Mineralization, origin and age classification of ferruginized sandstone in the Bahariya Oasis, Western Desert, Egypt: A contribution to the origin of red beds

Lithos, 22 (1988) 59-73 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

59

Mineralization, origin and age classification of ferruginized sandstone in the Bahariya Oasis, Western Desert, Egypt: A contribution to the origin of red beds A. M O C K E ~ and CH. AGTHE 2

:Minerah)gi,sch-Petrologisches lnstitut, Georg-A ugust Universitiit. G6ttingen, Goldschmidtstrasse l, D-3400 (;:Jltingen (t:ederal R~Tmblic of German)9 :lnstitut jiir Mineralogie und Kristallographie der Technischen Universitiit Berlin, Hardenbergstrasse 42, D- l O00 Berlin- 12 (federal Republic of German)~)

LITHOS

Mticke, A. and Agthe, Ch., 1988. Mineralization, origin and age classification of ferruginized sandstone in the Bahariya Oasis, Western Desert, Egypt: A contribution to the origin of red beds. kithos., 22: 5973. The Bahariya Oasis is a depression and the topography of it is mainly controlled by anticlines. It lies in the Egyptian Western Desert and is characterized by isolated, cone-shaped residual hills which are aligned parallel to anticline axes in the southern part of the depression. These hills are capped by ferruginized sandstone of nearly black colour with an average iron content of 20 wt.%. The existence of a humid, warm climate with heavy rainfalls and a strong influence of vegetation during the Cretaceous-Tertiary transition provided reducing and weakly acid conditions. They enabled the mobilization of Fe and Mn out of manganese-bearing ilmenite and magnetite, primarily present in the sandstone. The existence of an oxidation barrier accompanied by an increase in pH value resulted in the precipitation of these elements. There is a definite correlation between the position of iron enrichment and tectonics in that area. The fracture planes, formed in the crestal region of the anticlines, exposed the deeper parts of the sandstone. As a consequence the oxygen of the air gained access to the subjacent sandstone strata, resulting in the formation of oxidation barriers in relatively deep-seated parts of the sandstone. The enrichment of iron and manganese was furthermore controlled by: ( 1 ) The process of up-arching during the formation of the anticlines, the thick sedimenta~ sequences in the crestal region of the fold, thus facilitating the penetration ofsupergene solutions. (2) Apart from the predominant supergene nature of the iron enrichment, a lateral one, though on a subordinate scale, also took place. (3) The oxidation barrier is supposed to be coincident partly with an evaporation barrier. The iron horizons therefore constitute an epigenetically mineralized sandstone of the Bahariya Formation. The former term Radwan Formation for this type of ferruginized sandstone, now preserved in the form of residual hills, is therefore invalid. (Received May 12, 1986; accepted March 21, 1988)

Introduction

F e r r u g i n i z e d s a n d s t o n e s are p a r t i c u l a r l y o f t e n o b s e r v e d in s a v a n n a a n d d e s e r t r e g i o n s as a result of favourable weathering conditions of exposure. F e r r u g i n i z e d s a n d s t o n e s , d i s c u s s e d in this p a p e r a p p e a r to be i d e n t i c a l w i t h t h e s o - c a l l e d c o n t i n e n t a l red beds. T h e o n l y d i f f e r e n c e b e t w e e n b o t h is seen

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in t h e i r i r o n c o n t e n t a n d c o l o u r i m p r e s s i o n . Acc o r d i n g to W a l k e r ( 1 9 6 7 a ) a n d v a n H o u t e n ( 1968 ) t h e total i r o n a m o u n t o f red s a n d s t o n e ranges f r o m 1.7 to 3.5 wt.%. In e x c e p t i o n s t h e iron a m o u n t rises up to 1 0 - 1 2 w t . % ( T u r n e r , 1 9 8 0 ) . In o u r case, the i r o n c o n c e n t r a t i o n in t h e s a n d s t o n e is by far h i g h e r a n d it lies b e t w e e n 14 a n d 27 w t . % [ e x c e p t s a m p l e 3 ( T a b l e 1 ) w i t h 35.1 w t . % ] (see T a b l e 1), result-

60 TABLE 1 Element concentrations o f T o p h i l l iron layers analyses 1-14 (for localities c o m p a r e Fig. 2 ); ( i f not separately i n d i c a t e d data arc in p p m ) El Heiz Analysis Sample Ca (wt.%) Ti Cr Mn Fe(wt.%) Ni Cu Zn Sr Zr Hf Ba Pb Nb

1 138/8I 1.1 3395 39 295 17.5

34 380 205

Radwan 2 142/81 1.2

S a n d s t o n e Hill

El Ris

3 143/81

4 129/8l

5 i34/81

6 122/81

7 128/81

8 118/8l

9 121/81

10 130/81

11 155/81

12 165/81

13 136/81

14 137/81

1,4

0.7

1.8

1.7

1.4

1.4

0.8

1.5

1.4

1.9

1.3

0.9

330 64

2185 87

910 37

7575 107

285 17.5 . 46 825 -

1070 35.1 . 75 290 330 -

260 19.0 . 11 54 29 70

1885 27.0 . 17 29 102 275 770

121

1975 69

1530 56

215 22.8

2065 41

220 18.1

. 26 100 160

-

9305 88

5385 15.2 15

66 140

40 56 210

600

2120

460 140

17

.

. .

. .

600 21.4 16 11 13 15 590 119 . .

1210 45

2660 35

1930 51

5045 20.1 62 140 1700 . .

2080 15.1

540 70

605 14.1 -

60 320

62 200

1090

210

-

1125 22.4 13 28 245 315

1180 58 580 19.8

15 12 130 475

. .

E]ements with a t o m i c n u m b e r s s m a l l e r than 19 were not analyzed.

ing in a brown-black colour impression. However, a strong relationship between the ferruginized Bahariya sandstone and continental red beds is obvious in their mechanism of formation, which is in general still only poorly understood (Turner, 1980). The epigenetic process of ferruginization presented in this paper is identical with the process of late-diagenetic reddening of red beds in the sense of Turner (1980), so far as the enrichment of iron is considered to be of secondary origin (Walker, 1967a; Schlugar and Roberson, 1975; Walker et al., 1978). Therefore this paper can be regarded as a contribution to the origin of red beds. However, there are two main differences between red beds and ferruginized sandstone: ( I ) exceptional geological conditions in the area of the Bahariya Oasis led to an extremely strong enrichment of iron in the form of amorphous Fe (OH) 3, goethite and lepidocrocite; (2) the ferruginized sandstone of the Bahariya Oasis is of deep brown-black colour, indicating the lack of the transformation process of iron hydroxides into hematite. Therefore it appears not appropriate to use the term "red beds", the formation of which is concerned with a post-depositional process during which sandstones became red due to the fact that newly formed Fe(OH)3 and goethite (lepidocro-

cite) convert into hematite (Walker, 1967a, b; van Houten, 1968). Hematite is responsible for the colour impression of red beds. In the literature of the past the ferruginized sandstones of the Bahariya Oasis were treated with special interest due to their close geographical association with three stratiform iron-ore deposits (e.g., E1 Shazly, 1962; E1 Akad and Issawi, 1963) lying together at the northern border of the depression. But these stratiform iron-ore deposits as shown by Agthe (1985) are of submarine-hydrothermalsynsedimentary origin. Therefore they have no genetic relationship to the ferruginized sandstones as will be demonstrated in this paper.

Geological setting The Bahariya Oasis is located in the northern part of the Western Desert of Egypt, about 350 km SW of Cairo (Fig. 1). Like the other depressions located further southwards (Farafra, Dakhla and Kharga), it is a basin without drainage. It was formed by wind erosion affecting only moderately folded strata of the surrounding plateau. The tendency to form depressions in this area is enhanced by the existence of several anticlines. In contrast to the other depressions, Bahariya is surrounded by

61

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Fig. 1. Geographical setting of the Bahariya Oasis and the position of the Syrian fold axis (according to Pfannenstiel, 1953). escarpments (locally up to 50 m high). The sedimentary series, incised by the depression, are Cenomanian to Eocene in age. The Bahariya depression lies on the Abu Roash axis which belongs to a system of various fold axes ( = S y r i a n fold axis; Pfannenstiel, 1953). The southern end of this axis is situated approximately 50 km SW of Farafra in the depression of the Oasis Abu Munghar. From there it strikes in NE direction into the Farafra Oasis and continues in the same direction up to Bahariya, and extends further northwards (Fig. l ). In the area of the Bahariya Oasis the

Abu Roash axis splits into several more or less parallel anticlines (Fig. 2). The up-arching of the former plateau began after the Cenomanian and continued through the Maastrichtian until the Oligocene, The resulting ductile folds and extension joints in the crestal area of the anticlines offered favourable conditions for erosion, which had already begun in the Paleocene and continued up to the beginning of the Miocene (Dominik, 1985), The stratigraphic units outcropping in the Bahariya Oasis have been described in detail by Domi-

62 28 °

30'

28~40'

29000,

28" 50'

29°10'

Nasser Hills 35' El- Gedida

('Dj.

Maysera

[

Mandishai~

BAWITI 28° 20'

i r L.~./~

Dj. El ~

H

~roO ~ 25 °

~- Dj.Tobog, Dj. Hamrr Dj. Hefhuf

Basalt Hill~ ~,

a

28 °

28' o

Oa °

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IJ 28°

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BAHARIYA ~Tophill

ANTICLINE

! J~JJAnalysed iron layer ji]Radwan J~E[- Heiz I [ ~ Iron deposit j [~]EI- Ris ~ Basalt ~]SandstoneHill ~/ ~ Limestone/Dolomite[~ Ghorobi J [] Naqb EI-Horrc [ ] El Tibnia 0 5 10 15 Kilometres

P 27'

5~' ' ~. 28°30 ,

OASIS

iron layer

28 40"

28 :~50'

i 29°00,

27 °

29°10 '

Fig. 2. Distribution of anticlines and ferruginized residual hills in the Bahariya Oasis.

nik ( 1985 ). In this paper only those relevant to our subject will be discussed briefly. The Bahariya Formation is Cenomanian in age. It forms the base of the depression, the flanks of the residual hills, and parts of the surrounding escarpment. According to E1 Akad and Issawi (1963) and Basta and Amer ( 1968 ) the iron cappings of E1 Heiz represent another formation of Oligocene age which the authors called the Radwan Formation. The Radwan Formation, the main subject of this paper, is identical with ferruginized sandstones. In aver-

age they show a thickness of up to 25 m and occur at many localities of the depression (Fig. 2).

Previous works E1 Akad and Issawi (1963) dealt comprehensively with the iron-rich sandstone. The following can be gathered from the results of their investigation: The ferruginized sandstones ( = Radwan Forma-

63 tion) overlap the Bahariya Formation. The Bahariya Formation was folded prior to the sedimentation of the younger series. Once the folding process was finished, the sedimentation of the younger series began (i.e. El Heiz Formation, E1 Hefhuf Formation, and sediments of the Plateau Group) mainly in the troughs and on the flanks of the folded Bahariya Formation. The crests of the fold, however, were already denuded during the process of sedimentation. In this way the relief, which was developed during folding, was flattened to a great extent. The Radwan Formation was subsequently deposited in the Mid-Eocene or presumably only in the Oligocene, in a freshwater environment.

Occurrence of iron layers in the Bahariya Oasis The capping of the hills can be of different kinds: ( 1 ) Cappings of limestone or dolomite is mainly observed on elongated and tabular hills (Fig. 2 ). (2) Basalt forms, as a rule, plateau-shaped hills (Fig. 2 ), but in addition, hills with a shape o f a pyramide can also be capped by basalt (Basalt Hill) (Fig. 2). (3) The majority of the residual hills have coneshaped tops consisting of iron-bearing sandstone, which overlies the barren sandstone (Fig. 2). Ball and Beadnell ( 1903 ) mentioned these ironmineralized hills for the first time. E1 Shazly ( 1962 ) estimated the iron reserves of these hills to be approximately 100.106 t*. The distribution of the iron mineralizations is not arbitrary. They occur mostly parallel to the anticlines and mainly in the south and the central parts of the oasis along the Radwan, E1 Heiz, El Ris, and Sandstone Hill anticlines (Figs. 2 and 3 ). But they also occur in the north, and here, they are linked to the Ghorabi anticline where they are either covered by thick deposits of iron ores or limestones and dolomites. Iron layers within the Nubian Series of Bahariya are not only limited to tophill positions (Tophill iron layer). They often occur along the flanks of the residual hills as various Foothill iron layers. Both Tophill and Foothill iron layers are underlain by fine-grained sediments (e.g., silt and clay). Furthermore, the base of the depression is locally cov*1 t = 1 m e t r i c t o n n e = 10 3 kg.

Fig. 3. Alignment of residual hills along the El Heiz anticline in the southern part of the depression. ered by a basal iron layer (Groundfloor iron layer) (Fig. 4) the thickness of which is in the range of decimetres. All three types of iron layers are connected with coarse-grained sandstones. The difference in type of iron layer also suggests that such mineralizations could have been formed at different times.

Petrographic characteristics of the barren sandstones Apart from rounded grains of quartz and silicates, oxides and occasionally sulphides may be found in the barren sandstones. In most cases they are heavy minerals. Silicates (very rare) are zircon, tourmaline, kyanite, staurolite, garnet, plagioclase, microcline and titanite. The oxidic minerals are ilmenite (FeTiO3) and its alteration products pseudorutile (Fe2Ti3Og) and leucoxene (TiO2), as well as magnetite, magnesioferrite, pleonaste, chromite, rutile, anatase and columbite. Other minerals are apatite and graphite. Ilmenite is only rarely preserved as relics. Mostly it has been changed into leucoxene with all its intermediate products as a result of Fe leaching. By microprobe analysis unaltered ilmenite contains in average 12 mole% pyrophanite, 5 mole% hematite and occasionally minor amount of MgO in the range of 0.5 mole%. Apart from relics of ilmenite, pseudorutile and leucoxene predominate. Ilmenite and pseudorutile have very similar lattices; for this reason, ilmenite changes pseudomorphically into pseudorutile (Fig.

64

Fig. 4. Schematic position of different types of iron layers in the Bahariya Oasis, 5 ). With increasing iron leaching pseudorutile becomes unstable and the development of leucoxene (which is identical with rutile) starts. Rutile and ilmenite have no structural analogies and therefore, the original ilmenite crystal decays into a polycrystalline aggregate of microscopic rutile crystals. The strong iron leaching results in the development of shrinkage cracks (Fig. 6). The final product of the whole process of transformation is a polycrystalline, fine-grained rutile (=leucoxene). The iron content of these leucoxenes can be traced largely to FeOOH on grain boundaries. The primary rutiles can be easily distinguished from the leucoxenes as they are stable during weathering processes. They occur in twinned individual crystals and coarsegrained aggregates (Fig. 7 ). Hemoilmenite is mostly decomposed. The original hematite exsolution bodies are limonitized; the host ilmenite is transformed into pseudorutile or leucoxene (Fig. 8) Magnetite is observed only in relics because of its nearly completely alteration (Fig. 9). Sometimes titanomagnetite aggregates with newly formed rutile, which had resulted from former ilmenites, were observed and had occurred as trellis type in the original magnetite (Fig. 10), Magnesiospinel (MgA1204) and pleonaste ( ( Mg, Fe ) (AI,Fe) 204 ) are stable against weathering solutions and preserved (Fig. I1), whereas

Fig. 5. Ilmenite (i) surrounded by goethite (on grain boundaries) is partially decomposed into pseudorutile (ps); quartz (qu). Reflected light, oil immersion:diameter of the ilmenite crystal: 100/~m;Foothill iron layer. chromite shows peripheral leaching (Fig. 12 ). Cr203 contents in the range of 0.15-2.02 wt.% are found in altered ilmenite, particularly in the strongly leached forms, the leucoxenes; whereas the primary ilmenites (sometimes preserved in relics), are free of chromium. The Cr203 content is, therefore, without doubt, infiltrated and stems from decomposed chromite. Under the given weathering conditions, sulphidic minerals are not stable. Therefore they are only ob-

65

Fig. 6. llmenite with characteristic shrinkage cracks has completely decomposed into leucoxene (=polycrystalline rutile). Reflected light: oil immersion; diameter of the grain: 150 ~m. Tophill iron layer (concentrate of heavy minerals).

Fig. 8. The original host crystal ilmenite is decomposed into pseudorutile (ps), whereas the guest phase hematite is completely transformed into goethite (g). Reflected light; oil immersion: length of the crystal: 180/tm. Tophill iron layer: concentrate of heavy minerals.

served, when they occur as inclusions in stable quartz or stable rutile. These are chalcopyrite, pyrrhotite and pyrite. Graphite occurs in association with quartz and the heavy minerals in the form of coarse-grained, lamellar crystallites with strongly feathered ends as a result of mechanical strain developed during transport (Fig. 13). Occasionally fine-grained graphite inclusions in quartz and ruffle may be observed.

Petrographic characteristics of ferruginized sandstone

Fig. 7. Twinned rutile crystal and inhomogeneously decomposed ilmenite (i= mixture of pseudorutile and leucoxene). Both crystals are surrounded by goethite. Reflected light; oil immersion; length of the rutile crystal: 130/~m. Tophill iron layer.

Minerals occurring in the barren sandstones are also found in the ferruginized ones. The difference, in this case, is that the original cements (quartz, carbonate, kaolinite, anhydrite, and bituminous substance) have partly or completely been replaced by amorphous Fe(OH)3, goethite and lepidocrocite (Fig. 14) without replacing or corroding the quartz grains of the sandstone. Sometimes the ironrich cement consists only of lepidocrocite. In this case, quartz grains are either decomposed and dis-

Fig. 9. Pseudomorphic magnetite replaced by goethite, which was secondary transformed into hematite. Reflected light; oil immersion; diameter of the crystals: about 25/~m. Foothill iron layer.

66

Fig. 10. The intergrown minerals titanomagnetite and ilmenire (trellis-type) are strongly and differently altered (crystal in the center ): titanomagnetite is completely transformed into a mixture of unknown phases (possibly FeOOH + SiO2-aqua ), ilmenite into pseudorutile. Reflected light; oil immersion; diameter of the crystal: 80/xm. Tophill iron layer (concentrate of heavy minerals). appear almost completely, or their former shapes are obscured (Fig. 15). The lepidocrocite-dominated layers show the highest iron content (up to 35 wt.%; analysis 3, see Table 1 ). In transition zones of ferruginized to barren sandstones or in flossary cavities (of stalactitic shape) the cement of the mineralized sandstones consists only of the manganese minerals, e.g. cryptomelane and pyrolusite (Fig. 16). Corroded and decomposed quartz grains are present here, indicating that manganese-bearing solutions are obviously much more aggressive against quartz than those in

Fig. 11. Spinel inclusion in mineralized sandstone. Reflected light: oil immersion; diameter of the crystal: 60 pro. Foothill iron layer.

Fig. 12. Chromite crystal which is partially altered (along the grain boundary) included in ferruginized sandstone (ru=rutile). Reflected light; oil immersion; length of the chromite crystal: 60pro. Tophill iron layer. which usually iron predominates and out of which Fe ( O H ) 3 and goethite mostly precipitate (Fig. 17 ). Occasionally corroded quartz grain boundaries can also be recorded besides goethite and Fe(OH)3. In the immediate neighbourhood of ferruginized sandstones, barren areas show that the quartz grains have already been affected due to corrosion (Fig. 18). Relics of organic substances are frequently found in the sandstones and are generally replaced pseudomorphously by F e O O H (Figs. 19 and 20). This fact is of great significance for the mobilization of iron and manganese contained in the barren sandstones in the form of heavy minerals. The mobili-

Fig. 13. Strongly disturbed and mechanically strained graphite sheets included in ferruginized sandstone. Reflected light; oil immersion; length of the graphite aggregate: 270 pro. Groundfloor iron layer.

67

Fig. 14. The original cement of the sandstone is nearly completely replaced by goethite. Note the smooth grain boundaries of quartz crystals. Reflected light; normal objective; length of the largest quartz crystal: 0.8 mm. Tophill iron layer. zation of iron and manganese within the sandstones might have been caused by the organic material, resulted from the decomposition with a subsequent formation of reducing environment by aggressive rainwater infiltrating into the sandstones. In some parts of the Bahariya Oasis pigeon-like concretions occur at the basis of the denuded sandstone bed. At the centers of these bodies pseudomorphous replacement of primarily organisms by F e O O H can be observed (Fig. 21 ). These concretionary bodies demonstrate that organic material is not only involved within the process of dispersion - as discussed above - they were also of great importance for the concentration of iron. This concentration originates from the decomposition of organic ma-

Fig. 15. Lepidocrocite-mineralized sandstone with relics of strongly corroded quartz. Reflected light; oil immersion; length of the horizontal quartz relic: 80/~m. Tophill iron layer.

Fig. 16. The original cement is completely replaced by cryptomelane (cr), which is partially transformed into pyrolusite (p). The quartz crystals are strongly corroded and they show solution fissures. Reflected light; oil immersion; diameter of the larger quartz crystal: 135 #m. Tophill iron layer.

terial resulting not only in a reducing but also in a sulphide-bearing environment. Thus for iron-bearing solutions a sulphide barrier must have occurred and the former volumes of the organism is pseudomorphously replaced by pyrite a n d / o r marcasite. Due to the strong pyritic (marcasitic) character of the cement the mineralized bodies, which represent the sulphide-bearing halo around the decomposed organic inclusion, are preserved dur.ing degradation. Pyrite (marcasite), now situated near the surface is unstable and is transformed into F e O O H (Fig. 21 ).

Fig. 17. Relics of quartz crystals strongly replaced by cryptomelane. Reflected light; oil immersion; length of the longer edge of the photo: 270 #m. Tophill iron layer.

68

Fig. 18. Partially mineralized sandstone in which all the quartz cry.stals are strongly corroded (also in parts of the barren sandstone). Reflected light; normal objective; length of the mineralized zone: 220/~m. Tophill iron layer.

Comparison of different types of iron layers Regarding the distribution of the syngenetic and epigenetic metallic minerals there are no differences between the three types of iron layers (Tophill, Foothill, and Groundfloor iron layers, see Fig. 4). Characteristically different features of the Groundfloor and Foothill iron layers in comparison to the Tophill iron layer are: ( 1 ) Quartz grains are usually larger and poorly sorted. (2) The thickness of the iron layer is much smaller ( < 1 m ) . (3) The average iron content is lower (about 12 wt.%).

Fig. 19. Limonitized inclusion of organic material in ferruginized sandstone. Reflected light; normal objective; length of the inclusion: 130/~m. Groundfloor iron layer.

Fig. 20. The inclusion of organic material is completely replaced by goethite. Reflected light; oil immersion; diameter of the body: 170/tin. Foothill iron layer. (4) The iron mineralization between quartz grain boundaries shows coarser crystals. (5) Goethite is mostly replaced by hematite (Fig. 22). The transformation of goethite into hematite is well known in red beds (Walker, 1967a, b; Berner, 1969) and led to the characteristic red colour impression of the sandstones. Berner (1969) inferred a m i n i m u m temperature o f 4 0 ° C for the formation of hematite by this transformation process. As a sign of their higher iron concentration and the absence of the transformation of Fe ( O H ) 3 and goethite into hematite, the Tophill iron layers have a brown-black colour impression. Therefore the transformation ofhydroxidic iron minerals into he-

Fig. 21. The central part of ferruginized nodules consists of goethite which had replaced the organic inclusion (org). The organic material itself is embedded in ferruginized sandstone (sa); (g= graphite ). Reflected light; normal objective; length of the graphite crystal: 0.6 mm.

69

Fig. 22. The cement (goethite, light grey) of the ferruginized sandstone is partially transformed into hematite (white). Reflected light; oil immersion; length of the longer edge of the photo: 270 l~m. Foothill iron layer.

Fig. 23. The cement of the mineralized sandstone consists dominantly of cryptomelane (white) and subordinate goethite (grey). Reflected light; oil immersion; length of the cryptomelane mineralization: 250/tin. Tophill iron layer.

matite is restricted to porous iron-rich sandstones of lesser thickness as can be observed in the red-colouted Groundfloor and Foothill iron layers. (6) There are more unaltered ilmenite grains. (7) Sulphidic minerals are found more often (as inclusions in quartz). (8) Graphite is found both in Groundfloor and Foothill iron layers and is, therefore, characteristic of these two types. In the Tophill iron layers graphite is detected only in exceptions. ( 9 ) Cryptomelane was not found but lithiophorire which had replaced it. These differences suggest modified conditions of formation and also indicate variability in the composition of the sandstones (e.g., graphite content) of the two presumably older iron layers.

manganese mineral occurs. As a component of groutite ( M n O O H ) , manganese was integrated into the lattice ofgoethite by isomorphous replacement. However, if manganese content is exceeding 1000 ppm, manganese occurs independently as cryptomelane besides F e O O H (Fig. 23). The trace elements in the sandstone (except iron and manganese) reflect the accessory minerals. They do not correlate with the secondary iron a n d / o r manganese mineralization (Table 1 ). The contribution of elements belonging to minerals of two overlapping parageneses may explain why there is no correlation in element concentrations. Whereas most trace elements are linked to heavy minerals, the concentration of iron and manganese depends on the concentration of the same elements in the circulating waters, on the existing pore volume of the sandstone and the mechanism of precipitation as discussed in the following.

Geochemical analysis Fourteen samples, collected exclusively from the Tophill layers were analyzed (see Fig. 2). The iron content varies [except for one sample ( 3 ) with 35.1 wt.%: Table 1 ] within the range of 14-27 wt.%, witl~ an average of 19.2 wt.%. In all samples (with the exception of sample 3) F e ( O H ) 3 and goethite clearly dominate in comparison with the lepidocrocite. In sample 3 only lepidocrocite is present. This lepidocrocite, due to its replacing habit towards quartz, has gained a larger volume. In the samples with low manganese concentration, varying from 215 to 600 ppm, no independent

The origin of the iron layers The genetic development of the mineralization was first observed at an exposure of the Foothill iron layer in the area of E1 Ris. Several layers (with thicknesses varying from 20 to 50 cm ) are interbedded with barren sandstone, and have moulded out of it by weathering. At the base of such iron mineralization, intercalations of clay or silt with effiorescences of gypsum and anhydrite were found. The layers of the sandstone, ferruginized on one side, can

70

Fig. 24. The original cement consisting of bituminous substances was partially replaced by goethite which is completely transformed into hematite. Reflected light; oil immersion; diameter of the quartz crystal in the center: 50 pro. Foothill iron layer. be traced along a continuous horizontal distance of about 6-8 m to the other side, where no trace offerruginization could be observed. Compared with the neighbouring sandstone these sandstone layers appear to be bleached heavily. Thus ferruginization is strata-bound and is projected tongue-like into the sandstone. Only those parts of the sandstone that are porous and are underlain by, either a less porous sandstone or impervious horizon (e.g., silt), are ferruginized. Percolating rainwaters through the sedimentary rocks cause the separation of certain groups of elements, depending on the prevailing Eh and pH conditions. In the present case iron and subordinate manganese must have been mobilized from the ilmenite leaving behind an enrichment of the immobile titanium. It is evident that iron is also derived from magnetites occurring in the formation. Mobilization of iron and manganese took place under reducing conditions in the form of Fe 2÷, Mn 2+, 3+. In almost all sandstone samples, remains of plant materials as well as enrichments of bituminous substances on grain boundaries were found (Fig. 24). Similar features were observed by Kallenbach (1972) in iron layers from Lybya. The presence of these plant materials confirms the attainment of reducing conditions. The supergene solutions circulating through the sandstone leach the iron and manganese from the rock and move downwards in response to gravity. If these solutions encounter an impervious layer, their downward-directed movement must be con-

verted into a lateral movement. Once a sandstone which has been percolated in this way gets into contact with the oxygen of the air, an oxidation barrier for iron and manganese will be developed. This barrier penetrates into the sandstone in a tongue-like manner; the mineralization associated with this area affects the sandstone in the form of a roll-front. A mobilization of iron and manganese could also be possible in an acidic environment while the precipitation of the same elements may proceed in an area of predominating alkaline conditions. However, there are no indications that this type of precipitation predominates. This can be supported by the uncorroded quartz crystals occurring in ferruginized sandstone (see Fig. 14). Silicon belongs to the stronger migrants in an alkaline environment, whereas iron can hardly migrate and is usually precipitated. If ferruginization had taken place at an alkaline barrier, the quartz grains should have become corroded parallel to the ferruginization process. Alteration of quartz grains, as mentioned earlier, could only be registered in presence of lepidocrocite a n d / o r cryptomelane. The extension of this model in regard to the ferruginization of the Foothill to the Tophill layers of the residual hills entailed some difficulties in the first instance. The residual hills are nearly always covered with ferruginized caps and coarse rubble formed at the surface. Residual hills of this type were found in the southern part and in the south-central portion of the oasis. In the north-central part, where residual hills occur very rarely, the erosion of the ferruginized tophill zones has advanced to such an extent so that the ferruginization process could be clearly recognized. At the center of the tophill, where the iron mineralizations are less thick, barren parts are faintly visible as a result of the strong degree of erosion. The outer zones of these hills show more intensive ferruginization. Thus the Tophill layers are not continuous ferruginized horizons but are arranged concentrically around the peaks. Hence they should be classified like the Foothill layers, which have formed by strata-bound oxidation. From this it can be concluded that a continuous ferruginized cover never existed. Iron-rich layers could only have developed in distinct parts, where the formation of an oxidizing environment in the strata was possible. The definite correlation between the formation of iron layers and tectonics is apparent. According to the model proposed, tectonic activities constitute an

71 essential requirement for the simultaneous formation of the Tophill iron layers in many places. It is apparent that the fractures were developed in the crestal area during the formation of the anticlines. The oxygen of the air could migrate to the deeper sandstone horizons along these fractures. Oxidation barriers then appeared, along which the sandstone became ferruginized (Fig. 25). It seems that certain sandstone beds were strongly infiltrated with iron- and manganese-rich solutions. As a result of the presence of the oxygen which has infiltrated the bed, the metal content of these solutions became oxidized and precipitated. The formation of fractures did not only facilitate the movement of the supergene solutions, but also increases the quantities of solution moving into the beds. On the one hand, this situation consequently led to a stronger iron leaching of the sandstone and on the other, a greater enrichment in the precipitation zone occurred. Both the solubility and the resulting strong leaching of iron and manganese were certainly also favoured by the faintly acidic properties of the reducing solutions. Oxidation barriers alone were probably not sufficient to cause the formation of such large amounts of iron in the layers. It is apparent that besides oxidation, the process of evaporation played an eminent role for the precipi-

tation of iron and manganese (Fig. 25). This cannot only be supported by the occurrence of gypsum outcropping at the boarders of ferruginized bodies, but also by sandstones, exclusively mineralized with manganese, precipitated in stalactitic forms. Thus the strong iron impregnation of the sandstones required, in addition to the type of lithology (porous rocks) and tectonics (deep fractures), humid periods characterized by high rates of precipitation as well as warm, humid periods in which vegetation could develop and flourish. The iron content of the horizons were derived from the detrital iron-bearing minerals contained in the former capping which is now eroded, with the subsequent enrichment after its mobilization in the present ferruginized zone. No detailed information could be given about the thickness of the original capping. On the basis of the observed modal distribution of 3 vol.% ilmenite and 1 vol.% magnetite in the barren sandstone (which corresponds to a total iron amount of 1.8 wt.%) and considering the average iron content of 19.2 wt.% (the average of 13 analyses, see Table 1 ) in the ferruginized beds, an enrichment factor of 10.66 ( = 19.2: 1.8) could be established. Based on the field observation of an average thickness of 20 m for the ferruginized beds, the original capping is calculated to be 213 m

Fig. 25. Geological-geneticaldevelopment of residual hills presented in four stages, from stage I [beginning of iron (and/or manganese) impregnation of sandstone along oxidation and subordinate along evaporation barriers] to stage IV (formation of residual hills after the process of ferruginization). Sandstone of stages II-IV is presented in a simplified manner, leaving out the heavy minerals, their relics and the organic material.

72 ( = 2 0 x 10.66). The weight of such a thick sedimentary capping, however, could not have given rise to the isolated quartz grains observed in the sandstone (see Fig. 14). The estimated thickness would reduce considerably if iron-rich solution may also have been injected laterally. Furthermore it has to be noted that glauconite horizons, containing considerably higher amounts of iron than sandstone, must have existed in the eroded capping. These arguments favour for a former sedimentary capping of about 100 m, which seems to be sufficient to account for the formation of iron horizons. Our model of syngenetic iron enrichment in sandstone is therefore partially different to the process of diagenetic reddening described by Walker (1967a). His model was based in analogy to his observation by an iron enrichment in circulating groundwater, which decomposes detrital mafic silicates (contained in the sandstone) by progressive alteration. Precipitation occurred, according to Walker (1967a), when Eh-pH conditions changed and entered the stability field of Fe (OH)3, goethite and lepidocrocite. In the Bahariya Oasis the ferruginization of sandstone by iron precipitation from iron-rich groundwater is only of subordinate significance. The main source of iron enrichment is due to a per descendum mechanism of iron-bearing solutions. Also contrary to the observation of Walker (1967a) detrital ferromagnesian silicate minerals were not responsible for the original iron concentration in the sandstone of the Bahariya Oasis. In the barren sandstone among the heavy minerals ilmenite and magnetite and their respective relics are dominating.

Sequence of events and age relationships The events leading to the formation of the ferruginization of sandstone can be outlined in a chronological order as follows: ( 1 ) Deposition of terrestrial-fluviatile sandstone with argillaceous-siltic horizons and intercalations of glauconite (Bahariya Formation ). (2) Development of an extensive vegetation cover in a warm, humid climate. (3) Development of a reducing and faintly acid environment in the substrata by decomposition of dead organic substances. (4) Mobilization of iron and manganese out of

the heavy minerals present in the barren sandstone. ( 5 ) Re-activation of old anticlines resulting in the formation of extension joints in the crestal area which provided access for oxygen to deeper parts of the sandstone. (6) Precipitation of iron and manganese at oxidation barriers, mostly localized at impervious horizons, associated with iron-rich solutions of predominately descendent character and whose movement is subordinate lateral. (7) Formation of residual hills by erosion of the sandstone during which the ferruginized parts largely resisted the process of weathering. All the sandstones belong to the Bahariya Formation and are of Cenomanian age. Thus the ferruginization of sandstone could have occurred after the Cenomanian. Since the depression was eroded during the Oligocene, the iron layers must have existed already by that time. The Eocene mainly showed a marine facies and must, therefore, be excluded as a period of origin for these layers. During the Eocene the so-called Plateau Limestone was formed in the Bahariya Oasis (Dominik, 1985). Consequently, the ferruginization must have been developed between the Cenomanian and the lower Middle Eocene ( = oldest unit of the Plateau Limestone). The Cretaceous-Tertiary transition was most probably the period in which the process of ferruginization took place in coincidence with the development of anticlines. Furthermore, the great stratigraphic gap between the Turonian to the Paleocene except for the Campanian (Dominik, 1985 ) is characterized by a period of terrestrial conditions lasting long enough for the development of a dense vegetation. The depositional conditions favourable for ferruginized sandstones seem to be identical with the area, where this stratigraphical gap occurs. The pre-Eocene age for the ferruginized layers, as established above, can be supported by the occurrence of isolated outcrops of the ferruginized layers which are covered by Eocene iron ores at Djebel El Ghorabi.

Acknowledgements We thank H. Kallenbach (Geologisches Institut Technische Universit~it Berlin) and G. Shukry (Geological Survey, Cairo, Egypt) for valuable advice during our fieldwork. We are grateful to U.

73 N e u m a n n for carrying out some analyses with the m i c r o p r o b e (type ARL, G e o c h e m i s c h e s I n s t i t u t of the Universit~it G S t t i n g e n ) a n d to E. Wartala for m a k i n g the drawings. For the critical revision of our m a n u s c r i p t we are grateful to U. Hein. T h e financial support for the project was given by the Deutsche F o r s c h u n g s g e m e i n s c h a f t ( D F G ) within the framework of the " S o n d e r f o r s c h u n g s bereich ( S F B ) 69", which is established at the Technische Universit~it, Berlin.

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