Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau, Nigeria

Journal of African EarthSciences,Vol. 5, No. 5, pp. 535-550, 1986

0731-7247/86 $3.00 + 0.00 Pergamon Journals Ltd.

Printed in Great Britain

Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau, Nigeria IDA VALETON a n d HUBERT BEII3NER Geologisch-Paliiontologisches Institut und Museum der Universitht Hamburg, Btmdesstrai~ 55, D-2000 Hamburg 13, Germany

(Received,for publication 26 February 1986)

Alntract--A geochemical investigation was initated on 24 samples from four different weathering profiles--inselbergs--of the Jos Plateau, Nigeria. Results are presented on field-occurrence, macro- and micro-textures, normative mineralogical composition, kaolinite crystallinity, porosity and density measurements and major and trace element compositions. The polygenetic overprinted profiles, up to 80 m thick, show vertical zonation in soil horizons with very strong Fe- or Al-accumulation in the Box-horizonthat classifythem as fossiloxisolswhose derivation is related to coastal plains during Lower Tertiary times. The fabriques of the in situ weathering profiles show relic textures according to basaltic or magmatic parent rocks in the lower parts. The upper parts of the sections, where relic textures also partly occur, are mainly characterized by neoformed textures like vesicular, concretionary and spongy textures. In the Box-horizons,which are in all sections of basaltic origin, lateral facies change is developed. In profiles 9 and 10, this horizon is developed as an iron crust (Box,~) and in profiles 12 and 13 as bauxite (Box,a). Partly or completely maintained relictic minerals in the lower parts of the sections are quartz, zircon, monazite, rutile and opaque heavy minerals. Later eolian transport led to infiltration by illuviation in the upper parts of the profiles with some of these minerals. The mineral composition of the profiles is mainly determined by newly formed minerals like kaolinite, gibbsite, hematite, goethite and anatase. Kaolinite crystallinityvalues are mostly high except for the B/C-horizons of profiles 9 and 10 as well as for the upper part of the Bfhorizon in profile 9. Porosity and density in the B/C- and Bfhorizons are relatively monotonous. In the Box-horizonsporosity is reduced and density is mainly in relation to AI- or Fe-minerals. Chemical analysisshows enrichment of elements like Al, Fe, Cr, V and Hf in the Box-horizons,whereas soluble elements like alkalis and earth-aikalis have been leached out. Beside the preserved relic textures of the parent rocks, the origin of the weathering products is determined geochemicallyby the discrimination diagram 'Zr/TiO2: Nb/Y' after Floyd and Winchester (1978, Chem. Geol. 21,291-306).

1. I N T R O D U C T I O N TEXTURE properties, mineralogy and geochemistry of four sections with deep supergene laterite alteration were examined which are exposed on relictic inselbergs south of Jos on the Jos Plateau (Figs. 1 and 2). T h e aims of this study are exact descriptions of the weathering sections, their relation to a certain planation plain, the characterization of the p a l e o e n v i r o n m e n t of their formation and their correlation with similar ferallitic cretes in West Africa.

2. G E O L O G Y AND G E O M O R P H O L O G Y OF T H E JOS PLATEAU The deeply eroded Precambrian B a s e m e n t Complex of the Jos Plateau was p e n e t r a t e d during the Late Jurassic by the ' Y o u n g e r Granites' (160-170 Ma, Turner 1976, Bowden et al. 1984). This sequence of alkaline ring complexes is related to intraplate tectonics along a n o r t h - s o u t h lineament from the Hoggar/Air to the Jos Plateau. The creation of the triple junction South A t l a n t i c Benue T r o u g h - N i g e r - B i d a Basin during Early Cretace-

ous produced the first down- and uplift of an old preTertiary peneplained land surface. The consequences were deeply incised rivers, strong erosion of the elevated parts and resedimentation in the depressions. Cretaceous m a g m a t i s m is only known in the Benue Trough (Bowden et al. 1984). The c o m m e n c e m e n t of Tertiary magmatism, related to the C a m e r o o n line, began in the Jos Plateau r e # o n with the 'fluvio-volcanic series' ( M a c K a y et al. 1949). The surface of this series is deeply altered to ferallites. M a c L e o d et al. (1971) described L o w e r to Middle Tertiary plant remains in younger fluviatile to lacustrine cassiterite-bearing sands and clay deposits which intersect these 'oldest laterites' on the plateau. These bauxitebearing ferallites have to be placed into the L o w e r Tertiary (Table 1). The younger Tertiary and Q u a t e r n a r y in the Jos area are characterized by tectonic and volcanic activities with basic lava flows accompanied by coarse (lapilli) to fine (tufts) volcanic ejections. C o n t e m p o r a n e o u s weathering, reworking, erosion and resedimentation in river beds and lakes led to a topographical differentiation of the land surface since the Cretaceous time. T h e elevated Jos area today corresponds to an uplifted tectonic block with preserved old parts of a f o r m e r land surface and its weathering sections (Zeese 1983).

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3. LATERITES IN NIGERIA The studies on laterites in Central and Northern Nigeria made by several scientists (Table 2) are very preliminary and have led to very controversial ideas about their kind of formation and their age. Most of them agree, however, that there are several types of ferallitic and siallitic in situ laterite, differing in age. In Table 1. Sequence of volcanic activities on the Jos Plateau (after Wright 1976 and Bowden et al. 1984)

Omtexmmj Holocene 'Newer Basalt'

Alkali-Basalt (4.6 _ O.1Mato 0.5 + 0.2Ma)

Pleistocene

4. GEOLOGY, MINERALOGY AND CHEMISTRY OF THE LOWER TERTIARY LATERITES ON THE

Ternary Neogene Pliocene

' Older Basalt'

Miocene Paleogene Oligocene Eocene Paleocene

addition, mechanically reworked or chemically reprecipitated younger iron crusts are developed. Tectonical displacement of in situ laterites in the Iullemmeden Basin by subsidence, uplift or inclination is also described by Somebroek (1971). Boulang~ and Eschenbrenner (1971) published a few chemical, mineralogical and textural data about the ferallites on the 'fluvio-volcanic series' of the Jos Plateau. On the Jos Plateau a sequence of planation plains is developed starting with a pre-Tertiary deeply altered surface (Fig. 3), which is partly covered by the 'fluviovolcanic series' preserved by the Lower Tertiary weathering crust (MacLeod et al. 1971) and partly uplifted exposing the fresh rock due to younger erosion.

'Lateritized Older Basalt'

(= Fluvio-volcanic series)

Alkali-Basalt (11.1 + 0.4Mato 4.9 + 0.SMa)

JOS PLATEAU In this paper, four sections south of Jos are studied in detail: Section 9 Kwi Hill, Section 10 Werram, Section 12 west of Mbar and Section 13 south of Mbar (Figs. 1, 2 and 4). They are preserved on relictic inselbergs of a former planation plain in an altitude of + 1400 m NN today. The preserved thickness of the weathering profiles is 50--80 m (Fig. 5). The very thick profile no. 12 presents a lower saprolitic part originating from granite

G e o c h e m i s t r y and m i n e r a l o g y o f the L o w e r T e r t i a r y in situ laterites on the Jos Plateau, Nigeria

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Table 2. Laterites of various age and their possible stratigraphic position in Nigeria (BeiBner 1985) Author Du Preez (1956)

DeSwardt(1964) Burke and Durotoye (1972) Kogbe (1978) Somebroek (1971) Boulang6 and Eschenbrenner (1971) Hill and Rackham (1976)

Locality Nigeria

West- and EastAfrica (Nigeria and Uganda) Nigeria Sokoto-Basin IullemmedenBasin Jos Plateau

Laterites (a) 'Higher-lying' or 'Peneplain laterite' (b) 'Lower-lying laterite' (a) 'Older laterite' (b) 'Younger laterite' (a) 'Older latvrite' (b) 'Younger laterite' Crusty laterites and Fe-rich sandstones Six different Plinthit-formations (a) Laterite of the 'fluvio-volcanic series'

(b) Bauxite-crust Jos Plateau

(a) 'Primary laterite' (b) 'Secondarylaterite'J

Age of laterites Pliocene after the erosion of the 'Higher.lying laterite' older than Miocene is lying in Uganda under deepest Pleistocene end of Tertiary Quatemary post-Miocene 2 Upper Cretaceous 4Eocene Pliocene (curiasses ferrugineuses) Eocene (nivean bauxitique) Plincene to early Pleistocene

537

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I. VALETONand H. BmBNER

,-~1320m

Fig. 3. Sequence of planation plains between Kwi Hill and Werram area. ~ + 1400 m a.s.l.: relic 'inselbergs' of Old Tertiary laterite (Fe-crusts: black). ~ +1320 m a.s.l. (in the front): younger peneplain and the Werram valley (Valeton 1983).

Composed weathering profiles with lower preTertiary saprolitic parts on Precambrian gneisses and upper bauxitic parts on Cretaceous basaltic parent rocks, described from the Bagru Hills, Bihar, India by Valeton (1984), belong to the Lower Tertiary weathering period. Special attention has therefore been given to the question of different origins of the lower and upper part of the described weathering sections herein.

porphyries and an upper ferallitic part originating from basalt, whereas the saprolite of profiles 9 and 10 originates from basaltic parent rocks. The parent rock of profile 13, which is only 6 m thick, is a basalt too. The difference in kind and age of the parent rocks and their transformation leads to the presumption that the thickness of the presently exposed weathering crust could be the result of two different periods of supergene alteration (Boulang6 and Eschenbrenner 1971).

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Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau, Nigeria

profile Box- H.-; . . . . . . . . . . . .

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4.1 Description of the vertical sections, their polygenetic alteration and their texture elements 4.1.1. Description of the sections. The sections which arc exposed in quarries or on slopes of inselbergs are covered with talus material and blocks of iron crusts derived from the reworked surface of the profiles. Therefore the A-horizon and uppermost part of the B-horizon are not preserved leading to truncated sections. Details of polygenetic overprints will be described below. The fresh rock is nowhere exposed (Fig. 4). The sections can be subdivided into B/C-horizon, Br-horizon and Box-horizon. In the B/C-horizon--always situated on the Jurassic alkaline suite--weathering follows a downward directed alteration along the cleavage pattern, producing a very characteristic alteration texture with fresh cores in the central parts, which are surrounded by a sequence of outer shells with increasing alteration from the fresh center towards the fissures. In many places, later erosion of the weathering crust left behind the fresh blocks of the residual core stones (Fig. 6, Thomas 1974). The Br-hodzon was developed in Section 12 in granite porphyrite, in Sections 9, 10 and 13 in the 'fluviovolcanics'. It is characterized by well preserved relict textures of the former parent rock and by preservation of some stable heavy minerals which show more or less strongly etched surfaces. Neoformation of kaolinite by pseudomorphic replacement of former leucocrate minerals is the main alteration (Fig. 7). The very homogeneous saprolitic B,-horizon possesses a thickness of 70 m at least. The boundary between Br- and Box-horizon is, in most of the observation points, limited to more or less one level of altitude. The thickness of the saprolite is unusual for bauxitic ferallites. Only in bauxites near sea-level areas with synpedogenetic subsidence and rising groundwater level during allitization are considerable saprolites formed. However, due to the reduced drainage, the lower part of AES 5:5-G

those saprolites is normally characterized by high contents of smectites, as in the western part of Gujarat, India (Valeton 1983). The Box-horizon is exclusively related to the basaltic parent rock of the 'fluvio-volcanics' which covers the Jurassic granites. In the field, the Box-horizon forms a 6-10 m thick indurated hard layer with very steep slopes. Therefore, mapping of the distribution pattern of the Box-hOrizon from aerial photos is possible (Fig. 3). The Box-horizon still contains some basaltic relic textures (Fig. 8), but a large range of neoformed textures is predominant, which are the result of dissolution and reprecipitation. Deformation plans in basaltic relic textures prove a plastic to soft consistence of this horizon during its reorganization. So-called gel-like textures are very common, like vesicular, concretionary, pisolitic and spongy textures (Figs. 9 and 10).

4.1.2. Polygenetic alteration. The Box-horizon with its predominance of neoformed textures was dated as Lower Tertiary in age. It is therefore overprinted by younger alteration processes which started with beginning uplift, intersection of the former planation plain by rivers and drying out by lowering of the groundwater level. Younger weathering led to truncation of the ferallitic profiles by: 1. Dissolution and mechanical decomposition of the upper part, leaving behind recemented residual breccias in the uppermost part of the sections and big blocks of iron crust or bauxite on the surface of the plateau. Sometimes residual breccias are also developed in the basal part of the saprolites by strong leaching and washout. 2. Remobilization and reprccipitation of iron, silica and others along the slopes as fossil groundwater marks and in the lower planation plains as iron pans. 3. Lateral erosion of the soil sections producing the 'inselbergs' of today.

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Fig. 6. A schematic diagram to show one mode of development and decay for domed inselbergs and residual corestones (Thomas 1974).

Younger mechanical and chemical mobilization is represented also within the weathering profiles by a downward displacement of clay-siltisized particles by (a) eluvation in the upper and illuvation in the lower parts, and (b) by filling of the pore space with successive precipitation of goethite and gibbsite.

4.1.3 Porosity and density. These are the result of a very complex alteration by steps. Primary and secondary densities are mainly determined by density of the minerals and their quantity. Porosity of the parent rocks was nearly zero. By leaching and reprecipitation, it changed continuously in the various soil horizons. The porosity in the B/Chorizon and in the lower Br-hofizon was very high-between 25 and 55%---due to strong leaching and mechanical wash-out (Figs. 13-15). In the Box-horizon, rich in iron, the primary porosity was high but it was secondarily reduced by breakdown of the texture, leading to residual breccias and successive reprecipitation of iron. It varied between 20 and 35%. The Box-horizon, rich in AI, which had probably a high primary porosity, only posssessed a low secondary porosity between 15 and 27%, due to secondary Al-precipitates like pisolites, nodules and spongy textures.

4.2 Mineralogy Minerals have to be subdivided into 1. Stable relic minerals: quartz, zircon, rutile, monazite and opaque minerals; 2. neoformation: kaolinite, gibbsite, goethite, hematite and anatase. The normative mineral composition is given in Table 3. 4.2.1. Kaolinite. In B/C- and Br-horizon kaolinite (content from 1 to 95%), it is the only layer silicate. Its crystallinity was determined by the Hinckley's index (1963). The kaolinite in the B/C-horizon of Section 9 (9) and of Section 10 (15) is b-axis-disordered. In the Brhorizon its crystallinity is very high (Table 5). In principle, there are two generations of kaolinite; the first generation of small crystals (<2 /~m ~b) replaces the feldspar. The second coarser-grained generation (> 2 /~m 40, with idiomorphic outlines sometimes as booklets, is precipitated in fissures, pore spaces and hollows and might be related to a much later formation. 4.2.2. Gibbsite. It is the only AI mineral ranging between 2 and 79% which is mainly restricted to the Box-horizon and originates from basaltic parent rocks.

G e o c h e m i s t r y a n d m i n e r a l o g y o f t h e L o w e r T e r t i a r y in situ l a t e r i t e s o n t h e Jos P l a t e a u , N i g e r i a

Fig. 7. Basaltic relic textures, clearly showing the outlines of the former feldspars---nowkaolinite--in Br-horizon, profile 9, sample 5 (Beil3ner 1985). Fig. 8. Basaltic relic texture, showing gibbsite pseudomorph after plagioclase, profile 13, sample 5 (Beil3ner 1985). Fig. 9. Neoformed spongy texture with high porosity formed from a basaltic parent rock in BoxAi-horizon,profile 13, sample 5 (Bei6ner 1985).

541

542

I. VALETON and H. BEII3NER

Fig. 10. Residual conglomerate in the topmost part of Box.F~-horizon,resulting from breakdown of vesicular textures, profile 9, sample 1 (BeiBner 1985). Fig. 11. Twinned gibbsite crystal growing from solutions in an open pore space, profile 12, sample 3 (BeiBner 1985). Fig. 12. Crystallization of idiomorphic crystals of hematite as late fillings of pore space, profile 9, sample 4 (Beil3ner 1985).

Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau, Nigeria WERRAM

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Table 3, Mineralogical composition of the ferrallites of the Jos area calculated from chemical analysis and X-ray diagrams in Vol. % (BeiBner 1985)

Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Rhyolite Basalt Basalt Basalt Basalt Basalt Rhyolite Basalt Granite-p. Granite-p. Basalt Basalt Basalt Basalt Basalt Basalt

Section

Sample

Kaolinite

Gibbsite

Fe-min.

Anatase

Quartz

9 Kwi Hill

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 3 2 1 6 5 4 3 2 1

28.3 30.8 33.3 78.3 76.2 95.3 94.1 61.0 93.3 22.6 18.7 26.3 26,3 81.0 93.2 5.7 91.3 46.2 2.2 1.4 1.1 14.0 83.3 74.3

5.8 6.6 -~ --~ --5.4 4.5 3.0 6.9 ~ ~ 60.8 ~ -77.7 79.7 77.8 67.3 2.0 ~

63.7 60.4 65.5 17.4 20.6 1.0 2.6 36.2 2.1 70.1 75.0 69.4 64.9 15.4 1.5 31.2 8.3 3.4 17.5 16.4 18.4 16.2 12.6 22.7

2.2 2.2 1.2 4.3 3.2 3.7 3.3 2.8 1.5 1.9 1.8 1.3 1.9 3.6 1.9 2.3 0.4 0.2 2.6 2.5 2.7 2.5 2.1 3.0

---

10 Werram area

12 West of Mbar 13 South of Mbar

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Table 4. Chemical composition of the laterites (Bei6ner 1985) and main element composition of the rhyolite (after MacLeod et al. 1971) Section 9 Sample 1

2

3

4

5

Section 10 6

7

8

9

10

42.87 33.98 1.94 0.11 0.01 0.02 0.21 0.06 1.36 0.09 0.00 12.91 5.55 99.11

9.95 14.18 57.02 0.02 0.03 0.06 0.11 0.00 1.79 0.20 0.00 12.60 2.27 98.23

11

12

13

Section 12 14

15

3

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Section 13 1

6

5

4

3

2

I

6.37 48.58 15.35 0.04 0.03 0.02 0.01 0.00 2.50 0.09 0.00 26.33 0.58 99.90

37,64 33,27 11.82 0.14 0.02 0,04 0.00 0.00 2.00 0.06 0.00 13.74 0.60 99.33

32.05 27.78 20.76 0.12 0.02 0.03 0.00 0.00 2.74 0.15 0.00 12.00 0.80 96.45

SiO2 74.51 A 1 2 0 3 11.36 Fe203 1.70 FeO 2.38 MgO 0.17 CaO 0.45 bia20 3.80 K20 4.66 H2O+ 0.31 H200.11 Cox 0.08 TiO2 0.16 P2Os 0.03 CI 0.02

0 62 220 31 37 0 41 36 25 21 17 7 25 2 0 320 8 46 278

7 122 264 20 23 0 123 26 41 68 17 3 24 0 0 233 7 23 164

0 375 632 31 21 0 386 35 102 77 50 7 44 0 1 442 8 34 226

F

Rhyolite

Main elements (Wt.%) SiO2 AI20~ Fe203 MgO MnO CaO Na20 K20 TiO2 P2Os SO3 H2O+ H20-

12.91 16.03 55.18 0.06 0.03 0.06 0.23 0.00 2.13 0.16 0.00 12.41 1.01 Total 100.84

13.73 17.04 52.09 0.06 0.03 0.06 0.12 0.00 2.07 0.12 0.00 12.15 1.23 98.70

14.00 12.34 59.25 0.07 0.21 0.08 0.19 0.00 1.12 0.04 0.00 11.64 0.66 99.60

34.28 30.09 16.31 0.09 0.03 0.03 0.13 0.00 4.06 0.05 0.00 12.38 1.83 99.28

33.18 40.21 29.36 35.27 18.10 0.94 0.12 0.09 0.04 0.00 0.07 0.04 0.11 0.14 0.00 0.00 3.04 3.36 0.1)8 0.03 0.00 0.00 13.24 13.89 0.89 3.60 98.23 97.57

40.58 25.78 35.79 23.23 2.39 30.36 0.10 0.07 0.130 0.01 0.04 0.06 0.09 0.10 0.00 0.00 3.05 2.50 0.04 0.09 0.00 0.00 14.23 12.98 1.32 2.70 97.63 97.88

20 88 315 56 34 0 88 41 29 72 18 2 16 0 0 588 14 55 532

30 42 179 57 30 0 27 31 28 69 6 3 16 0 0 330 5 51 21i

5

0

18 281 20 56 0 24 24 25 64 13 3 8 3 0 189 9 26 257

29 297 37 48 0 47 23 19 39 15 3 7 0 0 269 12 5 236

8.41 12.11 11.42 12.38 13.56 15.84 63.80 62.17 58.29 0.04 0.07 0.06 0.03 0.03 0.04 0.06 0.05 0.06 0.06 0.14 0.09 0.00 0.00 0.00 1.78 1.32 1.74 0.17 0.26 0.17 0.00 0.00 0.00 11.50 10.44 11.71 1.00 1.10 1.74 99.23101.25101.16

34.65 46.21 2.58 30.97 36.38 40.98 14.23 1.50 29.47 0.09 0.14 0.04 0.03 0.02 0.03 0.04 0.03 0.03 0.05 0.00 0.01 0.01 0.02 0.00 3.28 1.86 2.22 0.05 0.07 0.14 0.00 0.00 0.00 13.54 13.45 24.59 1.10 0.85 0.85 98.04100.53100.94

41.05 70.83 1.03 0.65 0.53 34.84 18.05 51.31 51.59 52.25 7.83 3.03 17.47 16.07 18.15 0.12 0.26 0.03 0.02 0.04 0.01 0.01 0.02 0.03 0.02 0.03 0.07 0.03 0.04 0.02 0.10 0.00 0.00 0.00 0.00 0.00 0.03 0.130 0,00 0.00 0.40 0.18 2.58 2.47 2.68 0.09 0.01 0.06 0.05 0.05 0.00 0.00 0.00 0.00 0.00 14.25 7.40 27.60 28.18 27.39 0.49 0.25 0.33 0.32 0.35 99.21100.12100.46 99.42101.48

21 28 142 44 40 0 12 32 26 46 7 4 10 0 0 372 11 25 252

5

0

37 858 273 24 30 34 35 34 70 12 4 20 0 0 582 14 130 350

76 67 21 81 0 59 102 57 27 15 5 43 49 0 58 17 41 421

Trace elements (ppm) Ba

11

6

0

Ce Cr Cu Ga Hf La Nb bid Ni Pb Rb Sr Th U V Y Zn Zr

0 1121 46 26 55 75 26 25 18 44 14 20 0 0 879 3 50 450

13 812 41 19 57 26 35 18 30 8 10 30 0 0 800 5 56 500

36 163 36 2 60 83 23 27 160 0 8 26 0 0 242 31 350 140

40 6 331 72 21 30 16 29 23 50 0 3 12 0 0 517 3 30 320

138 2 274 10 45 2711 28 54 87 6 0 57 238 0 160 30 104 7 48 46 56 20 6 10 60 34 81 0 9 0 70 845 88 0 83 78 1313 332

0

0

18

5 1405 65 10 64 0 24 25 40 4 I0 16 0 0 989 0 74 268

13 613 89 7 62 0 24 29 80 2 6 16 0 0 712 11 94 200

40 711 31 11 58 0 26 38 30 4 10 20 0 0 823 0 42 288

96 222 77 24 73 0 147 205 75 47 77 4 60 65 4 84 76 83 1580

0 50 18 23 54 0 10 68 13 9 28 5 5 37 1 9 11 58 454

0 37 172 16 30 0 0 33 20 1 12 6 8 0 0 348 4 11 169

0 26 111 16 26 0 0 25 15 8 9 4 9 0 0 265 6 9 171

0 15 135 14 24 0 1 25 15 5 5 5 10 0 0 262 3 8 180

S MnO

0.10

0.02 0.21 100.07 LessO 0.04 Total 100.03 (average of 14 analyses)

Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau, Nigeria

MBAR s o u t h - w e a t h e r e d

basalt

545

hill I

profile 13 m sample . ,~:o Io~:

°~

5

u5

no

/,

Io~ i _~

iiLL ~' i

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)

4e

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j

i

/ I

! !

2

/ -

/

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/

¢

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le

,

0

•

•

,

,

•

40 60 Si02 AI203Fe203 [ w t % l

Bo. x

2"0

30

0

porosit y IVoI.%]

density[g/cm ]

TiO 2 [Wt%]

IIII.

b

,

20

III1 20 40 60 SiO2AI203 Fe203[wt%]

0

2

IVoI.I()0%}

hill II

III II111 II1!

6 • LHHHH.'.'.'~'HHHH~.'.'~

0

5"0

mineral composition

20 30 0 porosity[Vol%l

50 100 mineral composition[Vol.%]

density [g~m~

Ti 02 [Wt.%]

Fig. 15. Porosity (dotted line), density (continuous line) mineralogical and chemical composition of profile 13 (BeiBner 1985).

Table 5. Porosity, density and Hinckley-index of the laterites (BeiBner 1985)

Section 9

Porosity Horizon Sample (Vol. %) Box,Fe

Kwi Hill Br

10

B/C Box,Fe

Werram area

12

Westof Mbar 13

Br B/C Box,Al Br B/C Box.Ai

Southof Mbar Br

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 3 2 1 6 5 4 3 2 1

n.d. = Not determined.

28.6 28.2 35.1 44.0 53.5 42.7 50.3 35.4 41.9 21.9 23.8 20.6 24.9 56.2 43.8 27.5 9.9 42.2 16.4 14.4 16.2 16.8 22.5 24.2

Density (g/era 3)

Hinckley-index

3.20 3.05 3.49 2.82 2.86 2.56 2.63 2.94 2.61 3.44 3.28 3.51 3.33 2.80 2.54 2.80 2.64 2.65 2.63 2.64 2.70 2.61 2.75 2.76

n.d. n.d. n.d.

b-axis disordered 1.17 1.08 1.13 1.35

b-axis disordered n.d. n.d. n.d. n.d. 1.13

b-axis disordered n.d. 0.96 1.12 n.d. n.d. n.d. n.d. 0.76 0.76

Its crystallinity is high in X-ray diagrams as well as in thin sections. Successive phases of mobilization have led to several generations of gibbsite: (i) pseudomorphous after plagioclase, normally very thin crystals (2-30/~m ~b, max. 50--60/~m 40, partly with twins (Fig. 8); (ii) recrystallization of gibbsite from a gel-like material by crystal growth with 20-150 /~m 4>, and destruction of the outlines of feldspar. These crystals rarely show inclusions of Fe minerals; (iii) very pure gibbsite crystallization from solution fills pore space, fissures and desiccation cracks (Fig. 11). The pseudomorphs of gibbsite after feldspar clearly indicate a direct one-phase-desilication without kaolinite as intermittent. During the initial ferallitization the following steps of transformation are observable: 1. feldspar ---> gibbsite 2. feldspar --> gel-like material --> gibbsite 3. migration of aluminium in solution and later reprecipitation as gibbsite cutane in the pore space. The solutions were obviously undersaturated in silica which was drained out of the system without forming intermediate reaction products with the aluminium. The contemporaneous presence of iron in solution will be discussed later on.

546

I. VALETONand H. BEII3NER

4.2.3. Iron minerals, hematite and goethite. Neoformation of iron minerals took place as hematite (a-Fe203 and as goethite a-FeOOH). Determination even after destruction of Al minerals (after Norrish and Taylor 1961) was sometimes difficult or only possible by microscope. The quantity of secondary iron minerals is mostly low in the saprolites originating from rhyolitic as well as from basaltic parent rocks. In the Box-horizon originating from basaltic rocks the content of iron minerals is always above 16%. However, in addition, there is an alternative concentration of iron and aluminium minerals. Sections which are rich in gibbsite possess less than 31% of iron minerals and those which are poor in gibbsite contain between 60 and 75% of iron minerals. Hematite appears in three varieties: (i) Massive hematite which replaces not only the former pyroxene but which impregnates, in addition, the whole 'parent rock', tracing the outlines of feldspars and other texture elements by supply of iron in solution from the exterior. (ii) Submicroscopically fine-dissiminated, freegrained hematite is not related to soil horizons. It mainly appears in kaolinitic matrix as small red rosettes with a crystal size of 5-8/zm (Fig. 12). These two types of hematite belong to the initial phase of neoformed iron minerals during ferallitization, but parts of the fine dissiminated hematite may belong to a later polygenetic phase. (iii) Hematite as rhythmic precipitates fills fissures of 100/.tm. It might be a later precipitate in relation to the initial ferallitization. Here, it can alternate with goethite. Goethite which, due to the iron content, is enriched in the Box-horizon, appears as: Yellow-brown secondary formation in relation to the initial hematite in various neoformed texture elements of the Box-horizon. A negative correlation between hematite and goethite with a topward-rising content of goethite is clearly detected by X-ray diffraction. A younger neoformation of goethite starts from the surface of the sections downward along fissures and pore space of vesicular, tubular or spongy elements during a later polygenetic overprint. The Fe-rich cortex of pisolites is formed either by alternating goethite and relictic (?) hematite or by pure goethite. Superficial crusts of goethite show cauliflowerlike 'Glaskopf' textures. The crystallinity of this goethite is always very high and the replacement of the Fe by Al ranges between 5 and 16% (after Norrish and Taylor 1961). This low Al-substitution is related to hydromorphous, slightly acidic soils (Fitzpatrick and Schwertmann 1982). Submicroscopically fine-dissiminated goethite is always pseudomorph after hematite, with sizes of 5/zm. In addition it forms dust rings in gibbsite rims. Cutane of rhythmic precipitation is partly together with hematite, in any kind of pore space. It often grows as fine needles perpendicularly to the walls or radiately from the whole pore space filling.

In relation to hematite the formation of goethite is always a secondary superimposing process. It might have been active during a long time period starting at the final phase of ferallitization until today. This kind of superimposed goethitization during later times of hematite instabilization in laterites and bauxites has been described before by Valeton (1967) and Kfihnel et al. (1975). 4.2.4. Anatase. Anatase certainly is a neoformed mineral. In rhyolitic and granite porphyry parent rocks with low concentrations of titanium, the content of anatase is below 2%, whereas in the ferralites from basaltic parent rocks it varies between 2 and 4%.

4.3. Lateral change of facies in the Box-horizon As already mentioned, the saprolite is very homogeneous and therefore it does not show any marked lateral facies change. In contrast to this, the differentiation of either iron or aluminium (bauxite)-dominated sections expresses a pronounced lateral change of facies within the Boxhorizon. This lateral variation in textural, mineralogical and chemical composition shall be demonstrated by comparing the Sections 9 and 10, forming a compact Fe-crust, and Sections 12 and 13, where the Box-horizon is developed as bauxite. The facies, high in iron, is characterized by a primary association of hematite and kaolinite, accompanied by traces of gibbsite and low contents of anatase. The predominating fabric elements are vesicular textures with vertical hematitic tubes filled by kaolinite, indicating a former profound root horizon. The uppermost part is strongly influenced by subsequent intense solution processes leading to the formation of residual breccias by mechanical wash-out of the softer kaolinite, by chemical leaching of hematite and goethite precipitation as 'Glaskopf'. The Br-horizon may be influenced by the subsequent iron solution and reprecipitation under changing redoxconditions and may develop a 'mottled zone' in the upper part by impregnation of iron. The facies, high in aluminium, forms a high-quality bauxite with less than 31% iron minerals, 1-14% kaolinite, more than 2% anatase and 60-80% gibbsite. The fabric is dominated by spongeous textures. Unfortunately, in the investigated area, only ferallites on inselbergs are preserved. Therefore it is no longer possible to reconstruct the three-dimensional distribution of the facies pattern within the Box-horizon. In any case, this kind of facies differentiation in iron laterite on the one hand and bauxite on the other is the result of a strong mobilization of all elements, not only in a vertical but also in a horizontal direction. Iron migration demands a reducing environment in a hydromorphic environment.

Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau, Nigeria 4.4. Chemistry The aim of a more detailed examination was to characterize: (i) the chemical properties of the various horizons, (ii) the mobility of the elements in different environments and (iii) the chemical relationship between parent rocks and alteration products. 4.4.1. Chemical properties of the main and trace elements in the various horizons (they are shown in Table 4). The B/C-horizon of Sections 9, 10 and 12 was formed from the Jurassic alkaline suite, as well as the Br-horizon of Section 12. The chemistry of these parts of the sections therefore shows no cosanguinity with the Br-horizon of Sections 9, 10 and 13 and the Box-horizons which originate from the basaltic fluvio-volcanics of the Lower Tertiary. However, in the Br-horizon of all sections, the chemical activities during weathering ran in the same direction, leading to very similar geochemical composition by siallitization (kaolinization). In the Box-horizon the main geochemical differentiation took place by desilication and lateral separation of iron and aluminium. A clear relationship between elements which became depleted with silica or enriched either with Al or with iron is documented. A similar migration pattern for groups of elements will be discussed exemplarily.

4.4.2. Mobility of the elements in different environments. The change of elements took place in three ways: Relative enrichment of elements under more or less isovolumetric conditions by chemical extraction of highly mobile elements, like alkaline and earthalkaline elements, which is the case in the Br-horizon. Additional absolute enrichment by vertical or lateral migration and precipitation from solution rich either in Al and associated elements or in Fe and accompanying elements in the Box-hOrizon. In relation to the siallitic saprolite in bauxites (allites) only Al became more or less strongly enriched; elements like Zr, Ga and Nb are slightly enriched and Ni and Cr are strongly or slightly depleted, whereas in Fe-iron crusts elements like Cr, V and Hf can be enriched and Ti and Ga depleted. La, Ce, Ba, Rb and Sr are strongly extracted from both the AI- and the Fe-rich facies (see Figs. 16-18). Mechanical eluvation from the upper part and illuvation into the lower part of the weathering sections. This process is not active during ferallitization but becomes more and more important during the younger polygenetic overprint and degradation of laterites. The mechanical downward-washed material is not only of autochthonous origin but is mostly an allochthonous dusty to sandy aeolian sediment which covers the surface of the laterites during dry periods. Grains of clay aggregates, quartz, zircon, rutile and others pay for chemical irregularities along vertical drainage channels.

MBAR south hill I

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...........

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0

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/ • I ~ J '~

.C ,

[ppm|

4(]0 0 0 20 40 0 30 60 200 eV •Y • Z r x l 0 moo eCu mZn oCrxl0 aNi

~

.

• .

,,

,

.

,

,

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l()°Rb

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,

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,

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547

.

,

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o •Y

iNb

,

.

,

.

,

,

.

,

20 40 0 200 ~00 eBo mSr oTh eLa raCe oNd

6

~

lb aRb

Fig. 16. Chemical composition of trace elements in profile 13 on basaltic parent rocks showing the comportment of stable (Zr, Ga, Y, Nb), metastahle (V, Ba, Sr) and very instable (Cu, Zn, La, Ce, Nd) elements in Br- and Box-horizon (BeiBner

1985).

548

I. VALETONand H. BEIBNER

~,,~v III ~o ,,, O

O o

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M.

,N, \,~, ~v .

.

',(3; . .. . . ~I~ . .

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.

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.

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.

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~,0

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."

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,,~ ."/o ;ill

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~-'" ,' O/

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~,0 FezO 3

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[Wt%]

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parent rock:

section 10: ~ B/C-. rl Br _. • Box _ horizon

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section

I / : allito

40

Fe z O~ [Wt%]

Ai20~ lWt%]

III:

saprolite

section 12: ~ B/C-.O Br - . • Box- horizon

QO granite porphyrite

IV:

B/C-horizon

section 13:

O•OI•Z~&

& Br-. • Box- horizon

basalt

Fig. 17. Relationship between Fe203/AI203, SIO21A1203, TiO2/AI203,Ga/AI203, V/Fe203 and HffFe203 in the different horizons of the profiles 9-13 (BeiBner 1985).

4.4.3. Chemical relationship between parent rocks and alteration products. From a method by Floyd and Winchester (1978) for chemical interpretation of volcanic tufts by relationship of stable trace elements, it is also possible to determine the type of parent rock of laterites. Especially, the ratio Zr/TiO2 seems valuable for evidence of parent-rock chemistry. Nb is strongly enriched during weathering, giving high Nb/Y rates. The values of samples from the B/C-horizon and from Br of Section 12, which show rhyolitic and granite porphyry relic textures, are situated in the upper right quadrangle of Fig. 19, whereas the values of the samples

originating from basaltic rocks are placed in the field of the basalts.

5. DISCUSSION OF THE RESULTS The laterites are characterized by: Origin in the B/C-horizon from Jurassic igneous, and, in the upper part, from Tertiary basaltic rocks, very thick profiles with expressed vertical differentiation in a kaolinitic saprolite and a ferallitic soil horizon, a marked lateral differentiation of the Box-horizon in a

{or,el(hma~t factor]

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:"

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:-~

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,

~ .~ ::

I / "

i',!.

--

:,

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..

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Ni

Pb

Rb

,o

-' ~

"IT,; ., ~1' ~.

!:

,,

;, ,,:

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Th

U

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elements

Fig. 18. Main and trace element enrichment factors of all samples showing the behavior of the elements in the different horizons, for legend: see Fig. 17 (BeiBner 1985).

Zn

Zr

Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau, Nigeria tOOC÷P

F\

ph

o,lo

"~....

RO*D

0.~0 C+P ~ Ph' T

TA ! A R RO ,I- O AB ', Sub-AO B +N!

: cometclites ÷ pont~terites : ohonotites

: t~e~

: troChyo~esites : ondeMtes :' rhyotites :[rhyodocites * docites :'Otkoti t:W3sOttsl I:' sub-atkotine bosmts I : bosonltes + neohet~ites

ta "-.../_.__

1.()0

10.00

Nb

y

Fig. 19. Relationship Zr/TiO2 to Nb/Y in the different soil horizons of the profiles indicating the chemical composition of the various rocks (BeiBner 1985).

mainly spongeous gibbsite (bauxite) and a mainly vesicular hematitic fersiallite. The question of a possibly different age of an older siallitic and a younger ferallitic weathering crust--proposed by Boulang6 and Eschenbrenner (1971)--is difficult to prove because the parent rocks were transformed to saprolite without any sharp boundary in between. However, as the fluviovolcanics recover a relieved land surface, a period of intense weathering and reworking must have been in between and therefore we support a one-phase ferallitization. Because of the Eo-Oligocene plants in the covering sediments, this ferallitic weathering has to be dated as Lower Tertiary in age. A comparable vertical but mainly lateral separation of iron and aluminium in a Box-horizon under periodical reducing, hydromorphic conditions creating this bauxite-Fe-laterite facies pattern was found in western Gujarat, India (Valeton 1983). There the paleoenvironment of the ferallitization can easily be reconstructed as a periodically flooded coastal plain during Late Paleocene and Early Eocene times. Considering the laterite sections described previously as parts of an adequate paleo land surface, the conclusions concerning the environmental conditions would be: (1) topographic position near sea level (2) periodically inundated coastal plain (3) extreme climatic conditions with high temperatures and extremely wet periods (4) dense vegetation with deep vertical penetration of roots, at least in the areas rich in iron. Accepting a similar paleoenvironment for these Early Paleogene ferallites in the Jos Plateau, phases of important younger tectonic uplift led to the actual elevation of

549

the horst in relation to the graben and basins (Burke 1976). A whole sequence of younger basalts, laterization, reworking and erosion recovers this old planation plain in the northern and northeastern direction. The Jos Plateau therefore is the key area for relative and absolute age of specific events like peneplanation, deep chemical weathering, uplift and erosion, by dating the volcanic activities. The dating of similar ferallitic cretes in Equatorial Africa is still not very easy. The idea that this type of vertically and laterally differentiated ferallites is related to the specific paleoenvironment of flooded plains during phases of regression leads to the question of the age of the laterites on top of the Paleocene marine iron oolithes in the Socoto Basin and on the borders of the Benue Trough. From many countries in Equatorial Africa, extensive and thick ferallitic crusts are well known from Daccar in the West to the Cameroons in the East and Angola in the south. Boulang6 and Eschenbrenner (1971), Sombroek (1971) and Thomas (1974, 1980) apply names like 'cuirasses bauxitiques' and 'niveanx bauxitiques' and use them as Paleogene time markers for dating the events of the continental history in Equatorial Africa (Table 2). This time scale is in good accordance with other deep ferallitic weathering crusts in North and South America, the European Mediterranean area, India and Australia. Acknowledgments--Special thanks are directed to Prof. Kayode from the Department of Geology, University of Ire, Nigeria who invited I. Valeton as a guest professor in Spring, 1983, where many stimulating discussions on laterite genesis with Dr B. Durotoye and a very impressive field excursion with Dr G. Tietz took place. The laboratory work was done by H. BeiBner who passed his diploma examination on these laterites. Financial support by the University of Ire and the D A A D is gratefully acknowledged.

REFERENCES BeiBner, H. 1985. Geochemie und Mineralogie lateritischer Verwitterungsdecken des Jos Plateaus, Nigeria. Thesis, Univ. Hamburg. Boulang~ and Eschenbrenner 1971. Note stir la presence de cuirasses t6moins des niveanx bauxitiques et interm6diaires Plateau de Jos (Nigeria). Bull. Ass. S~n~gal et Quatern. Quest Aft'., Bull. Liaison S~n~ga131, 83-92. Bowden, P., Kinnaird, J. A., Abaa, S. I. Ike, E. C. and Turaki, U. M. 1984. Geology and mineralization of the Nigerian anorogenic ring complexes. Geol. Jb, Hannover B 56, 3-65. Burke, K. 1976. Neogene and quaternary tectonic, of Nigeria. In Geology of Nigeria (Edited by Kogbe, C. A.), pp. 363-369. Elizabethan, Lagos. Burke, K. and Durotoye, B. 1972. The Quaternary in Nigeria. In African Geology (Edited by Dessauvagie, T. F. J. and Whiteman, A. J.), pp. 32.5-348. Ibadan Univ. Press, Ibadan. Du Preez, J. W. 1956. Origin, classification and distribution of Nigerian laterities. Proc. l l l lnt. W. African Conf., Ibadan, pp. 22.3-234. De Swardt, A. M. J. (1964). Lateritisation and landscape development in parts of equatorial Africa. Z. Geomorph. 8, 313-333. Fitzpatrick, R. W. and Schwertmann, U. 1982. Al-substituted 8oethite---an indicator of pedogenic and other weathering environments in South Africa. Geoderma 27,335-347. Floyd, P. A. and Winchester, J. A. 1978. Identification and discrimination of altered and metamorphosed volcanic rocks using immobile elements. Chem. Geol. 21,291-306. Hill, I. D. and Rackham, J. L. 1976. The characteristics and distribution of ironpan on the Jos Plateau, Nigeria. Savanna 5, 79-82.

550

I. VALETON a n d H. BEII3NER

Hinckley, D. N. 1963. Variability in 'cristallinity' values among the Kaolin deposits of the coastal plain of Georgia and South Carolina. Clays and Clay Minerals, 11th National Conference, pp. 229-235. Kogbe, C. A. 1978. Origin and composition of the ferruginous oolites and laterites of North-Western Nigeria. Geol. Rdsch. 67,662-674. Kiihnel, R. A., Roorda, H. J. and Steensma, J. J. 1975. The crystallinity of minerals---a new variable in pedogenetic processes: a study of goethite and associated silicates in laterites. Clays and Clay Minerals 23, 349-354. MacKay, R. A., Greenwood, R. and Rockingham, J. E. 1949. The geology of the Plateau tinfields resurvey 1945-48. Bull. geol. Surv. Nigeria 19. MacLeod, W. N., Turner, R. M. and Wright, E. P. 1971. The geology of the Jos Plateau--general geology. Bull. geol. Surv. Nigeria 32, 1-119. Norrish, K. and Taylor, R. M. 1961. The isomorphous replacement of iron by aluminiumin soil goethites. J. Soil Sci. 12,294-306. Somebroek, W. G. 1971. Ancient levels of plinthisation in RimaSokoto River Basin. In Paleopedology: Origin, Nature and Dating of Paleosols (Edited by Yaalon, D. H.), pp. 329-338. Int. Soc. of Soil Science and Israel Univ. Press. Thomas, M. F. 1974. Tropical Geomorphology. Macmillan Press.

Thomas, W. G. 1980. Timescales and landform development on tropical shields---a study from Sierra Leone. In Timescales in Geomorphology (Edited by Cullingford, R. A., Davidson, D. A. and Lewin, 3.), pp. 333--354. Turner, M. F. 1976. Structure and petrology of the Younger Granite Ring Complexes. In Geology of Nigeria (Edited by Kogbe, C. A.), pp. 142-158. Elizabethan, Lagos. Valeton, I. 1967. Laterite und ihre Lagerst~itten. Fortschr. Mineral. 44, 67-130. Valeton, I. 1983. Palaeoenvironment of lateritic bauxites with vertical and lateral differentiation. In Residual Deposits, Surface Related Weathering Processes and Materials. Geol. Soc., London (Spec. Publ.) 11, 77-90. Valeton, I. 1984. The Cretaceous--Tertiary history of the Bagrn Hill Area, Bihar, and the palaeoenvironment of Bauxite Formation. In Products and Processes of Rock Weathering (Edited by Sinha Roy, S. and Ghosh, S. K.), Hindustan, India. Rec. Res. Geol. 11, 74-81. Wright, J. B. 1976. Volcanic rocks in Nigeria. In Geology of Nigeria (Edited by Kogbe, C. A.), pp. 93-142. Elizabethan, Lagos. Zeese, R. 1983. Reliefentwicklung in Nordost Nigeria, Refiefgenerationen oder morphogenetische Sequenzen. Z. Geomorph. 48, 225-234.