Isotope Studies and Source of Brines in The Middle Benue Trough, Nigeria

Isotope Studies and Source of Brines in The Middle Benue Trough, Nigeria

Magashi, U. B. and Goki, N. G

Department of Geology and Mining, Nasarawa State University Keffi, Nigeria

Abstract – Isotope studies on brines in the Middle Benue Trough, Nigeria, reveal signatures (¹O and ²H) that indicate evaporative enrichment and mixing with isotopically distinct deep waters, modelled by structurally focused brine migration and stratigraphic confinement within permeable Keana sandstones Formation. The synthesis of geological, geochemical, and isotopic data in the MBT brine system may be best explained by a hydrothermal-evaporative model: connate and geothermal fluids ascend through fault-controlled pathways, mingled with meteoric inputs and dissolved evaporite minerals, leading to the formation of highly saline brines. Recurrent cycles of fluid migration and surface evaporation result in significant halite and gypsum accumulations, structurally trapped along fault corridors. The geochemical and isotopic evidence points to a predominant meteoric origin for the brines in the study area, modified by intense evaporation and interaction with host rocks. Structural features and sandstone aquifers significantly control the migration and accumulation of these brines. The presence of both cold and hot brines can be attributed to differential geothermal gradients and fault-controlled fluid pathways. The elevated concentrations of dissolved

solids, particularly at Ribi and Keana, indicate long-term geochemical evolution, making these brines potential sources of industrial salts but also posing risks if used untreated for domestic purposes.

Keywords: Brine, Migration, Middle Benue Trough, Isotopic Evidence, Geothermal Gradient.

1. INTRODUCTION

   Brine refers to water that has a total dissolved solids (TDS) content or salinity of greater than 50 g/L. Brines usually contain cations such as K+, Ca2+, Na+, Mg2+, Li+, B3+, Sr2+, Rb2+, and Cs2+, as well as anions such as SO42, Cl, HCO3, CO3 2, Br, and I. Some of these are economically valuable, such as Li2CO3 and KCl; halite has been an important compound for humans. Humanity has a long history of using natural brines to produce Na salts and other minerals. For example, to produce edible salt in Zigong in the southwestern province of Sichuan, China, ancient people drilled deep boreholes and used wooden derricks to harvest brine as early as nearly 2000 years BP (before present); the most deeply drilled borehole in the Jinliu area reached a depth of 1,365.48 m. Brines are usually classified into three

   different types according to their chemical compositions: sulfate-, chloride-, and carbonate-type brines. In the Earth's crust, brines are distributed mainly in Quaternary salt lakes and are stored in the pores of sedimentary rocks in salt lakes and pre-Quaternary strata in depositional basins. Some brines are also formed in fractures in crystalline basement rocks and even on the deep seafloor, such as in the Red Sea.

   This study provides integrated isotopic and geological evidence for the origin, migration, and concentration of brines in the Middle Benue Trough. It documents stable isotope signatures (18O, 2H, d-excess) from multiple sites, combines those data with stratigraphy and structural mapping, and proposes a hydrothermal-evaporative model in which fault-focused upflow, permeable Keana sandstones, meteoric recharge, and surface evaporation jointly produce and trap saline accumulations. The paper adds regionally specific data for Nasarawa State, highlights contrasts between cold and hot brines, and identifies Keana and Ribi as sites of advanced geochemical evolution and potential industrial salt resources.

   It provides one of the few site-specific isotopic datasets for brines within the Middle Benue Trough, improving empirical constraints on brine provenance in the region. It also, advances a coherent conceptual model that links tectonic structure, reservoir sandstone properties, and surface evaporation to brine genesis and trapping, demonstrates practical implications for resource potential and public water safety, connecting geoscience results to applied outcomes and thermally distinct brines, which are relevant to geothermal and economic mineral discussions and to broader basin saline fluid persistence literature.

   Clear dataset of 18O and 2H measured on a reliable instrument with stated reproducibility a logical integration of isotopic plots, d-excess analysis, cluster classification and regional geology.

   1. Study Area

Nigeria has a wealth of brine deposits (Uma, 1998), especially in the Wuse-Akiri and Azara regions of Nasarawa State. There is a growing interest in utilizing brine water with high mineral content as an alternative source of geothermal energy (Membranes, 2023). Brines are abundant in various geothermal fields worldwide source and their exploitation could significantly contribute to our energy needs (Dobson et al., 2021; & Raybach & Abdulhaq, 2023).

The study area, Akiri, Ribi, Awe, and Keana, is located within the Middle Benue Trough in Nasarawa state, north central Nigeria (figure 1.1), an intracontinental rift basin that developed during the Cretaceous because of the separation of South America from Africa (figure 1.2) (Wright, 1968; Burke, et al., 1972; Azuka et al., 2023). Geologically, the area is underlain predominantly by Cretaceous sedimentary sequences, which reflect alternating marine and continental depositional environments. The stratigraphic succession begins with the Asu River Group of Albian age, characterized by dark shales, limestones, and sandstones, representing initial marine transgression. This is overlain unconformably by the Awe Formation (Cenomanian), comprising coarse-grained sandstones, siltstones, and minor shales, indicating fluvio-deltaic conditions. The overlying Keana Formation (Turonian) consists mainly of cross- bedded sandstones interbedded with shales and mudstones, interpreted as fluvial to shallow marine deposits. The Ezeaku Formation, which follows, represents a deeper marine phase, composed of calcareous shales and limestones. Structurally, the region is influenced by faults and folds associated with the NE-SW trending Benue Trough (Abubakar, 2014) which has controlled sedimentation patterns and hydrothermal fluid migration. The presence of brines and hydrothermal alteration features in the study area are indicative of significant post- depositional geochemical processes, often linked to tectonic reactivation and deep fluid circulation within the basin.

(

Figure 1.2: (a) Map showing Stages of Origin of the Benue

2. GEOLOGICAL SETTING

The Benue Trough of Nigeria is a tectonic feature that is part of the West Africa rift system (Binks and Fairhead, 1992; Genik, 1992, 1993; Akande et al.,

2011; Nwajide, 2013; Brownfield, 2016). The trough consists of a long stretch of sedimentary formations running from the northern tip of the Niger Delta Basin and terminates under the Chad Basin and sandwiched by the Basement Complex areas in the north and south of River Benue, filled with sediments that are Middle- Late Albian in age (Offodile, 1976). The trough has a NE-SW strike, and its narrowest part is about 130 km and the broadest part about 200 km. Based on the different structures and stratigraphic framework, the trough had been partitioned into three parts, namely the Southern, Central and Northern (Nwajide, 2013).

( 3. MATERIALS AND METHODS

This study involved sampling and isotopic investigations to evaluate the stuctural control on brine migration in the Middle Benue Trough. Isotopic

analysis (e.g., ¹O and ²H) was performed on

( selected samples to provide insights into the origin,

evolution, and hydrological interactions of the brines.

Stable isotopes of water (18O and 2H) were analysed at the Ghana Atomic Energy Commission (GAEC)

St Isotope Hydrology Laboratory using the Los Gatos

Figure 1.1: (a) Structural framework of the Middle Benue

Research Liquid Water Isotope Analyzer (LWIA-45-

EP), with reproducibility of ±0.2 for 18O and

±1.0 for 2H. The stable isotopes were reported in parts per mil () relative to the Vienna Standard Mean Ocean Water (V-SMOW) using the delta () notation (Equation 1):

sample 1000 () Equation 1

where Rsample and Rstandard represent the isotopic ratios of 2H/1H and 18O/16O for the samples and VSMOW.

1. RESULTS AND DISCUSSION

   1. Geology of the Study Area

      Geologically, the MBT is underlain by a thick succession of Cretaceous sediments, predominantly composed of alternating marine and continental facies. These include, in ascending order, the Asu River Group (Albian), characterized by marine shales, micaceous siltstones, and mudstones; the Awe Formation (Late AlbianCenomanian), comprising feldspathic sandstones and carbonaceous shales; the Keana Formation (CenomanianTuronian), noted for its massive, poorly sorted cross-bedded sandstones and subordinate shales; and the Ezeaku Formation

      Figure 4.1: Geologic Map of the Study Area.

      4.2. Isotopic Analysis

      The isotopic data, ¹O and ²H values (table 1), were plotted on standard isotopic correlation diagrams such as the Global Meteoric Water Line (GMWL) to trace the origin of the brines, assess evaporation trends, and differentiate between meteoric and connate water sources.

      Table 4.1: Hydrogen and Oxygen Isotopic Values of the Study Area

      (Turonian), consisting of calcareous shales, shelly limestones, and friable sandstones (Offodile, 1976). These formations are exposed across various brine- bearing towns, including Keana, Awe, Akiri, and Ribi.

      The geological map of the study area (Figure 4.1) illustrates the surface distribution of these lithostratigraphic units and their spatial relationships. Importantly, field investigations indicate that most brine occurrences are associated with formations containing interbedded shales and sandstones, which provide both sources of salinity and conduits for fluid flow.

      NG-461	Akuri pond salt (hot)	-32.83	0.75	-4.84	0.16
      NG-462	Ribi cold salt pond	-28.88	0.72	-3.99	0.10
      NG-463	Awe pond 1cold saltwater	-33.72	0.72	-5.48	0.16
      NG-464	Awe hot pond saltwater	-34.92	0.78	-5.28	0.14
      NG-465	Keana saltwater	10.91	0.37	2.43	0.06
      NG-466	Akiri pond salt	-32.58	0.60	-5.13	0.11

      LAB ID Sample ID 2H

      Std

      Dev 18O

      Std Dev

      Figure 4.2: Plot of ²H vs ¹O with Global Meteoric Water Line (GMWL) (Craig, 1961). Note: APC (Akiri Pond Cold), APH (Awe Pond Hot), APS hot (Awe Pond Salt).

      The red dashed line on the plot (Figure 4.2) represents the Global Meteoric Water Line (GMWL), which serves as a global reference for evaluating the isotopic composition of natural waters. Water derived from meteoric sourcessuch as precipitation from rainfall or snow – typically plots along or near this line. Several samples, including Akiri Pond Cold (APC), Awe Pond Hot (APH), and APS hot Awe Pond Salt (APS hot), plot below the GMWL, indicating isotopic enrichment likely due to evaporation or mixing with isotopically heavier fluids such as brine. In contrast, the Keana sample exhibits highly enriched ²H and ¹O values and deviates significantly from the GMWL, which is uncharacteristic of typical meteoric water. This marked deviation suggests that the Keana water sample may have undergone intense evaporation or has interacted with deep-seated, saline, or geothermal fluids. The presence of positive isotopic values, such as those recorded in the Keana sample, strongly implies a dominant influence from non- meteoric sources, highlighting significant external geochemical processes likely involving brine or geothermal contributions.

      Below is the d-excess versus ¹O Plot; d-excess plot (Figure 4.3), which is used to differentiate between meteoric origins, evaporation, and mixing processes in water samples.

      Figure 4.3: d-excess versus ¹O plot

      The d-excess is calculated using the formula d-excess

      = ²H 8 × ¹O. In global meteoric waters, typical d- excess values are around +10, represented by the red dashed line. Significant deviations from this benchmark suggest non-equilibrium processes, such as evaporation or interaction with saline (brine) water, which alter the isotopic signature of the samples.

      Key observations from the d-excess versus ¹O plot reveal important isotopic trends: Keana exhibits a notably low d-excess value (\~ -8.49), strongly suggesting significant brine influence or intense evaporation.

      Samples such as Awe Pond hot (APH), Akiri pond cold (APC), and Awe Pond salt (APS Hot) show d-excess values below the typical meteoric threshold of 10, indicating enrichment through evaporative processes. In contrast, Ribi cold pond (RCP) aligns more closely with the meteoric water range, pointing to a potentially less altered or fresh water source. These isotopic patterns are consistent with brine infiltration or geothermal mixing, particularly evident in the distinct behavior of the Keana sample. To further validate these interpretations, cluster analysis was conducted on the isotopic data, grouping similar samples and confirming the presence of clusters potentially impacted by brine. Below is the cluster analysis of the isotope data.

      Figure 4.4: Plot of Plot of ²H Vs ¹O for

      cluster analysis.

      The cluster analysis (Figure 4.4) revealed three distinct groups based on the isotopic composition of the water samples, providing valuable insights into their origin and evolution. The clustering effectively differentiates between brine-impacted waterssuch as Keana, which displays highly enriched isotopic values and low d- excess, indicating influence from deep-seated brine or geothermal fluidsevaporated meteoric waters (e.g., APH, APC, and APS hot), and relatively fresh water with signatures closer to meteoric origin (e.g., RCP). Specifically, Cluster 0 comprises samples that are isotopically depleted, likely representing meteoric waters with minor evaporation; Cluster 1 includes isotopically enriched samples, such as Keana, indicative of brine or geothermal influence; and Cluster 2 exhibits intermediate characteristics, suggesting possible mixing zones between meteoric and saline sources. This clustering pattern supports the interpretation that Keana's water sample originates from a distinctly different source, likely impacted by brine infiltration or geothermal processes.

      Stable isotopes of hydrogen (²H, deuterium) and oxygen (¹O) are essential tools in hydrogeology for tracing the origin, movement, and mixing processes of groundwater. These isotopes help distinguish between different water sources and assess modifications due to environmental or geochemical processes. The Global Meteoric Water Line (GMWL), expressed as ²H = 8 × ¹O + 10 (Craig, 1961), serves as a global reference for identifying meteoric water, which oriinates from

      precipitation. Groundwater samples that plot significantly below or above this line typically indicate the influence of non-meteoric processes such as evaporation, waterrock interaction, or mixing with saline or brine-rich fluids. Such deviations are critical in identifying and understanding the presence of brine or geothermal inputs in aquifer systems.

      The deuterium excess (d-excess), calculated as d- excess = ²H 8 × ¹O, serves as a valuable parameter for identifying isotopic deviations from typical meteoric water. Values significantly lower than the global average of approximately +10 are indicative of non-equilibrium processes, such as evaporative enrichment (Gat & Gonfiantini, 1981) or mixing with brine and geothermal waters (Clark & Fritz, 1997). In this study, the Keana sample exhibits notably enriched -values and a low d-excess, characteristics that are consistent with isotopic signatures influenced by brine infiltration or geothermal input.

      Cluster analysis effectively classified the water samples into three distinct groups: fresh meteoric water evaporated meteoric water and brine-influenced water. This statistical grouping supports and reinforces interpretations derived from isotopic plots and d- excess values, providing a coherent framework for distinguishing between different hydrochemical processes and sources influencing the water samples.

      According to Rollinson (1993), the diagram provides insight into the source of hydrogen in

      natural waters, offering a comparative framework for interpreting hydrogen isotopic composition. The supporting diagram that follows illustrates this relationship.

      Figure 4.5: Natural Hydrogen isotope reservoirs for samples of the study area

      Sub-system (modified from Rollinson, 1993)

      Figure 4.6: Natural Oxygen isotope reservoirs for samples of the study area

      Sub-system (modified from Rollinson, 1993)

      The hydrogen isotopic composition of the studied samples indicate a meteoric origin, reflecting input from precipitation (Figure 4.5). Similarly, the oxygen isotopic signatures align with characteristics typical of meteoric water sources (Figure 4.6).

      Isotopic Insights on Brine Genesis suggests that stable isotope data (¹O and ²H) provide crucial evidence for fluid provenance and mixing. Samples plot below the Global Meteoric Water Line (GMWL), with Keana brine exhibiting a low d-excess (~8.49), a signature of pronounced evaporative enrichment and deep-seated brine input.

      Such isotopic characteristics, displayed in standard diagrams (Figure 4.7), indicate that MBT brines are hybrids of meteoric water, fossil connate fluids, and of geothermal inputs. The brines distinctive -values and d-excess suggest substantial mixing below the surface, while structural traps and permeability barriers facilitate storage and further concentration through repeated fluid cycles.

      Figure 4.7: D-Excess versus ¹O Plot. APC (Akiri Pond Cold), APH (Awe Pond Hot), APS Hot (Awe Pond Salt).

      A conceptual model for brine migration synthesizing geological, geochemical and isotopic data; and for the MBT brine system, may be best explained by a hydrothermal-evaporative model thus; connate and geothermal fluids ascend through fault-controlled pathways, mingled with meteoric inputs and dissolve evaporite minerals, leading to the formation of highly saline brines. Recurrent cycles of fluid migration and surface evaporation result in significant halite and gypsum accumulations, structurally trapped along fault corridors. This model is best visualized using the below conceptual diagram figure 4.8.

      Figure 4.8: Conceptual Model of Brine Emergence in the Middle Benue Trough.

2. CONCLUSION

The geochemical and isotopic evidence points to a predominantly meteoric origin for the brines in the study area, modified by intense evaporation and interaction with host rocks. Structural features and sandstone aquifers significantly control the migration and accumulation of these brines. The presence of both

cold and hot brines can be attributed to differential geothermal gradients and fault-controlled fluid pathways. The elevated concentrations of dissolved solids, particularly at Ribi and Keana, indicate long- term geochemical evolution, making these brines potential sources of industrial salts but also posing risks if used untreated for domestic purposes.

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