene - Late Miocene Early Miocene-Late Eocene Early Eocene - Late Cretaceous

I

J.Azreg

Early Cretaceous Jurassic-Triassic-Permian

II

II

J.Hammam

J.Stah

Hammam Zriba

Fluorite deposits referred to in the text

J.Guebli

City Faults

II

I

Mediterranean Sea

II

El Kohol I

II

Type-II fluorite Type-I fluorite

Fig. 1. Geological setting of mineral deposits in northern Tunisia. (a) Geological and geodynamic setting of the working area at southern edge of the Mediterranean Sea (modified from Zouaghi et al. (2011)). (b) Mineral deposits in northern Tunisia (circle). Those deposits mentioned in the text are given by name of site. A: Nappe Zone, B: Zone of Triassic salt domes with Triassic rocks exposed and shelf zone, C: Jurassic Mountains and platform (modified from Sainfeld et al. (1953), Perthuisot (1981), Burrolet (1991), and Jemmali et al. (2011)). (c) Geological map of the study area (geology modified from Nicolini et al. (1966)).


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mineral assemblages in the eastern Mediterranean region. In the course of this comparison, emphasis has been placed on rare-earth elements (REE), Nd isotopes and the Sr isotope ratios for constraining the formation in relation to the basin subsidence and evaporation in the eastern Mediterranean Region. The evolution of this particular type of Sr–F mineralization has been described in terms of a hydraulic model, depicting the sedimentary source and host rocks in the immediate surroundings of the salt domes. This particular salt-dome-related process is compared with concentration of sulfate in the eastern Neo-Tethyan basin. 2. Geological setting The geological setting of the fluorite deposits has been described below, using the compilations and maps produced by Nicolini et al. (1966), Bishop (1975), Perthuisot (1981), Office National des Mines (1985), Burrolet (1991), Anderson (1996), Souissi et al. (1997), Patriat et al. (2003) and by Jemmali et al. (2011) (Fig. 1a–c). The late Triassic evaporitic series in Tunisia form part of the stratigraphic series laid down in the vast Saharan Basin extending in the SW of the country (Fig. 1a). Considering the lithology and the paleontological inventory, these sedimentary units belong to the so-called Germanic Triassic Facies typical of an epeiric basin (Dill et al., 2012). Large salt domes with halite and gypsum are surrounded by mineral deposits bearing Pb, Zn, F, Ba and Sr in northern Tunisia (Fig. 1b and c). The halokinetic motion was initiated throughout the early Jurassic along zones of structural weakness extending from Jebel Stah towards the Mediterranean Sea in the NE (Pohl et al., 1986). The rising dome structures and their related mineralizations stand out as morphological heights and are well expressed in the landscape, although the Triassic core sediments are poorly exposed (Figs. 1b, c and 2a, b). In central and eastern Tunisia, the Jurassic sedimentary series are made up of platform limestones and dolostones. The early Cretaceous saw some deltaic deposits prograding from the northern Sahara into a shallow marine depositional environment (Fig. 1a). The Cenomanian is transgressive all over Tunisia with a thin blanket of dolostones and evaporites. These transgressive phases lasted until the Turonian–Maastrichtian. Paleogene series overlie Maastrichtian limestones in a neritic and lagoonal to littoral facies which is abundant in phosphorite seams (Belayouni and Trichet, 1980; Dill and Kantor, 1997). From the Oligocene through the Early Miocene, the marine series receded and the marine sedimentation was gradually replaced by thick continental deposits. By the beginning of the Middle Miocene, another transgressive phase of the Langhian Ain Grab formation developed in northern Tunisia. It was succeeded during the Middle and Late Miocene by continental, epineritic and fluviolacustrine deposits. The transition from the Miocene to the Pliocene is characterized by fluvio-lacustrine deposits. The Tunisian fluorite deposits under consideration occur in sedimentary units as old as early Jurassic, celestite, ankerite, and Pb–Zn-bearing sediments may be traced down to the Triassic (Fig. 2c, d and Table 1). The youngest rock series hosting fluorite is Miocene (Burdigalian) in age. 3. Methods and analytical techniques Besides the routine mineral-optical analyses with the petrographic microscope, the samples were investigated by XRD and SEM–EDX. The scanning electron microscope FEI QUANTA 600 FEG (FEG = field emission gun) was used for mineral identification and image analysis for morphological studies. The SEM QUANTA 600 FEG was linked with a EDX System (GEMINI) operated under

three different pressure regimes: (1) High vacuum (ca 106 mbar), (2) LowVac (approx. 1–10 mbar), and (3) ESEM (=Environmental scanning electron microscope) (approx. 10–30 mbar). The use of low-vac conditions allowed refraining from sputtering prior to measurement these minute hand-picked particles of fluorite ore. Powdered samples were analyzed by XRF for their chemical composition using a PANalytical Axios and a PW2400 spectrometer. The trace elements in fluorite were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). An Agilent 7500ce equipped with a standard peristaltic pump, a MicroMist concentric nebulizer, a Peltier-cooled spray chamber, the Plasma Forward Power and the Shield Torch System was used. Rb–Sr and Sm–Nd analyses were performed on the Thermo-Finnigan TritonÓ TIMS at the isotope geology department of the University of Göttingen. Rb, Sr and REE were separated from one single rock digest using standard cation exchange procedures. Subsequent separation of Sm and Nd was achieved using the reverse ion chromatographic procedure with HDEHP resin (=di-(2-ethylhexyl)phosphoric acid). Prior to digestion, samples were mixed with a tracer solution enriched in 149Sm–150Nd. Concentrations were calculated using the ID-TIMS technique. Long-term reproducibility for NBS 987 (n = 14) are 0.71027 ± 5 and 0.05651 ± 3 for 87Sr/86Sr and 84Sr/86Sr, respectively. Instrumental mass bias was corrected with 88Sr/86Sr of 0.1194 using exponential law. Instrumental mass bias correction for Rb measurements was achieved via repeated analyses of SRM 984 yielding a 85Rb/87Rbraw of 2.6014 ± 42 (n = 12) resulting in an exponential mass bias of 2.6‰/amu (atomic mass unit). Sample measurements were performed under the same conditions and corrected with the exponential mass bias derived from the standard measurements. Long-term reproducibility for a Nd in-house standard (n = 11) are 0.511801 ± 39 and 0.348409 ± 41 for 143Nd/144Nd and 145Nd/144Nd, respectively. This 143Nd/144Nd value corresponds to a value of 0.511830 for 143Nd/144Nd of La Jolla (Nolte et al., 2011). Analytical mass bias was corrected with 146Nd/144Nd of 0.7219 using exponential law.

4. Results 4.1. Lithology, structure and texture of host rocks The Jebel Azreg, Jebel Hammam, Jebel Stah and Jebel El Kohol fluorite deposits are hosted by early Jurassic platform limestones, Jebel Guebli and Hammam Zriba by platform limestones of late Jurassic age and Jebel Mokhta by Cenomanian shelf limestones ´ tek fluorite deposit is bound (Anderson, 1996). Only the Qued M to fluvial sandstones of Burdigalian age (Anderson, 1996). The fluorite-bearing mineral assemblages occur in a great variety of ore structures (Table 1 and Fig. 1a, b) while the ore minerals were emplaced in cavities, veinlets, brecciated zones and developed zebra ore textures (Fig. 2c and d). Dark fluorite alternate with white barite, celestobarite and minor quartz conducing to a rhythmic banding (zebra ore). Similar mineral textures have been recorded from several sediment-hosted F–Ba–Sr deposits often associated with dolomite (Harpøth et al., 1986; Wallace et al., 1994; Cornwell et al., 2001; Strmic´ et al., 2009). The zebra rocks owe their typical rhythmic banding to different minerals (polymineralic) or different grain sizes of the same sort of mineral (monomineralic) (Fig. 3a, b and Table 1-type I, IIa). These rhythmic ore bodies are conformably interbedded with calcareous rocks or they mark an unconformity as exemplified at Hammam Zriba, where Upper Jurassic (Titonian) sedimentary rocks are separated from Campanian rocks by these zebra-type rocks (Fig. 2d). Locally such zebra-type host rocks may grade into wrigglites as demonstrated by the hematite and siderite-bearing


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a

c

Triassic

Campanian Eocene

Oligocene

Campanian limestone

b Tithonian limestone

d

Fig. 2. Host rock lithology and fluorite at outcrop in northern Tunisia. (a) Geomorphologic and geological expression of a Triassic salt dome of the Bou Khil Diapir Zone. The various lithological units are shown by their age of formation from the edge towards the core zone of the dome structure. (b) The inset provides a close-up view of the stratabound mineralization at the boundary between Campanian and Eocene limestones in the Colonne Dante Mine. The room-and-pillar-mining operation denotes the stratabound mineralization gently dipping towards the right at the same angle as the surrounding country rocks. (c) Flooded opencast mine within pervasively karstified and hematized Liassic limestones of a fluorite cavity fill at Hammam Jedidi. (d) Stratabound zebra ore composed of fluorite, barite, celestobarite and quartz (see hammer for scale). The inset shows a close-up view of the zebra ore (see biro for scale). Hammam Zriba.

Aptian limestones at Jerissa (Fig. 3c and Table 1-type IIb). This term was coined by (Kwak and Askins, 1981) when studying the F–Sn–W (–Be–Zn) Skarn at Moina, Tasmania, Australia. Fracturing is a common process observed in many sediment-hosted mineral deposits in NE Tunisia. Some types may be described as fitting breccias in massive and well-bedded limestones (Fig. 3e and Table 1-type IIIa). Stockwork-like mineralization with numerous thin mineralized fissures is encountered near unconformities and representative type IIIb of fault-bound mineralization (Fig 3f and Table 1-type IIIb). Type IIIc consists of veins originating from stockwork-like structures. They gradually convert into amygdules (Fig. 3g). Types IIIa and IIIb are unconformity related mirroring a more or less wide temporal gap between the enclosing country rocks. Type IIIc, unlike its predecessors is polyphase. A couple of mineralizations at Hammam Zriba, Hammam Jedidi and in the El Kef area at Sekarna display spinifex-like textures in veinlets and nests, which, in places, are only visible as relic ghost structures, e.g., Gran Halfaya (Table 1-type IV). All of these textures consist of thin tabular crystals mainly of barite and celestite that are related in time and space with unconformities. The greatest hiatus between Mesozoic host sediments known from Tunisian lasted from the Triassic through the late Cretaceous (Fig. 3d and Table 1). Vein-like cavities and fracture fills underneath unconformities and disseminated mineral assemblages are polymineralic and common to the Jebel Salta and Sekarna deposits (Fig. 3h and Table 1-type V).

4.2. Mineralogy of fluorite deposits 4.2.1. Fluorite Fluorite was selected for an in-depth mineralogical and chemical investigation for its trace elements amenable for comparison and

chronological studies. Under polychromatic light, the fluorite samples from Tunisia display bluish, reddish and white colors which under the ultraviolet lamp may turn into all shades of blue and white (Fig. 4a and b). Compared to fluorite samples from vein-type deposits the color of sediment-hosted fluorite varies only within a narrow color spectrum (Dill and Weber, 2010a). This is also the case as far as the crystal morphology of sediment-hosted fluorite is concerned whose faces are given in this paper by the common notation, using Miller’s indices (Table 2). The hexahedron {1 0 0} is the most common morphological form in the sediments (Table 2 and Fig. 4c). At Hammam Zriba, discrete morphological stages with {1 1 1} faces having an equal share as the {1 0 0} faces in the buildup of the crystal aggregates may be recorded. The most complex intergrowth of fluorite crystals was observed at Jebel Mokhta where the various morphological types may be placed in decreasing order as follows: {1 0 0} {1 1 1} > {1 1 0} (Fig. 4d). It is the only site under study in Tunisian fluorite deposits where in fluorite aggregates dominated by the hexahedron, a transition from the octahedron into the rhombic dodecahedron, may be recognized.

4.2.2. Strontium- and barium-sulfate The second most important mineral group in these fluorite deposits are Sr- and Ba-bearing sulfates. Three principal groups may be distinguished from each other using the Sr contents for subdivision of sulfate minerals. Low-Sr barite may contain Sr contents of as much as 4.9 wt.% Sr. Its tabular crystals are found in an intimate oriented intergrowth with fluorite (Fig. 4h). Celestobarite has Sr contents in the range 7.9–25.9 wt.% Sr. A cobweb of thin tabular crystals of celestobarite was encountered along the unconformity between Triassic and early Cretaceous limestones at Hammam Jedidi (Fig. 4e). At outcrop, white calcite and


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H.G. Dill et al. / Journal of African Earth Sciences 92 (2014) 76–96 Table 1 Structure and texture of sediment-hosted mineralization in NE Tunisia (site of mineralization, age(s) of host rocks), structuretexture, minerals encountered in the pertinent structures and textures, origin of mineralized structures and textures.

Site Bou Grine

Age of host rock Triassic

Bou Khil Doghra

Hammam Jedidi

Triassic vs. Early Cretaceous

Hammam Zriba, (plus fluorite deposits)

Early to late Jurassic vs. Late Cretaceous

Jerissa

Aptian

Doghra

Triassic

Hammam Jedidi ((plus fluorite deposits) Jebel Slata

Structure+ texture

Mineral association

Zebra rocks dolomitic stromatolite within limestones, rhythmic banding caused by different minerals (polymineralic)

Galena, sphalerite

Zebra rocks dolomitic wellbedded limestones rhythmic banding caused by different grain sizes (monomineralic) Zebra rock unconformity related, with calcareous footwall and hanging wall rocks

Celestite

Zebra rock unconformity related, within marls, argillaceous rocks, and calcareous footwall and hanging wall rocks Zebra rocks dolomitic wellbedded limestones rhythmic banding caused by different grain sizes (monomineralic) Wrigglite

Celestite, fluorite, quartz, calcite, galena, sphalerite

Fitting breccias in massive and well-bedded limestones

Celestite

Early-stage replacement (sabkha environment still recognizable)

Type I

Dolomite, ankerite, celestite

Celestite, celestobarite, calcite

Late-stage replacement (no primary textures preserved, along unconformity)

IIa

Siderite

Hematite, siderite

Epigenetic replacement (deformation ?) Hydraulic fracturing (initial stage)

IIb IIIa

Calcite, barite, galena, fluorite

Early Cretaceous Stockwork-like mineralization near unconformities

Galena, ankerite, calcite, barite Celestite, galena, sphalerite

Bou Khil

Eocene vs. Late Cretaceous

Bou Grine Hammam Jedidi

Triassic vs. Late Cretaceous Triassic vs. Early Cretaceous

Hammam Zriba (plus fluorite deposits) Hammam Jedidi

Late Jurassic vs. Late Cretaceous

Upper Jurassic vs. Late Cretaceous

Celestite, calcite, barite, galena

Sekarna

Late Jurassic vs. Late Cretaceous

Garn Halfaya

Triassic vs. Late Cretaceous

Spinifex (in parts as relic or ghost structures)

Celestite, calcite, barite, galena, sphalerite, fluorite Celestite, barite, smithsonite

Jebel Slata

Aptian vs. late Albian conglomerate separated by an unconformity Eocene

Vein-like cavities and fracture fills underneath unconformities

Barite, galena

Dissemination, cavity filling

Smithsonite, Zn silicate

Sekarna

Origin

Unconformity related vein, stockwork-like, polyphase (rhythmic banding), amygdules

Spinifex texture in veinlets and nests

celestobarite develop the so-called zebra rocks (Fig. 4f). A closer look at the zebra rocks under the SEM reveals an oriented intergrowth of fluorite with laminated celestobarite (25.9 wt.% Sr) (Fig. 4g). The most elevated Sr contents were analyzed from celestite (41.7 wt.% Sr, 9.4 wt.% Ba, 2.2 wt.% Ca) that is found in a regular intergrowth of thin plates of celestite with fluorite at Jebel Azreg (Fig. 3i and Table 2). Some celestite samples are also enriched in barium (up to 16.6 wt.% Ba) and calcium (up to 6.3 wt.% Ca). The latter Sr-enriched sulfate minerals are rather uncommon

Galena, marcasite, calcite Celestobarite

Celestite, calcite, barite, fluorite

Hydraulic fracturing (advanced stage)

IIIb

Hydraulic fracturing (waning stage) transitional into hydrothermal veins and fracture filling. There is more space created than fluids supplied to be filled it with minerals (space >>>fluid supply) Remobilization underneath or above unconformity

IIIc

IVa

Remobilization underneath or above unconformity with supergene alteration Karst-related mineralization (reducing > oxidizing)

IVb

Karst-related mineralization (oxidizing)

Vb

Va

constituents in fluorite deposits but widespread in the Tunisian sediment-hosted deposits (Dill, 2010a). 4.2.3. Pb-bearing sulfate, carbonate, phosphate, and arsenate minerals Among the complex sulfate, phosphate and arsenate s.s.s., Pb is the predominating cation as demonstrated by the minerals hedyphanite, hinsdalite, mimetite, plumbojarosite, drugmanite, plumbogummite, pyromorphite (Fig. 5a–d and Table 2). Rarely, the pure end member types were found among these APS minerals


81


a

b

c

d

e

f

g

h ba

gn

3 2 1 1

ca

Fig. 3. Mineral structures and textures. (a) Polymineralic zebra-type textures made up of celestite (white), ankerite, and ferroan dolomite (brown), which is in parts replaced by Mn–Fe oxide hydroxides. The basal contact of the ore beds is sharp whereas the contact on top is irregularly-shaped. Bou Khil/Saint Pierre. (b) Monomineralic to oligomineralic zebra-type textures made up of bands of celestite (white) of different grain size. Doghra. (c) Wrigglite made up of hematite replacing ferroan carbonate. Jerissa. (d) Spinifex relic (ghost) structures in Cenomanian through Turonian limestones at Gran Halfaya. The tiny tabular crystals were primarily celestite, barite or celestobarite. (e) Fitting breccia in well-bedded lagoonal Triassic dolomites. The space created by the fractures are accommodated by celestite. Doghra. (f) Thin veinlets randomly penetrate calcareous rock underneath the Triassic marls developing a stockwork celestite mineralization at Bou Khil. The geomorphological and geological setting is given in Fig. 2a. (g) Unconformity-related, polyphase (three distinct generations showing rhythmic banding) celestobarite mineralization ending in an amygdule. It reflects the waning stage of hydraulic fracturing with veins transitional into hydrothermal veins and fracture filling. There was more accommodation space created than the amount of fluids supplied to be filled the voids. (h) Karst cavity within Aptian limestones filled with calcite (white) plus ankerite (brown at the selvage of the veinlets) (ca), barite (ba) and galena (gn) near the unconformity at Jebel Slata. The inset provides an overview of the veinlike cavity-filling which is accentuated by the abandoned opencast mining operations and the adits at various altitudes of Jebel Slata. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Fig. 4. Fluorite, Sr- and Ba-sulfate on a macroscopic and microscopic scale. (a) Aggregates of fluorite hexahedra displaying a reddish blue fluorescence under the UV lamp. Locality: Mokhta. (b) Fragments of colorless fluorite displaying a pinkish fluorescence under the UV lamp Locality: Hammam. (c) Hexahedral crystals of fluorite under the SEM. Locality Jebel Stah. (d) Complex intergrowth of hexahedra, octahedra and dodecahedra of fluorite. The relative abundance of morphological types is as follows: {1 0 0} {1 1 1} > {1 1 0}. Locality: Jebel Mokhta. SEM. (e) Thin tabular crystals of celestobarite along the unconformity between Triassic limestones below and early Cretaceous limestone above. Locality: Hammam Jedidi. (f) Zebra rock made up of white calcite alternating with dark celestobarite. Hammam Jedidi. For scale see field note book. (g) Oriented intergrowth of fluorite (dark) with laminated celestobarite (25.9 wt.% Sr). Locality: Hammam Zriba. SEM. (h) Alignment of tabular crystals (white) of barite (Sr free) within fluorite (dark). Locality: Jebel Mokhta. SEM. (i) Regular intergrowth of thin plates of celestite (41.7 wt.% Sr, 9.4 wt.% Ba, 2.2 wt.% Ca) with fluorite (dark matrix). Locality: Jebel Azreg. SEM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(=aluminum-phosphate sulfate) (Dill, 2001). Mimetite from Hammam Zriba contains minor amounts of Zn and some samples from Jebel El Kohol have part of the Pb substituted for by Ca2+ (Fig. 5d). Cerussite and anglesite were only met in intimate intergrowth with galena as it is undergoing weathering. These oxidic minerals developed coatings on fluorite and infill corrosion cavities within fluorite. 4.2.4. Zn-bearing carbonate and hydrosilicates Zinc-bearing carbonates and hydrosilicates are less widespread than equivalent Pb-bearing minerals. Hemimorphite has been

spotted in samples from Hammam Zriba and Jebel El Kohol (Fig. 5e). Smithsonite is common among the late-stage minerals at Jebel Guebli, Hammam Zriba, and Jebel El Kohol (Fig. 5f). At ´ tek, Mg substituted for part of the Zn in smithsonite giving Qued M rise to Mg-bearing smithsonite (Table 2). 4.2.5. Cu, Zn and Pb sulfides Chalcopyrite and sphalerite are sporadically present, whereas galena was mined together with fluorite. Elevated arsenic contents leading to mimetite may be accounted for by the presence of tennantite, reported inter alia at Jebel Stah (Souissi et al., 1997).


83


a

b

c

d

e

f

sm

´ tek. (b) Plumbojarosite– Fig. 5. Minerals associated with fluorite and Ba- and Sr-sulfate minerals under the SEM. (a) Plates of hedyphanite coating fluorite. Locality: Qued M ´ tek. (c) Slender prisms of Ca-bearing pyromorphite developed stellate aggregates on fluorite. Locality: hinsdalite s.s.s. grew in a corrosion cavity on fluorite. Locality: Qued M ´ tek. (d) Zincian mimetite intergrown with fluorite. Locality: Hammam Zriba. (e) Rosette of hemimorphite crystals on fluorite. Locality: Jebel El Kohol. (f) Smithsonite Qued M (sm) on drusy fluorite. Locality: Hammam Zriba.

4.2.6. Silicates Apart from quartz and its microcrystalline modification chalcedony, there are only phyllosilicates that play a significant part in the fluorite mineralization. In the insoluble residue of the calcareous rocks we identified chlorite, kaolinite, and degraded white mica (illite) (Table 2). Glauconite, observed among the phyllosilicates did not form part of the fluorite mineralization and was only mentioned in the sequence of mineralization (Fig. 6 and Table 2). 4.3. Geochemical fluorite characterization 4.3.1. REE geochemistry The trace-element composition of fluorite provides an in-depth view of the sedimentological and mineralogical processes. On that

account numerous papers have been published by different authors to identify, e.g., the sources of fluids or to assist during the correlation of deposits (Richardson and Holland, 1979; Hein et al., 1990; Möller et al., 1994; Gramaccioli et al., 1999; Hill et al., 2000; Kolonin and Shironosova, 2007). In the current study the trace element and REE composition of the various sites under study was determined and listed in Table 3. In comparison with mineral color (Table 2) the minor element content from silver (Ag) through zinc (Zn) show a rather random distribution with no correlation between mineral color and element contents, while the REE and Y contents show a regular change. The colorless or white fluorite is poor in REE plus Y, whereas the blue and reddish types of fluorite are significantly enriched in both element groups. The LREE (light REE) do not significantly differ from each other but


84


Fig. 6. Paragenetic scheme of minerals in the fluorite deposits in NE Tunisia.

marked difference exist between the three different color types as far as the HREE and Y contents are concerned. The blue fluorite stands out by its anomalously high HREE and Y contents. To unravel the relationship between the chemical composition of fluorite and the host rock sediments, some trace elements, the total amount of REE and the La/Lu ratio, were plotted vs. the age of host rocks (Fig. 7). REE and Y share the same shape of distribution pattern, reflecting a strong increase towards the Neogene (Fig. 7a). The La/Lu ratio behaves totally different from the amount of REE and Y and shows an opposite trend (Fig. 7a). The base metals although very different much with regard to the total amount, share a similar trend with a pronounced low during the upper Cretaceous (Fig. 7b). Mercury and selenium exhibit the same trend as the REE but less strongly enriched than the REE (Fig. 7a and c). Lithium, cesium and beryllium which are markedly different as far as their contents are concerned display a common trend (Fig. 7c). Based on the REE content of the various samples, two types of fluorite can be distinguished. Type 1 fluorites (Fl-1) are characterized by high REE content (4.2–60.9 lg/g) and type 2 fluorites (Fl-2) show low REE content (2.7–7.4 lg/g). The REE contents of the various fluorite types were PAAS normalized after McLennan (1989).

Fig. 8a and b shows that the PAAS-normalized REE patterns may be split into the above mentioned two groups. The sandstone´ tek show the most pronounced skewhosted deposits at Qued M ness (defined as TbN/((LaN + LuN) 0.5)) characterized by a strong ‘‘hump’’ around Tb (Fig. 8a; Fl-1). A less distinct increase of the skewness ratios may be observed in one sample of El Kohol (Fig. 8a) and Jebel Azreg (Fig. 8a, 802-1). The cerium-anomalies, calculated as log(CeN/(LaN + PrN)), are weakly negative for the majority of samples (Fig. 8a and b). Only three samples at Qued ´ tek (799-3), El Kohol (797-1) and Jebel Azreg (802-1), that also M stand out for their abnormally high skewness ratios, display a marked positive Ce-anomaly (Fig. 8a). The Eu anomalies, calculated as EuN/(SmN GdN)0.5, of all samples under consideration are positive, some of which take abnormally high values (Fig. 8b). All samples display positive Y anomalies (Fig. 8) and the Y/Ho ratios ranges between 40 and 80 typical for open sea-water but is strongly reliant on salinity (Lawrence et al., 2006). The La/Lu ratio is