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Micromorphological and chemical complexities of a lateritic profile from basalt (Jos Plateau, Central Nigeria)
Chemical Geology 170 Ž2000. 81–93 www.elsevier.comrlocaterchemgeo
Micromorphological and chemical complexities of a lateritic profile from basalt žJos Plateau, Central Nigeria/ Z. Horvath ´ ) , B. Varga, A. Mindszenty Applied Geology, EotÕos krt.4 r a, Hungary ¨ ¨ L. UniÕersity, H-1088 Budapest Muzeum ´ Received 3 January 1998; accepted 22 June 1999
Abstract A ferricrete-capped lateritic profile developed on basaltic saprolite is described. Within both ferricrete and the immediately underlying soft laterite, two types of sesquioxidic cement are distinguished: the isopachous goethitic intergranular cement which is predominant and is interpreted as an indication of past phreatic conditions; and the Fe-stained meniscus cement which is interpreted as a sign of vadose conditions. The observed features are suggested to reflect changing drainage conditions related to morphological rejuvenation of the profile, in full accordance with previous hypotheses developed on the basis of geomorphological consideration. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ferricrete; Laterite; Sesquioxidic cement
1. Introduction The West African Bauxite Province ŽBardossy ´ and Aleva, 1990. consists of several localities of economy grade bauxite; however, only a few minor occurrences were described from Nigeria ŽBoulange´ and Eschenbrenner, 1971; Mindszenty, 1976., most recently from the Mambilla Plateau ŽSchwarz, 1994a,b.. This latter formation developed on various parent rocks in the Cameroon Volcanic Zone. The Tertiary volcanism of the Jos Plateau is also related to the above-mentioned zone ŽMackay et al., 1949 in Valeton and Beissner, 1986.. The two plateaux ŽMambilla and Jos. are situated at similar latitudes;
)
Corresponding author. Tel.: q36-1-267-0820; fax: q36-1266-0162.
nevertheless, bauxite minerals occur only in low concentrations in the latter as detected by Boulange´ and Eschenbrenner Ž1971., Valeton and Beissner Ž1986., Valeton Ž1988., and Becker Ž1992.. In 1995, we visited several localities on the Jos Plateau and in the surrounding area. This paper describes a gibbsite-bearing lateritic profile developed on basaltic parent rock 30 km to the South of Jos close to the village of Barakin Ladi — a site not yet mentioned by the above cited authors. The investigation focused on the micromorphological features and chemical composition of the laterite. Fieldwork and laboratory observations were undertaken in cooperation with the Institute of Geography of the University of Cologne and the Institute of Economic Geology of the University of Berlin. The main goal of the study was to provide new data relevant to the Tertiary geomorphological evolution of the Plateau.
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Z. HorÕath ´ et al.r Chemical Geology 170 (2000) 81–93
2. Location of the Jos Plateau The Jos Plateau is an outstanding feature in Northeastern Nigeria situated at latitudes 10811X and 8855X and longitudes 8821X and 9830X , covering a total surface area of 9400 km2 . It is bounded by the Kaduna Plains to the North and West, by the Bauchi Plains to the East, and by the Benue Plains to the South. The highest peak rises to 2010 m asl to the East of Jos, the Capital of Plateau State. The northern part of the plateau shows a gradual transition to the Bauchi Plain, whereas the margins facing the plains to the South and South–West are marked by steep escarpments.
3. Previous research In 1971, Macleod et al. published the geology of the Jos Plateau. In the same year, Boulange´ and Eschenbrenner Ž1971. discussed the morphological and mineralogical characteristics of a ferruginous crust on the flat topped hills. Valeton and Beissner Ž1986. studied the geochemistry and mineralogy of in situ laterites on the Jos Plateau and described the observed weathering profiles. They suggested that these weathering profiles were related to a particular planation surface; they characterized the paleoenvironment of the formations and they correlated the profiles with similar ferrallitic cretes in West Africa. Using characteristic features of paleosoils, Zeese Ž1991. pointed out the connection between the characteristic features and age of paleosoils in Central and Northeast Nigeria.
4. Geology The Jos Plateau is a part of the Basement Complex of Nigeria ŽFig. 1. consisting of migmatite, gneiss, undifferentiated igneous Žolder granite., and metamorphic rocks of Pre-Cambrian to MidCambrian age and the so called younger granite complex including granites, syenites, gabbros, anorthosites, and volcanic rocks of Mid-Jurassic in age ŽWright, 1974.. The granites are of intrusive or of metasomatic origin.
The Mid-Jurassic intrusive event is related to the break up of Gondwana and the beginning of the separation between Africa and South America. Famous tin deposits are connected to these ring-shaped formations, mostly on the Jos Plateau. The creation of the triple junction South Atlantic – Benue Trough – Niger-Bida Basin during Early Cretaceous, resulted in the first faulting event of the old pre-Tertiary peneplained land surface. Progressing from SW to NE, the Cretaceous transgression gradually invaded the northeastern part of the country. The Early Cenozoic is characterised by intense erosion of the elevated parts and accumulation of the resedimented material in the depressions. On the Jos Plateau, the first known record of the Cenozoic volcanics is the so called Fluvio Volcanic Series, which is probably older than Early Oligocene ŽMcleod et al., 1971; Zeese, 1991.. This formation and the younger Tertiary and Quaternary products of volcanic activity serve as the parent rocks of many of the weathered lateritic profiles on the Jos Plateau. Contemporaneous or later faulting led to displacement and tilting of lateritized Older and Newer Basalt and the basement. Lateral erosion of the soil sections produced the characteristic tablehill topography.
5. Methodology Samples were collected from along the scarps of the Barakin Ladi plateau and from the covering ferruginous hardpan and were investigated by using quantitative XRF and XRD analyses. The XRF measurements were carried out on a Philips instrument, type PW1410-10, using the program ‘‘oxiquant’’. XRD measurements were carried out on a Philips instrument, Cu-anode type PW1710 and countgoniometer type PW1820. XRD measurements were controlled by the software PC-APD, version 3.5. The use of these two methods facilitated the calculation of the quantitative mineralogical composition of the samples, as they contain only a limited number of mineral phases such as kaolinite, goethite, hematite, and gibbsite. Thin sections were described using a terminology partly derived from Brewer Ž1964., partly from
Z. HorÕath ´ et al.r Chemical Geology 170 (2000) 81–93
83
Fig. 1. Geological sketch map of the Jos Plateau Žreworked after Dessauvagie, 1974..
Stoops Ž1983., Bullock et al. Ž1985., and Bardossy ´ and Aleva Ž1990., always using the one best suited to the material in question. Munsell-colour of the dried samples was also recorded.
saprolite showing the residual structure of basalt and the more altered upper part of the profile. At the bottom of the profile, there is the saprolite with Liesegang banded structure.
6. Profile description
7. Micromorphology
On the top of the studied weathering profile, a vesicular, concretionary, pisolitic hardpan iron-cap can be observed. Below, there is a goethite-rich gibbsitic horizon. It is followed by a clayey horizon with subangular clasts, showing chemical brecciation Žmottled zone.. This is a transition zone between the
A summary of the most important features recognized is as follows. Ž1. The relict texture of the basaltic rock is clearly preserved not only at the bottom of the profile, but in weathered clasts of the upper, oxic horizon as well. The borders of the basaltic clasts appear to be worn,
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serrated, andror mammilated. Basaltic phenocrysts Žformer olivine, pyroxene, and feldspar. are easy to identify by their shape and cleavage, the latter is always filled by Fe-oxides ŽPlate 1a.. Ž2. XRD analytical observations were used to confirm and quantify the presence of gibbsite, observable also with the optical microscope Žless than 5%.. Gibbsitization is closely related to the iron coated, tightly intergrown feldspar laths of the groundmass. On the other hand, macrocrystalline gibbsite can be seen along some cracks, biogalleries,
and in voids as well. Gibbsite also occurs as ‘‘porphyroblasts’’ in mosaic-like groups of often twinned ŽPlate 1b., sometimes palmate crystals. Gibbsitic-cutans can be found around some goethitic nodules throughout the uppermost 1 m of the profile. Ž3. Traces of bioturbation are abundant in the upper part of the profile. Bioturbation is obvious because of the homogenous matrix and the elongate, rounded channels, which are tapering and branching in some cases. These macro and micropores are usually coated by iron oxide and filled by clay
Plate 1. Ža. Olivine phenocryst. Relict texture in a weathered basaltic clast. Scale bar: 100 mm, qN. Žb. Fibrous gibbsite crystals in a pore. Scale bar: 100 mm, qN.
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cutans and younger yellowish soil material, which consists of homogenous clay with scattered silt-size quartz grains and exhibits a clear micropedality ŽPlate 2a..
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Ž4. Examining the thin sections from the middle and upper part of the profile, we recognized a characteristic granular microstructure consisting of granules and ferritic micropeds ŽMuller, 1977 in Stoops,
Plate 2. Ža. Cross-section of a biogallery, coated by clay and by ferritic micropeds. Scale bar: 200 mm, qN. Žb. Kaolinitic–hematitic boxwork structure. Scale bar: 100 mm, qN.
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1983.. Ferritic micropeds appear in the oxisol-type homogenous matrix, along cracks, and in clay-coated voids which are supposed to be result of bioturbation ŽPlate 2a.. Ferritic micropeds postdate most of the clay coatings.
Ž5. Several types of clay cutans can be distinguished in the intergranular space mainly in the upper part, but in trace amounts also at the bottom of the profile. The amount of these clay coatings is around 0–5%, as shown by point counting. The
Plate 3. Ža. The isopachous cement is a sign of phreatic conditions. Scale bar: 200 mm, 1 N. Žb. Meniscus cement idicates vadose conditions. Scale bar: 100 mm, 1 N.
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Table 1 TirFe and ZrrTi ratios along the profile
Fig. 2. ZrrTi plot of the Barakin Ladi laterite samples clearly displaying their basaltic origin. Discrimination fields after Hallberg Ž1984..
colour of the cutans varies from brown to yellowish brown to yellow and red. Usually, cutans follow the walls of the voids impregnated by iron oxide and were precipitated clearly before the accumulation of the ferritic micropeds ŽPlate 2a.. According to Soil Taxonomy ŽSoil Survey Staff, 1975., in oxisols, less than 1% clay cutans should be present. Thin cutans near the surface may be the result of local Že.g., lateral. clay translocation, rather than vertical eluviation and illuviation process, and are probably related to the drying and wetting cycles of the soils as suggested by Stoops Ž1968. Žin Stoops, 1983.. Ž6. One of the most conspicuous parts of this profile is the concretionary Žpisolithic. ferricrete which forms a horizon about 30-cm thick. It consists
Depth Žm.
TirFe Žwt.%rwt.%.
ZrrTi Žwt.%rwt.%.
0.0 0.4 0.6 0.8 1.0 2.5 3.5 11.7
0.0392 0.0121 0.1875 0.1400 0.0589 0.1233 0.1311 0.1777
0.0228 0.0285 0.0204 0.0105 0.0140 0.0091 0.0097 0.0089
almost entirely of pisoids ŽPlate 3b. and ooids with very little powdery matrix. About 100–500 mm sized pisoids and ooids show several phases of septaria formation. The ooids are cemented by a goethitic– ferruginous material, which displays either meniscus-like or isopachous ring-morphologies. Ž7. Within both ferricrete and the immediately underlying soft laterite, two types of cement could be distinguished in macropores and in the intergranular space. The isopachous, orange coloured, goethitic intergranular cement is predominant in some cases and is particularly abundant in the saprolite. This type of cement is built up of thin, goethitic–clayey zones parallel with the surface of the irregular shaped voids ŽPlate 3a.. We suggest that isopachous cement is an indication of precipitation from solutions having completely filled the available porespace and
Fig. 3. TirFe and ZrrTi values along the profile.
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therefore can be considered as a sign of past phreatic conditions. Fe-stained cement of meniscus morphology occurs only inbetween the ooids of the iron cap. It is considered to be the sign of vadose conditions precipitated from solution adherent to the grain surfaces when most of the porespaces were aerated ŽPlate 3b.. We think that the presence of both isopachous and meniscus cements in the concretionary ferricrete is the direct result of changing conditions of saturation in the profile. This confirms the idea of
the profile having been formed in a position near to the groundwater table. Ž8. The top of the lateritic profile is capped by a vesicular, concretionary laterite conglomerate consisting of redeposited older, partly ferralitic soil material. Abundant clay cutans coating some of the pore-spaces suggest a non-oxisol type overprint of ferrallitization. Ž9. Boxwork textures were detected in one single sample only, right underneath the concretionary iron-
Fig. 4. Chemical and mineralogical changes along the vertical profile.
Depth Žm.
SiO 2 Ž%.
Al 2 O 3 Ž%.
Fe 2 O 3 Ž%.
TiO 2 Ž%.
Zr Žppm.
Kaolinite Ž%.
Gibbsite Ž%.
Hematite Ž%.
Goethite Ž%.
L.O.I. Ž%.
Sum of elements
Sum of minerals
0.0 0.4 0.6 0.8 1.0 2.5 3.5 11.7
19.30 30.65 36.81 38.69 31.98 33.91 34.58 37.36
20.07 27.80 35.74 35.43 29.81 29.84 28.17 31.21
46.52 26.91 9.05 9.61 23.73 21.76 21.24 14.90
2.13 0.38 1.98 1.54 1.63 3.13 3.25 3.09
291 65 242 141 137 171 189 165
41.46 65.85 79.08 83.12 68.71 72.85 74.29 80.27
5.64 2.73 6.88 3.97 4.08 1.62 0.00 0.00
25.28 12.83 5.91 6.31 22.48 20.72 20.55 14.02
23.62 15.67 3.49 3.67 1.39 1.15 0.76 0.98
12.38 14.86 16.47 14.96 13.32 12.41 12.86 13.41
100.73 100.95 100.20 100.42 100.72 101.25 100.48 100.20
98.15 97.46 97.35 98.61 98.29 99.48 97.06 97.59
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Table 2 Chemical and mineralogical composition of the Barakin Ladi Profile
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crust, within the kaolinitic–gibbsitic zone, where cracks and fissures of the weathered kaolinite rich material were filled by iron oxide ŽPlate 2b.. Ž10. In addition, a few quartz grains could also be detected mainly from the top of the profile pointing to wind-blown contribution or some other process of transportation and deposition in a topographically low position.
8. Changes in the chemical and mineralogical composition The saprolite is of basaltic origin, which is clearly shown by its texture. We might assume a basaltic origin for the entire profile according to the ZrrTi plot of the samples ŽFig. 2.. Fig. 3 shows the changes of the TirFe ratio vertically in the profile. Its upward decreasing trend in the saprolite is followed by an abrupt increase in the clayey horizon, and above it these values are changing erratically. These TirFe ratios do not fit into the trend of a typical in-situ weathering profile. The upward decreasing TirFe ratio in the saprolite can be explained by the formation in the zone of the water–table fluctuation where, due to repeated dissolution and precipitation, lateral translocation of the elements takes place. The erratic changes of the TirFe ratio above the saprolite suggest sedimentation of some allochthonous material. This observation seems to be confirmed by the trend of the ZrrTi ratio ŽFig. 3, Table 1.. The chemical composition ŽFig. 4, Table 2. displays that silica is accumulated in the clayey horizon, but it becomes less abundant toward the top, while the amount of Fe increases. The mineralogical composition is more informative. It displays very clearly the variation of gibbsite in the vertical profile. Also, the two Fe-minerals can be separately observed. The amount of kaolinite in the transition zone is larger than that in the saprolite. Toward the top, kaolinite becomes less abundant. Kaolinite crystallinity can be estimated by measuring its D-value and peak-width at the half height of its 001 basal reflection. An increase of these values corresponds to poorer crystallinity. At the transition zone, these
values also show discontinuity. Toward the top, both values increase ŽFig. 5, Table 3.. The transition zone can be assumed as an illuvial horizon, with the accumulation of kaolinite and the removal of ironminerals. Gibbsite occurs in small quantities at the top of the profile; downward its amount decreases. The changes in distribution of the iron-minerals is also connected to the boundary of the saprolite and the clayey horizon. Toward the top, goethite becomes predominant, while in the lower part hematite is the main Fe-mineral. The distribution of the Fe-minerals is unusual. The predominance of hematite would have been expected at the very top of the profile, where due to free drainage, oxidizing conditions prevail, while in the lower parts, under more phreatic conditions, oxihydroxides should have been present. Our results show the contrary: hematite is predominant at the bottom, whereas goethite is abundant on top. To resolve the problem, the possible genesis of Fe-oxides should be taken into consideration
Fig. 5. D-values and peak-width at half height of the Ž001. plane of kaolinite along the profile Ž ; index of crystallinity..
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Table 3 Selected values of XRD analyses were used in order to estimate the crystallinity of kaolinite Depth Žm.
2Q of the Gibbsiter
Deviation
2Q Kaolinite
˚. D kaolinite ŽA
0.0 0.4 0.6 0.8 1.0 2.5 3.5 11.7
18.291 18.292 18.294 18.311 26.651 26.630 26.643 26.609
Kaolinite
˚. peak-width ŽA
quartz peak y0.02 y0.02 y0.02 0.00 y0.01 y0.03 y0.02 y0.06
ŽFig. 6a.. As it is well known from Schwertmann’s investigations, the first precipitate is normally an amorphous, metastable mineral, ferrihydrite ŽSchwertmann and Fitzpatrick, 1992.. Because of its metastability, with time it is converted to other Feoxides or oxihydroxides. This transformation is controlled by the conditions under which it takes place. Such conditions are water activity and the presence of organic matter. In the presence of organic matter, the formation of
12.270 12.312 12.280 12.317 12.315 12.314 12.308 12.264
7.227 7.202 7.220 7.188 7.195 7.208 7.204 7.204
0.345 0.244 0.330 0.269 0.273 0.230 0.285 0.260
hematite is hampered ŽTardy and Nahon, 1985; Schwertmann and Fitzpatrick, 1992. and goethite is favored. Such may be the situation on properly forested hilltops. Where organic matter is absent, hematite may form Že.g., hilltops without a forest cover or, alternatively, deeper but still well-drained horizons of the lateritic profile.. In areas of groundwater discharge with high water-activity, goethite is expected ŽTardy and Nahon, 1985.. We think that the hard, fine grained, homoge-
Fig. 6. Tentative explanation for precipitation and dissolution of iron-minerals under different environmental conditions Ža. before and Žb. after base-level drop.
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nous, highly goethitic material exposed by the nearby gully may represent such a situation. More so, it shows no signs of resedimentation so it could not have been originally formed at foothill position. Based on its mineralogical composition Žabsence of Al-minerals!., it cannot have been formed as an isalterite in hilltop position, either, but rather close to the groundwater–table. When the water-table falls ŽFig. 6b. Žeither because of uplift or because of climatic change. and vegetation becomes poorer, hematite may also form on the top. According to Schwertmann Ž1985., goethite may form in two different ways. In the case of high water-activity, it precipitates directly from solution. These crystals are normally birefringent and show a columnar habit, whereas goethite formed on alteration of hematite is, as a rule, isotropic. ŽSchwertmann, 1985..
9. Conclusion In the upper part of the studied profile, we have found a partly isalteritic–kaolinitic saprolite in the process of gibbsitization. It is covered by a partly concretionary–partly pisolitic ferricrete. The micromorphology of the ferricrete — particularly the presence of isopachous and meniscus cement — clearly reflects that it formed under alternately vadose and phreatic conditions, probably in a low-level position Žvalley bottom or valley terrace.. This is in full accordance with the idea of concretionary laterites being formed in gentle topographic depressions within the zone of the water-table Žsee also Boulange´ and Eschenbrenner, 1971.. In topographic depressions, the possibility of the sedimentation of some allochtonous material is always high. Geochemical data suggest that the upper part of the studied profile, namely the gibbsitic horizon and the iron cap, might be partly of such an origin Žthey show that some minor contribution from a nonbasaltic source was a possibility to reckon with.. The present uplifted position of the profile and its complex structure must be the result of repeated morphological rejuvenations ŽBoulange, ´ 1984.. The observed progressive gibbsitization of the kaolinitic
isalterite we assign to these rejuvenations is most probably related to the uplift of the Jos Plateau. The recognition of various ferruginous cementmorphologies reflecting the changing hydrological environment in the crust deserves special attention. It shows that micromorphological observations may reinforce the hypotheses developed on the basis of geomorphology alone. 10. Uncited reference Schwarz, 1997 Acknowledgements We acknowledge the financial support of the Volkswagen Foundation and OTKA-T-019309. We are grateful to Dr. Reinhard Zeese, cordinator of the project. Thanks are due to Dr. Laszlo ´ ´ Fodor for his contribution during the fieldwork and to Dr. Torsten Schwarz for the useful discussions. The XRF analisys were made by the second author at laboratories of TU Berlin. References Bardossy, G., Aleva, G.J.J., 1990. Lateritic bauxites. Develop´ ments in Economic Geology No. 27 Elsevier, 624 pp. Becker, A., 1992. A time–space model for the genesis of Early Tertiary laterites from the Jos Plateau, Nigeria. J. Afr. Earth Sci. 15 Ž2., 265–269. Boulange, de ´ B., 1984. Les Formations Bauxitiques Lateritiques ´ Cote-D’ivoire Travaux et Documents de l’Orstom No. 175, ˆ 341 pp. Boulange, ´ B., Eschenbrenner, V., 1971. Note sur la presence de cuirasses temoins des niveaux bauxitiques et intermediaires Plateau de Jos ŽNigeria.. Assoc. Senegal. Etude. Quat. Afr., Bull. Liaison No. 31, 83–92. Brewer, R., 1964. Fabric and Mineral Analysis of Soils. Huntington, New York, 482 pp. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T., Babel, U., 1985. Handbook for Soil Thin Section Description. Waine Research Publications, England, 152 pp. Dessauvagie, T.F.J., 1974. The Geological Map of Nigeria, Scale: 1:1 000 000. Hallberg, J.A., 1984. A geohemical aid to igneous rock type identification in deeply weathered terrain. J. Geochem. Explor. 20, 1–8. Macleod, W.N. et al., 1971. The geology of the Jos Plateau: Vol. 1. General geology. Geol. Surv. Niger., Bull. 32 Ž1., 119 pp.
Z. HorÕath ´ et al.r Chemical Geology 170 (2000) 81–93 Mindszenty, A., 1976. Some remarks on the laterites of Nigeria. Travaux du ICS No. 13, pp. 185–190, Zagreb. Schwarz, T., 1994a. Ferricrete formation and relief inversion. Catena 21, 257–268. Schwarz, T., 1994b. Bauxite formation on different parent rocks on the Mambilla Plateau ŽSE Nigeria.. In: Smith, B.J., Warke, P.A. ŽEds.., Eurolat ’94, Laterites, Paleoweathering and Paleosurfaces, pp. 16–20, Belfast. Schwarz, T., 1997. Distribution and genesis of bauxite on the Mambilla Plateau, SE Nigeria. Appl. Geochem. 11, 1–13. Schwertmann, U., 1985. Properties of goethites of varying crystallinity. Clays Clay Miner. 33, 369–378. Schwertmann, U., Fitzpatrick, R.W., 1992. Iron minerals in surface environments. Catena Suppl. 21, 7–30. Stoops, G., 1983. Micromorphology of the oxic horizon. Soil Micromorphology. Soil Genesis A,B Vol. II, pp. 419–440.
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Tardy, Y., Nahon, D., 1985. Geochemistery of laterites, stability of Al-goethite, Al-hematite, and Fe 3q-kaolinite in bauxites and ferricretes: an approach to the mechanism of concretion formation. Am. J. Sci. 285, 865–903. Valeton, I., 1988. Verwitterung und Verwitterungslagerstatten. In: ¨ Fuchtbauer, H. ŽEd.., Sedimente und Sedimentgesteine. Sedi¨ mentpetrologie Teil II Vol. 2, pp. 11–68, Stuttgart. Valeton, I., Beissner, H., 1986. Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau. Niger. J. Afr. Earth Sci. 5 Ž5., 535–550. Wright, J.B., 1974. Volcanic rocks in Nigeria. In: Kogbe, C.A. ŽEd.., Geology of Nigeria, Section 2, pp. 93–143, Lagos. Zeese, R., 1991. Paleosols of different age in Central and Northeast Nigeria. In: Lang, J. ŽEd.., Sedimentary and Diagenetic Dznamics of Continental Phanerozoic Sediments in Africa. J. Afr. Earth Sci. 12 Ž1r2., pp. 311–318.
Micromorphological and chemical complexities of a lateritic profile from basalt žJos Plateau, Central Nigeria/ Z. Horvath ´ ) , B. Varga, A. Mindszenty Applied Geology, EotÕos krt.4 r a, Hungary ¨ ¨ L. UniÕersity, H-1088 Budapest Muzeum ´ Received 3 January 1998; accepted 22 June 1999
Abstract A ferricrete-capped lateritic profile developed on basaltic saprolite is described. Within both ferricrete and the immediately underlying soft laterite, two types of sesquioxidic cement are distinguished: the isopachous goethitic intergranular cement which is predominant and is interpreted as an indication of past phreatic conditions; and the Fe-stained meniscus cement which is interpreted as a sign of vadose conditions. The observed features are suggested to reflect changing drainage conditions related to morphological rejuvenation of the profile, in full accordance with previous hypotheses developed on the basis of geomorphological consideration. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ferricrete; Laterite; Sesquioxidic cement
1. Introduction The West African Bauxite Province ŽBardossy ´ and Aleva, 1990. consists of several localities of economy grade bauxite; however, only a few minor occurrences were described from Nigeria ŽBoulange´ and Eschenbrenner, 1971; Mindszenty, 1976., most recently from the Mambilla Plateau ŽSchwarz, 1994a,b.. This latter formation developed on various parent rocks in the Cameroon Volcanic Zone. The Tertiary volcanism of the Jos Plateau is also related to the above-mentioned zone ŽMackay et al., 1949 in Valeton and Beissner, 1986.. The two plateaux ŽMambilla and Jos. are situated at similar latitudes;
)
Corresponding author. Tel.: q36-1-267-0820; fax: q36-1266-0162.
nevertheless, bauxite minerals occur only in low concentrations in the latter as detected by Boulange´ and Eschenbrenner Ž1971., Valeton and Beissner Ž1986., Valeton Ž1988., and Becker Ž1992.. In 1995, we visited several localities on the Jos Plateau and in the surrounding area. This paper describes a gibbsite-bearing lateritic profile developed on basaltic parent rock 30 km to the South of Jos close to the village of Barakin Ladi — a site not yet mentioned by the above cited authors. The investigation focused on the micromorphological features and chemical composition of the laterite. Fieldwork and laboratory observations were undertaken in cooperation with the Institute of Geography of the University of Cologne and the Institute of Economic Geology of the University of Berlin. The main goal of the study was to provide new data relevant to the Tertiary geomorphological evolution of the Plateau.
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2. Location of the Jos Plateau The Jos Plateau is an outstanding feature in Northeastern Nigeria situated at latitudes 10811X and 8855X and longitudes 8821X and 9830X , covering a total surface area of 9400 km2 . It is bounded by the Kaduna Plains to the North and West, by the Bauchi Plains to the East, and by the Benue Plains to the South. The highest peak rises to 2010 m asl to the East of Jos, the Capital of Plateau State. The northern part of the plateau shows a gradual transition to the Bauchi Plain, whereas the margins facing the plains to the South and South–West are marked by steep escarpments.
3. Previous research In 1971, Macleod et al. published the geology of the Jos Plateau. In the same year, Boulange´ and Eschenbrenner Ž1971. discussed the morphological and mineralogical characteristics of a ferruginous crust on the flat topped hills. Valeton and Beissner Ž1986. studied the geochemistry and mineralogy of in situ laterites on the Jos Plateau and described the observed weathering profiles. They suggested that these weathering profiles were related to a particular planation surface; they characterized the paleoenvironment of the formations and they correlated the profiles with similar ferrallitic cretes in West Africa. Using characteristic features of paleosoils, Zeese Ž1991. pointed out the connection between the characteristic features and age of paleosoils in Central and Northeast Nigeria.
4. Geology The Jos Plateau is a part of the Basement Complex of Nigeria ŽFig. 1. consisting of migmatite, gneiss, undifferentiated igneous Žolder granite., and metamorphic rocks of Pre-Cambrian to MidCambrian age and the so called younger granite complex including granites, syenites, gabbros, anorthosites, and volcanic rocks of Mid-Jurassic in age ŽWright, 1974.. The granites are of intrusive or of metasomatic origin.
The Mid-Jurassic intrusive event is related to the break up of Gondwana and the beginning of the separation between Africa and South America. Famous tin deposits are connected to these ring-shaped formations, mostly on the Jos Plateau. The creation of the triple junction South Atlantic – Benue Trough – Niger-Bida Basin during Early Cretaceous, resulted in the first faulting event of the old pre-Tertiary peneplained land surface. Progressing from SW to NE, the Cretaceous transgression gradually invaded the northeastern part of the country. The Early Cenozoic is characterised by intense erosion of the elevated parts and accumulation of the resedimented material in the depressions. On the Jos Plateau, the first known record of the Cenozoic volcanics is the so called Fluvio Volcanic Series, which is probably older than Early Oligocene ŽMcleod et al., 1971; Zeese, 1991.. This formation and the younger Tertiary and Quaternary products of volcanic activity serve as the parent rocks of many of the weathered lateritic profiles on the Jos Plateau. Contemporaneous or later faulting led to displacement and tilting of lateritized Older and Newer Basalt and the basement. Lateral erosion of the soil sections produced the characteristic tablehill topography.
5. Methodology Samples were collected from along the scarps of the Barakin Ladi plateau and from the covering ferruginous hardpan and were investigated by using quantitative XRF and XRD analyses. The XRF measurements were carried out on a Philips instrument, type PW1410-10, using the program ‘‘oxiquant’’. XRD measurements were carried out on a Philips instrument, Cu-anode type PW1710 and countgoniometer type PW1820. XRD measurements were controlled by the software PC-APD, version 3.5. The use of these two methods facilitated the calculation of the quantitative mineralogical composition of the samples, as they contain only a limited number of mineral phases such as kaolinite, goethite, hematite, and gibbsite. Thin sections were described using a terminology partly derived from Brewer Ž1964., partly from
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Fig. 1. Geological sketch map of the Jos Plateau Žreworked after Dessauvagie, 1974..
Stoops Ž1983., Bullock et al. Ž1985., and Bardossy ´ and Aleva Ž1990., always using the one best suited to the material in question. Munsell-colour of the dried samples was also recorded.
saprolite showing the residual structure of basalt and the more altered upper part of the profile. At the bottom of the profile, there is the saprolite with Liesegang banded structure.
6. Profile description
7. Micromorphology
On the top of the studied weathering profile, a vesicular, concretionary, pisolitic hardpan iron-cap can be observed. Below, there is a goethite-rich gibbsitic horizon. It is followed by a clayey horizon with subangular clasts, showing chemical brecciation Žmottled zone.. This is a transition zone between the
A summary of the most important features recognized is as follows. Ž1. The relict texture of the basaltic rock is clearly preserved not only at the bottom of the profile, but in weathered clasts of the upper, oxic horizon as well. The borders of the basaltic clasts appear to be worn,
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serrated, andror mammilated. Basaltic phenocrysts Žformer olivine, pyroxene, and feldspar. are easy to identify by their shape and cleavage, the latter is always filled by Fe-oxides ŽPlate 1a.. Ž2. XRD analytical observations were used to confirm and quantify the presence of gibbsite, observable also with the optical microscope Žless than 5%.. Gibbsitization is closely related to the iron coated, tightly intergrown feldspar laths of the groundmass. On the other hand, macrocrystalline gibbsite can be seen along some cracks, biogalleries,
and in voids as well. Gibbsite also occurs as ‘‘porphyroblasts’’ in mosaic-like groups of often twinned ŽPlate 1b., sometimes palmate crystals. Gibbsitic-cutans can be found around some goethitic nodules throughout the uppermost 1 m of the profile. Ž3. Traces of bioturbation are abundant in the upper part of the profile. Bioturbation is obvious because of the homogenous matrix and the elongate, rounded channels, which are tapering and branching in some cases. These macro and micropores are usually coated by iron oxide and filled by clay
Plate 1. Ža. Olivine phenocryst. Relict texture in a weathered basaltic clast. Scale bar: 100 mm, qN. Žb. Fibrous gibbsite crystals in a pore. Scale bar: 100 mm, qN.
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cutans and younger yellowish soil material, which consists of homogenous clay with scattered silt-size quartz grains and exhibits a clear micropedality ŽPlate 2a..
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Ž4. Examining the thin sections from the middle and upper part of the profile, we recognized a characteristic granular microstructure consisting of granules and ferritic micropeds ŽMuller, 1977 in Stoops,
Plate 2. Ža. Cross-section of a biogallery, coated by clay and by ferritic micropeds. Scale bar: 200 mm, qN. Žb. Kaolinitic–hematitic boxwork structure. Scale bar: 100 mm, qN.
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1983.. Ferritic micropeds appear in the oxisol-type homogenous matrix, along cracks, and in clay-coated voids which are supposed to be result of bioturbation ŽPlate 2a.. Ferritic micropeds postdate most of the clay coatings.
Ž5. Several types of clay cutans can be distinguished in the intergranular space mainly in the upper part, but in trace amounts also at the bottom of the profile. The amount of these clay coatings is around 0–5%, as shown by point counting. The
Plate 3. Ža. The isopachous cement is a sign of phreatic conditions. Scale bar: 200 mm, 1 N. Žb. Meniscus cement idicates vadose conditions. Scale bar: 100 mm, 1 N.
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Table 1 TirFe and ZrrTi ratios along the profile
Fig. 2. ZrrTi plot of the Barakin Ladi laterite samples clearly displaying their basaltic origin. Discrimination fields after Hallberg Ž1984..
colour of the cutans varies from brown to yellowish brown to yellow and red. Usually, cutans follow the walls of the voids impregnated by iron oxide and were precipitated clearly before the accumulation of the ferritic micropeds ŽPlate 2a.. According to Soil Taxonomy ŽSoil Survey Staff, 1975., in oxisols, less than 1% clay cutans should be present. Thin cutans near the surface may be the result of local Že.g., lateral. clay translocation, rather than vertical eluviation and illuviation process, and are probably related to the drying and wetting cycles of the soils as suggested by Stoops Ž1968. Žin Stoops, 1983.. Ž6. One of the most conspicuous parts of this profile is the concretionary Žpisolithic. ferricrete which forms a horizon about 30-cm thick. It consists
Depth Žm.
TirFe Žwt.%rwt.%.
ZrrTi Žwt.%rwt.%.
0.0 0.4 0.6 0.8 1.0 2.5 3.5 11.7
0.0392 0.0121 0.1875 0.1400 0.0589 0.1233 0.1311 0.1777
0.0228 0.0285 0.0204 0.0105 0.0140 0.0091 0.0097 0.0089
almost entirely of pisoids ŽPlate 3b. and ooids with very little powdery matrix. About 100–500 mm sized pisoids and ooids show several phases of septaria formation. The ooids are cemented by a goethitic– ferruginous material, which displays either meniscus-like or isopachous ring-morphologies. Ž7. Within both ferricrete and the immediately underlying soft laterite, two types of cement could be distinguished in macropores and in the intergranular space. The isopachous, orange coloured, goethitic intergranular cement is predominant in some cases and is particularly abundant in the saprolite. This type of cement is built up of thin, goethitic–clayey zones parallel with the surface of the irregular shaped voids ŽPlate 3a.. We suggest that isopachous cement is an indication of precipitation from solutions having completely filled the available porespace and
Fig. 3. TirFe and ZrrTi values along the profile.
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therefore can be considered as a sign of past phreatic conditions. Fe-stained cement of meniscus morphology occurs only inbetween the ooids of the iron cap. It is considered to be the sign of vadose conditions precipitated from solution adherent to the grain surfaces when most of the porespaces were aerated ŽPlate 3b.. We think that the presence of both isopachous and meniscus cements in the concretionary ferricrete is the direct result of changing conditions of saturation in the profile. This confirms the idea of
the profile having been formed in a position near to the groundwater table. Ž8. The top of the lateritic profile is capped by a vesicular, concretionary laterite conglomerate consisting of redeposited older, partly ferralitic soil material. Abundant clay cutans coating some of the pore-spaces suggest a non-oxisol type overprint of ferrallitization. Ž9. Boxwork textures were detected in one single sample only, right underneath the concretionary iron-
Fig. 4. Chemical and mineralogical changes along the vertical profile.
Depth Žm.
SiO 2 Ž%.
Al 2 O 3 Ž%.
Fe 2 O 3 Ž%.
TiO 2 Ž%.
Zr Žppm.
Kaolinite Ž%.
Gibbsite Ž%.
Hematite Ž%.
Goethite Ž%.
L.O.I. Ž%.
Sum of elements
Sum of minerals
0.0 0.4 0.6 0.8 1.0 2.5 3.5 11.7
19.30 30.65 36.81 38.69 31.98 33.91 34.58 37.36
20.07 27.80 35.74 35.43 29.81 29.84 28.17 31.21
46.52 26.91 9.05 9.61 23.73 21.76 21.24 14.90
2.13 0.38 1.98 1.54 1.63 3.13 3.25 3.09
291 65 242 141 137 171 189 165
41.46 65.85 79.08 83.12 68.71 72.85 74.29 80.27
5.64 2.73 6.88 3.97 4.08 1.62 0.00 0.00
25.28 12.83 5.91 6.31 22.48 20.72 20.55 14.02
23.62 15.67 3.49 3.67 1.39 1.15 0.76 0.98
12.38 14.86 16.47 14.96 13.32 12.41 12.86 13.41
100.73 100.95 100.20 100.42 100.72 101.25 100.48 100.20
98.15 97.46 97.35 98.61 98.29 99.48 97.06 97.59
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Table 2 Chemical and mineralogical composition of the Barakin Ladi Profile
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crust, within the kaolinitic–gibbsitic zone, where cracks and fissures of the weathered kaolinite rich material were filled by iron oxide ŽPlate 2b.. Ž10. In addition, a few quartz grains could also be detected mainly from the top of the profile pointing to wind-blown contribution or some other process of transportation and deposition in a topographically low position.
8. Changes in the chemical and mineralogical composition The saprolite is of basaltic origin, which is clearly shown by its texture. We might assume a basaltic origin for the entire profile according to the ZrrTi plot of the samples ŽFig. 2.. Fig. 3 shows the changes of the TirFe ratio vertically in the profile. Its upward decreasing trend in the saprolite is followed by an abrupt increase in the clayey horizon, and above it these values are changing erratically. These TirFe ratios do not fit into the trend of a typical in-situ weathering profile. The upward decreasing TirFe ratio in the saprolite can be explained by the formation in the zone of the water–table fluctuation where, due to repeated dissolution and precipitation, lateral translocation of the elements takes place. The erratic changes of the TirFe ratio above the saprolite suggest sedimentation of some allochthonous material. This observation seems to be confirmed by the trend of the ZrrTi ratio ŽFig. 3, Table 1.. The chemical composition ŽFig. 4, Table 2. displays that silica is accumulated in the clayey horizon, but it becomes less abundant toward the top, while the amount of Fe increases. The mineralogical composition is more informative. It displays very clearly the variation of gibbsite in the vertical profile. Also, the two Fe-minerals can be separately observed. The amount of kaolinite in the transition zone is larger than that in the saprolite. Toward the top, kaolinite becomes less abundant. Kaolinite crystallinity can be estimated by measuring its D-value and peak-width at the half height of its 001 basal reflection. An increase of these values corresponds to poorer crystallinity. At the transition zone, these
values also show discontinuity. Toward the top, both values increase ŽFig. 5, Table 3.. The transition zone can be assumed as an illuvial horizon, with the accumulation of kaolinite and the removal of ironminerals. Gibbsite occurs in small quantities at the top of the profile; downward its amount decreases. The changes in distribution of the iron-minerals is also connected to the boundary of the saprolite and the clayey horizon. Toward the top, goethite becomes predominant, while in the lower part hematite is the main Fe-mineral. The distribution of the Fe-minerals is unusual. The predominance of hematite would have been expected at the very top of the profile, where due to free drainage, oxidizing conditions prevail, while in the lower parts, under more phreatic conditions, oxihydroxides should have been present. Our results show the contrary: hematite is predominant at the bottom, whereas goethite is abundant on top. To resolve the problem, the possible genesis of Fe-oxides should be taken into consideration
Fig. 5. D-values and peak-width at half height of the Ž001. plane of kaolinite along the profile Ž ; index of crystallinity..
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Table 3 Selected values of XRD analyses were used in order to estimate the crystallinity of kaolinite Depth Žm.
2Q of the Gibbsiter
Deviation
2Q Kaolinite
˚. D kaolinite ŽA
0.0 0.4 0.6 0.8 1.0 2.5 3.5 11.7
18.291 18.292 18.294 18.311 26.651 26.630 26.643 26.609
Kaolinite
˚. peak-width ŽA
quartz peak y0.02 y0.02 y0.02 0.00 y0.01 y0.03 y0.02 y0.06
ŽFig. 6a.. As it is well known from Schwertmann’s investigations, the first precipitate is normally an amorphous, metastable mineral, ferrihydrite ŽSchwertmann and Fitzpatrick, 1992.. Because of its metastability, with time it is converted to other Feoxides or oxihydroxides. This transformation is controlled by the conditions under which it takes place. Such conditions are water activity and the presence of organic matter. In the presence of organic matter, the formation of
12.270 12.312 12.280 12.317 12.315 12.314 12.308 12.264
7.227 7.202 7.220 7.188 7.195 7.208 7.204 7.204
0.345 0.244 0.330 0.269 0.273 0.230 0.285 0.260
hematite is hampered ŽTardy and Nahon, 1985; Schwertmann and Fitzpatrick, 1992. and goethite is favored. Such may be the situation on properly forested hilltops. Where organic matter is absent, hematite may form Že.g., hilltops without a forest cover or, alternatively, deeper but still well-drained horizons of the lateritic profile.. In areas of groundwater discharge with high water-activity, goethite is expected ŽTardy and Nahon, 1985.. We think that the hard, fine grained, homoge-
Fig. 6. Tentative explanation for precipitation and dissolution of iron-minerals under different environmental conditions Ža. before and Žb. after base-level drop.
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nous, highly goethitic material exposed by the nearby gully may represent such a situation. More so, it shows no signs of resedimentation so it could not have been originally formed at foothill position. Based on its mineralogical composition Žabsence of Al-minerals!., it cannot have been formed as an isalterite in hilltop position, either, but rather close to the groundwater–table. When the water-table falls ŽFig. 6b. Žeither because of uplift or because of climatic change. and vegetation becomes poorer, hematite may also form on the top. According to Schwertmann Ž1985., goethite may form in two different ways. In the case of high water-activity, it precipitates directly from solution. These crystals are normally birefringent and show a columnar habit, whereas goethite formed on alteration of hematite is, as a rule, isotropic. ŽSchwertmann, 1985..
9. Conclusion In the upper part of the studied profile, we have found a partly isalteritic–kaolinitic saprolite in the process of gibbsitization. It is covered by a partly concretionary–partly pisolitic ferricrete. The micromorphology of the ferricrete — particularly the presence of isopachous and meniscus cement — clearly reflects that it formed under alternately vadose and phreatic conditions, probably in a low-level position Žvalley bottom or valley terrace.. This is in full accordance with the idea of concretionary laterites being formed in gentle topographic depressions within the zone of the water-table Žsee also Boulange´ and Eschenbrenner, 1971.. In topographic depressions, the possibility of the sedimentation of some allochtonous material is always high. Geochemical data suggest that the upper part of the studied profile, namely the gibbsitic horizon and the iron cap, might be partly of such an origin Žthey show that some minor contribution from a nonbasaltic source was a possibility to reckon with.. The present uplifted position of the profile and its complex structure must be the result of repeated morphological rejuvenations ŽBoulange, ´ 1984.. The observed progressive gibbsitization of the kaolinitic
isalterite we assign to these rejuvenations is most probably related to the uplift of the Jos Plateau. The recognition of various ferruginous cementmorphologies reflecting the changing hydrological environment in the crust deserves special attention. It shows that micromorphological observations may reinforce the hypotheses developed on the basis of geomorphology alone. 10. Uncited reference Schwarz, 1997 Acknowledgements We acknowledge the financial support of the Volkswagen Foundation and OTKA-T-019309. We are grateful to Dr. Reinhard Zeese, cordinator of the project. Thanks are due to Dr. Laszlo ´ ´ Fodor for his contribution during the fieldwork and to Dr. Torsten Schwarz for the useful discussions. The XRF analisys were made by the second author at laboratories of TU Berlin. References Bardossy, G., Aleva, G.J.J., 1990. Lateritic bauxites. Develop´ ments in Economic Geology No. 27 Elsevier, 624 pp. Becker, A., 1992. A time–space model for the genesis of Early Tertiary laterites from the Jos Plateau, Nigeria. J. Afr. Earth Sci. 15 Ž2., 265–269. Boulange, de ´ B., 1984. Les Formations Bauxitiques Lateritiques ´ Cote-D’ivoire Travaux et Documents de l’Orstom No. 175, ˆ 341 pp. Boulange, ´ B., Eschenbrenner, V., 1971. Note sur la presence de cuirasses temoins des niveaux bauxitiques et intermediaires Plateau de Jos ŽNigeria.. Assoc. Senegal. Etude. Quat. Afr., Bull. Liaison No. 31, 83–92. Brewer, R., 1964. Fabric and Mineral Analysis of Soils. Huntington, New York, 482 pp. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T., Babel, U., 1985. Handbook for Soil Thin Section Description. Waine Research Publications, England, 152 pp. Dessauvagie, T.F.J., 1974. The Geological Map of Nigeria, Scale: 1:1 000 000. Hallberg, J.A., 1984. A geohemical aid to igneous rock type identification in deeply weathered terrain. J. Geochem. Explor. 20, 1–8. Macleod, W.N. et al., 1971. The geology of the Jos Plateau: Vol. 1. General geology. Geol. Surv. Niger., Bull. 32 Ž1., 119 pp.
Z. HorÕath ´ et al.r Chemical Geology 170 (2000) 81–93 Mindszenty, A., 1976. Some remarks on the laterites of Nigeria. Travaux du ICS No. 13, pp. 185–190, Zagreb. Schwarz, T., 1994a. Ferricrete formation and relief inversion. Catena 21, 257–268. Schwarz, T., 1994b. Bauxite formation on different parent rocks on the Mambilla Plateau ŽSE Nigeria.. In: Smith, B.J., Warke, P.A. ŽEds.., Eurolat ’94, Laterites, Paleoweathering and Paleosurfaces, pp. 16–20, Belfast. Schwarz, T., 1997. Distribution and genesis of bauxite on the Mambilla Plateau, SE Nigeria. Appl. Geochem. 11, 1–13. Schwertmann, U., 1985. Properties of goethites of varying crystallinity. Clays Clay Miner. 33, 369–378. Schwertmann, U., Fitzpatrick, R.W., 1992. Iron minerals in surface environments. Catena Suppl. 21, 7–30. Stoops, G., 1983. Micromorphology of the oxic horizon. Soil Micromorphology. Soil Genesis A,B Vol. II, pp. 419–440.
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Tardy, Y., Nahon, D., 1985. Geochemistery of laterites, stability of Al-goethite, Al-hematite, and Fe 3q-kaolinite in bauxites and ferricretes: an approach to the mechanism of concretion formation. Am. J. Sci. 285, 865–903. Valeton, I., 1988. Verwitterung und Verwitterungslagerstatten. In: ¨ Fuchtbauer, H. ŽEd.., Sedimente und Sedimentgesteine. Sedi¨ mentpetrologie Teil II Vol. 2, pp. 11–68, Stuttgart. Valeton, I., Beissner, H., 1986. Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau. Niger. J. Afr. Earth Sci. 5 Ž5., 535–550. Wright, J.B., 1974. Volcanic rocks in Nigeria. In: Kogbe, C.A. ŽEd.., Geology of Nigeria, Section 2, pp. 93–143, Lagos. Zeese, R., 1991. Paleosols of different age in Central and Northeast Nigeria. In: Lang, J. ŽEd.., Sedimentary and Diagenetic Dznamics of Continental Phanerozoic Sediments in Africa. J. Afr. Earth Sci. 12 Ž1r2., pp. 311–318.