Petrography and geochemistry of ferricrete near Shire, northern Ethiopia

Bheemalingeswara Konka * , Solomon Gebreselassie and Ebrahim Nesro Hussen Department of Earth Sciences, CNCS, P.O.Box. 3066, Mekelle University, Mekelle, Ethiopia (*kbheema@hotmail.com) ABSTRACT A detailed petrographic and geochemical study was conducted on ferricrete (laterite) developed on Mesozoic (?) ferruginous sandstone near Shire, Tigray region, northern Ethiopia. 30 rock samples were analyzed for major and minor elements and studied for petrographic details. Ferruginous sandstone overlying the Neoproterozoic low grade basement rocks dominantly contains quartz followed by orthoclase feldspar and iron oxides/hydroxides as cement. Residual enrichment process has resulted in the development 2-3m thick ferricrete horizon and also subhorizons: mottled, mixed nodular and psuedo-pisolitic. Mineralogy of the ferricrete includes limonite, goethite and hematite. Arid conditions and dehydration reactions seem to have produced hematite from goethite. The ore minerals show replacement, cavity and fracture filling, remobilization and colloform textures. Si>Al>Fe is the mobility pattern observed in the ferricrete horizon though presence of secondary quartz and kaolinite are also common. Development of ferricrete is related to the climatic condition that existed during Eocene and is comparable with similar deposits in Arabian Nubian Shield (ANS).


INTRODUCTION
In Ethiopia, minable iron deposits reported so far are very few.Three different types of deposits are reported from Ethiopia, though small in size.They include magmatic iron (Fe-Ti type) of Precambrian age from Bikilal, Melka Arba areas, banded iron formation (BIF type) of Precambrian age from Koree, Gordoma, Chago areas, and lateritic (also gossan related) iron deposits (residual type) from Melka Sedi, Garo, Gato, Billa, Gambo, and Gammalucho areas.

Regional Geologic Setting
Northern Ethiopia forms part of southern Arabian-Nubian Shield (ANS) and consist of Precambrian low-grade volcanic, volcano-sedimentary, mafic and ultramafic rocks of ophiolitic character and are intruded by syn-and post-tectonic plutonic rocks (Tadesse, 1997).The rocks have experienced different phases of deformation.The tectonic structures include the fold-thrust domains with associated shear zones of predominant sinistral sense of shear which are attributed to major collision orogeny during the amalgamation of the Arabian-Nubian Shield (Tadesse et al., 1999).The basement Precambrian rocks are divided into: (i) the metavolcanic/volcaniclastic dominated Lower Tsaliet Group (~850 Ma; Teklay, 1997), (ii) metasedimentary rocks containing Upper Tambien Group (835-740 Ma, Alene et al., 2006;Avigad et al., 2007), (iii) Dolomite dominated Didikama Formation, and (iv) younger diamictites and metasandstone/conglomerate containing Negash diamictites and Shiraro molasses (Avigad et al., 2007).The intrusive felsic plutons (syn-and post-tectonic granitoids) ranging in age from 800 to 520 Ma have altered the Precambrian rocks and acted as source of heat for the hydrothermal fluids that resulted in base metal and gold mineralization in the region (Tadesse et al., 1999;Asfawossen et al., 2001;Kuester et al., 2009;Bheemalingeswara et al., 2012).

Geology of the study area
It forms part of lithostratigraphy of NW Tigray and are dominarted by low grade Tsaliet Group rocks and Mesozoic sedimentary rocks (Fig. 2).Tsaliet Group consists of metavolcanic (MV) rocks of mafic to intermediate composition.They are intruded by younger granitic plutons and overlain by Mesozoic sedimentary rocks.The sedimentary rocks are dominated by ferruginous sandstone with thin bands of siltstone and claystone.Ferricrete is a conspicuous unit in the area developed on ferruginous sandstone (Ebrahim, 2011) which is related to Adigrat Sandstone (Tadesse, 1997).Alluvial deposits mostly on sedimentary rocks present in northern and eastern  (Tadesse, 1997).It is dominated by coarse (>3mm) orthoclase feldspar and quartz and with development of kaolinite along the intrusive contact.

Ferruginous Sandstone
It shows red color due to high content of iron and is intercalated with bands of claystone/siltstone.Tadesse (1997) interpreted it equivalent to Adigrat Sandstone.It covers large area, overlies unconformably the Precambrian basement rocks and is characterized by flat lying morphology on both sides of gorgesand is undergoing erosion.In the west side of the study area the rock shows angular unconformity with MV unit making it to have an outlier feature.It varies in thickness from about 30-35m in SW to 10m in NE and shows a gradational contact with the overlying lateritic unit (Fig. 3).The sandstone is thick, bedded and reddish brown in color having medium to coarse grain size quartz as dominant mineral and followed by orthoclase feldspar and iron hydroxides as cement.Alteration of feldspars to kaolinite is seen at many places and being soft it is eroded leaving depressions on the top of the profile.The resistant quartz minerals remain in situ and show alignment fabric indicating sedimentary process (Fig. 3A & B).
Development of secondary structures such as cracks and joints common in sandstone which are later filled by iron hydroxides and chert (Fig. 3C).In the northern part, there are small patches of pink colored thinly laminated siltstone beds showing conchoidal fracture.

Ferricrete
It is about 2-3m thick, developed on ferruginous sandstoneas part of weathering/ lateritization and extensively exposed in the study area.The vertical section exposed in the quarry site (416000 and 1558000m, Fig. 2) shows the transformation with gradational contact from source ferruginous sandstone to iron-rich ferricrete (horizon) and also sub-horizons within ferricrete.
Ferricrete varies in thickness from 3m to less than a meter particularly in the northern part.At places it is absent because of its removal due to erosion and filling the depressions and gentle slopes downstream.Mixed mottled sub-horizon is characterized by the breakdown of nodular structures into smaller size spherical, irregular or concretionary shapes with increasing coating by brown color iron oxides.Compared to other sub-horizons, banded nature of iron oxide cement is common (Fig. 4).This sub-horizon is followed by <1m thick pseudo-pisolitic sub-horizon which is characterised by presence of pisolites, occur as independent and welded variety.The nucleus, mainly quartz, is coated by iron oxides with varying thickness.The rounded and sub-rounded red/brown color pisolites grows thicker due to coating and strongly indurated and welded together producing pseudo-pisolite and colloform textures.With time, the welded ones are breaking down producing independent pisolites which get eroded and accumulated in the stream channels.Occurrence of independent pisolites or welded variety seems depend on the varying degree of fragmentation and oxidation (Fig. 4).Though, the iron-rich cement strongly binds the

Petrography of ferruginous sandstone and ferricrete
Fine to medium grained ferruginous sandstone typically shows red color in thin section.It is mainly composed of irregular, sub-rounded to angular shaped quartz grains (~70%), orthoclase feldspar (~10%)and ferruginous oxides as cement (~ 20%) (Fig. 5).Quartz shows low relief and low birefringence but does not show wavy extinction.Iron oxide/hydroxides serve as a binding material (cement) between mineral grains and rock fragments and seems to have remobilized during the chemical weathering process (Fig. 5).Presence of chert as lenses, fracture filling and irregular bodies in sandstone and ferricrete is related to the dissolution of primary quartz and precipitation (Figs. 5 and 6). the principal ore mineral in these transformations in this zone.Association of this nodular variety (NV) with residual zones of the simple mottled variety defines the iron crust with mixed subhorizon.Brown-red nodules with yellowish brown rings haves changed to brown pseudopisolites with banded outer shells.The rings developed in a centripetal way at the expense of the cores in the nodules (Fig. 6).This evolution proceeds as follows: isolation of scales of opaque matrix from the core; diminution of these scales of matrix as soon as the outer shell develops; obliteration of the micro-porosity observed in the core; and replacement of the opaque matrix by hematite.Such changes have resulted in the development of white and brown alternating bands in the pisolites (Fig. 6; Ebrahim, 2011).
Table .1.Range of major and minor oxide values for ferricrete and ferruginous sandstone.
Table .2. Correlation matrix for major oxides in ferricrete (a total of 16 surface samples).

Geochemistry of ferricrete
Major and trace element data for ferruginous sandstone and ferricrete are presented in table Al; and positive relationship though not very strong between Si and Al.Relatively higher mobility of Si compared to Al is related to secondary quartz produced due to dissolution of primary quartz (Si), its removal and precipitation (Fig. 6A) compared to kaolinite (Al) which is partially removed by erosion not by dissolution (Fig. 3F).pits data.Compared to others, mottled sub-horizon varies in thickness from less than a meter to more than 3m (Fig. 7).Among trace elements, vanadium shows significant values > 1300 ppm in ferrictere compared about 300ppm in source sandstone.These values also tallies well with Ezana data in which vanadium shows values > 2500ppm.Apart from V, zirconium values range upto 222 ppm and Co upto 129 ppm (Ebrahim, 2011).

Development of ferricrete
Petrographic together with field data indicate that ferruginous sandstone forms the source for the development of ferricrete as part of lateritization and residual enrichment.Different sub-horizons noted in the ferricrete profile based on mineralogy and textures compare well with the laterites (ferricretes) reported elsewhere developed on ferruginous sandstone (Nahod et al., 1977;Tardy and Nahod, 1985;Ramakrishan and Tiwari, 2006;Ramanaidu et al., 1996).

Ferruginous sandstone to mottled sub-horizon
Transition from red colored ferruginous sandstone to mottled sub-horizon lateritization is progressive and is indicated by a) increase of coloration and indurations; b) preservation of the primary sedimentary structure; and c) complete and/or partial epigenesis of the components of parent rock minerals (quartz and orthoclase feldspar).The ferricrete with mottled sub-horizon is originated directly from the ferruginous sandstone with about 15-20% iron oxides/hydroxides as cement and iron oxides supplied by the percolating solutions during weathering.The major process in the transformation of ferruginous sandstone to mottled sub-horizon is the breakdown of the rock into lithic fragments and chemical breakdown of orthoclase feldspar mineral structures and partial dissolution of quartz.This will produce a network of channels and large tubular voids of large diameters (cm) in which kaolinite produced due to breakdown of feldspar and iron-rich cement material can accumulate.

Mottled to nodular sub-horizon
Continued weathering of mottled sub-horizon has produced indurated nodular sub-horizon.It Voids developed by leaching of the quartz serve as secondary "reception" structures for kaolinite.This is because the original ferruginous sandstone is devoid of kaolinite and the latter is the result of the chemical breakdown of orthoclase feldspar present in sandstone (Fig. 6).
Kaolinite is later replaced and progressively enriched in iron thus producing fine-grained iron ore mineral characterized by small pore size (<< 0.1 mm).In addition, in this sub-horizon, there is an accumulation of secondary quartz developed by precipitation from the percolating silica-rich solutions from the upper parts of the profiles (Fig. 6).

Nodular to pseudo-pisolitic sub-horizon
The mixed nodular sub-horizon further evolved into pseudo-pisolitic possibly due to the following geochemical changes.Removal of Si in the mixed nodular sub-horizon and the subsequent modifications affected the ferruginous components (iron oxides/hydroxides) and During which the iron oxide/hydroxides have modified from hydroxide (limonite, goethite) to hematite (iron oxide).Thus, by the combination of absolute and relative accumulations of iron produced ferricrete from the ferruginous sandstone.But during lateritization, precipitation of the silica-rich solutions derived from dissolution of primary quartz produced secondary quartz, chert periodically, thus reducing the size regularly and finally reaches to smaller sizes in the pseudopisolitic sub-profile.Al on the other hand oscillates between kaolinite and iron oxy-hydroxides where it will be a substitute and Fe oscillates between dissolved and crystallized forms only.

Mineral Paragenetic Sequence
The mineral that are involved in the lateritization of sandstone to iron crust (ferricrete) are hematite, goethite, limonite, ochre, quartz, feldspar, kaolinite, chert and iron oxy-hydroxide cement.On the basis of field observation and petrographic study following paragenetic sequence is proposed for the lateritic iron deposit and shown in table 3.
Table 3. Paragenetic sequences of the minerals in ferricrete formation thickness of the line indicates concentration of the mineral).

Comment on genesis
Hematite, goethite, limonite and ochre are common minerals in the ferricrete.Goethite is seen converted to hematite instead of hematite to goethite as hematite does not show any alteration to goethite as expected in an oxidizing weathering condition.Paragenetic sequence of iron minerals seems to have developed in the following manner.The ferruginous sandstone originally may be devoid of hydroxides of iron (like goethite) because of the transformation of metastable goethite formed in water into hematite during long burial digenesis of sediments of Tertiary to Paleozoic in age (Tardy and Nahod, 1985) and this is noticed by not well developed goethite in the the crystal of goethite, hematite is stable than goethite in water but in the reverse situation goethite is stable but in the case concretion and nodules in iron crust (ferricrete) formation both minerals do not from large crystals and appear as a very tiny particle size (Tardy and Nahod, 1985).So, the governing condition for stability of these minerals mainly is dehydration and water activity compared to the particle size.
If iron is released from silicates and from other primary source in water then it will form goethite; if the solubility product of goethite is excess than that of ferri-hydrite.Ferri-hydrite is formed and transformed to hematite through dehydration or to goethite through dissolution; the factor that favors ferri-hydrite formation will also favor the formation of hematite by high temperature by dehydration.The factors controlling the formation of ferri-hydrite in solution are: a) rapid release of Fe; b) low concentration of organic compound which complexes iron (allowing concentration of inorganic Fe 3+ ); c) iron release in ferruginous sandstone being high because the large pore size (with iron hydroxide matrix) favor increasing activity of water; d)presence of negligible amounts of organic matter (or organic compounds) in thesource ferruginous sandstone could be one of the reasons for the stability of hematite over goethite during ferricrete formation; and e) equilibrium condition involving water activity, pore size and nodule formation (Tardy and Nahod, 1985;Nahod et al., 1977;Tardy et al., 1991).

CONCLUSION
Ferricrete developed on ferruginous sandstone (Adigrat? of Mesozoic) is composed of a succession of different sub-horizons, namely mottled, mixed nodular and pseudo-pisolitic.Bheemalingeswara, K., Solomon, G and Ebrahim, N.H (MEJS) Volume ):32-50, 2013 © CNCS, Mekelle University 33 ISSN:2220-184X related to iron-rich Paleozoic Enticho Sandstone (Gebresilassie et al., 2012);and 4) presence of iron-rich bands within Adigrat Sandstone of Mesozoic age near Wukro, Hawzein, Dugum, Adigrat etc.At present among the reported iron deposits of lateritic type are the major resources in terms of tonnage (>100mt) and hence are attracting attention of companies like Ezana Mining Company PLC (EMD) and Universities like Mekelle University to conduct research.Present paper is the result of one such effort on ferricrete near Shire, northern Ethiopia.
, foliations, folds are common in the basement rocks compared to sandstone and related ferricrete.The latter commonly show structures like primary bedding, fractures and cross bedding especially in sandstone unit.The fractures are dense, dip vertically, show northsouth and east-west trend and are filled by quartz, chert and iron oxides.

Figure 2 .
Figure 2. Geological map of the study area.

Figure 3 .
Figure 3. A) Dark brown colored ferricrete on top of ferruginous sandstone, B) weathered ferruginous sandstone with white colored aligned quartz minerals, C) fresh ferruginous sandstone with thin iron and silica -rich bands, D) and E) pseudo-pisolitic texture in ferricrete, and F) kaolinite development in ferricrete.

Figure 4 .
Figure 4. Ferricrete profile showing different sub-horizons in the ferricrete from quarry exposure.
may be related to two processes.1) The massive iron crust with strong iron-rich cement (mottled sub-horizon) gradually develops into nodules or concretions by partial dissolution of quartz grains and remobilization of iron-rich cement.This is well indicated in the petrographic study where the voids created by the partial dissolution of quartz and breakdown of feldsparsare partially replaced by kaolinite-ferruginous material(Figs.3, 4& 6).Thus the nodular and concretionary type sub-horizon is produced with colloform and open space filling textures.2)Bheemalingeswara, K., Solomon, G and Ebrahim, N.H kaolinite in the iron and kaolinite matrix.The re-organization of indurated mixed nodular subhorizon takes place by insitu centripetal way of accumulation of iron oxide and removal of kaolinite and development of pisolitic structures.Continuous reorganization into pseudo-pisolite is accompanied by the gradual removal of kaolinite and further dissolution of the nodules and mobilization of iron oxide/hydroxides.This leads to the reduction in size of the coarse nodular structure into smaller size pisolites and thus producing the pseudo-pisolitic textures and pseudopisolitic sub-horizon.Presence of poor matrix in this horizon compared to other sub-horizons exposed the pisolites to erosion and accumulated in the stream channels.Behavior of Fe, Al and Si in the vertical sections suggests that residual enrichment of iron is significant.Movement of iron oxides downward from the top sub-horizons facilitates coating and thus accumulation of iron around the nodular fragments.Mobility rate of iron being low compared to silica and aluminum gradually accumulated with time and favorable conditions.
polished sections; b) Goethite does not replace hematite because the temperature is relatively high and do not favor hydration of hematite to goethite in the iron crust particularly that occurs at the top of the profile; and c) The relative stability of goethite and hematite depends on many factors such as grain size effect.If the crystal of hematite is grater or equal to the grain size of Fe 2 O 3 content in ferricrete varies from 30-50 wt%, Al 2 O 3 upto 25% and SiO 2 upto 30%.On the basis of the field and petrographic data, the successive sub-horizons in the ferricrete are noted as: massive structure (iron pan) and dismantled pisolites upper mixed horizon (medium nodular)lower mixed horizon (coarse nodular)  mottled horizon (saprolitic) the ferruginous sandstone.Ferricrete horizon shows gradational contact with the source ferruginous sandstone.Gradual removal of kaolinite and silica facilitated enrichment of iron oxide/hydroxides in the form of coating around lithic fragments, nodular, pisolitic etc. Iron hydroxides, limonite and goethite with time are changing to hematite due to dehydration.Hematite is fine grained and shows textures like colloform banding, fracture filling, dissemination etc. Geochemical behavior of major elements clearly indicates the role of climate, parts of the area (e.g. about 2m thick in the May-Imblay River valley).Development of hexagonal / polygonal mud cracks filled with fine sand and with downward tapering are also common.The Shire area is well connected by asphalt road from Mekelle via Adigrat-Axum

Fe 2 O 3 Al 2 O 3 +SiO 2 SiO 2 Al 2 O 3 Al 2 O 3 +Fe 2 O 3 Fe 2 O 3 1 Al 2 O 3 +SiO 2
variation in concentration compared to Al value, which remain almost constant in both sandstone and ferricrete.Correlation matrix (Table2) indicates negative relationship for iron with Si and Table 2) of surface ferricrete samples data also correlate well with the test (Ebrahim, 2011)ctivity etc factors in the development of laterite development.According to an approximate estimate the tonnage of the reserve is above 100mt with an average grade of about 35-40wt% iron and among trace elements, V, Zr and Co shows relative enrichment upto 0.3wt%(Ebrahim, 2011).Ibrahim Nesro is grateful to Ministry of Education, Ethiopia for partial fund granted as part of post graduate research.Authors duly acknowledge Mekelle University for granting a research fund through the project on "Geological and Geochemical study of Lateritic Iron Deposit near Mentebteb, Shiraro, Northern Tigray, Ethiopia (CNCS/RB/34/2011)" which helped to analyze few samples from Shire.We are also very thankful to Ezana Mining Plc for providing us borehole data.Authors duly acknowledge the reviewers Dr. N. Siddaiah and Dr. Anbarasu for providing useful comments and helping to improve the paper.