Economic Aspects of Carbonates of the Albian Asu River Group in Tse-Kucha Near Yandev, Middle Benue Trough, Nigeria

The limestone-shale sequence at Tse-Kucha belongs to the Albian Asu River Group deposited during the first marine transgressive cycle in the Benue Trough. They however vary compositionally in mineralogy and texturally from fine to coarse grained. The compositional disparities between the samples of the limestone imposes corresponding differences in their industrial utilization. Assessment of the limestone suggested a wider range of industrial utilization beyond the present day application. For example, the highly calcitic limestone (60.04%) which is characterized by high CaO (43.8-53.3%) and low amount of MgO (0.44-1.06%) constitute suitable raw materials for the cement production, agriculture, metallurgical purification in blast furnace, lime manufacture, chalk, plaster and other filler applications. Apart from silica which varies from 2.1-10.0%, other impurities in the limestone are less than 1%. In view of the critical need of limestone in more strategic sectors of the economy, such as, iron and steel, agriculture, metallurgical purification/processes in blast furnace, lime manufacture industries, it is recommended that Government should exercise control measures over their exploitation for less profitable ventures. High quality burnt bricks and polished rock slabs could adequately substitute for sandcrete blocks produced from cement, which in many cases unaffordable to the peasants in the community.


Introduction
The study area is located in Tse-kucha quarry of Dangote Cement Company Limited Gboko in Benue State of Nigeria (Figure 1).The area falls within the Middle Benue Trough, where the Albian Sediments of the Asu River Group are exposed (Figure 2).It lies within longitudes 8 o 56′ and 9 o 00′ E and latitudes 7 o 20′and 7 o 30′, part of Gboko sheet 271.For many years, several authors have been working on the Cretaceous sediments of the Benue Trough.The Trough is geographically divided into three portions: Lower, Middle and Upper Benue Trough, (Akande et al., 1992; Figure 2).The Trough generally consists of both continental clastic deposits and marine sediments.The origin/evolution and stratigraphy of the trough have been discussed in details by Burke et al. (1970), Grant (1971), Nwachukwu (1972), Olade (1976), Adjihige (1979), Ofoegbu (1984), Maurin et al. (1986).The rift hypothesis was widely favoured in the evolution of the basin (Benkhelil, 1989).
Unlike in the Lower Benue Trough, very few studies have been carried out in the Middle Benue Trough.Published works on the economic aspect of the limestone in the Middle Benue Trough are rare.Petters (1982) worked on the paleontology of the Albian sediments exposed at Yandev.Nair and Ramanathan (1984) studied the sedimentology, stratigraphy and paleogeographic significance of the Albian limestone at Yandev.Abimbola and Akande (1996) studied the petrology, geochemistry and stable isotopes of the carbonate rocks of Arufu-Akwana, Middle Benue Trough while Adekeye and Akande (2002) determined the depositional environment of the carbonates of the Albian Asu River Group around Yandev, Middle Benue Trough.
The strategic location of these carbonate rocks in the Middle Benue Trough where there are towns and cities with high population and industrial conurbations made it important to study the limestone deposit and evaluate it for uses other than cement production, which is the ongoing project in the area.

Geology of the Middle Benue Trough
Previous workers (Offodile, 1976;Offodile & Reyment, 1976;Petters, 1982;Obaje et al., 1994) described some aspects of the Upper Cretaceous lithostratigraphic formations in the Middle Benue Trough.Sedimentation began in the Albian with the deposition of interbedded marine shale-limestone sequence of the Asu River Group (Figure 2).These sediments are interpreted as sediments of the first transgressions in the Benue Trough.The sediments are fossiliferous, containing numerous ammonites (Offodile & Rayment, 1976).Many agglutinated and few calcareous foraminifera taxa were described by Petters (1982) and Ramanathan and Nair (1984).The Asu River Group in the Middle Benue Trough was said to have attained a thickness of ca.200 m.This formation is overlain by the Awe Formation (Late Albian-Early Cenomanian regressive phase of the first marine transgression).The Awe Formation is ca.100 m thick (Offodile, 1976) and consists of carbonaceous shale, flaggy, medium to coarse grained calcareous sandstones and clay with a coarsening upward sequence.The sediments were deposited in a transitional environment.Overlying this formation is the Keana Formation (Figure 2).It consists of cross bedded, coarse grained, feldsparthic sandstones.The sandstones are generally poorly sorted.According to Offodile (1976) conglomerates, bands of shales and limestones are intercalated towards the top.This Formation is generally not fossiliferous, and the sediments were deposited in fluviatile environments during the Cenomanian regression in the Middle Benue Region.The Makurdi Formation is a lateral equivalent of the Keana Formation in the study area (Figure 2).
The Late Cenomanian-Early Turonian, Eze-Aku Formation overlies the Keana.The Eze-Aku Formation marks the beginning of the second depositional cycle.The sediments deposited consist of calcareous shales, micaceous, fine to medium-grained, friable sandstones and beds of shaly limestone.Elsewhere, outcrops of this formation consists of shales with beds of limestone of variable thickness and colour (Offodile, 1976; Figure 2).
Overlying the Eze-Aku Formation in the upsection is the Awgu sediments which represent the last shallow marine deposits of the second depositional cycle (Turonian-Coniacian).This sediment consists of bluish-grey to dark-black carbonaceous shales, calcareous shale, shaly limestones, siltstones and coal seams (Obaje et al., 1994).The presence of coal suggested a rapid change in the depositional environment.Spores, pollen and dinoflagellates have been recorded from the intercalating coals, coaly shales and carbonaceous shales.The sediments are reported as being fossiliferous consisting predominantly of arenaceous foraminifera and few calcareous taxa and ostracods (Offodile & Reyment, 1976).
The youngest formation in the Middle Benue Trough is the Lafia Formation.This Formation represents sediments of the third depositional cycle in the region.The formation unconformably overlies the Agwu Formation (Figure 2).The sediments were deposited in a continental environment and consist of ferruginised sandstones, red loose sands, flaggy mudstone, lignite bands and clays.The Lafia Formation thickens in the southwest direction and is thought to be the lateral equivalent of the Campano-Maastrichtian Enugu Nkporo Formation of the Lower Benue Trough (Offodile, 1976).The Lafia Formation is generally not fossiliferous (Figure 2).

Materials and Methods of Study
Fifteen (15) representative samples were collected from different parts of the benches as exposed at a quarry site in Tse-Kucha village (Figures 4 and 5).The number of samples was determined by the outcropping pattern in the respective bench.
Samples were prepared for petrographic studies at the Department of Geology, University of Ibadan, Ibadan, Nigeria.The minerals present in the thin sections were identified (Figure 6).Powdered samples were prepared for X-ray diffraction studies.These samples were analysed at ACME Analytical Laboratories Ltd, Vancouver.Canada.Diffraction peaks of the minerals present were identified using their characteristic set of d-spacings.The relative proportions of the minerals were calculated using the area method as described by Carroll and Dorothy (1970).
Major and trace element composition of the limestone samples were determined using Induced Coupled Plasma-Emission Spectrometry (ICP-ES) and Induced Coupled Plasma-Mass Spectrometry (ICPMS) at ACME Analytical Laboratories Ltd, Vancouver.Canada.

Mineralogy
Petrographic examinations and interpretation of diffractograms (Figures 6,7 and 8) show that calcite (CaCO 3 ) is the dominant mineral in the Tse-Kucha limestone.It constitutes about 60.04%.Quartz and pyrite are about 16.0 and 4.0% respectively.The compositional disparities between the benches of limestone impose corresponding differences in their petrogenesis and industrial utilization.For example, the highly calcitic limestone is a suitable raw material in cement, agriculture (liming of soils and fish ponds), paint, poultry and metallurgical purification processes in the steel industry (Figure 7).
A comparison of these results with the chemical specifications of a number of industries that utilize raw limestone is discussed below.

Iron and Steel Industry
Limestone can be mixed with iron ore to remove the impurities in the iron ore.Limestones form a substance known as slag when mixed with iron ore.The slag is separated from the iron, and taken away, leaving pure molten iron which can be made into steel alloy.

Blast Furnace (B.F.)
The initial reaction is the thermal decomposition of calcium carbonate to form Calcium oxide (Lime): CaCO 2 (s) → CaO (s) + CO 2 (g) This is followed by the reaction of lime with sandy impurities (SiO 2 ) in the iron ore to form calcium silicate: Comparing the results of the test samples from Tse-kucha to the industrial specifications on 2-Gboko limestone (Bejide, 2000).
2-Specification of limestone for Blast furnace after Umeshwar, 2003.3-Specification of limestone for Steel Melting Shop after Umeshwar, 2003.4-Specification of limestone for India glass industry after Umeshwar, 2003.5-Specification of limestone for flat glass manufacturing after Emefurieta and Ekuajemi, 1995.6-Specification of limestone in % for container glass after Harben, 1995.

Steel Melting Shop (SMS)
The chemical compositions of most of the limestone samples analyzed were not within the range of the specifications for steel melting shop.The SiO 2 content is too high though the CaO content is within the recommended value (Table 4).

Glass Industry
The Tse-Kucha limestone could not serve as a raw material in the production of glass because of the lower amount of CaO compared to the recommended value (Table 4).
For colourless glass high purity of limestone is needed with organic matter of 0.3 at maximum.
As per Indian standard 997-1937, FeO content should not be more than 0.50% in calcite, 0.01 in limestone.

Chemical Industry
Coral limestone and limeshell, being comparatively purer, are preferred in the chemical industry as a mineral supplement in cattle feed.The carbonates must be low in silica and alumina.Extremely low in fluorine and contains no arsenic (Department, Mineral and Energy, Republic of South Africa, 2003).

Fertilizer Industry
For the production of fertilizer, organic matter and sulpur should be negligible (Umeshwer, 2003).The total CaCO 3 content however should be at least 84.0%.It is believed that the SiO 2 content of the Tse-kucha limestone could be reduced through appropriate beneficiation from 6.3% to less than 5.0% which is the maximum recommended value for fertilizer production (Table 6).

Lime Manufacture
For the production of chalk and plaster, SiO 2 may be 10% or more but MgO should be negligible.The chemical composition of the Tse-kucha limestone is within the industrial specification for both chalk and plaster.However solid and highly compact variety would be required.

Agriculture
Nitrogen compounds are the source of soil acidity.In addition nitrates are a major factor in the leaching of other nutrient salts from soils.These are then replaced by the available hydrogen ions, rendering the soil even more acidic.Apart from supplying nutrients, a desirable fertilizer should offset changes in soil acidity brought about by nitrification.Calcium carbonate neutralizes soil acidity when applied on the soil surface (Blevins et al., 1978;Conyers et al., 2003;Caires et al., 2005).
The relatively impure samples which could not meet the requirement for chemical and other applications earlier stated can be used in the liming of soils (Emofurieta & Ekuajemi, 1995).
It is therefore, apparent that the use of limestone for cement production only may not be entirely beneficial to the industrial growth of other sectors, such as, steel making shop, chemical, fertilizer and lime production.More fertilizer production companies should be setup to utilize the considerable reserves of limestone and phosphate in the Cretaceous and Tertiary sedimentary basins of Nigeria.2-Specification of limestone for fertilizer production after Umeshwar, 2003.3-Specification of limestone for textile production Umeshwar, 2003.

Conclusions
The limestone deposit of Tse-kucha area in the Middle Benue Trough was found to be calcitic in composition.This was supported by the chemical composition which showed the dominance of CaO and loss on ignition (LOI) which is mainly CO 2 .
Industrial evaluation of the limestone showed that it can be used as a raw material for the production of fertilizer, lime, chemicals, metallurgical purification in blast furnace, as well as, chalk and plaster apart from the conventional use in cement production.This sundry application would promote economic growth in the area of occurrence and the country at large.Government should therefore exercise control over the use of limestone for less profitable ventures.

Figure 1 .
Figure 1.Map of Nigeria showing the Study Area Figure 2. a) Map of Nigeria showing the Middle Benue Trough and other sedimentary basins of Nigeria; b) Idealized N-S stratigraphic cross-section across the Chad Basin-Benue Trough-Niger Delta depicting a connected trans-Atlantic Sea way between the South Atlantic and the Tethys Sea during the Coniacian-Turonian (After Obaje, 2009)

Figure 7 .
Figure 7. Diffractogram of the limestone from DE South Bench of the open quarry at Tse-Kucha near Yandev in Gboko

Figure 9 .Figure 10 .
Figure 9. Percentage composition of the Oxides in the limestone of Tse-Kucha area

Table 2 .
Trace elemental concentrations of Tse-Kucha Limestone Table4, most of the samples were found to meet the specifications for use in steel The CaO content is high (43.8-53.3%)while the SiO 2 (2.11-10%) and sulphur contents are low.The specification for limestone to be used in blast furnace include about 90% CaCO 3 , 6.0-11.5% total SiO 2 + Al 2 O 3 and 4% MgO.Sulphur and phosphorous contents are expected to be negligible.

Table 4 .
Industrial specifications of limestone (%) for blast furnace, steel melting and glass industries

Table 6 .
Industrial specifications of limestone (%) as raw materials for fertilizer, textile and ceramic industries Results of analysed limestone samples from Tse-kucha village (This Study).