Journal of South American Earth Sciences 16 (2003) 167–178

www.elsevier.com/locate/jsames

Dolomitization patterns of the Liassic platform of the Tazekka Pb–Zn

district, Taza, eastern Morocco: petrographic and geochemical study
J. Auajjara,*, J. Boulègueb
a
Ecole Mohammadia d’Ingénieurs Département Génie Minéral—Avenue Ibn Sina BP 765, Agdal 10 000, Rabat, Morocco
b
Laboratoire de Géochimie et Métallogénie Université de Paris VI—Tour 26/16 Etage 5, 4 Place Jussieu 75 252, Paris Cedex 05, France
Received 1 May 2001; accepted 1 October 2001

Abstract
Dolomitization is the most significant diagenetic process within the Liassic carbonates of the Tazekka district. It is more extensive in the
Middle Atlas Causse than in the Middle Atlas domain.
In the Middle Atlas Causse, the dolomites may be syndepositional, early diagenetic, epigenetic (with a paleogeographical control), or
hydrothermal. Three hydrothermal dolomites are associated with the ore deposits. Two early phases, type 1 and 2, are separated in time from
a phase of saddle dolomite. Saddle dolomite represents a late stage diagenesis post-dating Pb– Zn mineralization. Early dolomites and saddle
dolomite were precipitated from similar fluids during three distinct phases but formed by two mechanisms, by a continuum of replacement
during Carixian (early hydrothermal type 1 dolomite) and the Toarcian (early hydrothermal type 2 dolomite) and by cement precipitation
(saddle dolomite). Precipitation of saddle dolomite cement followed a phase of fracture generation.
In the Middle Atlas the sequential character of changes in dolomite characteristics in the middle formation of the Lower Lias confirms the
synsedimentary nature of early diagenesis and seawater is regarded as the source of the Mg. Euhedral dolomite crystals indicate a minor late
diagenesis.
The d13C values of the dolomites range from 2 0.83‰ to þ2.37‰ PDB. There is a general decrease in d18O values and a corresponding
increase in diagenetic intensity from the laminated dolomite (2 2.43‰) to the dolomitized calcarenite at the base of reduced series of the
Middle Lias (2 9.77‰).
There is a decrease in both d18O and d13C values in the ore deposits passing from sterile to mineralized rocks. Mineralized limestones show
d18O values of around 2 7.66‰. The dolomites of Bou Khalifa average 27.20‰, those of Aı̈n Hallouf 27.96‰, Asdi Ben Zerhla 2 7.86‰
and Aı̈n Tarselt 28.21‰ have similar average values, but those of Sidi Abdellah have values of around 29.79‰. d18O values of the
hydrothermal dolomites differentiate two groups: the first contains the stratiform ore deposits and the second the orebody of Sidi Abdellah.
Saddle dolomites have an average d18O values of 29.39‰ PDB. These phases possess differing isotopic signatures and apparently formed at
different temperatures.
Several different waters were involved in the formation of the dolomites of the Tazekka district and that contrasting processes were
involved in these episodes of dolomitization.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: Dolomitization; Diagenesis; Lias; Tazekka Pb –Zn district; Morocco

1. Introduction waters, and mixed marine and meteoric waters (Land, 1980;
Hardie, 1987; Moore, 1989; Purser et al., 1994; Morrow,
Dolomites are believed to form in a variety of different 1998). Models used to explain dolomitization range from
settings, from the surface shortly after deposition, to the those driven by surficial processes such as evaporative
subsurface burial regime. A wide variety of natural waters pumping (Hsu and Siegenthaler, 1969), seepage reflux
have been suggested as involved in the dolomitization (Adams and Rhodes, 1960; Nielsen et al., 1994), self
process, including evaporite related waters, normal marine drainage, or mixing zone processes (Hanshaw et al., 1971;
Badiozamani, 1973), to burial effects brought about
* Corresponding author. Tel.: þ 212-3768-7150; fax: þ 212-3777-8853. compaction (Woo and Moore, 1996; Drivet and Mountjoy,
E-mail address: auajjar@emi.ac.ma (J. Auajjar). 1997; Steinhauff et al., 1999), geothermal circulation or
0895-9811/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0895-9811(03)00051-8
168 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178

rising hydrothermal fluids (Braithwaite, 1991; White and The Paleozoic basement, which outcrops in the Tazekka
Al-Aasml, 1997; Boni et al., 2000). massif, consists of Lower Ordovician epimetamorphic
In this study petrographical and geochemical features schists (Rauscher et al., 1982), an Upper Visean-Namurian
have been used to investigate the origin and diagenetic volcano-sedimentary complex (Chalot-Prat and Vachard,
history of dolomites in the Liassic carbonate platform of 1989), and magmatic rocks (granite and microdiorite)
the Tazekka Pb-Zn district where we have a many surrounded by contact metamorphic aureoles.
different dolomitization processes operating in such small The petrographical and geochemical studies of volcanic
area. rocks of the volcano-sedimentary complex show the space –
time coexistence of basic magmas of different affinities
(calco-alkaline, ‘intermediate’ alkaline) with basic magma-
2. Geologic setting mixing products, and rhyolitic and ignimbritic magma. The
existence of numerous ‘acid –basic’ magma mixing rocks
The Tazekka district lies in northeastern Morocco, and various out-pouring distributions within volcanic pile
southwest of Taza, at the northern end of the Middle Atlas are ascribed to eruptions triggered by the injection of basic
chain. It extends across two structural domains, the Middle magma into acid magma (Chalot-Prat and Cabanis, 1989).
Atlas Causse to the northwest and the Middle Atlas to the The cover comprises Triassic and Liassic rocks (Fig. 2).
southeast (Fig. 1). The Trias, resting unconformably on the Paleozoic

Fig. 1. Localization of the Pb –Zn Fe ore deposits in the Tazekka District. 1, Paleozoic basement; 2, Middle Atlas; 3, Middle Atlas Causse; 4, Detroit sud-rifain;
5, Pb– Zn vein in the basement; 6, Veins and Stockwork of quartz and barite; 7, Stratiform Pb ore deposit; 8, Pb ore deposit; 9, Zn ore deposit; 10, Fe ore
deposit; 11, Fe vein; A.B.Z., Asde Ben Zerhla, A.KH., Aı̈n Khebbab; A.T., Aı̈n Tarselt; K.L., Koudiat Lakhâa; S.A., Sidi Abdellah.
 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178 169

Fig. 2. Lithostratigraphy of the Lower and Middle Lias in Middle Atlas (A) and the Middle Atlas Causse (B).

basement, includes two formations of red argillites separ- a continental domain in the northwest. Distension is
ated by a volcanic formation. The Lower Lias includes three associated with sub-aqueous doleritic flows. During the
formations, a lower unit of fine-grained dark massive Lower Lias, deposition of lower and middle formations
dolomites, a middle unit consisting of repeated sequences (Hettangian-Sinemurian) occurs under a low bathymetry. A
of dolomitic breccia, laminated dolomites and crystalline restricted marine and evaporitic environment are marked by
dolomites (from bottom to top), and an upper unit made up early dolomitization of calcareous mud. With the deposition
of a lower pale-grey fine-grained limestone and an upper of the Lotharingian, the marine character strengthens; the
dark-grey limestone and intervals of oolitic and bioclastic higher frequency of oolitic and bioclastic limestones at
limestone. The Middle Lias is a series of interbedded the top of the unit indicate a coastal environment. Since the
limestones and marls, with the marls more numerous at the Carixian, a renewed subsidence immerses a part of the
top of the series (Robillard, 1981; Salomé, 1984; Auajjar, sector in an extern platform domain, until the Aalenian-
1987). Lower Bajocian. Subsidence fluctuations are indicated by
The structural framework of the Paleozoic formations the development of argillaceous sediments. Two particular
resulted from two tectonic-metamorphic phases and associ- layers, a siliciclastic interval at the summit of Domerian and
ated fracturing. The first phase is considered to be intra- a Fe- and Mn-rich level of black marls at the summit of
Visean, whereas the second phase and the Late Hercynian Lower Toarcian, attest of an epirogeny in the external
fractures are attributed to the post-Westphalian (Hoepffner, platform domain which affects sedimentation of the internal
1978, 1987). part of the basin.
The Tazekka district corresponds to the northern edge of The differentiation of the district into two paleogeo-
the Middle Atlas basin which comprises rhombohedral graphic domains began in the Middle Lias (Robillard, 1981;
depocenters filled with predominantly carbonate rocks of Auajjar, 1987).
Middle Jurassic age and bounding ridges forming synsedi-
mentary anticlines (Fedan et al., 1989). The Mesozoic cover
of the Tazekka district is diagonally truncated by a series of 3. Methods
NNE –SSW to NE – SW reverse faults, associated with some
frontal overthrusting (Colo, 1961; Robillard, 1981). The About 150 samples for petrographic examination and
Middle Atlas Causse was affected by a major tectonic geochemical analysis were collected from mineralized
episode during the Middle Lias (Robillard, 1981; Auajjar, limestone and all dolomitic facies recognized within the
1987; Auajjar and Macquar, 1992). Pb – Zn Tazekka district. All samples were examined using
During the Mesozoic, the paleogeographic evolution of standard transmitted light microscopy. Thin sections were
the district is as follows (Auajjar, 1987). During the stained with alizarin-red-S and potassium ferricyanide
Triassic, the district is a sabkha-playa domain, surrounded mixture following the method of Dickson (1966). Cath-
by evaporites and red argillites to the southwest and odoluminescence microscopy (CL) was carried out for 30
170 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178

thin sections using a Technosyn cold cathod luminoscope at circular oncolites in which the cores may be recrystallized
15 –20 kV and gun currents of 400 –500 mA. calcite or saddle dolomite. The original limestone was an
Microsamples were analyzed for carbon and oxygen oopelmicrite with rare sponge spicules. Gypsum pseudo-
isotopes at the Laboratory of Stable Isotopes at the Paris VII morphs and pyrite crystals are also present, together with a
University. Powdered carbonate samples ranging from 0.2 few mud cracks.
to 0.5 mg were drilled from polished slabs using drill bits (2) The layered dolomite is finely to medium crystalline
100 mm in diameter. These samples were roasted at 380 8C (50 –100 mm), mostly nonplanar, unimodal dolomite with a
under vacuum for 1 h to remove organic matter. Samples xenotopic texture (Fig. 3A). It may show inclined or cross-
were then reacted with anhydrous phosphoric acid at 55 8C bedding, but at the base traces of laminites and stromatolitic
for about 10 min for calcite and about 1 h for dolomite. This structures are seen. At Sidi Abdellah, these dolomites show
was done in an online gas extraction system connected to an alternating levels of dolomite crystals of two different sizes.
inlet of a VG602E ratio mass spectrometer. Results are (3) The white vuggy Lotharingian dolomite is massive,
expressed in standard d notation as per mil relative to the coarse-grained (medium-grained crystals), and cross-strati-
PDB standard. Routine corrections for 17O was applied. fied. Medium-grained crystals pervasively replace the
Overall reproducibility ð1sÞ of results for both d18O and oolitic host leaving relics of pre-existing allochems.
d13C is better than 0.1‰. Crystals are planar-e to planar-s; grey-brown to grey, and
range in size from 250 to 400 mm. Nonplanar crystals
exhibit sweeping extinction whereas planar crystals show
4. Petrography normal extinction (Fig. 3B). There is significant (5 –15%)
intercrystalline porosity.
Dolomitization is the most significant diagenetic process (4) The dolomitized calcarenites at the base of a reduced
within the Liassic carbonates of the Tazekka district. Middle Lias are dark and coarsely crystalline (150 –
Petrographical and field characteristics allow several 250 mm) with a xenotopic texture. There is significant
different types of dolomite to be recognized in the Liassic, (10%) intercrystalline porosity. The distribution of this
defining two domains. dolomite seems to be linked with that of paleorelief and/or
small NE –SW horsts resulting from tectonism of the
4.1. The Middle Atlas Causse Middle Lias (Auajjar and Macquar, 1992).
At Asdi Ben Zerhla, an epigenetic dolomite front
The dolomites of the Middle Atlas Causse include a abruptly cross-cuts the Middle Lias limestones but does
number of different lithologies (Fig. 2). not affect the top of the Domerian.
(1) The fine-grained dolomite of the Lower Lias is a grey (5) The early hydrothermal dolomite type 1 hosting the
to black dolomicrite. It contains irregular, elongated and mineralization of the Bou Khalifa stratiform deposit is

Fig. 3. (A) Photomicrograph of thin section of the layered dolomite showing that dolomite is finely to medium crystalline (50–100 mm) mostly nonplanar,
unimodal dolomite with a xenotopic texture (scale bar is 250 mm). (B) Photomicrograph of thin section of the white vuggy of the Lotharingian dolomite
showing that nonplanar crystals exhibit sweeping extinction wheareas planar crystals show normal extinction (scale bar is 250 mm).
 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178 171

sphalerite (of 2 –3 cm) and of galena are associated with

chalcopyrite and pyrite. Together these commonly form
massive aggregates in both early and saddle dolomites. In
the selvage of the orebody the Domerian limestones are
dolomitized, but sterile.
In thin section, dark organic matter envelops beige
rhombohedral dolomite crystals 250 mm in diameter
(Fig. 5B). These are inclusion-filled, and sometimes zoned
(with dark cores) and are associated with sprays of barite
replaced by dolosparite, and 100 –500 mm euhedral crystals
of quartz with inclusions of carbonates.
Cathodoluminescence analysis reveals that the rhombo-
Fig. 4. Divisions of the dolomite at Bou Khalifa ore deposit. 1, crystalline hedral dolomite crystals are generally zoned, with dull red
dolomite of the Lower Lias; 2 and 3, hydrothermal dolomite; 4, Lower and
cores and brightly luminescent red rims (Fig. 5B).
Middle Toarcian; 5, unconformity at the top of the Lower Lias; 6,
unconformity at the top of the Lower Carixian. (7) The coarsely crystalline saddle dolomite is
associated with Pb– Zn sulfides in the ore deposits. It
occurs filling dissolution breccias, in faults and veins,
grey, coarsely crystalline, and rich in brachiopods forming cement in fractures and lining vugs and geodes
(terebratulids and rhynchonellids). It reflects replacive of many centimetres diameter. In some geodes it is
dolomitization of a basal Carixian limestone lumachelle. It overlain by bitumen.
overlies the dolomites of the Lower Lias formation and Centimeter-sized crystals have the characteristic sweep-
fills small karstic pockets above the unconformity (Fig. 4). ing extinction of saddle dolomite and are generally zoned,
with alternating of limpid and cloudy bands (Fig. 5C). They
Bedding within the dolomites may be regular or contorted,
overlie sulfides and cement a fractured crystals of galena
showing load casts, pinching and compaction features.
and sphalerite. They fill most of any remaining stylolites,
Large elongated elements of the underlying crystalline
vugs and breccias, indicating that they formed later post-
dolomites of the Lower Lias stand vertically in the
dating the Pb– Zn mineralization phase. Euhedral dolomite
sediment, and are interpreted as fragments detached from
crystals also occur within crystals of galena, cross-cutting
the walls, suggesting deposition in fractures and cavities
the cleavage.
due to surficial dissolution. Deflection of the bedding
Saddle dolomite fills voids and fractures cross-cutting the
around large crystals of sphalerite indicates that mineral-
early hydrothermal dolomite hosting the mineralization
ization predates compaction (Fig. 5A). Areas containing
(Auajjar, 1994), indicating a time gap.
abundant brachiopods include geodes filled by saddle Under Cathodoluminescence saddle dolomite crystals
dolomite. The cauliflower-like shapes of these suggest that are generally zoned, with dull red cores and brightly
they reflect replacement of Ca or Ba sulfate nodules. luminescent red rims (Fig. 5D). Microprobe analyses reveal
Some silicified fossils indicate a more littoral setting than a general homogeneity of trace elements, although the cores
that of typical Carixian facies. They reflect inter- to of crystals are relatively rich in Mg.
supratidal deposits filling irregularities of the emerging Saddle dolomites reflect late stage hydrothermal diagen-
surface of the Lower Lias, where crystalline dolomite esis. They post-date Pb– Zn mineralization (Auajjar, 1994).
forms early karst. Dolomite of this kind is common in Mississippi-Valley-type
In thin section, the early hydrothermal dolomite is a Pb – Zn mineralization and usually forms later than the main
xenotopic dolospar mosaic containing areas of micro- mineralization (Leach and Sangster, 1993; Braithwaite and
dolospar, residual zones, and micro-geodes lined with Rizzi, 1997).
zoned euhedral dolomite crystals with dark cores. Pyrobitu-
men occludes intercrystalline porosity of the coarse crystal- 4.2. The Middle Atlas
line dolomite.
(6) Another lithology is the early hydrothermal The dolomitization in the Middle Atlas results in several
dolomite type 2 hosting the mineralization of the Sidi different lithologies (Fig. 2):
Abdellah ore deposit. In the Sidi Abdellah open space
filling orebody, mineralization is developed in Domerian 1. The fine-grained dolomite of the Lower Lias is grey to
limestones, along a N70-70SSE fracture with low vertical black and micritic. It contains irregular, elongated and
throw. The ore body ranges in thickness from 1.2 m to a circular oncolites in which the cores may be recrys-
few cm at the bottom of the gallery. It is formed by banks tallized calcite or saddle dolomite. The original
of folded, mineralized Domerian limestones, replaced by limestone was an oopelmicrite with rare sponge
black coarsely crystalline dolomite, and crossed by veins spicules. Gypsum pseudomorphs, pyrite crystals and
of saddle dolomite up to 15 cm thick. Crystals of mud cracks are also present locally.
172 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178

Fig. 5. (A) Deflection of the bedding around large crystals of sphalerite, at Bou Khalifa ore deposit. (B) Cathodoluminescence of the early hydrothermal
dolomite type 2 hosting the Sidi Abdellah orebody showing that dark organic matter envelops beige rhombohedral dolomite crystals 250 mm in diameter, and
the zoned rhombohedral dolomite crystals with dull red cores and brightly luminescent red rims (scale bar is 250 mm). (C) Photomicrograph of thin section
showing that the saddle dolomite with centimeter-sized crystals have the characteristic sweeping extinction and are generally zoned, with alternating of limpid
and cloudy bands (scale bar is 500 mm). (D) Cathodoluminescence showing zoned saddle dolomite crystals with dull red cores and brightly luminescent red
rims (scale bar is 500 mm). (E) Photomicrograph of thin section of laminated dolomite showing regular alternations of microsparitic and micritic layers of
variable thickness (scale bar is 500 mm). (F) Photomicrograph of thin section of Idiotopic dolomite of the Lotharingian limestones showing that euhedral
dolomite crystals truncate the ooids (scale bar is 500 mm).
 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178 173

2. The dolomitic sequence is formed by the repetition of 3. The idiotopic dolomite of the Lotharingian limestone.
25 –50 cm units, comprising an intraformational brec- Idiotopic crystals are extensively developed in
cia at the base, laminated dolomites in the middle, and mineralized oobioclastic limestone, cross-cutting the
crystalline dolomites at the top. ooids (Fig. 5F). Microprobe analysis reveals that the
2.1. The intraformational breccia has a dolomicrite cores of these are dolomitic while the rims are
matrix containing beige millimetre to centimetre magnesian calcite.
microspar intraclasts, together with pellets, grape-
stones, and oncolites. Birds-eye fenestrae in the
matrix contain geopetal fillings, with a dolomi- 5. Results of d18O and d13C isotopes
crite base (sometimes coarsening to microspar)
and dolospar top. Table 1 summarizes the results of isotopic analyses of
2.2. The laminated dolomite shows regular alterna- sterile and mineralized carbonates from the Liassic platform
tions of dark and clear layers of variable thickness of the Tazekka Pb –Zn district. The d13C values of the
(Fig. 5E). The clear layers are microspar while the dolomites range from 2 0.83 to þ 2.37‰. These are similar
dark layers are micritic. The dark layers contain to marine carbonate values.
pellets, and sponge spicules, some foraminifera.
They also include bipyramidal quartz crystals. 5.1. Sterile facies
Mud cracks and birds-eye fenestrae in the micritic
laminae contain geopetal fillings. The contacts There is a general decrease in d18O values and a
between the dark and clear layers may be marked by corresponding increase in diagenetic intensity from the
planar birds-eyes (Shinn 1968). The laminae may be laminated dolomite (2 2.43‰) to the dolomitized calcar-
gently undulating and are sometimes fragmented by enite at the base of reduced series of the Middle Lias
desiccation and apparently reworked. They are thought (2 9.77‰) (Table 2 and Fig. 6).
to reflect deposition in a supratidal environment. Late diagenetic dolomites have depleted d18O values.
2.3. The crystalline dolomite has a xenotopic texture There are no significant differences between the d18O values
with crystals about 150 mm. There is little of the late diagenetic dolomites (2 9.77‰) and those of the
porosity and no primary structures are preserved. saddle dolomites (2 9.39‰) of the Tazekka district or other

Table 1
d18O and d13C isotopic data of the early hydrothermal dolomite, saddle dolomite (SD), mineralized limestone of the Lower Lias (ML), and sterile facies
(Domerian black limestone (DO) and Carixian black limestone (CA)) of the Liassic platform of the Tazekka district

Ore deposit Early hydrothermal dolomite and SD Sterile facies

18 18 13
d O Mean of d O d C d18O d13C

Sidi Abdellah 29.61 29.79 þ 1.28 24.60 (DO) þ1.73 (DO)

29.98 þ 0.83 24.80 (DO) þ1.66 (DO)
25.15 (CA) þ1.13 (CA)
212.58 (SD) 2 0.83 (SD) 23.77 (CA) þ1.70 (CA)
Bou Khalifa 27.69 þ 0.30
27.90 27.26 þ 1.83
26.21 þ 1.49
27.88 (SD) þ 1.27 (SD)
Aı̈n Hallouf 27.56 þ 0.12
27.64 þ 0.29
28.71 27.76 þ 0.52
27.48 þ 0.28
27.39 þ 0.44
29.97 (SD) þ 1.16 (SD)
Asdi Ben Zerhla 28.48 27.86 þ 1.59 23.02 (CA) þ1.84 (CA)
27.24 þ 0.72 25.14 (DO) þ1.95 (DO)
27.57 (SD) þ 2.37 (SD)
Aı̈n Tarselt 28.21 28.21 þ 0.48
28.93 (SD) 20.16 (SD)
Col Bab Mtik 27.66 (ML) 20.75 (ML) 25.85 (DO) þ1.29 (DO)
174 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178

Table 2 Thus, although the hydrothermal dolomites have a similar

d18O and d13C isotopic data of dolomitic facies of the Liassic platform of petrography they do not have the same d18O values.
the Tazekka district
Saddle dolomites are more depleted d18O values than
Dolomitic facies d18O d13C Origin the early hydrothermal dolomites. Values vary from 2 7.57
to 2 12.58‰ with an average of 2 9.39‰, and are similar to
Dolomitized calcarenite 29.29 þ 2.54 those reported for saddle dolomite from other areas (2 9 to
at the base of reduced 29.60 þ 1.57 Late diagenesis 2 11‰, (Mattes and Mountjoy, 1980; Taylor and Sibley,
series of the Middle Lias 29.95 þ 1.70
1986; Mountjoy and Qing, 1992; Nielsen et al., 1994;
210.24 þ 1.92
Braithwaite and Rizzi, 1997; Boni et al., 2000)).
Layered dolomite 26.60 þ 0.53 Early diagenesis
27.39 þ 0.44
Laminated dolomite 22.43 þ 2.00 Syndepositional 6. Discussions
diagenesis
6.1. The genesis of the dolomites
late diagenetic cements (Mattes and Mountjoy, 1980;
Zenger, 1983; Woronick and Land, 1985; Lee and Fried- The variations in the petrographic features described
man, 1987; White and Al-Aasm, 1997). reflect differences in the conditions of dolomitization.

6.1.1. The Middle Atlas Causse

5.2. Mineralized facies
The fine-grained dolomite at the base of Lower Lias is a
penecontemporaneous dolomite deposited in an inter- to
In the ore deposits of Sidi Abdellah and Asdi Ben Zerhla, supratidal environment within a confined internal platform
where Total Organic Carbon (TOC) values are high with an evaporitic tendency.
(Auajjar, 1994), d13C values seem to be independent. The layered dolomite is also formed early in diagenesis
There is a decrease in both d18O and d13C values in the ore by recrystallization linked to a short period of exposure in
deposits passing from sterile to mineralized rocks (Table 1). the supratidal environment of the internal platform.
Mineralized limestones show d18O values of around Except for these two lithologies, all of the dolomites of
2 7.66‰. The dolomites of Bou Khalifa average 2 7.20‰, this domain have a late diagenetic origin.
those of Aı̈n Hallouf 2 7.96‰, Asdi Ben Zerhla 2 7.86‰ The coarse-grained character and cross-bedding of the
and Aı̈n Tarselt 2 8.21‰ have similar average values, but white vuggy dolomite of the Lotharingian indicates the
those of Sidi Abdellah have values of around 2 9.79‰ dolomitization of an oolitic calcarenite deposited in a high-
(Table 1). energy environment. The localization of these dolomites at
The d18O values of the early type 2 hydrothermal the base of the Lotharingian, their widespread distribution in
dolomites of the Sidi Abdellah open space filling the Middle Atlas Causse, and their relationship to the
orebody are different from those of early type 1 unconformity at the top of the Lower Lias, suggests a
hydrothermal dolomites hosting other stratiform ores. paleogeographical control on their formation. This, together
They differentiate two groups; the first contains the with their high porosity, suggests a possible origin in an
stratiform ore deposits of the Tazekka Pb – Zn district upstream area by the mixing meteoric and marine waters
(Aı̈n Hallouf, Aı̈n Tarselt, Bou Khalifa and Asdi Ben developed preferentially at the base of the formation.
Zerhla) and the second the orebody of Sidi Abdellah. The relationship of the dolomitized calcarenites at the
base of the reduced Middle Lias to horsts and paleorelief
features demonstrates the paleogeographic control on this
dolomitization. The discontinuity that separates these rocks
from the Upper Domerian black limestones above shows
that dolomitization occurred prior to deposition of the Upper
Domerian during the emergence of the Lower Lias.
The xenotopic texture of the early hydrothermal
dolomite type 1 of the Bou Khalifa deposit suggests that it
formed by replacement of the lumachelle at a relatively high
temperature (Gregg and Sibley, 1984; Zenger 1976), at least
above the critical roughening temperature that ranges from
50 to 80 8C (Gregg and Sibley, 1984). The dolomite formed
Fig. 6. Plot of C and O stable isotope compositions of dolomitic facies of during the emergent phase between the end of the
the Liassic platform of the Tazekka district. HD, Hydrothermal dolomite;
SD, Saddle dolomite; ML, Mineralized limestone; DCRML, Dolomitized
Domerian, and the Lower Carixian, but after the Pb – Zn
calcarenite of the reduced series of the Middle Lias; LD, Layered dolomite; mineralization. The age is therefore earliest Carixian
LMD, Laminated dolomite. (Auajjar, 1994).
 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178 175

The formation of the early type 2 hydrothermal dolomite dolomite results from a short exposure in the supratidal
that hosts the mineralization of Sidi Abdellah orebody is environment. The repetition of these dolomite lithologies is
linked directly to that of the mineralization, and mineraliz- attributed to the syndepositional character of diagenesis.
ing solutions are thought to be the cause of the dolomitiza- Evidence of minor late diagenesis in the Middle Atlas is
tion. Dolomitization fabrics progressively disappear away seen in the euhedral zoned dolomite crystals in the
from the orebody. The dolomitization event was post- Lotharingian oobioclastic limestone. These formed by
Domerian. neoformation in a confined and calm subtidal environment.
The dolomite hosting Pb –Zn Mississippi Valley-type In the Middle Atlas syndepositional and early diagenetic
mineralization typically forms an integral part of the dolomitization is much more common than late diagenesis.
evolution of the sedimentary basin concerned. The ore
deposits are formed by brines produced during the burial of 6.2. Isotopic implications
basin sediments. These migrate through aquifers, depositing
metals in the carbonate rocks of the platform (Sverjensky, 6.2.1. Sterile facies
1984; 1986; Leach and Rowan, 1986; Anderson and The d13C values of the dolomites are similar to marine
Macqueen, 1988; Leach and Sangster, 1993). Dolomitiza- carbonate values, indicating that the dolomite retained the
tion and mineralization events commonly occur several carbon isotope values of the original limestones. The level
times in Mississippi Valley-type districts, linked to pulses in of values and their narrow range does not appear to reflect
fluid flow caused by local tectonic and sedimentary events significant organic diagenetic processes, such as sulfate
within the basins (Leach and Rowan, 1986; Oliver, 1986; reduction, methanogenesis, or thermal decarboxylation.
Qing and Mountjoy, 1992; Boni et al., 2000). The general decrease in d18O values and a corresponding
Saddle dolomite commonly reflects a late stage hydro- increase in diagenetic intensity from the laminated dolomite
thermal diagenesis. It post-dates the Pb– Zn mineralization to the dolomitized calcarenite at the base of reduced series
(Auajjar, 1994). Saddle dolomite has been taken to indicate of the Middle Lias (Table 2 and Fig. 6), is similar as in other
formation in a burial environment by hot (60 –150 8C), and areas (Dickson and Coleman, 1980; Meyers and Lohmann,
saline (2 – 6 times seawater salinity) hydrothermal fluids 1985; Moore, 1985; Woronick and Land, 1985).
(Radke and Mathis, 1980; Gregg, 1983; Searl, 1989). Generally, depleted oxygen values may reflect either an
Machel (1987) suggested that saddle dolomite may be a increase in temperature or the influence of meteoric water.
product of chemical compaction and thermochemical The paleogeographical control of the late diagenetic
sulphate reduction. Finally, Mountjoy and Halim-Dihardja dolomites suggests the intervention of meteoric waters
(1991) and Dix (1993) indicated that saddle dolomite may rather than any increase in temperature.
also form during late in diagenesis at shallow depth by At the dolomite front, dolomites are impoverished in
hydrothermal fluids rising along faults. d18O relative to adjacent limestone. So dolomitization is
There are three principal phases of hydrothermal accompanied by a decrease in d18O and by only slight
dolomite in the Tazekka district, early type 1 and 2 phases, variation in d13C.
and saddle dolomite. The formation of the saddle dolomite It appears that several different waters were involved in
appears to be separated from that of early hydrothermal the formation of the dolomites of the Tazekka district and
dolomites by an unconformity (fracture phase), indicating a that contrasting processes were involved in these episodes
time gap. It is suggested that saddle dolomite and of dolomitization.
hydrothermal dolomites precipitated from similar fluids
during three phases and by two mechanisms, by replace- 6.2.2. Mineralized facies
ment (the hydrothermal dolomites) and by cement precipi- The average values of d18O for both the mineralized
tation (the saddle dolomite). limestone and for the hydrothermal dolomites are essen-
In the Middle Atlas Causse dolomite has formed tially the same, suggesting that the generating solutions
penecontemporaneously, as a result of early diagenesis, were at similar temperatures.
epigenetically with a paleogeographical control, and by the The decrease in both d18O and d13C values in the ore
circulation of hydrothermal mineralized solutions. deposits passing from sterile to mineralized rocks, can be
attributed to water – rock isotopic exchange in an
6.1.2. The Middle Atlas alteration zone. Outside this zone, d18O values indicate
The fine-grained dolomite at the base of the Lower Lias that the host rocks have not been exposed to any
is a penecontemporaneous dolomite deposited in the inter- abnormally high geothermal gradient, and have not been
to supratidal environment of a restricted platform with an subject to transformation by any thermal effect. This can
evaporitic tendency. be attributed either to a decrease in fractionation linked
The intraformational breccia and laminated dolomite in to an increase in temperature towards the ore deposits
the middle formation of the Lower Lias are sedimentary (Hall and Friedman, 1969; Moritz et al., 1996), or to an
in origin. The limestones that they replace were deposited in increase in the water/rock ratio (Sverjensky, 1981). An
the intertidal environment of the platform. The crystalline increase in water/rock ratio might be correlated with
176 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178

mineralogical change such as dolomitization and sulfide and Mountjoy, 1997). The diagenetic transformation of
deposition. smectite to illite will liberate Mg and as pore fluids migrate
The d18O values of the early type 2 hydrothermal upwards they are responsible for the dolomitization of
dolomites of the Sidi Abdellah ore deposit are different overlying carbonates.
from those of the type 1 hosting other stratiform ore The red argillites of the Triassic of the Tazekka district
deposits of the Tazekka district. They differentiate two show a progressive evolution of clay minerals from the
groups; the first contains the stratiform ore deposits of Middle Atlas Causse to the Middle Atlas. There is a
the Tazekka Pb – Zn district (Aı̈n Hallouf, Aı̈n Tarselt, transition from chlorite-montmorillonite, to swollen chlor-
Bou Khalifa and Asdi Ben Zerhla) and the second the ites, and well-crystallized chlorites. This mineralogical
deposit of Sidi Abdellah. Thus, although the dolomites evolution reflects a spatial zoning of the clays, with Mg-rich
have a similar petrography they do not have the same chlorites to the east and Mg-poor to the west. The Mg was
d18O values. The Bou Khalifa deposit (Carixian) and Sidi expelled during late diagenesis which has been recognized
Abdellah (late-Domerian) orebody have both been in the lower red argillites by the abundance of the 2 M
affected by different hydrothermal fluids. Isotope values polytype of illite (70 – 100%) and by the presence of chlorite
therefore confirm field observations suggesting two with a cristallinity index which varies from 3 to 3.75
phases of mineralization (Auajjar, 1994). (Benchekroun, 1985). The Mg was used in the dolomitiza-
The saddle dolomites have more depleted d18O values tion of adjacent limestones.
than the early hydrothermal dolomites. Such depletion
indicate that the saddle dolomites precipitated at higher
temperatures (60 – 150 8C (Radke and Mathis, 1980); 90– 7. Conclusions
215 8C (Machel, 1987)), and from more saline (2 –6 times
that of sea water) hydrothermal fluids (Radke and Mathis, 1. Dolomitization is more extensive in the Middle Atlas
1980; Gregg, 1983; Searl, 1989). Zenger (1983) indicated Causse than in the Middle Atlas domain.
that the higher temperatures typical of burial environments, 2. In the Middle Atlas Causse dolomitization may be
may aid dolomitization by increasing thermodynamic synsedimentary, early or late diagenetic (with paleogeo-
activity and reducing kinetic constraints. Machel (1987) graphical control) or hydrothermal.
suggested that the saddle dolomite may be a product of 3. The dolomites in the Middle Atlas are synsedimen-
chemical compaction and thermochemical sulfate tary. The sequential changes in character of the
reduction. Finally, Mountjoy and Halim-Dihardja (1991) dolomites of the middle formation of the Lower
and Dix (1993) indicated that the saddle dolomite may also Lias confirm the syndepositional origin. Euhedral
form during late diagenesis at shallow depths by hydro- crystals suggest minor late diagenesis.
thermal fluids rising along faults. 4. The diversity of dolomite types is attributed to
The formation of the saddle dolomite appears to be variations in genesis. The first phase of dolomitization
separated from that of early hydrothermal dolomites by an with the formation of fine-grained dolomite is the
unconformity (fracture phase), indicating a time gap. It is same in all districts and occurs in a restricted
suggested that saddle dolomite and hydrothermal dolomites environment of an evaporitic platform interior.
precipitated from similar fluids during three phases and by 5. The early diagenetic dolomites resulted from a brief
two mechanisms, by replacement (the early hydrothermal phase of exposure in a subaerial environment, perhaps
dolomites) and by cement precipitation (the saddle dolo- in a supratidal area.
mite). These phases possess differing isotopic signatures 6. The epigenetic dolomitization is attributed to burial
and apparently formed at different temperatures. Field data, diagenesis. Hydrothermal dolomite formation was
petrographic and stable isotope results suggest a continuum linked to the passage of mineralizing solutions.
of replacement, during the Carixian for the early type 1 Three hydrothermal dolomites are associated with
dolomite, and the Toarcian for the early type 2 dolomite. the ore deposits of the Middle Atlas Causse. Two
Precipitation of saddle cement followed a phase of fracture early phases, type 1 and 2, are separated in time from
generation. a phase of saddle dolomite. Saddle dolomite rep-
resents a late stage diagenesis post-dating Pb – Zn
6.3. Sources of the Mg mineralization. Early dolomites and saddle dolomite
were precipitated from similar fluids during three
Seawater was probably the source of the Mg for the early distinct phases and but formed by two mechanisms, by
diagenetic dolomites (Land, 1985). replacement (early type 1 and type 2 dolomites) and
The Mg required for the deep burial dolomitization may by cement precipitation (saddle dolomite). These
have come from the compaction of Triassic argillites of the phases possess differing isotopic signatures and
Tazekka district. Similar sources have been suggested for apparently formed at different temperatures. Field
other areas (Mattes and Mountjoy, 1980; Schofield and data, petrographic and stable isotope results suggest
Adams, 1986; Lee and Friedman, 1987; Dix, 1993; Drivet a continuum of replacement, during the Carixian for
 J. Auajjar, J. Boulègue / Journal of South American Earth Sciences 16 (2003) 167–178 177

the early type 1 dolomite, and the Toarcian for the Braithwaite, C.J.R., Rizzi, G., 1997. The geometry and petrogenesis of
early type 2 dolomite. Precipitation of saddle cement hydrothermal dolomites at Navan, Ireland. Sedimentology 44,
421– 440.
followed a phase of fracture generation. Chalot-Prat, F., Cabanis, B., 1989. Découverte, dans les volcanites
7. Seawater was the source of Mg for the syndeposi- carbonifères du Tazekka (Maroc oriental), de la coexistence de diverses
tional and early diagenetic dolomites while the Mg for séries basiques, d’une série acide et d’importants phénomènes de
late diagenetic dolomites is thought to have been mélanges. Comptes Rendus de l’Académie des Sciences de Paris 308
derived from waters released by compaction of (Série II), 739–745.
Chalot-Prat, F., Vachard, D., 1989. Découverte de Foraminifères
Triassic argillites. serpoukhoviens (Namurien inférieur) dans la série volcano-sédimen-
8. Several different waters were involved in the for- taire du Tazekka (Maroc oriental). Comptes Rendus de l’Académie des
mation of the dolomites of the Tazekka district and Sciences de Paris 308 (Série II), 1157–1160.
that contrasting processes were involved in these Colo, G., 1961. Contribution à l’étude du Jurassique du Moyen Atlas
episodes of dolomitization. septentrional. Notes du Service Géologique du Maroc, 139 and 139 bis,
226pp.
Dickson, J.A.D., 1966. Carbonate identification and genesis as revealed by
staining. Journal of Sedimentary Petrology 36, 491– 505.
Acknowledgements Dickson, J.A.D., Coleman, M.L., 1980. Changes in carbon and oxygen
isotope composition during limestone diagenesis. Sedimentology 27,
107– 118.
We thank the Director of the Laboratory of Stable Isotopes
Dix, G.R., 1993. Patterns of burial and tectonically controlled dolomitiza-
the Professor Marc Javoy at the University Paris VII (France) tion in an Upper Devonian fringing-reef complex: Leduc Formation,
for allowing O and C isotopes facilities. Helpful comments Peace River Arch area, Alberta, Canada. Journal of Sedimentary
and critical reviews on an early version of this paper by the Petrology 63, 628–640.
Professors Don Zenger (University of Pomona, USA) and Drivet, E., Mountjoy, E.W., 1997. Dolomitization of the Leduc Formation
(Upper Devonian), Southern Rimbey-Meadowbrook Reef Trend,
Colin Braithwaite (University of Glasgow, UK) is greatly
Alberta. Journal of Sedimentary Research 67, 411– 423.
appreciated. This paper benefited greatly from the helpful Fedan, B., Laville, E., El Mezgueldi, A., 1989. Le bassin jurassique
comments of the two Journal reviewers, Professors Francisca du Moyen Atlas (Maroc): exemple de bassin en relais de
Martinez-Ruiz and Javier Elorza. décrochement. Bulletin de la Société Géologique de France 8
(VI), 1123–1136.
Gregg, J.M., 1983. On the formation and occurrence of saddle dolomite-
discussion. Journal of Sedimentary Petrology 53, 1025–1026.
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