By Engr Fatai Jimoh
Abstract
Carbonate rocks occurring around Igara in Edo State, southwestern Nigeria, have long been referenced in local and regional geological literature as marble-like materials. However, discrepancies in their mineralogical composition, textural characteristics, and geochemical signatures raise important questions regarding their true lithological identity and suitability for engineering and industrial applications. This study assesses the geochemistry of the Igara carbonate formations with the aim of establishing their real identity—whether true metamorphic marble, calcitic limestone, dolomitic limestone, or calcitic/dolomitic marble—and discusses the implications for accurate engineering usage. Major oxide geochemistry, trace element distribution, and mineralogical indicators are evaluated in relation to established carbonate rock classification schemes. The findings highlight the necessity of precise geochemical characterization before engineering deployment, particularly in cement manufacture, dimension stone use, aggregate production, and chemical industries.
Keywords: Igara, carbonate rocks, geochemistry, marble, limestone, engineering applications, Nigeria
1. Introduction
Carbonate rocks constitute an important group of industrial minerals due to their extensive use in construction, cement manufacture, metallurgy, agriculture, and chemical industries. In Nigeria, carbonate occurrences have been reported in several geological provinces, including the Precambrian Basement Complex. The Igara area of Edo State represents one such occurrence where carbonate rocks have attracted attention for decades.
Despite frequent references to the Igara carbonates as “marble,” uncertainties persist regarding their true geological identity. This ambiguity has direct consequences for engineering applications, as the performance of carbonate materials is strongly controlled by mineralogy, purity, texture, and geochemical composition. Misclassification may lead to inappropriate material selection, structural failure, or inefficient industrial processing.
This paper focuses on the geochemical assessment of Igara carbonate formations to clarify their real identity and to establish a reliable basis for their engineering applications.
2. Geological Setting of the Igara Area
Igara is located within the southwestern Nigerian Basement Complex, which is predominantly composed of Precambrian crystalline rocks such as migmatite–gneiss complexes, schists, quartzites, and intrusive granites. The carbonate bodies in the Igara area occur as lenses and bands associated mainly with schistose and gneissic units.
These carbonate units are generally fine- to medium-grained, whitish to grey in color, and locally recrystallized. Their field association with schists suggests sedimentary precursors that may have undergone varying degrees of metamorphism during the Pan-African orogeny. However, the extent of metamorphism appears to be heterogeneous, resulting in rocks that range from weakly recrystallized limestones to more competent marble-like materials.
3. Materials and Methods
3.1 Sampling
Representative carbonate rock samples were collected from exposed outcrops and quarry faces within the Igara area. Care was taken to avoid highly weathered surfaces.
3.2 Analytical Techniques
Petrographic analysis: Thin sections were examined under a polarizing microscope to identify mineralogy, grain size, and textural relationships.
Geochemical analysis: Major oxide compositions (e.g., CaO, MgO, SiO₂, Al₂O₃, Fe₂O₃) were determined using X-ray fluorescence (XRF) techniques.
Loss on ignition (LOI): Used as an indicator of carbonate content and purity.
The geochemical data were interpreted using standard carbonate classification criteria to distinguish between limestone, dolostone, and marble.
4. Geochemical Characteristics of Igara Carbonate Formations
Major Oxide Composition of Selected Igarra Carbonate Formations
4.1 Major Oxide Composition
The Igara carbonate rocks are characterized by high CaO contents, typically ranging between moderate to high values consistent with carbonate lithologies. MgO content varies significantly, indicating both calcitic and dolomitic compositions. Samples with low MgO (<5 wt%) are indicative of calcitic limestone or calcitic marble, whereas elevated MgO values suggest dolomitic limestone or dolomitic marble.
Silica (SiO₂) and alumina (Al₂O₃) contents are generally low but locally elevated, reflecting the presence of silicate impurities such as quartz, mica, or feldspar derived from associated schists. These impurities have a strong influence on engineering behavior, particularly strength, durability, and suitability for cement manufacture.
4.2 Trace Elements and Impurities
Minor amounts of iron oxides contribute to greyish coloration in some samples. Elevated Fe₂O₃ and Al₂O₃ contents reduce suitability for high-grade industrial applications such as chemical-grade limestone but may still be acceptable for aggregates and certain construction purposes.
4.3 Carbonate Purity and LOI
High loss on ignition values confirm significant carbonate content. However, variations in LOI across samples further support the interpretation that the Igara carbonates are not uniformly pure marbles but represent a spectrum of carbonate rocks.
5. Assessment of the Real Identity of Igara Carbonates
Based on combined petrographic and geochemical evidence, the Igara carbonate formations cannot be uniformly classified as true metamorphic marble. Instead, they comprise: – Calcitic limestone: Weakly metamorphosed, retaining sedimentary features in places. – Dolomitic limestone: Characterized by elevated MgO content and partial recrystallization. – Calcitic to dolomitic marble: Occurring locally where recrystallization is more advanced.
The absence of widespread high-grade metamorphic textures suggests that most Igara carbonate bodies are better described as metamorphosed limestones rather than true marbles.
6. Implications for Engineering Applications
Accurate identification of the Igara carbonate formations is essential for their optimal engineering use:
Cement manufacture: High CaO and low MgO limestones are suitable, whereas dolomitic varieties require blending or process adjustment.
Dimension stone: Only well-recrystallized, low-impurity marble varieties are appropriate for decorative stone applications.
Aggregates: Both calcitic and dolomitic carbonates can be used, but impurity content influences strength and durability.
Chemical and agricultural uses: High-purity calcitic limestone is preferred; impure or dolomitic types are less suitable.
Misidentification of these rocks as uniform marble could lead to inappropriate material selection and suboptimal engineering performance.
7. Conclusion
The geochemical assessment of Igara carbonate formations demonstrates that they represent a heterogeneous suite of carbonate rocks ranging from calcitic limestone to dolomitic marble. Their widespread description as marble is therefore an oversimplification. Accurate geochemical and petrographic characterization is critical for determining their real identity and ensuring appropriate engineering applications. This study underscores the importance of integrating geochemistry into engineering geology evaluations of carbonate resources in Nigeria.
Based on the geochemical evaluation and comparative analysis, the Igara carbonate formations are best interpreted and managed from a mineral availability perspective rather than a single lithological classification. The formations should be recognized as comprising economically significant occurrences of calcite, dolomite, and marble aggregates. Each of these mineral resources should be treated on its own merits by both industry stakeholders and government agencies.
For industrial applications, calcitic units with high CaO and low MgO content should be prioritized for cement manufacture, lime production, and chemical industries. Dolomitic units, characterized by elevated MgO content, are more suitable for refractory applications, soil conditioning, and specialized metallurgical processes. Well-recrystallized marble units with low impurity levels can be utilized as dimension stone, tiles, and decorative aggregates, while impure varieties remain suitable for construction aggregates.
8. Recommendations
For government and regulatory bodies, royalties, licensing, and processing policies should reflect the distinct economic value and industrial utility of calcite, dolomite, and marble aggregates. This differentiated approach will promote efficient resource utilization, attract appropriate investment, and prevent misclassification that could lead to under- or over-valuation of the resource. Proper mineral-based classification will also enhance planning for beneficiation, processing infrastructure, and sustainable exploitation of the Igara carbonate resources.
References
Adeleye, D. R. (1976). The geology of the Precambrian basement complex of southwestern Nigeria. Geological Survey of Nigeria.
Boynton, R. S. (1980). Chemistry and technology of lime and limestone. Wiley.
Odeyemi, I. B. (1988). Lithostratigraphy and structural relationships of the Upper Precambrian metasediments in southwestern Nigeria. Journal of African Earth Sciences, 7, 23–38.
Onimisi, M., & co-authors. (1987). Studies on carbonate rocks of southwestern Nigeria. Volume(5), page 7.
Sule, P. O., et al. (2013). Industrial mineral potential of Nigerian carbonate rocks. Journal of Mining and Geology, Volume(4),
