Journalof AfricanEarthSciences,Vol. 6, No. 1, pp. 1-8, 1987
0731-7247/87 $3.00+ 0.00 PergamonJournalsLtd.
Printedin Great Britain
Paleozoic oolitic ironstones of the Algerian Sahara: a review SALAHGUERRAK* Ecole Nationale Polytechnique d'Alger et Centre Armoricain d'Etude Structurale des Socles, CNRS, Rennes, France
(Receivedfor publication 26 February 1986) Abstract--The Paleozoic sediments are present over the greatest part of the Saharan Platform, from the border of Morocco, Rio de Oro and Mauritania in the W, to the Libyan frontier in the E. They outcrop in the Ougarta chains, the Gourara and the Touat, the Northern border of the Reguibat Shield and constitute a sedimentary girdle around the Touareg Shield, namely the Azzel Matti, the Ahnet, the Mouydir, the Ajjers Tassilis, the Tafassasset Tassilis and the Ouan Ahaggar Tassilis. In all these formations, numerous oolitic ironstones occur, particularly in Ordovician, Devonian and Lower Carboniferous rocks. We can distinguish essentially two types of oolitic ironstones (OIS): OIS with constant thin beds and horizontal extension: they will be named EXID type (Extensive Iron Deposition), and OIS of local extension and irregular thickness: they will be named LOID type (Local Iron Deposition). The EXID type is located in the Ordovician, the Middle Devonian and in the very Lower Carboniferous (Tournaisian) sedimentary rocks. The LOID type occurs in the Lower and Upper Devonian rocks and appears as the economically interesting type. From the Zemmour to the Fezzan, a real Paleozoic oolitic iron belt appears 3000 km long, determining a N Gondwanian oolitic iron province, formed during cold to temperate climates. R~sumr----Lessrries palrozoiques de la plateforme saharienne algrrienne ont 6t6 reconnues depuis ies frontirres marocaine, du Rio de Oro et de la Mauritanie,/~ l'Ouest, jusqu~ la frontirre libyenne ~ l'Est. Elles affleurent essentiellement dans les chaines d'Ougarta, le Gourara, le Touat et constituent une vrritable guirlande autour du bouclier Touareg, en particulier l'Azzei Mani, l'Ahnet, le Mouydir, les Tassilis des Ajjers, les Tassilis du Tafassasset et de l'Ouan Ahaggar. Dans toutes ces formations sont interstratifi6es de nombreuses occurences de minerais de fer oolithiques, en particulier dans l'Ordovicien, le Drvonien et le Carbonifrre infrdeur. Nous pouvons distinguer deux types de minerals de fer oolithiques: les minerais constitu6s par des lits d'rpaisseur constante et ayant une extension horizontale importante: ce sera le type extensif; les minerais ayant une extension locale et une 6paisseur variable: ce sera le type local. Le type extensif est localis6 dans l'Ordovicien, le Drvonien moyen et la base du Carbonifrre (Tournaisien). Le type local, quant h lui, est uniquement situ6 dans le D6vonien infrrieur et moyen: il peut 6tre consid6r6 comme le type ayant le plus d'intrrrt 6conomique. Depuis le Zemmour jusqu'au Fezzan, une vrritable ceinture palrozoique de fer oolithique a 6t6 mise en 6vidence. Elle se dessine sur 3000 km de long, drterminant ainsi une province ferrifrre Nord-Gondwanienne, formre durant les climats froids ~ temprrrs du Palrozoique.
In the northern part of Africa the oolitic ironstones a r e f r e q u e n t in t h e P a l e o z o i c s e d i m e n t s (Fig. 1). I n Algeria, these sediments are present from boundaries of M o r o c c o , R i o d e O r o a n d M a u r i t a n i a in t h e W , to t h e L i b y a n f r o n t i e r in t h e E. T h e y o u t c r o p in O u g a r t a r a n g e s , t h e G o u r a r a a n d t h e T o u a t , t h e n o r t h e r n b o r d e r o f t h e R e g u i b a t shield (Tin° d o u r B a s i n ) a n d c o n s t i t u t e a s e d i m e n t a r y girdle a r o u n d t h e T o u a r e g Shield: t h e A z z e l M a t t i , t h e A h n e t , t h e M o u y d i r a n d t h e A j j e r s Tassilis.
Definition THE STUDY o f o o l i t i c i r o n s t o n e s ( O I S ) in N A f r i c a , p a r t i c u l a r l y in t h e A l g e r i a n S a h a r a , allows o n e to distinguish t w o m a j o r t y p e s o f d e p o s i t s . (1) T h i n b e d d e d O I S o f large g e o g r a p h i c a l e x t e n s i o n : it will b e n a m e d E X I D t y p e ( E x t e n s i v e I r o n D e p o s i tion). T h i s t y p e c a n e x t e n d f r o m at least 20 k m to m o r e t h a n 100 o r 200 k m . (2) O I S o f r e s t r i c t e d e x t e n s i o n a n d i r r e g u l a r t h i c k n e s s : it will b e n a m e d L O I D t y p e ( L o c a l I r o n D e p o s i t i o n ) . It can e x t e n d to 20 k m m a x i m u m a n d its t h i c k n e s s can e x c e e d 30 m. Deposits of economic importance are generally of L O I D t y p e . A l l t h e s e O I S o c c u r r e n c e s a r e c o n n e c t e d to the genetic Clinton type or SCOS-IF (sandy, clayey a n d o l i t i c , s h a l l o w - i n l a n d sea f o r m a t i o n ) o f K i m b e r l e y (1978).
SOUTHERN FLANK OF THE TINDOUF
T h e T i n d o u f B a s i n is a large syncline o r i e n t e d W S W E N E a n d l o c a t e d b e t w e e n t h e A n t i - A t l a s to t h e N a n d t h e R e g u i b a t S h i e l d to t h e S. It is b o u n d e d to t h e E b y t h e E r g C h e c h d e p r e s s i o n a n d t h e O u g a r t a r a n g e s a n d to the W, by the El Aioun Basin and the Mauritanides r a n g e . I n this a r e a , t h e d e p o s i t i o n o f i r o n s t o n e s s t a r t e d d u r i n g t h e Silurian a n d e n d e d d u r i n g t h e F a m e n n i a n . O n l y t h e L O I D t y p e is r e p r e s e n t e d . It d e v e l o p s two
* Present address: Centre Armoricain d'Etude Structurale des Socles, Institut de Grologie, Universit6 de Rennes, Campus de Beaulieu, 35042 Rennes Cedex, France. AES 6.'1-^
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Fig. 1. Oolitic ironstones of the Algerian Sahara. a: Cambrosilurian sediments; b: Devonian sediments; c: Carboniferous sediments. 1: Gara Djebilet W; 2: Gara Djebilet Center; 3: Gara Djebilet E; 4: Gara Sefra W; 5: Gara Sefra E; 6: Oguilet Laroussi; 7: Mecheri Abdelaziz; 8: Tguililet El Hamra; 9: Nba; 10: Fedj Mtaigat; 11: Fedj Mlehas; 12: Gour Jiffa; 13: Djebel Meremda; 14: Gara El Haouia; 15: Gara El Haouia South; 16: Djebel Naidjat; 17: Djebel Atinim West; 18: Djebel Atinim South; 19: Djebel Atinim center; 20: Guelb Berrezouk; 21: Djebel Reboub; 22: DrA El Kelba; 23: Djebel Zerhamra; 24: Hassi Feguaguira; 25: Bled El Mass; 26: Gara Raha; 27: Djebel Mouima; 28: Foum Belrem; 29: Meredoua; 30: Gara Rghya; 31: In Heguis; 32: 'Attafaitafa Formation'; 33: 'Srrie des Trottoirs' and 'Talus &Tigillites', 34: Orsine Formation; 35: Djebel Illerene.
large economic deposits (Gara Djebilet and Mecheri Abdelaziz) and a great number of occurrences.
LITHOLOGY AND SEDIMENTARY STRUCTURF~
This deposit is located in the western part of the Tindouf Basin. It is interbedded in the Lower Devonian sediments (Gevin 1960), which are characterized by sandy-argillaceous successions (Fig. 2). Gara Djebilet consists of three separate main bodies, named Gara West, Gara Center and Gara East. These bodies correspond to large lenses of different sizes (7-90 km2), the whole deposit being extended by about 60 km. The thickness of these lenses is not constant and varies from 0 to about 30 m. The actual mineralogy of the deposit results from numerous and complementary factors: depositional conditions, diagenetic transformations and weathering processes. Non-ferriferous and ferriferous minerals have been detected as: apatite, siderite, quartz, bavalite, magnetite, maghemite, hematite, goethite and pyrite. The relationships between the main minerals can be summarized as follows: magnetite is well developed in oolites, and very rarely in the matrix; hematite is located in the matrix and oolites; bavalite can constitute a part of oolites and matrix and is linked to hematite; siderite is late diagenetic and invades all the components of the ironstone; apatite occurs in ferriferous facies, but does not contain all phosphorus included in the iron ore. About
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Paleozoic oolitic ironstones of the Algerian Sahara: a review 40% of phosphorus appears to be included in chlorites, goethite, siderite and magnetite. Some ferrugineous crusts and paleosoils constitute lenses in the iron-rich deposit and could be interpreted as the result of emersion periods; maghemite is particularly well developed in this deposit and it results from weathering processes which have affected the magnetite. The arrangement of the different particles (oolites, intraclasts and detrital grains) allows one to distinguish three petrographical facies: a facies with cemented oolites; a facies with detrital matrix; a facies with non-detrital matrix. The lenticular morphology of each iron oolitic body, coarsening-upwards sequences of the sedimentary envelope, and the development of bioturbation, suggest a deltaic or subdeltaic environment for the deposition of this iron formation. Then, the three garas would be considered as three different fans of the deltaic accumulation. The total geological reserves can be estimated as 4.5 billion tons, with an iron concentration between 38 and 57% (Matheron 1955). Mecheri A bdelaziz
The Mecheri deposit is located 250 km in the E of Gara Djebilet. It is interbedded in upper Famennian sediments as shown in Fig. 3. The 'Mineral Formation' overlies an argillaceous and silty 'Inframineral Formation' and is overlain by a silty-clayey 'Supramineral Formation'. These Famennian sediments are cut across by a doleritic intrusion producing local mineralogical reactions and mechanical effects resulting in an upraising of the central part of the deposit, and thus dividing it into three zones: Western, Central and Eastern. The deposit appears to be constituted by 16 iron lenses included within sedimentary lenticular bodies (Guerrak 1985). The lithology is characterized by its fine grain size, except for the 'Mineral Formation'. As for Gara Djebilet, the mineralogical composition results from numerous and complementary effects. Three main parageneses are present: maghemitemagnetite, bavalite-hematite and bavalite-magnetite. Apatite, quartz, calcite, ripidolite and goethite have also been detected. Four petrographical facies have been observed: microconglomeratic facies; facies with cemented oolites; facies with detrital matrix; facies with non-detrital matrix. The conclusions about petrographical studies can be summarized as follows: the arrangement of the different particles (oolites, intraclasts, bioclasts and detrital grains) shows that the oolitic ironstone is a mixture of several and different granulometric fractions; numerous observations of broken oolites, multiple oolites, multiple nuclei, allow one to conclude that an intrasedimentary oolitization process, successive ooliti-
zation phases within temporary environments, predate the final deposit: similar phenomena have been described in Libya by Chauvel and Massa (1981). Total geological reserves are of about 2.5 billion tons with a mean iron concentration of 43% (Sonarem 1978). The morphology and the petrographical facies of the oolitic bodies, and their position in coarsening-upwards sequences, indicate that they are a result of prograding processes in platform deltaic deposits. Fedj Mlehas
The iron occurrences of Fedj Mlehas are located in the eastern part of the Tindouf Basin. Six beds of oolitic iron formation are interbedded in the Lower Devonian sediments, at the top of green shales (Menchikoff 1935, Gevin 1960, Guerrak and Chauvel 1985). The six beds are not of the same size and the thickness variations are very important: 0.5-8.3 m. The mineralogy of the iron formation is dominated by the hematite-chlorite paragenesis. Four petrographical facies can be observed: microconflomeratic facies; facies with cemented oolites; facies with detrital matrix; facies with non-detrital matrix. The lenticular aspect of the oolitic bodies and their position in coarsening-upwards sequences indicates that the sedimentary environment of Fedj Mlehas ironstone is deltaic or subdeltaic like the Djebilet and Mecheri deposits. Other occurrences
In Lower Devonian sediments of the southern flank of the Tindouf Basin, several small oolitic iron occurrences have been recognized. From the W to the E, between Gara Djebilet and Mecheri: Gara Sefra East near Aouinet Legraa: at the top of a Silurian argillaceous series, 1 m of ironstone is overlain by fine sandstones. Gara Sefra West: one oolitic bed of 1 m thickness is interbedded Lower Devonian green shales and the general dip is of about 2° N, as for the other formations of this part of the Tindouf Basin. One hundred kilometers to the W of Mecheri, in an area named Oguilet Laroussi, a very thin oolitic iron bed is located at the top of the Upper Devonian. This occurrence represents the outcropping of a large subsurface iron deposit which is expressed by an important aeromagnetic anomaly. Only drilling could confirm or invalidate this hypothesis. Tguililet El Hamra and Nba: these two occurrences are interbedded in fine sandstone and seem to be of Lower Devonian age. Fedj Mtaigat: probably more important than these last points, Fedj Mtaigat presents about six beds of oolitic ironstone interbedded in Devonian shales and siltstones. Unfortunately outcrops are covered up by dune sands which mask the extension of the oolitic
S. GUERRAK ,GE
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bodies. However, it is possible to describe trough crossbeddings in the oolitic facies and some conglomeratic ironstone with tabular cross-beddings and trough crossbeddings. This involves a high energy level for the deposition environment of oolites. The thickness of the beds varies from 0.8 to 2 m and their lenticular morphology is evident. In the S of Bou Bernous region, the area of Gour Jiffa (Gevin 1960) seems to be interesting. It presents at least two beds of oolitic iron formation interbedded in Lower Devonian fine sandy sediments. These beds do not exceed 2 m in thickness and present tabular cross-beddings. Recently, in three bore holes (Sonarem 1981) near the Mecheri area, several beds of DIS have been cut across. The paleontological study of the drill cores is just beginning but some data are available: one of the DIS beds is Lochkovian in age and another is Llandoverian.
OUGARTA In Ougarta mountains, stratigraphic subdivisions of Ordovician sediments are not easy because of the scarcity of fossils. Commonly (Arbey 1962, Gomes-Silva et al. 1963, Erem 1985, Legrand 1974), five lithostratigraphic formations are recognized, as shown in Fig. 4. Interbedded in these formations, about 12 beds of ironstone of EXID type have been observed. These beds are generally thin (0.25-3 m) but largely extended (more than 50 km). Lower Tremadoc Drfi
El Kelba: 1 m of DIS in sandy sediments.
Upper A renig Djebel Meremda: 2.4 m of OIS in quartzitic and sandy sediments, 0.25 m of DIS interbedded in sandstones.
Paleozoic oolitic ironstones of the Algerian Sahara: a review Djebel Naidjat: 3 m of OIS with pisolites in an argillaceous sequence. Upper Llanvirn Djebel Reboub: 0.5 m of microconglomeratic ironstone in a silty to sandy sequence. Gara Haouia East: 1.3 m of OIS in a silty to sandy sequence. Lower Caradoc Guelb Berrezouk South: 0.5 m of OIS in siltstones (2 beds), 0.5 m of OIS in silty sediments. Djebel Atinim: 0.30 m of OIS in silty to sandy formation, 0.30 m of sandstone with iron oolites. Gara Haouia South East: 0.20 m of OIS in silty to sandy formation. Top of the Lower Caradoc Dj ebel Atinim: 2.8 m of ironstone in sandy sediments. Guelb Berrezouk South: 0.5 m of ironstone in silty sediments. Upper Caradoc Djebel Atinim: 4 m of conglomeratic ironstone overlain by oolitic ironstone in silty to sandy sediments. tu COUPETYPE =
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T E S M r NO k OG r E INDICES ~ DE FER MEMBRES FORMATIONS OOLITH[QU} Argiles
E A U GNE sS £ TA E
The main minerals are: hematite, chlorite, goethite, calcite and quartz. Three facies can be described: microconglomeratic facies facies with detrital matrix facies with non-detrital matrix. Pisolites can be observed in some occurrences. These oolitic ironstones are associated with silty to argillaceous sandstones which developed in finingupwards sequences. The bottom position of the OIS in the sequences involves and outlines remnants of transgressive episodes during Ordovician times.
AHNET, AZZEL MATTI, BLED EL MASS AND GOURARA The stratigraphic succession has been studied by Moussine-Pouchkine (1976), Biju-Duval et al. (1968) and is represented on Fig. 5. Two types of EXID ironstone are interbedded in the Devonian sediments: a shaly-calcareous iron-rich sequence and a sandy argillaceous iron-rich sequence. Characteristics of the first one are as follows: high energy features are absent and the environment is supposed to be very quiet;
OOLITIC I IRONSTONE
FACI E S
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MIDDLE AND LOWER FAMENNIAN
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Fig. 4. Ougatria Mountains: Lithostratigraphic section (after GomesSilva et aL 1963).
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Fig. 5. Anhet Basin: Lithostratigraphic section (after MoussinePouchkine 1976).
benthic faunas are well preserved and not reworked; ferriferous oolites are mainly constituted by chlorite; the deposits constitute lenticular bodies with a maximum extension of 20 km and a thickness of 3-5 m; the elementary sequences are fining upwards, from oolites to limestones. Two occurrences are known: Meredoua in the Lower Eifelian and Foum Belrem in the Lower Frasnian. The second iron-rich sequence is more extended than the shaly-calcareous one and it outcrops along more than 50 km: the presence of conglomeratic beds and cross-bedding express the high level of energy during sedimentation; the bioturbation is well preserved, but the fossilized fauna is reduced to some shell fragments. This sequence is well represented: Siegenian: a very thin bed of oolitic ironstone (it does not exceed 10 cm in thickness) is interbedded in siltstones and shales of the Bled El Mass area. Lower Famennian (II and III): iron rich oolites are included in black shales. The oolitic beds are sometimes lenticular and two or three beds can be observed in some outcrops. This type is well developed in the S of the Ahnet region (in Heguis) and is extended more than 50 km. Upper Famennian (V): in the Lower Khenig Formation, in the center of the Ahnet basin, an oolitic bed (thickness: 2 m) is interbedded between siltstones (below) and shales (above) and shows well developed cross bedding and bioturbation. Famennian VI: in the Azzel Matti, two occurrences of ironstones have been observed: the first type is red, with a hematite-rich bed located in a restricted area of only some kilometers. Very Lower Devonian of the Gourara: in the Hassi Fegaguira syncline we can observe a thin bed of oolitic ironstone (5-10 cm) interbedded in argillaceous and silty sediments. It is probably extended to more than 100 km. The main sequences are fining-upwards, but thin sequences are coarsening-upwards from oolitic iron-rich sediments to conglomerate. Localization of oolitic beds in the large sequences suggests that they outline transgressive periods all through Devonian times.
Upper Ludlow: in clayey, silty and sandy sediments of the 'Atafaitafa Formation'; Gedinnian: in alternating silts and shales of the 'Talus Tigillites' (Dubois et al. 1967); Siegenian: in ferriferous shales of the 'Serie des Trottoirs' (Fabre 1976); Emsian: in the sandy-argillaceous sediments of the Orsine Formation (Latreche 1982); Strunian: in the sandy sediments of the Djebel Iller~ne (Latreche 1982). Actually, petrographical data from these occurrences are very rare, but a detailed study is planned during the forthcoming years. CONCLUSIONS
The iron concentrations, which are interbedded in different formations of Paleozoic sediments, are represented by oolitic or non-oolitic iron depositions. They are linked to conditions of iron transportation, the distance of the source, the nature of the ferriferous support, and physicochemical characteristics of the sedimentary environment. In the southern flank of the Tindouf Basin, the iron source was not too much distant, probably located in the Reguibat Shield which contains important Precambrian Banded Iron Formation (Bronner and Chauve11979). For the other areas we can imagine a remote source such as perhaps the south of the W African Craton, 'the Man uplift'. We know also (Beuf et al. 1971) that cristalline rocks of the Touareg Shield were totally recovered by sediments during all Paleozoic times: therefore, they cannot be considered as the probable source of the Paleozoic iron deposits of Ougarta and Tassilis. Added to these regional conditions, the influence of structural behaviour, emphasized by a stable, rigid and relatively elevated area, allows the building of prograding deltas with a possible iron concentration of LOID type. Sedimentary processes on the Saharan Platform had been emphasized by Bertrand-Sarfati et al. (1977). In that case, we can explain the OIS of EXID type which we include in a clastic typical continental shelf sedimentation. Then, the input of sediments may be highly episodic as a consequence of river flooding. ExceptionTHE TASSILI N'AJJERS ally, heavy flooding may deposit important tonnages in a matter of weeks, to be redistributed over the shelf The Ajjers Tassilis in the NE part of the Hoggar are during ensuing years: it could explain the important constituted by about 2000 m of sediments lying on concentrations of iron arriving in the basin. Iron-rich crystalline rocks of the Pan-African Chain. silicates (Carroll 1958) were precipitated in particular Basically we can distinguish (Fig. 6): conditions of pH and Eh (Garrels and Christ 1965). the Cambro-Ordovician of the Inner Tassili; Then, the oolitization can be developed in quiet condithe Silurian of Intra-Tassilian Depression; tions by intrasedimentary processes around detrital or the Lower Devonian external Tassilis; bioclastic nucleus (Guerrak and Chauve11985). If nuclei the Upper Devonian-Lower Carboniferous Pretas- were too scarce in the sediment, or if the chloritic mud silian Plain. was not abundant enough, the oolitization did not Studied in detail by petroleum geologists (Dubois et develop and ferrugineous sandstones, siltstones or al. 1967, Beuf et al. 1971), this large area presents oolitic shales were formed. These characters can explain the iron occurrences of EXID type at least at five levels of extraordinary extension of the iron rich deposits from the Paleozoic pile: the Zemmour in Mauritania (Sougy 1964) to the Libyan
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Fig. 6. Illizi Basin: Lithostratigraphic section (after Dubois et al. 1967, Beuf et al. 1971, Latr6che 1982).
Platform (Nakhla et al. 1978, Van Houten and Karasek 1981). The regional sedimentary conditions are guided by evident transgressive cycles or at least by some alternating episodes of subsidence and sedimentation. However the LOID type appears to be the result of a local equilibrium between sedimentation and subsidence. These conditions have allowed the edification of deltaic bodies which are known also in the northern part of the Tindouf Basin (Cavaroc et al. 1976). According to Morel and Irving (1978) and Scotese et
al. (1979), positions of Gondwanaland during Ordo-
vician and Devonian times indicate that N Africa was located in cold or temperate zones. Therefore we cannot consider the ironstones as the result of iron leaching processes under intertropical conditions. This Gondwanian iron-rich belt, more than 3000 km long, must be compared to the similar N American belt, extended from Newfoundland to Alabama. For the iron belt, the stable position of Laurentia, particularly during the Silurian, inferred dominant climatic factors in the formations of ironstones.
Acknowledgments The National Mining Research Company of Algeria (EREM) provided field support for this study. For this we thank A. Slougui. We benefited greatly from advice and criticism from J. J. Chauvel and A. Moussine-Pouchkine. The financial support was provided by the University of Rennes, by the National Center for Scientific Research (CNRS), and by the Ecole Nationale Polytechnique of Algiers.
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