ORIGINAL PAPER

Large euhedral quartz crystals in the Triassic dolomitesand evaporites of central Tunisia: implications for silica diagenesisin sulphate-rich and high-Mg environments

Mohsen Henchiri & Riadh Abidi & Nejib Jemmali

Received: 29 November 2013 /Accepted: 20 January 2015# Saudi Society for Geosciences 2015

Abstract The Triassic salt diapirs in central Tunisia showsplendid large multicoloured crystals of quartz. The study ofthese crystals and their host rocks sheds light on their originand, more specifically, the relationships between silicaauthigenesis and its host rocks. Petrographic analyses ofquartz crystals show strongly undulose euhedral tosubeuhedral crystals (generally rhombohedral crystals) withconcentration of many solid inclusions including sulphatesand dolomite. Voids, shelter pores and negative crystals ofgypsum, anhydrite and dolomite rhombs are found near thecentral parts of the crystals. Preserved anhydrite inclusionsand dissolved evaporite are also found in these crystals.XRD analyses of white and black quartz crystals show crys-talline alpha-quartz as the only silica phase present in mostsamples with a crystallinity index of 5. Accessory mineralsinclude small relict patches of unreplaced calcite, gypsum,dolomite and anhydrite that escaped replacement. Fouriertransform infrared (FTIR) spectra reveal the presence of abun-dant sulphate and organic compound included in the silicamasses. The quartz crystals occurring in the allochthonousTriassic salt bodies are typically authigenic owing to theeuhedral shape, the absence of any siliciclastic grains in thehost rocks of quartz crystals, and also, to the absence of anysedimentary, wind or water-induced controls on the crystaldistribution in their hosting rocks. Quartz growth in Triassicsalt diapirs is a complex multistage and possibly continuousmechanism. During the progressive uplift of Triassic evapo-rites and dolomites, the dissolution of evaporites is enhanced

and creates pores for the precipitation of the silica with devel-opment of larger quartz crystals. The colour variability in thequartz is ascribed to different ambient materials or fluids dur-ing crystallization. Remnant organic matter, before being al-tered and oxidised, could likely be included in the silicamasses of the quartz and may serve not only to catalyse theprecipitation and the growth but also to darken the quartz aswell. Silica necessary for the formation of the quartz is mainlyderived from two potential sources: (1) the presence of greenand red clays in the Triassic salts and (2) silica-rich diageneticfluids percolating from adjacent Lower Cretaceous sandyaquifers.

Keywords Quartz . Evaporites . Dolomites . Triassic .

Diapir . Silica diagenesis . Tunisia

Introduction

Authigenic double-terminated quartz has been reported in awide range of host sedimentary rocks and diagenetic settings(Table 1). The most common host rocks for authigenic quartzare likely the sulphate evaporites and limestones. The under-standing of the close relationship between silica and its bear-ing sedimentary rocks gives new insights into the trends ofsilica diagenesis in sulphate-rich and high-magnesium envi-ronments. Much has been written about the description of theoccurrence of euhedral β-quartz in the endogen realm, i.e.high-temperature and high-pressure silica forms (e.g. Flörkeet al. 1981) (β-quartz is stable in the crust up to T >573 °C andtransform into alpha-quartz at the conditions of the earth sur-face); however, few papers (Bontes 1953; Demangeon 1966;Stamatakis 1989; Chafetz and Zhang 1998) provide dataabout the origin of near surface authigenic quartz in

M. Henchiri (*) :N. JemmaliDepartment of Geology, Faculty of Science of Gafsa,Sidi Ahmed Zarrouk, 2112 Gafsa, Tunisiae-mail: mohsen.henchiri@fsg.rnu.tn

R. AbidiDepartment of Geology, Faculty of Science of Tunis,Al Manar II, 1060 Tunis, Tunisia

Arab J GeosciDOI 10.1007/s12517-015-1788-5

sedimentary sulphate evaporites and dolostones. The occur-rence of authigenic quartz has been mentioned in diapiric saltbodies in Tunisia (Perthuisot et al. 1978; Khessibi 1978) andworldwide (Hawkins 1918). However, the origin of the quartzin salt diapirs in central Tunisia has not been studied in detailand it was found that the published papers provide only con-troversial data about salt tectonics and dynamics with no ap-parent unanimity (Boukadi 1985; Zargouni 1986; Ouled-Ghrib and Sliman 1994; Villa et al. 1994; Hlaiem 1998;Hatira et al. 2001; Ghanmi et al. 2001; Hammami et al.2001; Chikhaoui 2002). With the exception of authigenic do-lomites in salt diapirs of central Tunisia (Al-Aasm andAbdallah 2006), there have been, so far, no attempts made todecipher the authigenesis of silica in sulphate -rich and high-magnesium rocks of Triassic salt diapirs, due perhaps to (i) thecomplex patterns of Triassic diapir emplacement and (ii) therelatively localised occurrences of these minerals.

The present study is a contribution to understand the rela-tionship between silica diagenesis and its hosting rocks (evap-orites and dolostones) by discussing the possible origins andsignificances of some well-developed euhedral quartz crystalsoccurring in Triassic salt diapirs in central Tunisia. This studyis carried out with field and lab-based investigations with op-tical, petrographic and mineralogical tools including X-raydiffraction (XRD) and Fourier transform infrared (FTIR).

Geological setting

The studied Triassic sedimentary rocks are located in JebelHamra and Jebel Rheouis in central Tunisia (Fig. 1), in thedomain of chaotic outcrops of the Triassic in form of diapiricbodies injected along conjugated normal faults with complexhalokinetic movements. These movements have controlled

the overall spatiotemporal distribution of salt diapirs in theregion and have also affected the geometry of the surroundingrocks since Jurassic and Cretaceous (Khessibi 1978). TheTriassic rocks, not in their normal stratigraphic position, arethe oldest outcrops in the field area with varicoloured patina.According to Khessibi (1978), the stratigraphic position of allthese rocks is the Norian age. The neighbouring localitiesshow outcropping anticline cores occupied by LowerCretaceous series (Burollet 1956; Burrollet et al. 1983;M’Rabet 1987). The Lower Cretaceous formations aremapped collectively as the BContinental Intercalaire^ (CI),which constitutes a huge sandstone-dominated aquifer in cen-tral and southern Tunisia and North Africa (Edmunds et al.2003).

Samples and analytical methods

Four hundred forty-three samples were selected from theTriassic host sedimentary rocks. The majority of quartz crys-tals collected in the field were cut into small slabs perpendic-ular to their c-axis. The cutting or sawing of larger quartzcrystals (Parrish 1945) was performed following the schemeof Gordon and Parrish (1945). Slabs were polished using abra-sive grit, and the polished surface was examined under a 20-power hand lens. Handpicking of specific small crystals wereoperated using binocular microscope. Hot hydrochloric acidwas used to separate quartz crystals from their dolomitic hostrocks. Thin sections were examined to identify the lithofaciesand the petrographic interpretations. The mineralogical iden-tification of all mineral species were determined using XRD,conducted on powdered samples mounted on a glass slide byusing Cu Kα X-ray radiation at an accelerating voltage of40 kV and electrical current of 20 mA in a Philips

Table 1 Authigenic quartz from Silurian through Pleistocene

Age Location Host rock lithology Reference

Pleistocene Jubayl, Saudi Arabia Dolomite (Mg-bearing carbonates) and evaporites Chafetz and Zhang (1998)

Miocene Greece Evaporites Stamatakis (1989)

Miocene Melbourne, Australia Coal Baker (1957)

Tertiary New Mexico Gypsum Tarr (1929)

Cretaceous Yoredal, England Limestones/dolostones Black (1949)

Triassic France Evaporites and dolomite Bontes (1953)

Triassic France Evaporites and dolomite Demangeon (1966)

Triassic Tunisia-Le Kef Evaporites and dolomite Perthuisot et al. (1978)

Permian Denmark Evaporites Fabricius (1984)

Devonian Liège, Belgium Limestones/dolostones Molenaar and De Jong (1987)

Devonian Liège, Belgium Limestones/dolostones Richter (1972)

Silurian New York, USA Evaporites Friedman and Shulka (1979)

Silurian N. Brunswick, Canada Carbonates Noble and Van Stempvoort (1989)

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PANanalytical diffractometer. Diffractograms were treated byHigh-Score® software for peaks search. Quartz samples werestudied by FTIR spectroscopy to detect the presence of water(OH), organic matter, carbonate and sulphate included in the

quartz. The FTIR spectra were obtained using KBr pellets.The pressed transparent pellets were obtained under vacuumpressure of 200 kg cm−2. The pellets were heated to 100 °Cand re-pressed before the FTIR run to expel adsorbed water

Fig. 1 Map of Tunisia showing the main structural elements and the location of the studied diapirs (modified after Perthuisot (1978))

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and obtain better spectra. The spectra were recorded using aBrucker Vector 22 apparatus. Spectra were treaded usingOpus® software.

Results and interpretations

Description of the Triassic sedimentary rocks

The Triassic sedimentary rocks are represented by variegateddeposits of sulphates (masses of gypsum and anhydrite, celes-tite, barite and carnallite), silicate minerals (phlogopite,phengite, talc, tourmaline, etc.), carbonates and clays of vari-ous kinds (Fig. 2a), minor sulphides and native sulphur, andsodium and potassium chlorides. High porosity and importantbrecciation and collapse structures are common sedimentaryfeatures (Fig. 2b) attributed to the poor lithification of these

Bfluidised diapiric rocks^. The sulphate evaporites appear tobe usually a very interesting combination of calcium sulphatein its anhydrous and hydrous forms. The great numbers offreely occurring quartz crystals were found sporadically em-bedded in the gypsiferous dolomites (Fig. 3a–c).

Description of the quartz crystals

The studied quartz crystals belong to low-temperature forms,i.e. α-quartz, that crystallise below 575 °C. The majority ofthe crystals are dark red, brown or black, and the remainder(not to exceed 5 %) are hyaline or colourless. The size rangesfrom 0.75 to more than 5 cm in length (c-axis) and up to 2 cmin width (b-axis). The majority are double-terminated (bipy-ramidal), and many of them are almost geometrically perfectin shape (Fig. 4). The crystallographic forms are typicallythose of the rhombohedral and trapezohedral classes. Themost common form is that of the unit prism, terminated atboth ends, often with equally developed faces of the positivefirst-order rhombohedron (h\nD) but occasionally with un-equally developed faces of the same form. The majority ofcrystals include rhombohedral prisms with hexagonal pyra-mids on each end.

Double-terminated individual forms

The greater numbers of these quartz crystals in Triassic saltdiapirs are double-terminated, lighter in colour and found ingypsum and beige dolomites (Fig. 4a, b). The dark colour ofquartz crystals, which disappeared during heating tests (up to350 °C), is interpreted to be derived from diffused organicinclusions. The majority of the crystals show highly etchedrough faces with millimetric cavities and etch figures of var-ious geometries. These cavities appear to be related to thedissolution of remnant dolomite and/or anhydrite solid inclu-sions embedded but not totally encased in these crystals.

Interpenetrating crystals

Interpenetration of two unequally developed crystals normalor parallel to their growth axes is very common (Fig. 4c).Other crystals grow out from various positions on larger crys-tals, principally from one of the prismatic faces (Fig. 4d). Theovergrowing crystals take two predominating forms, one, inwhich a single crystal predominates and has grown about andincluded smaller crystals, and the other, in which the crystalsare arranged in radiating clusters, often with one crystal,somewhat larger than the others. Some crystals are so crowdedthat the growing crystals obstructed one another, formingmasses of interlocking quartz crystals. The large crystals withhighly crowded small crystals are usually dark-coloured.

Breccia

Sulfates

a

b

Fig. 2 Triassic sedimentary rocks. a Variegated deposits of mineralsulphates, carbonates and clays of various kinds. b High porosity andimportant brecciation and collapse structures are common sedimentaryfeatures of Triassic sedimentary rocks in the field area

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Crystal clusters

Some crystal clusters were recognised with groupings ofquartz crystals that appear to diverge from a common point.Generally, these crystal clusters are lighter in colour with in-dividual crystals that have readily the same dimensions ofsingle crystals.

Solid inclusions

Small opaque particles of host rock, i.e. sulphates and dolo-mite, embedded but sometimes not totally encased in the crys-tals, were initially recognised in 45 % of analysed samples.

�Fig. 3 a Embedded authigenic quartz crystals in beige gypsiferousdolomites. The crystals appear to be haphazardly distributed withabsence of any alignment or concentration along or parallel to anystratigraphic horizon or lamina. b The quartz crystal is doublyterminated and almost geometrically perfect in shape. Large holes canbe observed on some of the crystal faces. c Large euhedral quartz crystalincluding host rock inclusions (gypsiferous dolomite)

a

b

c

d

Fig. 4 Forms of the quartz in the studied Triassic salt diapirs. aHexagonal rhombohedral symmetry forms with 1, doubly terminatedsimple form with rhombohedral prism and two pyramids (combinationof two rhombohedra, the plus r and the minus z), and 2 and 3 showunequal development of the rhombohedral faces resulted in thedistorted growth of the crystals. b Peculiar quartz crystals with distortedrhombohedral faces resulted from unequal development of the prismaticfaces. The crystals 2 and 3 show flattened shape. c Randomlyinterpenetrated crystals irrespective of the habitual twin laws mentionedin a, with 1, random parallel interpenetration of two unequally developedcrystals, and 2 and 3 show c-axis subnormal interpenetrations. dObviousJapan twin law between two or more unequally developed crystals

a

b

c

Large hole

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Their presence, which is an intrinsic feature of authigenicquartz crystals, indicates a correlation between the colour in-tensity and the growth rates. All around the embedded solidinclusions there is not only growth imperfections but also deepdefects resulting in an incomplete development of the crystal.

Petrography

Under the microscope (Fig. 5), strongly undulose euhedral tosubhedral authigenic quartz crystals (generally rhombohedralcrystal) show the concentration of many microstructures thatcan be easily distinguished and ascribed to gypsum and anhy-drite or dolomite inclusions (Fig. 5a). Voids, shelter pores andnegative crystals of gypsum, anhydrite and dolomite rhombs

are commonly found near the central parts of the crystals(Fig. 5b). Preserved anhydrite inclusions (bright yellow) anddissolved evaporite pseudomorphs (dark blue/black) are alsofound in these crystals. The majority of sectioned crystalsshow a clear last growth band separated from the main centralsilica masse with a sharp break in colour between them. Thesebands are a mixture of silica and host rock remnants, i.e. an-hydrite and dolomite, and may mark the last generation ofgrowth in the pore space. Thin sections of twinned crystalsreveal the abundance of anhydrite bright yellow and darkblue-coloured inclusions in the contact zone between thetwo or more crystals (Fig. 5c, d). This case gives rise to roleplayed by the solid inclusions embedded in the crystals duringtheir growth history.

(1)

(2)

(2)

(1)

a b

c d

Fig. 5 a c-Axis normal crosscutof euhedral quartz crystalshowing (1) a distinctive growthband of silica clearly separatedfrom (2) the main silica masse ofthe crystal. Note the relativescarcity of not only large hostrock solid inclusions, i.e.anhydrite (black arrows) but alsothe large internal cavities (whitearrow). Cross polarised light. bEuhedral quartz crystal with largeinternal cavities and negativecrystals some of which are stillfilled with host rock remnants.The external growth band (1) isalso clear in this thin section. Thelower right corner of the pictureshows a quartz crystal with neartop cut. Cross polarised light. dThe quartz crystal is likelytwinned with two other smallcrystals (white arrows). Note theprevalence of anhydriteinclusions in the direct contactbetween the twinned crystals(black arrows). Cross polarisedlight. d In this photomicrograph,the section is parallel to thegrowth axis of the crystal with anobvious twinning (Japan law)between two unequally developedcrystals. Note also the relativelarge void. Cross polarised light

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Mineralogy

XRD analyses

XRD analyses of white (Fig. 6a) and black (Fig. 6b) quartzcrystals show crystalline alpha-quartz (with its fingerprint3.34 (2θ), 4.25 (2θ), and 1.38 (2θ)) as the only silica phasepresent in most samples. The quartz has a crystallinity index(Murata and Norman 1976) of 5. Accessory minerals includesome small relict patches of unreplaced calcite (3.4 (2θ), 2.29(2θ)), gypsum (7.56 (2θ), 3.06 (2θ)), dolomite (2.89 (2θ),

2.19 (2θ)), and anhydrite (3.50 (2θ), 2.85 (2θ), 2.33 (2θ)) thatescaped replacement (solid inclusions). Dolomite XRD sam-ple (Fig. 6c) reveals the presence of ubiquitous quartz andanhydrite.

FTIR analyses

The stacked spectra (Fig. 7) were recorded in the limited rangeof 3800–400 cm−1. The range of 1700–3800 cm−1 that con-tains the broad bands of waters is not shown in the figurebecause water bands in our case seem to have various origins

(4.2

1)

(3.4

9)(3

.32)

(2.8

4 ) ( 2.4

5)

(2.2

7)(2

.22)

(2.1

2)

(1.9

7 ) ( 1.8

1)

(1.6

7)

(1.5

4 )

(4.2

1)

(3.3

2)

(2.8

4)

(2.4

5)

(2.2

7 )(2

.22 )

(2.1

2)

(1.9

7) (1.8

1 )

(1.6

7 )

(1.5

4 )

(2.4

5)

(2.6

6)(2

.53) (2.1

9 )

(2.0

1 )

(1.8

0 )(1

.78)

(1.5

4)

(3.6

9)

(4.0

2)

(3.3

3)

a

b

c

Fig. 6 a X-ray diffraction (XRD) of white quartz crystals showing theabundance of anhydrite inclusions. b XRD of black quartz crystalsshowing the scarcity of embedded host rock solid inclusions, only faintquantities of anhydrite and barite were detected. The black colouring

materials are not detected with the XRD analyses due perhaps to theiramorphous forms. c XRD of the dolomitic host rock showing thepresence of ubiquitous quartz and anhydrite

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such as H2O in gypsum, OH groups in some clays or evenadsorbed waters, which have no relevance regarding the stud-ied quartz. The spectra are determined by the high content ofquartz in the sample. The main bands at 1077 and 456 cm−1

are often disturbed in other spectra by overlapping in contrastto the small but isolated bands at 776 and 795 cm−1, which areindicators of quartz (Povarennykh 1978). The bands at 1145and 1120 cm−1 are sulphate bands. A carbonate-dominatedspectrum with characteristic bands is at 1427 and 874 cm−1.The bands indicating organic compounds are present (albeitsmall) in 1274 and 1448 cm−1 and a shoulder at 1710 cm−1.

Discussion

Authigenic origin of the quartz

As mentioned above, the interpretation of an authigenic originfor the quartz is based on the interpretative inferences from thesedimentological and petrographic evidences. The absence ofany siliciclastic material in the host rock where the quartz isfound, i.e. dolomite and evaporites, is interpreted as a strongargument of the autochthonous origin for the quartz. In addi-tion, crystals appear haphazardly distributed throughout theevaporitic and dolomitic facies, with absence of any alignmentor concentration along or parallel to any stratigraphic hori-zons. Similarly, the absence of any segregation of the crystalsin the host rock, irrespective of their sizes and their weights,strongly support an in situ origin for these crystals. If thequartz crystals were allochtonous and transported into the siteof deposition, then the c-axis of the majority of the crystalsshould display a parallel orientation and stacked-crystal accu-mulation. The absence of such a situation in the sulphate anddolomitic facies indicates no eventual sedimentary, wind orwater-induced accumulation patterns of these crystals.

Growth model for the quartz

The formation of well-developed euhedral quartz crystals insalt diapirs appears to be related to a combination of a com-plex set of conditions and components to be present at differ-ent stages of the diapiric evolution of salts. During the piercingand the progressive uplift of salts (Fig. 8), the percolation ofsilica-rich, for example orthosilicic acid H4SiO4 (Fournier1973), and relatively thermal fluids, i.e., >60 °C (Bouri et al.2007), from nearby sandy aquifer of the Lower Cretaceousformations (Mammou 1989; Edmunds et al. 2003; OSS2003; Bouri et al. 2007) (Fig. 9) may induced (i) the dilutionof the alkalinity of the evaporite pore fluids and (ii) the pre-cipitation and the stability of silica. Low-grade metamorphismand rock deformation could readily have resulted in an in-crease of Si in the hydrothermal fluids and the growth of thecrystals displacively by propagation of lineages (best seen inFig. 3c), in a system entirely dependent on silica concentra-tions in the ambient fluids and the availability of pore space.Silica precipitation is enhanced by elevation of the overalltemperature of the rocks driven by halokinetic uplift (Seligand Wallick 1966; Crampon 1973; Fabricius 1984). Duringthe progressive uplift and the deformation of Triassic rocks, anew diagenetic setting, with a major shift in pore fluid chem-istry, prevails in the pore space so that the lowered alkalinitycould readily yield the maturity of quartz crystals especiallywhen the pore fluids became progressively oxygenated. Therole of halokinetic movements, by the intrusion alongpreexisting faults in the hydrogeological basins in centralTunisia is implied by Chaibi et al. (2013), Tanfous et al.(2005) and Inoubli et al. (2006).

In this complex mechanism, the main acting factors of sil-ica diagenesis are summarised in (i) the immediate availabilityof abundant silica-rich and oxygenated flushing meteoric wa-ters from aquifer-bearing formations, (ii) the steady pace of

Quartz

Quartz

Sulfate

Sulfate

CarbonateOrganic compounds

Quartz

Quartz

Abs

orba

nce

1145

456

1077

776

797

8741274

14481710

1120

Dark red quartz

Grey quartzBlack quartz

Light brown quartz

Fig. 7 Fourier transform infrared(FTIR) analyses of quartzcrystals. The bands at 1077 and456 cm−1 are indicators of quartz.The bands at 1145 and 1120 cm−1

are sulphate bands. The carbonatebands are at 1427 and 874 cm−1.The bands indicating organiccompounds are present at 1274and 1448 cm−1 and a shoulder at1710 cm−1

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halokinetic uplift of salt diapir and (iii) the subsequent in-crease of the overall temperature of surrounding sedimentaryrocks and overburdens. The remnant organic matter, beforebeing altered and oxidised, could likely be included in thesilica masses of the quartz and may serve to catalyse the pre-cipitation and the growth (Baker 1957).

The relationship between the silica and sulphate-rich and/or high-magnesium sedimentary rocks is not yet well enough

understood and seems always problematic (Arbey 1980;Chafetz and Zhang 1998; Henchiri and Slim-S’Himi 2006).This relationship depends chiefly upon the thermodynamicequilibrium between the alkalinity degree of the pore fluidsduring silica precipitation and the amount of diffusive silica(H4SiO4) supply in the formational site, which may deeplyinfluence the silica stability trends either towards solubilityor polymerization (Dove 1994). In addition, the presence of

STABILITY OF EVAPORITES

SOLUBILITY OF SILICA

HIGHER

ALKALINITY

(pH ≥9)

MODERATE

ALKALINITY

(pH 7-9)

STABILITY OF SILICA

SOLUBILITY OF EVAPORITES

FROM:DEEP BURIED AKALINE AND

ANOXIC CONDITIONS (BRINES)

(BOTTOM OF TRIASSIC DIAPIR)

TO:NEAR SURFACEOXIC

CONDITIONS (METEORIC WATERS)

(TOPO F TRIASSIC DIAPIR)

H4SiO4(ORTHOSILICIC ACID)

A MAJOR SHIFT IN PORE FLUID CHEMISTRY

INCLUDED BY THE DIAPIRIC UPLIFT

EUHEDRAL FORMS OF

SILICA

SILICA PRECIPITATION

IN PORE SPACE

PORE SPACE CREATION

(1) ABUNDANCE OF SILICA-RICH AND

OXYGENATED FLUSHING METEORIC

WATERS

(2) HALOKINESIS AND SALT DIAPIRIC UPLIFT

(3) INCREASE OF OVERALL ROCK

TEMPERATURES

MAIN ACTING FACTORS:

Fig. 8 Conceptual model for the multistage mechanism of quartz growth and silica diagenesis

Fig. 9 Inferred schematic model of quartz precipitation in salt diapirs inthe study areas (see the text for details). In deep buried and reducedenvironment, where brines prevail, silica precipitation is inhibited dueto higher alkalinity (bottom of the Triassic diapir). During theprogressive uplift in near surface conditions and subsequent to the

introduction of abundant meteoric waters, a major shift in pore fluidchemistry occurs in a mixing zone; the aeration prevails in the porefluids so that the oxidation of sulphides and the subsequent loweredalkalinity could readily yield the maturity and the development of largerquartz crystals

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Mg-bearing host rocks (dolomites and high-magnesium cal-cites) is a factor of control of silica growth, since Mg ions areexcellent coagulating agents for silica and have no significanteffects on its solubility (Millot 1960).

Origin of silica, i.e. orthosilicic acid H4SiO4

Sources within the Triassic sedimentary rocks

Contrary to common belief, the presence of green and redclays in the Triassic sedimentary rocks (Kamoun et al.2001), even though they were reworked during uplift, theyare considered as a major source of orthosilicic acid in porefluids for the quartz formation. Clay mineral diagenesis wassuggested by Towe (1962), Heling (1978) and Boles andFranks (1979) as a possible source of silica for quartz forma-tion. Silica can be derived from the transformation of smectiteto hydrous mica or illite according to two possible reactionstoichiometries (Boles and Franks 1979). Also, the recyclingunder hydrothermal conditions of some authigenic silicateminerals and metamorphism-affiliated Bgreenstones^(Bajanik 1969; Crampon 1973; Perthuisot 1978) within thesalt body, such as phlogopite, phengite, green tourmalineand talc, all have been proved to be present and disseminatedin the Triassic sedimentary rocks and some of them morehighly soluble than quartz, could have contributed the silicathat precipitated as euhedral quartz. The origin of these sili-cates is debatable since Upper Cretaceous basaltic outpouringlavas and pyroclastic materials were highlighted in boreholesin eastern Tunisia (Mattoussi-Kort et al. 2007). Biosiliceousprecursors (albeit undetectable with XRD, since they consistof amorphous opal tests or fragments) cannot be discounted inthis scheme. The dissolution of biogenic silica is thought to bea major source of silicifying fluids during diagenesis(Henchiri 2007). Biogenic silica in the Triassic salt diapir,even though it was not investigated in detail, could have con-tributed sufficient enriched fluids to be precipitated as quartz.

Sources outside the Triassic sedimentary rocks

Dissolution of detrital grains of polycrystalline and monocrys-talline quartz and microcline feldspar in the aquifer-bearingfeldspathic sandstones of Lower Cretaceous Sidi Aïch andBoudinar formations is also advocated as a major source at-tributed to the high contents of SiO2 in these groundwaters(Edmunds et al. 2003; du Sahel) 2003; Chalbaoui 2001; Bouriet al. 2007) compared with other aquifers in the world (Drever1988; Thiry et al. 1988; MacFarlane et al. 1994; Thiry andRibet 1999; Gomèz-Grass et al. 2000). According to Holness(1997), the circulation of fluids is strongly enhanced and fa-cilitated by the presence of faults and fracture networks. Thediapiric pulling up of the low-density Triassic sedimentaryrocks through younger sedimentary cover, with the

subsequent elevation of the overall temperatures (Selig andWallick 1966; Crampon 1973), could have originated fold-ings, fractures networks and faults in the surrounding rocksbefore being pierced by salts (Perthuisot 1978).

Conclusion

Detailed sedimentological, petrographic and mineralogicalanalyses of well-developed euhedral quartz occurring inTriassic salt diapirs have led to the following conclusions:

1. The quartz occurring in the allochthonous Triassic saltbodies is typically authigenic owing to the euhedral shape,the absence of any siliciclastic grains in the host rocks ofquartz crystals, and also to the absence of any sedimenta-ry, wind or water-induced controls on the crystal distribu-tion and accumulation patterns in the host sedimentaryrocks. The quartz crystals are randomly distributed.

2. The growth of well-developed euhedral quartz crystals insalt diapirs implies a combination of complex sets of con-ditions via a multistage and possibly continuous processof silica diagenesis. During the progressive diapiric upliftof Triassic salts in near surface conditions, the aeratedconditions prevail in the pore fluids with notable decreasein their alkalinity to values that permitted the dissolutionof evaporites and dolomite, creation of pore space and theprecipitation of silica.

3. Silica could have been derived from two potential silicasources; the presence of Triassic green and red clays, eventhough they are reworked, likely constitutes, via clay dia-genesis, a possible intraformational source of orthosilicicacid (H4SiO2) for the quartz formation. In addition, thepercolation of silica-rich flushing waters deriving fromextraformational Lower Cretaceous siliciclastic thermalaquifer (CI) into the Triassic diapirs is thought to be be-hind the formation of the authigenic quartz.

References

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Arbey F (1980) Formes de la silice et l’identification des évaporites dansles formations silicifiées: bullettin des Centres de Recherches, ex-ploration - production. Elf-Aquitaine 1:309–364

Bajanik S (1969)Observations préliminaires sur les roches vertes du Triasde l’Atlas Tunisien. Ibid 30:5–10

Baker G (1957) Microcrystallin quartz crystals in brown coal, Victoria.Am Mineral 31:22–30

Black WW (1949) An occurrence of authigenic feldspar and quartz inYoredale limestones. Geol Mag 86:129–133

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