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Mineralization and fluid inclusion studies of the Aptian carbonate-hosted PbZnBa ore deposits at Jebel Hamra, Central Tunisia
Journal of Geochemical Exploration 128 (2013) 136–146
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Mineralization and fluid inclusion studies of the Aptian carbonate-hosted Pb\Zn\Ba ore deposits at Jebel Hamra, Central Tunisia Jaloul Bejaoui a, b,⁎, Salah Bouhlel b, Esteve Cardellach c, Àngels Canals d, Joaquim Perona c, Àngels Piqué d a
UR-MDTN, Centre National des Sciences et Technologies Nucléaires, Tunisia Université la Manouba, Laboratoire de Biotechnologie et Valorisation des Bio-Géoressources (LR11ES31) and Université de Tunis El Manar, Faculté des Sciences de Tunis, Ariana, Tunisie Departement de Geologia, Universitat Autònoma de Barcelona, Spain d Departament de Cristallografia, Mineralogia i Dipòsits Minerals, Universitat de Barcelona, c/ Martí i Franquès s/n, 08028 Barcelona, Spain b c
a r t i c l e
i n f o
Article history: Received 31 October 2011 Accepted 2 February 2013 Available online 10 February 2013 Keywords: Jebel Hamra Upper Aptian Fluid inclusion Thermochemical Basinal brines Tunisia
a b s t r a c t The Jebel Hamra, which is hosted in the Graben Zone of central Tunisia, is an epigenetic dolomite-hosted Pb\Zn\Ba deposit that replaces platform carbonates of Lower Cretaceous age (Upper Aptian). Mineralization occurs as stratiform lenses, and breccia- and paleokarst-fillings. Ore reserves are around 0.5 Mt and grades are 7% Zn and 3% Pb. Sulphide mineralization occurs as local replacement of dolomitized and silicified carbonate strata. Primary inclusion fluids in sphalerite have homogenization temperatures ranging from 110 to 152 °C with a mode at 137 °C. Final ice-melting temperatures range from −15 to −10 °C, corresponding to salinities of 13.5–18.5 wt.% NaCl equiv. Fluid inclusions in saddle dolomite homogenize to the liquid phase between 175°C and 193 °C, with a mode at 187 °C. Final ice-melting temperatures range from −23 to −15 °C corresponding to salinities between 18 and 24 wt.% NaCl equiv. δ34S values of sphalerite and galena vary between −3 and +6.3‰, compatible with a thermochemical reduction of sulfate from Triassic evaporites (δ34S=+15‰). Pb\Zn\Ba mineralization is spatially and possibly genetically related to E–W-trending fractures, which acted as conduits and/or depositional sites for circulating hydrothermal fluids. Fluid inclusion and sulfur isotope data suggest the contribution of basinal brines in ore formation. Combined field, sedimentological, mineralogical, isotopic and geochemical data of the Lower Cretaceous mineralization in Jebel Hamra is compatible with Mississippi-Valleytype mineralization. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Tunisia has been historically an important producer of fluorite, Zn, Pb and barite from MVT deposits, such as Hammam Zriba fluorite deposits (Bouhlel et al., 1988), Pb\Zn of Fedj Lahdoum deposit (Charef and Sheppard, 1987), Zn\Pb of Bou Grine mine (Orgeval, 1994), and Zn of Fedj Hassène (Bejaoui et al., 2011). In the Jebel Hamra area, Central Tunisia (Fig. 1), there are several potentially important Pb\Zn\Ba deposits. They are related to NW–SE and E–W trending fault systems and occur as veinlets, cavity fillings, disseminations and replacements of host rocks. Hydrothermal alterations correspond to dolomitization and silicification of Upper Aptian limestone. Ore minerals are galena and sphalerite, with minor chalcopyrite and barite. The Jebel Hamra Pb\Zn deposit was early studied by Fuchs (1973) and Amouri (1981). These authors associated the Jebel Hamra deposit to the aptian paleosurface of the central Tunisian platform. They considered that the mineralization is of syngenetic type and the age of formation is the end of Aptian. However, some aspects of its genesis, such as ⁎ Corresponding author at: UR-MDTN, Centre National des Sciences et Technologies Nucléaires, Tunisia. E-mail address: [email protected] (J. Bejaoui).
the precipitation mechanisms, the origin of the salinity or the relationship with dolomitization processes in Jebel Hamra ore deposit, are still poorly understood. In Tunisia only a few studies have been focused on mineralization associated to karst structures (Bejaoui and Bouhlel, 2007; Routhier, 1980; Rouvier et al., 1985). The aim of this paper is to present the mineralogical, textural and geochemical data (fluid inclusions and sulfur isotopes data) of the Jebel Hamra deposits as an example of karst-related mineralization, in order to determine the origin and evolution of the mineralizing fluids. Finally, the data are compared to other important Tunisian Pb\Zn MVT deposits. 2. Geological setting The Jebel Hamra area is located in Central Tunisia, 50 km NW from the town of Kasserine (Fig. 1). It hosts several Pb\Zn\Ba deposits and occurrences around Bir Ferza, Khanguet Zitoun, Ali Lakhdar and Khanguet Slougui localities (Fig. 2). The geology of the area is dominated by extensively folded massive limestones and dolostones interlayered with marl units of Lower Cretaceous age. Following NW–SE trending faults, graben structures, hosting Pb\Zn\Ba deposits developed during Miocene, Pliocene and Quaternary (Chihi, 1995). The main outcrops are
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Fig. 1. Location of the Jebel Hamra deposit in Tunisian geological map (1) Quaternary: alluvial deposits, clay sands and conglomerates. (2) Upper Miocene: interlayered marls and sandstone. (3) Upper Senonian: limestones with interlayered clays. (4) Lower Senonian: marls with limestones of the Kef formation. (5) Turonian: limestones of the Bireno formation. (6) Upper Cenomanian to Turonian: marls, and marly-limestones. (7) Albian to Cenomanian: marls and limestones of the Fahdene formation. (8) Aptian: reef limestones and dolomites of the Serdj formation. (9) Triassic: clays, dolomites and evaporites. (10) Faults. (11) Mineralization. (12) Border Faults.
Upper Aptian limestones and dolostones of the Serdj Formation, bordered by Albian marls of the Fahdene Formation. The sedimentation during this period was controlled by three active E–W-trending normal faults that divided the basin into four blocks (Fig. 3). The Jebel Hamra area was affected, from Late Eocene to Upper Miocene, by the NW–SE compression related to the Alpine orogeny (Ben Ayed, 1986; Chihi, 1995). Pb\Zn\Ba deposits are found along an unconformity surface (commonly known as unconformity below the Albian), developed within dolostones of Upper Aptian age, on the four blocks that resulted from the brittle deformation of the area of Miocene age. Several morphologies of the mineralizations can be distinguished: cavities, joints, breccia fillings, disseminations and stratabound replacements of the carbonate host rocks (Bejaoui and Bouhlel, 2007; Bouhlel, 1993; Bouhlel et al., 1988 and Routhier, 1980). In the Jebel Hamra area, the sulfide mineralization occurs as a stratabound body in a dolomitic horizon (Fig. 4), which replaces almost entirely the Upper Aptian limestones of the Serdi Formation (Gargasian to Gargasian–Clansayesian age; M'Rabet, 1981). The mineralization consists of coarse-grained sphalerite, galena and minor pyrite and chalcopyrite partially replacing blocks of the host dolostone, as lenticular pockets, or as concretionary infillings of cavities, with a thickness ranging from a few centimeters up to 3 m.
3. Analytical techniques Mineral and host rock samples were collected at surface outcrops and mine drift in the Bir Ferza, Khanguet Zitoun, Ali Lakhdar and Khanguet Slougui areas at Jebel Hamra (Fig. 3). The mineralogical study was carried out using transmitted and reflected light microscopy and cathodoluminescence (CL). CL studies were done with an ELM-3R Luminoscope device, coupled to a polarized light microscope. Some samples were also analyzed in a Stereoscan S-360 scanning electron microscope with an energy-dispersive spectrometer (SEM-EDS) at the Serveis de Microscopia, Universitat Autònoma de Barcelona.
Microthermometric measurements were performed on a Linkam THMSG 600 heating-freezing stage at the Department of Geology of Faculty of Sciences of the University Tunis Manar and at the Department of Geology of the Universitat Autònoma de Barcelona. Temperatures were calibrated from the melting of synthetic H2O\CO2 fluid inclusions (− 56.6 °C), melting of ice and critical homogenization at 220 bar (374.1 °C) of synthetic H2O inclusions in quartz. The precision of measurements below 0 °C was ±0.1 °C and of homogenization temperatures was ±2 °C. Salinity of fluid inclusions, reported in equiv. mass % NaCl equiv, was calculated using the Bodnar's (1993) equation. For the sulfur isotope analyses, sulfide minerals were converted to SO2 gas by combustion with cupric oxide (Fritz et al., 1974). Analyses were performed at the USGS (United States Geologic Survey) in Denver and at the SCT of the University of Barcelona (Spain) using a continuous flow method as described by Giesemann et al. (1994). Results are reported as per mil (‰) deviations relative to the Vienna Canyon Diablo Troilite (VCDT) standard. 4. Alteration Dolomitization and silicification have been identified in most of the tectonic units of the Jebel Hamra, especially in the mineralized areas. The dolomitization and silicification events affecting the Upper Aptian limestones and marls are especially related to an NW–SE trending normal fault that affected the Jebel Hamra area (Fig. 2). The Jebel Hamra area is mostly constituted by a dolomitic limestone containing abundant, well preserved, fragmental fossils including rudists, corals and bryozoans. Dolomitization affects the middle and the upper section of the Serdj Formation (Upper Aptian). Where the entire unit is present, the thickness of the Serdj Formation ranges from 80 m in the northern part of the Jebel Hamra, near the evaporitic Triassic outcrop of Henchir Nechla, to 150 m in the southern part (Figs. 1 and 4). Dolomitization develops near fractures and discontinuities related to bedding (Fig. 5B and E). Dolomite beds have thicknesses varying from
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Fig. 2. Geological map of the Jebel Hamra with the locations of studied sites. Location of the cross-section (Fig. 3) is also shown.
centimeters to a few meters (average 5 m) and a lateral continuity over twenty meters (Fig. 4). Zebra dolomite is typically composed of alternating horizontal layers of dark, fine-grained dolomite (dolomite 1) and white, coarse-grained, sparry, void-filling dolomite (dolomite 2; Fig. 5B and C). A zebra dolomite set consists of several of parallel disc shaped lenses of dolomite (dolomite 1 and dolomite 2). The spacing between bands is of the order of 10 to 20 mm. Two bands may merge into one (Fig. 5C). The boundaries between bands are mostly irregular and show rapid transitions from fine dolomite (dolomite 1) to coarse-crystals dolomite (dolomite 2; Fig. 6B and C). In zebra dolomites the set of equidistant bands may cut across the host rock's bedding, which implies that the regularity of spacing cannot be inherited and must therefore originate by some kind of self-organization, as was pointed out by Merino (1984) and Fontboté (1993).
The dolomitic zebra and breccia bands are displacive bands, that is, bands that forcibly make room for themselves as they grow. These bands do not need a preexisting open space to grow into (Lopez-Horgue et al., 2010 and Merino et al., 2006). The contacts between the white dolomite (dolo2) and dark dolomite (dolomite1) are in detail irregular, suggesting that they represent dissolution voids or fractures enhanced by dissolution, rather than simple fractures (Fig. 6). Fine inclusions of brown to black bitumen occur along fractures and stylolites (Fig. 6A). The transition from replacive dolomite (dolomite1) to white displacive bands means that there was no growth interruption between them; that the displacive dolomite 2 is not filling a prior void (but making room for itself by pushing the host replacive dolomite aside) Merino and Canals (2011). Two generations of dolomite have been distinguished in the zebra texture: 1) dolomite replacing the carbonate matrix (dolomite 1),
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Fig. 3. Cross section of Jebel Hamra. (A) Upper and Middle Albian : marls and limestones (B) and (C) Upper Aptian reef limestones and dolostones. (D) Lower Cretaceous: limestone clays and limestone, with Barremian sands. (E) The Jurassic constituted with limestones and dolostones (F) Triassic; evaporites, clays and dolostones and (G) sulfide mineralization.
consisting of euhedral to subhedral crystals of relatively uniform size (~ 100 μm) with intercrystalline porosity (Fig. 6D); and 2) a white, sparry dolomite (dolomite 2: saddle dolomite) with anhedral to subhedral crystals that have undulose extinction and cloudy centers surrounded by clear crystal growth rims (Figs. 5 and 6). Saddle dolomite has a deep-red cathodoluminescence (CL) color with no internal zoning. The sparry crystals of saddle dolomite form a xenotopic to hypidiopic mosaic (Fig. 6). In outcrops, saddle dolomite facies are white and locally present a pale pink color. It is found in several types of dolomite fabrics: (i) zebras dolomite with dol1 and dol 2 (Fig. 5)
and (ii) as cement in breccias. Saddle dolomite (dolomite 2) precipitation is frequently related to hydrothermal process and mineralization. The dolomitic facies (arranged in centimeter's bands thick of alternating fine and coarse grained) of the upper section of the Serdj Formation is affected by a widespread silicification, which occurs as disseminated on the borders of dolomite (dolomite 2) crystals, fine-grained quartz crystals (2–10 mm; Fig. 6E). Locally, sphalerite and galena crystals are associated with the fine-grained quartz. Quartz crystals are situated at the margin of dolomite rhombic crystals (Fig. 6E and F). Petrographic studies indicate that silicification started after dolomitization and was probably
Fig. 4. Lithostratigraphic columns of mineralized areas. (1) Limestones interlayered with clays. (2) Platform carbonate. (3) Dolomitized and silicified limestones. (4) Clays and limestone.
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Fig. 5. Distribution of dolomitized limestones of the Serdj formation (Upper Aptian). (A) limestones rich orbitolina, of Upper term of Serdj, (B) and (C) subhorizontal vuggy porosity within zebra dolomite. The vug margins are irregular and lined with white sparry dolomite (dol2), (D) coarsely crystalline galena filling large solution vugs, (E) zebra dolomite composed of alternating horizontal layers of dark grained and white, coarse grained sparry, void filling dolomite, and (F) limestone is replaced by dolostone and filling cavities with barite.
associated with dissolution (Figs. 6 and 7A), brecciation as well as with the main mineralizing event (see Table 1). 5. Ore mineralogy and petrography The principal ore mineralogy is composed of sphalerite with minor galena and barite, is enclosed within a dolomitized and silicified limestone. The paragenetic sequence of the Jebel Hamra deposits was established from outcrop observations and microscopic studies. According to the origin, two events of mineral formation have been distinguished: hypogene and supergene (see Table 1). Stage I, corresponding to the primary (hypogene) minerals consists of sphalerite and galena as major sulfides, and pyrite and chalcopyrite as minor
phases. Barite is also present in variable amounts. Dolomite and quartz occur as gangue minerals. Sphalerite is yellow to dark brown-colored, medium- to coarsegrained (0.5 cm to 3 cm), and occurs as disseminations and aggregates of anhedral crystals. Sphalerite is found filling fractures and vugs, or as crystals replacing the dolostone host rock. Galena is commonly present as coarse crystals (1 mm to 10 cm), disseminated and replacing dolostone and/or as vein fillings. It is found associated with pyrite and sphalerite. Pyrite is present as disseminated concretionary nodules of a few mm in size in the dolomite host rock. Chalcopyrite is found locally filling fractures or disseminated in dolostone. It can be altered to covellite and goethite. Barite crystals are euhedral, fibroradiated or coarse-grained (few mm to cm in size). Barite is found as massive, banded, and disseminated aggregates coexisting
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Fig. 6. Photomicrographs of undolomitized, dolomitized and silicified host rocks (Upper Aptian reef limestones). (A) Coarse bioclastic limestone and bitumen within stylolite. (B) Transition from the replacive Dol1to the cement dolomite (Dol2) phases; the size of the dolomite crystals increases towards the cavity (from Dol 1 to Dol 2), (C), (D), (E) and (F) dolomitzed and silicified limestone, the saddle dolomite (dol2) with irregular margin is collapsed by finely crystalline quartz. Abbreviations: Dol = dolomite. Qz = quartz P = porosity Cal = Calcite.
with calcite and is sometimes present replacing the dolostone matrix. In many veins, barite crystals were brecciated and the interstices were filled with later iron oxides and covellite. Stage II is constituted by pyrolusite, cerussite and iron oxides, formed after a supergene alteration. Pyrolusite fills voids in dolomite crystals and coating quartz crystals (Fig. 7E and F). Cerussite occurs within vugs. Iron oxides are common throughout the mining area, resulting in variable concentrations of goethite, hematite as alteration of pyrite, together with pyrolusite filling cavities in dolomite.
6. Fluid inclusion study A microthermometric study was performed on sphalerite and saddle dolomite samples from Ali Lakhdar and Khanguet Slougui mineralized areas (Fig. 3). Only primary fluid inclusions (following the criteria of Roedder, 1984) were selected for microthermometric measurements. Primary fluid inclusions in sphalerite are biphasic (L+ V) at room temperature with sizes between 20 μm and 300 μm and occur grouped or isolated, showing smooth to irregular shapes (Fig. 8). They are liquid-rich, with a small vapor bubble filling 5% to 10% of the inclusion volume. Microthermometric measurements of sphalerite-hosted fluid inclusions are illustrated in Fig. 9. The eutectic temperatures (Te) range from −49.8 °C to −22.1 °C, indicating the presence of other salts in addition to NaCl, likely divalent cation chlorides (Te (MgCl2) = −35 °C; Dubois and Marignac, 1997; Te (CaCl2) = −52 °C; Oakes et al., 1990). Final ice-melting temperatures (Tmi) range from −15 to −10 °C (n=95), corresponding to salinities from 13.5 to 18.5 wt.% NaCl equiv, with a mean of 15 wt.% NaCl equiv. All primary inclusions homogenized to the liquid phase at temperatures ranging from 110°C to 152 °C (n = 95). The density of the fluids was calculated from the equation of state for aqueous systems (Bodnar, 1993) and it is between 1066 and 1103 g/cm 3.
Saddle dolomite commonly hosts fluid inclusions uniformly distributed within the crystals, having frequently smooth contours. The dimensions of the inclusions observed in dolomite, range between 5 and 40 μm most of them are liquid rich two-phase (L + V). Only the largest inclusions (up to 20 μm), preferentially grouped in the crystal cores or along crystal growth zones, were considered as primary. They are two-phase, liquid-rich, with a small vapor bubble, filling 5 to 15% of the inclusion volume. The first melting was observed at a temperature comprised between − 43 °C and − 27 °C, with a mean of − 35 °C, lower than the NaCl\H2O (eutectic ~− 21.2 °C), indicating the presence of other salts such as MgCl2 and CaCl2 (Bodnar, 1993). Tmi range is between − 23 and − 15 °C (n = 21), which corresponds to salinities between 18 and 24 wt.% NaCl equiv. Fluid inclusions homogenize to the liquid phase between 175°C and 193 °C, with a mode at 187 °C (n = 21) (Fig. 9). 7. Sulfur isotopes Sulfur isotope compositions were determinated on five sphalerite and seven galena samples. δ34S values of sphalerite range from +1.2 to +6.3‰ and those of galena from −3 to +1‰ (Table 2) and (Fig. 10). The δ34S values are not diagnostic of a particular sulfur source, but considering the geological environment of the deposits and the presence of evaporites of Triassic age, these values would be compatible with a H2S source after reduction of evaporite sulfate (δ34S from +12 to +15‰; Claypool et al., 1980; a similar value is given for the Triassic sulphates in north Tunisia by (Charef, 1986 and Orgeval, 1994). 8. Lead isotopes 206 Pb/ 204Pb, 208Pb/ 204Pb, and 207Pb/ 204Pb ratios for nine galena samples from the ore karst fall in the range of 18.7021 to 18.7052, 15.7204 to 15.7246 and 38.0134 to 38.0273 (Table 3) and (Fig. 12). These data plot above the initial isotopic composition of lead from
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Fig. 7. Mineralogy of the sulfide dominant stage and main textural relationships in the Jebel Hamra ore deposits. (A) Dolomite and quartz crystals cemented by massive sphalerite, (B) sphalerite and dolomite association showing irregular contact, (C) microtextures of cogenetic sphalerite and galena, (D) replacement of saddle dolomite with sphalerite, and (E) a backscattered electron image “SEM”of Pyrolusite (MnO2) which is the last manganese mineral to form; infilling void spaces between looks ore appropriate dolomite rhombs and surrounding quartz crystals. (F) Pyrolusite (MnO2) fills small cavities in host rock, constituted with dolomite and quartz. (G) Chalcopyrite crossed by microfracture and filling by goethite and covellite, (H) galena altered into cerusite with remains of quartz, and (I) colloform goethite in dolomite. Abbreviations: Qz = quartz; Dol = dolomite; sp = sphalerite; Gn = galena; Cpy = chalcopyrite; Gth = goethite; Cv = covellite; Pyr = pyrolusite; sm = smithsonite.
Table 1 Paragenetic sequence of ore, gangue and supergene minerals in the Jebel Hamra sulfide deposits.
isotopic evolution of Big Field reservoir from Plumbotectonics (Zartman and Haines, 1988). The lead isotopic values of galena collected from Jebel Hamra are similar to the lead isotope data of a number of ore deposits in Tunisia and show a similarity with Upper Crust values (Bouhlel, 2005; Orgeval, 1994). 9. Discussion and conclusions Pb\Zn\Ba mineralization at Jebel Hamra is essentially stratabound and occurs as open space fillings, NW–SE trending fractures and brecciated zones and replacements within dolostones of Upper Aptian age. Ore minerals are constituted by sphalerite and galena with minor pyrite, chalcopyrite and barite. Mineralization is accompanied by a hydrothermal alteration of host rocks (dolomitization and silicification). A late, supergene alteration produced a secondary mineral paragenesis with pyrolusite, covellite, cerussite and Fe oxides (Fig. 11). Ore textures indicate that sulfide deposition was related to the dissolution of the dolomite host rock (Fig. 7B and D) evidenced by the presence of the relict of crystal dolomite in sphalerite. Fluids trapped in primary fluid inclusions in sphalerite and saddle dolomite are characterized by high salinities (19 to 25 wt.% NaCl equiv) and moderate homogenization
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Fig. 8. Photomicrographs showing primary aqueous fluid inclusions hosted in sphalerite. Th = temperature of homogenization; Tmi = temperature of melting of ice. The fluid inclusion in black colour are not measured (decrepitated or they reveal necking down).
temperatures (175 °C to 193 °C). Microthermometric data indicate that fluids involved in sphalerite precipitation were polysaline brines (13.5 to 18.5 wt.% NaCl equiv), probably MgCl2 and CaCl2-rich. The deposition of sphalerite took place between 110 °C and 150 °C. Saddle dolomite is assumed to record rapid precipitation from a high temperature and saline fluid (Radke and Mathis, 1980; Searl, 1989) and is taken as a common (although not diagnostic) indicator of hydrothermal Table 2 Sulphur isotopic compositions of Jebel Hamra ore deposits.
Fig. 9. Homogenization temperature versus Tmi (melting temperature) of primary fluid inclusions in dolomite and sphalerite from the Jebel Hamra deposit.
Sample ID
Location
Mineral
δ 34S (CDT) ‰
Sample ID
Mineral
δ 34S (CDT) ‰
HMPb1 HMPb2 HMPb3 HMPb4 HMPb5 HMPb6 HMPb7
Jebel Jebel Jebel Jebel Jebel Jebel Jebel
Galena Galena Galena Galena Galena Galena Galena
1 0.5 −1.8 −2.3 −1.2 −3 1.2
HMZn1 HMZn2 HMZn3 HMZn4 HMZn5 HMZn6 HMZn7
Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite
5.6 5.3 2 6.3 5.1 4.5 2.2
Hamra Hamra Hamra Hamra Hamra Hamra Hamra
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Fig. 10. The histogram of the sulfur isotopic compositions in the Jebel Hamra Pb–Zn–Ba deposit.
Table 3 Lead isotopic compositions of galena at Jebel Hamra ore deposits. Sample
206
Pb/204Pb
Jebel Hamra—Ali Lakhdar Jebel Hamra—Fedj Fekkat Jebel Hamra—Ali Lakhdar Jebel Hamra—Ali Lakhdar Jebel Hamra—Bir Ferza Jebel Hamra—Fedj Fekkat Jebel Hamra—Khanguet Slougui Jebel Hamra—Khanguet Slougui Jebel Hamra—Khanguet Zitoun
18.702 18.704 18.705 18.642 18.807 18.633 18.648 18.743 18.704
207
Pb/204Pb
15.720 15.724 15.723 15.730 15.740 15.734 15.718 15.722 15.740
208
Pb/204Pb
39.013 39.026 39.027 38.902 38.903 38.804 38.905 38.802 39.037
conditions (Davies and Smith, 2006). Calculated salinities (19 to 25 wt.% NaCl equiv) and temperatures (175 °C to 193 °C) of fluids trapped in the saddle dolomite are higher than those calculated for sphalerite. As saddle dolomite precipitated before sphalerite, the hydrothermal system evolved from higher to lower salinity and temperature with time. The wide ranges in both homogenization temperatures and salinity in Jebel Hamra ore deposits (Fig. 9) suggest that mixing of fluids occurred during ore formation at Jebel Hamra. δ 34S values for sphalerite and galena in the deposits, ranging between −3 and +6.3‰, point to an isotopically homogeneous source. Although the δ34S values are not diagnostic of a particular sulfur source, considering the geological environment of the deposits and the
presence of evaporites of Triassic age, these values would be compatible with a H2S source after reduction of evaporite sulfate (δ34S from +12 to +15‰; Claypool et al., 1980; a similar value is given for the Triassic sulphates in north Tunisia by Charef (1986) and Orgeval (1994). Assuming a formation temperature for sulfides between 150 and 200 °C, and considering the presence of abundant organic matter in the deposits, the reduction of sulfate was probably thermochemically driven (TSR). Therefore, sulfide precipitation at Jebel Hamra could have occurred when metal-bearing fluids mixed with H2S-rich solutions produced from reduction of sulfate which acted as “traps” for metal precipitation. The mixing hypothesis would also explain the contrasting temperatures and salinities found in fluids trapped in sphalerite and saddle dolomite. From the data obtained in the present work, we can envisage a conceptual model for the Pb\Zn mineralizations of Jebel Hamra as follows: (i) Saline solutions of unknown origin (seawater, evaporated seawater) were heated during deep circulation through faults and fractures within the sedimentary basement (Mesozoic), leaching metals (Zn, Pb) and Ba from the sedimentary rocks (Figs. 10 and 11); (ii) precipitation of sulfides would take place where the upwelling hydrothermal solutions would mix with H2S-rich fluids trapped in sulfate-rich rocks (such as Triassic evaporites in the North of Jebel Hamra) undergoing TSR. Thus, the presence of H2S-rich sites was probably the key factor in the formation of the Jebel Hamra deposits. Mixing between fluids could also cause the dissolution of carbonates, creating the space
Fig. 11. Schematic N–S cross-section of Tunisia, showing the distribution of ore deposits and potential fluid-flow from the tellian folded belt to the ore host rocks in Upper Miocene time (a) Upper Miocene and Pliocene (molasses, sandstone); (b) Lower Miocene and Oligocene, (sandstone); (c) Tellian and Numidian sheets (Jurassic–Cretaceous–Oligocene; sandstone, shale and limestone); (d) Upper Cretaceous–Paleocene (shale and marl); (e) Jurassic–Lower Cretaceous (sand, shale, limestone); (f) Jurassic–Lower Cretaceous (Platform facies); (g) Triassic salt; (h) Upper Miocene magmatism; (i) fault; (j) Pb–Zn ore deposits (from Bouhlel, 2005).
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Fig. 12. Galena isotope compositions (filled circles) of Jebel Hamra. Growth curves from Zartman and Doe (1981) give the composition of lead reservoirs at 400-Ma intervals (black squares).
(hydrothermal karst) needed to host the ores as suggested by Corbella et al. (2006). This study shows similar features compared to North Pennine Orefield (NPO) mineralization (Bouch et al., 2006) which demonstrates the importance of dolomitization in generating significant proportions of porosity, which host the stratabound mineralization, and also indicates that significant brecciation and dissolution accompanied the mineralization. The geological environments as well as the mineralogical, microthermometrical and isotope data are typical to those reported for MVT deposits. On the other hand, the temperatures and salinities found in Jebel Hamra are similar to other Tunisian carbonatehosted F\Pb\Zn\Ba deposits namely Zaghouan, Tajerouine (Bouhlel, 2005), Jebel Ajered (Bejaoui et al., 2008) and to Pb\Zn deposits related to salt diapirs as Fedj el Adoum (Charef and Sheppard, 1987 and Bouhlel, 2007) and to Pb\Zn deposits related to Triassic salt in nappes
zone as Fedj Hassène (Bejaoui, 2012 and Bejaoui et al., 2011) where microhermometric and isotope studies demonstrated the presence of basinal brines as mineralizing fluids. The late Alpine faulting, together with the reactivation of pre-existing basement faults, enhanced the fracture permeability of the carbonate rocks, thus favoring the possible circulation of the metal-bearing fluids that deposited the primary sulphide ores and providing the pathways for their ascent (Leach et al., 2005).
Acknowledgements We thank the editor and two anonymous reviewers, for their constructive comments, which helped us to improve the manuscript. We also thank Prof. M. Jebrak for reading the manuscript and helpful suggestions.
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Mineralization and fluid inclusion studies of the Aptian carbonate-hosted Pb\Zn\Ba ore deposits at Jebel Hamra, Central Tunisia Jaloul Bejaoui a, b,⁎, Salah Bouhlel b, Esteve Cardellach c, Àngels Canals d, Joaquim Perona c, Àngels Piqué d a
UR-MDTN, Centre National des Sciences et Technologies Nucléaires, Tunisia Université la Manouba, Laboratoire de Biotechnologie et Valorisation des Bio-Géoressources (LR11ES31) and Université de Tunis El Manar, Faculté des Sciences de Tunis, Ariana, Tunisie Departement de Geologia, Universitat Autònoma de Barcelona, Spain d Departament de Cristallografia, Mineralogia i Dipòsits Minerals, Universitat de Barcelona, c/ Martí i Franquès s/n, 08028 Barcelona, Spain b c
a r t i c l e
i n f o
Article history: Received 31 October 2011 Accepted 2 February 2013 Available online 10 February 2013 Keywords: Jebel Hamra Upper Aptian Fluid inclusion Thermochemical Basinal brines Tunisia
a b s t r a c t The Jebel Hamra, which is hosted in the Graben Zone of central Tunisia, is an epigenetic dolomite-hosted Pb\Zn\Ba deposit that replaces platform carbonates of Lower Cretaceous age (Upper Aptian). Mineralization occurs as stratiform lenses, and breccia- and paleokarst-fillings. Ore reserves are around 0.5 Mt and grades are 7% Zn and 3% Pb. Sulphide mineralization occurs as local replacement of dolomitized and silicified carbonate strata. Primary inclusion fluids in sphalerite have homogenization temperatures ranging from 110 to 152 °C with a mode at 137 °C. Final ice-melting temperatures range from −15 to −10 °C, corresponding to salinities of 13.5–18.5 wt.% NaCl equiv. Fluid inclusions in saddle dolomite homogenize to the liquid phase between 175°C and 193 °C, with a mode at 187 °C. Final ice-melting temperatures range from −23 to −15 °C corresponding to salinities between 18 and 24 wt.% NaCl equiv. δ34S values of sphalerite and galena vary between −3 and +6.3‰, compatible with a thermochemical reduction of sulfate from Triassic evaporites (δ34S=+15‰). Pb\Zn\Ba mineralization is spatially and possibly genetically related to E–W-trending fractures, which acted as conduits and/or depositional sites for circulating hydrothermal fluids. Fluid inclusion and sulfur isotope data suggest the contribution of basinal brines in ore formation. Combined field, sedimentological, mineralogical, isotopic and geochemical data of the Lower Cretaceous mineralization in Jebel Hamra is compatible with Mississippi-Valleytype mineralization. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Tunisia has been historically an important producer of fluorite, Zn, Pb and barite from MVT deposits, such as Hammam Zriba fluorite deposits (Bouhlel et al., 1988), Pb\Zn of Fedj Lahdoum deposit (Charef and Sheppard, 1987), Zn\Pb of Bou Grine mine (Orgeval, 1994), and Zn of Fedj Hassène (Bejaoui et al., 2011). In the Jebel Hamra area, Central Tunisia (Fig. 1), there are several potentially important Pb\Zn\Ba deposits. They are related to NW–SE and E–W trending fault systems and occur as veinlets, cavity fillings, disseminations and replacements of host rocks. Hydrothermal alterations correspond to dolomitization and silicification of Upper Aptian limestone. Ore minerals are galena and sphalerite, with minor chalcopyrite and barite. The Jebel Hamra Pb\Zn deposit was early studied by Fuchs (1973) and Amouri (1981). These authors associated the Jebel Hamra deposit to the aptian paleosurface of the central Tunisian platform. They considered that the mineralization is of syngenetic type and the age of formation is the end of Aptian. However, some aspects of its genesis, such as ⁎ Corresponding author at: UR-MDTN, Centre National des Sciences et Technologies Nucléaires, Tunisia. E-mail address: [email protected] (J. Bejaoui).
the precipitation mechanisms, the origin of the salinity or the relationship with dolomitization processes in Jebel Hamra ore deposit, are still poorly understood. In Tunisia only a few studies have been focused on mineralization associated to karst structures (Bejaoui and Bouhlel, 2007; Routhier, 1980; Rouvier et al., 1985). The aim of this paper is to present the mineralogical, textural and geochemical data (fluid inclusions and sulfur isotopes data) of the Jebel Hamra deposits as an example of karst-related mineralization, in order to determine the origin and evolution of the mineralizing fluids. Finally, the data are compared to other important Tunisian Pb\Zn MVT deposits. 2. Geological setting The Jebel Hamra area is located in Central Tunisia, 50 km NW from the town of Kasserine (Fig. 1). It hosts several Pb\Zn\Ba deposits and occurrences around Bir Ferza, Khanguet Zitoun, Ali Lakhdar and Khanguet Slougui localities (Fig. 2). The geology of the area is dominated by extensively folded massive limestones and dolostones interlayered with marl units of Lower Cretaceous age. Following NW–SE trending faults, graben structures, hosting Pb\Zn\Ba deposits developed during Miocene, Pliocene and Quaternary (Chihi, 1995). The main outcrops are
0375-6742/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gexplo.2013.02.001
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Fig. 1. Location of the Jebel Hamra deposit in Tunisian geological map (1) Quaternary: alluvial deposits, clay sands and conglomerates. (2) Upper Miocene: interlayered marls and sandstone. (3) Upper Senonian: limestones with interlayered clays. (4) Lower Senonian: marls with limestones of the Kef formation. (5) Turonian: limestones of the Bireno formation. (6) Upper Cenomanian to Turonian: marls, and marly-limestones. (7) Albian to Cenomanian: marls and limestones of the Fahdene formation. (8) Aptian: reef limestones and dolomites of the Serdj formation. (9) Triassic: clays, dolomites and evaporites. (10) Faults. (11) Mineralization. (12) Border Faults.
Upper Aptian limestones and dolostones of the Serdj Formation, bordered by Albian marls of the Fahdene Formation. The sedimentation during this period was controlled by three active E–W-trending normal faults that divided the basin into four blocks (Fig. 3). The Jebel Hamra area was affected, from Late Eocene to Upper Miocene, by the NW–SE compression related to the Alpine orogeny (Ben Ayed, 1986; Chihi, 1995). Pb\Zn\Ba deposits are found along an unconformity surface (commonly known as unconformity below the Albian), developed within dolostones of Upper Aptian age, on the four blocks that resulted from the brittle deformation of the area of Miocene age. Several morphologies of the mineralizations can be distinguished: cavities, joints, breccia fillings, disseminations and stratabound replacements of the carbonate host rocks (Bejaoui and Bouhlel, 2007; Bouhlel, 1993; Bouhlel et al., 1988 and Routhier, 1980). In the Jebel Hamra area, the sulfide mineralization occurs as a stratabound body in a dolomitic horizon (Fig. 4), which replaces almost entirely the Upper Aptian limestones of the Serdi Formation (Gargasian to Gargasian–Clansayesian age; M'Rabet, 1981). The mineralization consists of coarse-grained sphalerite, galena and minor pyrite and chalcopyrite partially replacing blocks of the host dolostone, as lenticular pockets, or as concretionary infillings of cavities, with a thickness ranging from a few centimeters up to 3 m.
3. Analytical techniques Mineral and host rock samples were collected at surface outcrops and mine drift in the Bir Ferza, Khanguet Zitoun, Ali Lakhdar and Khanguet Slougui areas at Jebel Hamra (Fig. 3). The mineralogical study was carried out using transmitted and reflected light microscopy and cathodoluminescence (CL). CL studies were done with an ELM-3R Luminoscope device, coupled to a polarized light microscope. Some samples were also analyzed in a Stereoscan S-360 scanning electron microscope with an energy-dispersive spectrometer (SEM-EDS) at the Serveis de Microscopia, Universitat Autònoma de Barcelona.
Microthermometric measurements were performed on a Linkam THMSG 600 heating-freezing stage at the Department of Geology of Faculty of Sciences of the University Tunis Manar and at the Department of Geology of the Universitat Autònoma de Barcelona. Temperatures were calibrated from the melting of synthetic H2O\CO2 fluid inclusions (− 56.6 °C), melting of ice and critical homogenization at 220 bar (374.1 °C) of synthetic H2O inclusions in quartz. The precision of measurements below 0 °C was ±0.1 °C and of homogenization temperatures was ±2 °C. Salinity of fluid inclusions, reported in equiv. mass % NaCl equiv, was calculated using the Bodnar's (1993) equation. For the sulfur isotope analyses, sulfide minerals were converted to SO2 gas by combustion with cupric oxide (Fritz et al., 1974). Analyses were performed at the USGS (United States Geologic Survey) in Denver and at the SCT of the University of Barcelona (Spain) using a continuous flow method as described by Giesemann et al. (1994). Results are reported as per mil (‰) deviations relative to the Vienna Canyon Diablo Troilite (VCDT) standard. 4. Alteration Dolomitization and silicification have been identified in most of the tectonic units of the Jebel Hamra, especially in the mineralized areas. The dolomitization and silicification events affecting the Upper Aptian limestones and marls are especially related to an NW–SE trending normal fault that affected the Jebel Hamra area (Fig. 2). The Jebel Hamra area is mostly constituted by a dolomitic limestone containing abundant, well preserved, fragmental fossils including rudists, corals and bryozoans. Dolomitization affects the middle and the upper section of the Serdj Formation (Upper Aptian). Where the entire unit is present, the thickness of the Serdj Formation ranges from 80 m in the northern part of the Jebel Hamra, near the evaporitic Triassic outcrop of Henchir Nechla, to 150 m in the southern part (Figs. 1 and 4). Dolomitization develops near fractures and discontinuities related to bedding (Fig. 5B and E). Dolomite beds have thicknesses varying from
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Fig. 2. Geological map of the Jebel Hamra with the locations of studied sites. Location of the cross-section (Fig. 3) is also shown.
centimeters to a few meters (average 5 m) and a lateral continuity over twenty meters (Fig. 4). Zebra dolomite is typically composed of alternating horizontal layers of dark, fine-grained dolomite (dolomite 1) and white, coarse-grained, sparry, void-filling dolomite (dolomite 2; Fig. 5B and C). A zebra dolomite set consists of several of parallel disc shaped lenses of dolomite (dolomite 1 and dolomite 2). The spacing between bands is of the order of 10 to 20 mm. Two bands may merge into one (Fig. 5C). The boundaries between bands are mostly irregular and show rapid transitions from fine dolomite (dolomite 1) to coarse-crystals dolomite (dolomite 2; Fig. 6B and C). In zebra dolomites the set of equidistant bands may cut across the host rock's bedding, which implies that the regularity of spacing cannot be inherited and must therefore originate by some kind of self-organization, as was pointed out by Merino (1984) and Fontboté (1993).
The dolomitic zebra and breccia bands are displacive bands, that is, bands that forcibly make room for themselves as they grow. These bands do not need a preexisting open space to grow into (Lopez-Horgue et al., 2010 and Merino et al., 2006). The contacts between the white dolomite (dolo2) and dark dolomite (dolomite1) are in detail irregular, suggesting that they represent dissolution voids or fractures enhanced by dissolution, rather than simple fractures (Fig. 6). Fine inclusions of brown to black bitumen occur along fractures and stylolites (Fig. 6A). The transition from replacive dolomite (dolomite1) to white displacive bands means that there was no growth interruption between them; that the displacive dolomite 2 is not filling a prior void (but making room for itself by pushing the host replacive dolomite aside) Merino and Canals (2011). Two generations of dolomite have been distinguished in the zebra texture: 1) dolomite replacing the carbonate matrix (dolomite 1),
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Fig. 3. Cross section of Jebel Hamra. (A) Upper and Middle Albian : marls and limestones (B) and (C) Upper Aptian reef limestones and dolostones. (D) Lower Cretaceous: limestone clays and limestone, with Barremian sands. (E) The Jurassic constituted with limestones and dolostones (F) Triassic; evaporites, clays and dolostones and (G) sulfide mineralization.
consisting of euhedral to subhedral crystals of relatively uniform size (~ 100 μm) with intercrystalline porosity (Fig. 6D); and 2) a white, sparry dolomite (dolomite 2: saddle dolomite) with anhedral to subhedral crystals that have undulose extinction and cloudy centers surrounded by clear crystal growth rims (Figs. 5 and 6). Saddle dolomite has a deep-red cathodoluminescence (CL) color with no internal zoning. The sparry crystals of saddle dolomite form a xenotopic to hypidiopic mosaic (Fig. 6). In outcrops, saddle dolomite facies are white and locally present a pale pink color. It is found in several types of dolomite fabrics: (i) zebras dolomite with dol1 and dol 2 (Fig. 5)
and (ii) as cement in breccias. Saddle dolomite (dolomite 2) precipitation is frequently related to hydrothermal process and mineralization. The dolomitic facies (arranged in centimeter's bands thick of alternating fine and coarse grained) of the upper section of the Serdj Formation is affected by a widespread silicification, which occurs as disseminated on the borders of dolomite (dolomite 2) crystals, fine-grained quartz crystals (2–10 mm; Fig. 6E). Locally, sphalerite and galena crystals are associated with the fine-grained quartz. Quartz crystals are situated at the margin of dolomite rhombic crystals (Fig. 6E and F). Petrographic studies indicate that silicification started after dolomitization and was probably
Fig. 4. Lithostratigraphic columns of mineralized areas. (1) Limestones interlayered with clays. (2) Platform carbonate. (3) Dolomitized and silicified limestones. (4) Clays and limestone.
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Fig. 5. Distribution of dolomitized limestones of the Serdj formation (Upper Aptian). (A) limestones rich orbitolina, of Upper term of Serdj, (B) and (C) subhorizontal vuggy porosity within zebra dolomite. The vug margins are irregular and lined with white sparry dolomite (dol2), (D) coarsely crystalline galena filling large solution vugs, (E) zebra dolomite composed of alternating horizontal layers of dark grained and white, coarse grained sparry, void filling dolomite, and (F) limestone is replaced by dolostone and filling cavities with barite.
associated with dissolution (Figs. 6 and 7A), brecciation as well as with the main mineralizing event (see Table 1). 5. Ore mineralogy and petrography The principal ore mineralogy is composed of sphalerite with minor galena and barite, is enclosed within a dolomitized and silicified limestone. The paragenetic sequence of the Jebel Hamra deposits was established from outcrop observations and microscopic studies. According to the origin, two events of mineral formation have been distinguished: hypogene and supergene (see Table 1). Stage I, corresponding to the primary (hypogene) minerals consists of sphalerite and galena as major sulfides, and pyrite and chalcopyrite as minor
phases. Barite is also present in variable amounts. Dolomite and quartz occur as gangue minerals. Sphalerite is yellow to dark brown-colored, medium- to coarsegrained (0.5 cm to 3 cm), and occurs as disseminations and aggregates of anhedral crystals. Sphalerite is found filling fractures and vugs, or as crystals replacing the dolostone host rock. Galena is commonly present as coarse crystals (1 mm to 10 cm), disseminated and replacing dolostone and/or as vein fillings. It is found associated with pyrite and sphalerite. Pyrite is present as disseminated concretionary nodules of a few mm in size in the dolomite host rock. Chalcopyrite is found locally filling fractures or disseminated in dolostone. It can be altered to covellite and goethite. Barite crystals are euhedral, fibroradiated or coarse-grained (few mm to cm in size). Barite is found as massive, banded, and disseminated aggregates coexisting
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Fig. 6. Photomicrographs of undolomitized, dolomitized and silicified host rocks (Upper Aptian reef limestones). (A) Coarse bioclastic limestone and bitumen within stylolite. (B) Transition from the replacive Dol1to the cement dolomite (Dol2) phases; the size of the dolomite crystals increases towards the cavity (from Dol 1 to Dol 2), (C), (D), (E) and (F) dolomitzed and silicified limestone, the saddle dolomite (dol2) with irregular margin is collapsed by finely crystalline quartz. Abbreviations: Dol = dolomite. Qz = quartz P = porosity Cal = Calcite.
with calcite and is sometimes present replacing the dolostone matrix. In many veins, barite crystals were brecciated and the interstices were filled with later iron oxides and covellite. Stage II is constituted by pyrolusite, cerussite and iron oxides, formed after a supergene alteration. Pyrolusite fills voids in dolomite crystals and coating quartz crystals (Fig. 7E and F). Cerussite occurs within vugs. Iron oxides are common throughout the mining area, resulting in variable concentrations of goethite, hematite as alteration of pyrite, together with pyrolusite filling cavities in dolomite.
6. Fluid inclusion study A microthermometric study was performed on sphalerite and saddle dolomite samples from Ali Lakhdar and Khanguet Slougui mineralized areas (Fig. 3). Only primary fluid inclusions (following the criteria of Roedder, 1984) were selected for microthermometric measurements. Primary fluid inclusions in sphalerite are biphasic (L+ V) at room temperature with sizes between 20 μm and 300 μm and occur grouped or isolated, showing smooth to irregular shapes (Fig. 8). They are liquid-rich, with a small vapor bubble filling 5% to 10% of the inclusion volume. Microthermometric measurements of sphalerite-hosted fluid inclusions are illustrated in Fig. 9. The eutectic temperatures (Te) range from −49.8 °C to −22.1 °C, indicating the presence of other salts in addition to NaCl, likely divalent cation chlorides (Te (MgCl2) = −35 °C; Dubois and Marignac, 1997; Te (CaCl2) = −52 °C; Oakes et al., 1990). Final ice-melting temperatures (Tmi) range from −15 to −10 °C (n=95), corresponding to salinities from 13.5 to 18.5 wt.% NaCl equiv, with a mean of 15 wt.% NaCl equiv. All primary inclusions homogenized to the liquid phase at temperatures ranging from 110°C to 152 °C (n = 95). The density of the fluids was calculated from the equation of state for aqueous systems (Bodnar, 1993) and it is between 1066 and 1103 g/cm 3.
Saddle dolomite commonly hosts fluid inclusions uniformly distributed within the crystals, having frequently smooth contours. The dimensions of the inclusions observed in dolomite, range between 5 and 40 μm most of them are liquid rich two-phase (L + V). Only the largest inclusions (up to 20 μm), preferentially grouped in the crystal cores or along crystal growth zones, were considered as primary. They are two-phase, liquid-rich, with a small vapor bubble, filling 5 to 15% of the inclusion volume. The first melting was observed at a temperature comprised between − 43 °C and − 27 °C, with a mean of − 35 °C, lower than the NaCl\H2O (eutectic ~− 21.2 °C), indicating the presence of other salts such as MgCl2 and CaCl2 (Bodnar, 1993). Tmi range is between − 23 and − 15 °C (n = 21), which corresponds to salinities between 18 and 24 wt.% NaCl equiv. Fluid inclusions homogenize to the liquid phase between 175°C and 193 °C, with a mode at 187 °C (n = 21) (Fig. 9). 7. Sulfur isotopes Sulfur isotope compositions were determinated on five sphalerite and seven galena samples. δ34S values of sphalerite range from +1.2 to +6.3‰ and those of galena from −3 to +1‰ (Table 2) and (Fig. 10). The δ34S values are not diagnostic of a particular sulfur source, but considering the geological environment of the deposits and the presence of evaporites of Triassic age, these values would be compatible with a H2S source after reduction of evaporite sulfate (δ34S from +12 to +15‰; Claypool et al., 1980; a similar value is given for the Triassic sulphates in north Tunisia by (Charef, 1986 and Orgeval, 1994). 8. Lead isotopes 206 Pb/ 204Pb, 208Pb/ 204Pb, and 207Pb/ 204Pb ratios for nine galena samples from the ore karst fall in the range of 18.7021 to 18.7052, 15.7204 to 15.7246 and 38.0134 to 38.0273 (Table 3) and (Fig. 12). These data plot above the initial isotopic composition of lead from
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Fig. 7. Mineralogy of the sulfide dominant stage and main textural relationships in the Jebel Hamra ore deposits. (A) Dolomite and quartz crystals cemented by massive sphalerite, (B) sphalerite and dolomite association showing irregular contact, (C) microtextures of cogenetic sphalerite and galena, (D) replacement of saddle dolomite with sphalerite, and (E) a backscattered electron image “SEM”of Pyrolusite (MnO2) which is the last manganese mineral to form; infilling void spaces between looks ore appropriate dolomite rhombs and surrounding quartz crystals. (F) Pyrolusite (MnO2) fills small cavities in host rock, constituted with dolomite and quartz. (G) Chalcopyrite crossed by microfracture and filling by goethite and covellite, (H) galena altered into cerusite with remains of quartz, and (I) colloform goethite in dolomite. Abbreviations: Qz = quartz; Dol = dolomite; sp = sphalerite; Gn = galena; Cpy = chalcopyrite; Gth = goethite; Cv = covellite; Pyr = pyrolusite; sm = smithsonite.
Table 1 Paragenetic sequence of ore, gangue and supergene minerals in the Jebel Hamra sulfide deposits.
isotopic evolution of Big Field reservoir from Plumbotectonics (Zartman and Haines, 1988). The lead isotopic values of galena collected from Jebel Hamra are similar to the lead isotope data of a number of ore deposits in Tunisia and show a similarity with Upper Crust values (Bouhlel, 2005; Orgeval, 1994). 9. Discussion and conclusions Pb\Zn\Ba mineralization at Jebel Hamra is essentially stratabound and occurs as open space fillings, NW–SE trending fractures and brecciated zones and replacements within dolostones of Upper Aptian age. Ore minerals are constituted by sphalerite and galena with minor pyrite, chalcopyrite and barite. Mineralization is accompanied by a hydrothermal alteration of host rocks (dolomitization and silicification). A late, supergene alteration produced a secondary mineral paragenesis with pyrolusite, covellite, cerussite and Fe oxides (Fig. 11). Ore textures indicate that sulfide deposition was related to the dissolution of the dolomite host rock (Fig. 7B and D) evidenced by the presence of the relict of crystal dolomite in sphalerite. Fluids trapped in primary fluid inclusions in sphalerite and saddle dolomite are characterized by high salinities (19 to 25 wt.% NaCl equiv) and moderate homogenization
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Fig. 8. Photomicrographs showing primary aqueous fluid inclusions hosted in sphalerite. Th = temperature of homogenization; Tmi = temperature of melting of ice. The fluid inclusion in black colour are not measured (decrepitated or they reveal necking down).
temperatures (175 °C to 193 °C). Microthermometric data indicate that fluids involved in sphalerite precipitation were polysaline brines (13.5 to 18.5 wt.% NaCl equiv), probably MgCl2 and CaCl2-rich. The deposition of sphalerite took place between 110 °C and 150 °C. Saddle dolomite is assumed to record rapid precipitation from a high temperature and saline fluid (Radke and Mathis, 1980; Searl, 1989) and is taken as a common (although not diagnostic) indicator of hydrothermal Table 2 Sulphur isotopic compositions of Jebel Hamra ore deposits.
Fig. 9. Homogenization temperature versus Tmi (melting temperature) of primary fluid inclusions in dolomite and sphalerite from the Jebel Hamra deposit.
Sample ID
Location
Mineral
δ 34S (CDT) ‰
Sample ID
Mineral
δ 34S (CDT) ‰
HMPb1 HMPb2 HMPb3 HMPb4 HMPb5 HMPb6 HMPb7
Jebel Jebel Jebel Jebel Jebel Jebel Jebel
Galena Galena Galena Galena Galena Galena Galena
1 0.5 −1.8 −2.3 −1.2 −3 1.2
HMZn1 HMZn2 HMZn3 HMZn4 HMZn5 HMZn6 HMZn7
Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite
5.6 5.3 2 6.3 5.1 4.5 2.2
Hamra Hamra Hamra Hamra Hamra Hamra Hamra
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Fig. 10. The histogram of the sulfur isotopic compositions in the Jebel Hamra Pb–Zn–Ba deposit.
Table 3 Lead isotopic compositions of galena at Jebel Hamra ore deposits. Sample
206
Pb/204Pb
Jebel Hamra—Ali Lakhdar Jebel Hamra—Fedj Fekkat Jebel Hamra—Ali Lakhdar Jebel Hamra—Ali Lakhdar Jebel Hamra—Bir Ferza Jebel Hamra—Fedj Fekkat Jebel Hamra—Khanguet Slougui Jebel Hamra—Khanguet Slougui Jebel Hamra—Khanguet Zitoun
18.702 18.704 18.705 18.642 18.807 18.633 18.648 18.743 18.704
207
Pb/204Pb
15.720 15.724 15.723 15.730 15.740 15.734 15.718 15.722 15.740
208
Pb/204Pb
39.013 39.026 39.027 38.902 38.903 38.804 38.905 38.802 39.037
conditions (Davies and Smith, 2006). Calculated salinities (19 to 25 wt.% NaCl equiv) and temperatures (175 °C to 193 °C) of fluids trapped in the saddle dolomite are higher than those calculated for sphalerite. As saddle dolomite precipitated before sphalerite, the hydrothermal system evolved from higher to lower salinity and temperature with time. The wide ranges in both homogenization temperatures and salinity in Jebel Hamra ore deposits (Fig. 9) suggest that mixing of fluids occurred during ore formation at Jebel Hamra. δ 34S values for sphalerite and galena in the deposits, ranging between −3 and +6.3‰, point to an isotopically homogeneous source. Although the δ34S values are not diagnostic of a particular sulfur source, considering the geological environment of the deposits and the
presence of evaporites of Triassic age, these values would be compatible with a H2S source after reduction of evaporite sulfate (δ34S from +12 to +15‰; Claypool et al., 1980; a similar value is given for the Triassic sulphates in north Tunisia by Charef (1986) and Orgeval (1994). Assuming a formation temperature for sulfides between 150 and 200 °C, and considering the presence of abundant organic matter in the deposits, the reduction of sulfate was probably thermochemically driven (TSR). Therefore, sulfide precipitation at Jebel Hamra could have occurred when metal-bearing fluids mixed with H2S-rich solutions produced from reduction of sulfate which acted as “traps” for metal precipitation. The mixing hypothesis would also explain the contrasting temperatures and salinities found in fluids trapped in sphalerite and saddle dolomite. From the data obtained in the present work, we can envisage a conceptual model for the Pb\Zn mineralizations of Jebel Hamra as follows: (i) Saline solutions of unknown origin (seawater, evaporated seawater) were heated during deep circulation through faults and fractures within the sedimentary basement (Mesozoic), leaching metals (Zn, Pb) and Ba from the sedimentary rocks (Figs. 10 and 11); (ii) precipitation of sulfides would take place where the upwelling hydrothermal solutions would mix with H2S-rich fluids trapped in sulfate-rich rocks (such as Triassic evaporites in the North of Jebel Hamra) undergoing TSR. Thus, the presence of H2S-rich sites was probably the key factor in the formation of the Jebel Hamra deposits. Mixing between fluids could also cause the dissolution of carbonates, creating the space
Fig. 11. Schematic N–S cross-section of Tunisia, showing the distribution of ore deposits and potential fluid-flow from the tellian folded belt to the ore host rocks in Upper Miocene time (a) Upper Miocene and Pliocene (molasses, sandstone); (b) Lower Miocene and Oligocene, (sandstone); (c) Tellian and Numidian sheets (Jurassic–Cretaceous–Oligocene; sandstone, shale and limestone); (d) Upper Cretaceous–Paleocene (shale and marl); (e) Jurassic–Lower Cretaceous (sand, shale, limestone); (f) Jurassic–Lower Cretaceous (Platform facies); (g) Triassic salt; (h) Upper Miocene magmatism; (i) fault; (j) Pb–Zn ore deposits (from Bouhlel, 2005).
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Fig. 12. Galena isotope compositions (filled circles) of Jebel Hamra. Growth curves from Zartman and Doe (1981) give the composition of lead reservoirs at 400-Ma intervals (black squares).
(hydrothermal karst) needed to host the ores as suggested by Corbella et al. (2006). This study shows similar features compared to North Pennine Orefield (NPO) mineralization (Bouch et al., 2006) which demonstrates the importance of dolomitization in generating significant proportions of porosity, which host the stratabound mineralization, and also indicates that significant brecciation and dissolution accompanied the mineralization. The geological environments as well as the mineralogical, microthermometrical and isotope data are typical to those reported for MVT deposits. On the other hand, the temperatures and salinities found in Jebel Hamra are similar to other Tunisian carbonatehosted F\Pb\Zn\Ba deposits namely Zaghouan, Tajerouine (Bouhlel, 2005), Jebel Ajered (Bejaoui et al., 2008) and to Pb\Zn deposits related to salt diapirs as Fedj el Adoum (Charef and Sheppard, 1987 and Bouhlel, 2007) and to Pb\Zn deposits related to Triassic salt in nappes
zone as Fedj Hassène (Bejaoui, 2012 and Bejaoui et al., 2011) where microhermometric and isotope studies demonstrated the presence of basinal brines as mineralizing fluids. The late Alpine faulting, together with the reactivation of pre-existing basement faults, enhanced the fracture permeability of the carbonate rocks, thus favoring the possible circulation of the metal-bearing fluids that deposited the primary sulphide ores and providing the pathways for their ascent (Leach et al., 2005).
Acknowledgements We thank the editor and two anonymous reviewers, for their constructive comments, which helped us to improve the manuscript. We also thank Prof. M. Jebrak for reading the manuscript and helpful suggestions.
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