Geology and Hydrogeophysical Investigation of Gurum and Environs, Lere Sheet 147SE, Bassa-Plateau, North-Central Nigeria

: Geology and hydrogeophysical investigation of Gurum and its environs was undertaken within Lere Sheet 147 SE. The geological studies revealed the presence of Precambrian rocks (undifferentiated migmatites and Older Granites) and Jurassic rocks (Younger Granite) of different varieties belonging to the Buji Complex. The Buji Complex is composed of two (2) super imposed ring complexes, the earlier volcanic and high-level hyperbyssal intrusion dominated by granitic rocks. Hydrogeophysical results showed depth to various geo-electric layers and the range of apparent resistivity values with lithological units and their water yielding potentials. The studies clearly show the aquifer thickness is sufficient and the resistivity value falls within the range of good water yield. Additionally, it can also be said that the aquifers in the study area including both weathered overburden and fractured crystalline rocks are capable of yielding significant amount of water to wells. The aquifer thickness appears to increase towards the north eastern part of Gurum.


Introduction
The study area is located around Gurum and its environs within Lere Sheet 147 SE.The study area is underlain by the Precambrian rocks of the Nigerian Basement Complex as well as rocks belonging to the Younger Granite Suite.Here, groundwater occurs within the weathered overburden, as well as fractured crystalline rocks.These aquifer proportions vary from place to place.Geophysical investigation was carried out using schumberger array method in the collection of data, 10 VES points were taken in Gurum and its environment.The study was carried out to collate detail information about the hydrogeology of the area from the locals, to understand water related issues and develop sustainable solutions in collaboration with the community.The data acquired provide a thorough understanding of the groundwater system on Gurum area, assisting in the sustainable development and management of water resources to meet the needs of the local population.
Additionally, geological and geophysical survey of Gurum and its environs was investigated, so as to ascertain the groundwater potential of the area in relation to its geology so as to achieve some of these specific objectives; to carry out detailed geological studies of the area, to determine resistivity values and thickness of various lithological rock units, and to also assess

Location and Accessibility
The area of study is Gurum and its environs.It falls within the Basement Complex of North Central Nigeria consisting mainly of migmatite and the Younger Granites consisting of biotite granites belonging to the Buji Complex.The study area is bordered in the south by Mista Ali and in the north by Rumfar Gwamna Village, which are the major villages surrounding the study area.The study area has an estimated landmass of 20km² and lies between Latitudes 10°01'00''N to 10°02'35''N and Longitudes 8°48'15"E to 8°51'00''E (Fig. 1), provides an aerial photograph of the location of study area.The research area is accessible through Jos-Zaria road.Access through the research area was also made possible through a good network of secondary roads and footpaths that traverse.The accessibility of some parts of the area was hindered by the terrain which is hilly with high features and steep sides and the presence of gully erosion in some areas.Figure 2 is the mean monthly rainfall of Jos area from 2010 to 2015.Table 1 also shows the mean monthly temperature of Jos-Plateau from 2010-2015.The earliest study of the Buji Complex was done by Falconer (1911) in the survey of the Jos Plateau tin fields.The study was carried out on a regional scale.Figure 4

Geology of Jos Plateau
The Jos Plateau area is a part of the Precambrian to Mid-Cambrian and Jurassic Northern Nigerian crystalline patches of Cenozoic Volcanic rocks, during the Pan-African as well as during the Early and Middle Cambrian, Older (Pan African) granites were emplaced in several places.
After a prolonged period of denudation, during the Jurassic, the second Phase of granites were intruded (Younger Granites) according to (Falconer, 1911).The Younger Granites of the Jos Plateau and surrounding areas are petrologically distinctive series of alkali feldspar granites associated with rhyolites and minor gabbros and syenites.They occur in subvolcanic intrusive complexes as ring dykes and related annular and cylindrical intrusions.They are richly mineralized with tin (Sn), niobium and columbite (Falconer, 1911).
The Younger Granites of the Jos Plateau occur as rocky hilly massif sharply differentiated from the smoother topography of the surrounding basement rocks.In extent they cover an area of about 8600km² in central Nigeria and are located within a N-S rectangle of 400km long and 60km wide (Buchanan et al.,1971).It extends from south to north between Latitudes 9°00'N' to 10° 30'N and from the west to east between Longitudes 8°30'E to 9°30'E.
The Basement Complex rock underlies some two-third of the Jos Plateau.Wright (1971) gave a division of the Basement Complex rocks of Jos Plateau to include Pre-migmatite rocks, migmatites, granite gneiss and Older Granites.
The description of the basement rock units presented below follows this division.Pre-migmatite rocks are divided into granulitic gneiss and intermediate rocks.The granulitic gneisses are fine grained rocks with a sugary texture, grey to brown in colour, distinctly foliated and composed largely of quartz, feldspars and biotite (Wright, 1971).The usual rock type is a quartzofeldspathic granulite with biotite either as dispersed flakes or aggregated in thin discontinuous hands or streaks parallel to the foliation.The gneisses may be uniform over extensive outcrops, modified only by thin veins of quartz or pegmatite.
Three groups of intermediate rocks occur on the Jos Plateau.These are the Rahaman diorite (located in extreme north-western part), the Toro diorite (located in the central part) and the Sho diorites located in south-western part.All the three possess a primary texture which compares with an igneous rock, modified by later crystalloblastic development chiefly of red brown biotite and microcIine (Wright, 1971).The Rahaman and Toro diorites are hypesthenes-bearing rocks and are wholly massive or almost so, while the Sho diorite contains hornblende and includes massive as well as foliated and strongly migmatised rocks with extensive neo-mineralization.The migmatites include rocks of varying lithologies, textures and structures, showing differing degrees of granitization and migmatization.They occupy mainly the western part of Jos Plateau.In general, exposure is limited and the outcrops are of low relief.Over large areas, the degree of migmatization may not change appreciably apart from local fluctuations, but generally there is a pronounced increase in the vicinity of the principal granites and granite gneiss masses.
Some of the migmatites are composite in nature, consisting of metamorphic host rock and acid injections which may be pegmatitic, feldspathic or granitic material (McCurry, 1989).In these composite migmatites, a discrete leucocratic component alternates with a more basic component (Wright, 1971).The migmatites are a varied group: ranging 'from coarsely mixed gneisses to more diffusely textured rocks of the variable grain size and frequently porphyroblastic.Foliation is usually not well marked but it is commonly shown by the streaky aggregates of dark minerals which wind around the porphyroblastic feldspars.Metamorphism here is high grade.Granite gneiss, The term granite gneiss generally applied to a heterogeneous group of rocks, predominantly of granodioritic composition, in which variable amounts of remnant streaks and larger relict bodies of original gneiss are recognizable, and in which subordinate granitic and tonalitic phases occur (McCurry, 1989).On the Jos Plateau they occur in north-central, north-eastern, eastern and south-eastern parts, Where the rock have contacts with migmatites, they are transitional and generally have a complete structural conformity with the foliation in both rocks trending parallel to the common boundary (Wright, 1971).The main component minerals of the granite gneisses are plagioclase, quartz, microcline, and biotite.The common accessories include apatite, epidote, muscovite, sericite sphene, allanite and iron ore.
Older Granites encompass rocks intruded during the Pan-African orogenic cycle (McCurry, 1989).Three main categories were identified, based largely on general texture and mineralogical characteristics, are recognized on the Jos Plateau area.The first group is most extensive and include medium to coarse grained biotite and biotite hornblende granites which are frequently porphyritic.These occur in patches as inselbergs on the Plateau and include and the Dass Hills granite (located at the eastern part).Others are the Zagun Granite (located in the western part), the Maijuju and the Sho Granite found in the central and south-western parts respectively.
The Second group is the medium to coarse grained biotite hornblende granites and quartz syenites.They are mostly non-porphyritic and outcrop mainly in the south-eastern and northcentral parts of the Jos Plateau as Gindiri Granite and part of the Toro Annular Complex, respectively.They are composed of feldspars, quartz, biotite and hornblende.
The third group is fine to medium-grained and biotite-muscovite granites.They are widely distributed but are mostly concentrated in the western and south-western parts of the Jos Plateau.These granites are found outcropping within the Maijuju and Gora Granite in the central part and the Gindiri Granite and Lere Granite in the south-eastern part.Other outcrops of this rock type include those of the Assob Granite (located in the south-western part), the Rafin Bauna Granite and the Mariri Granite all located in the western part of the Jos Plateau.They are composed mainly of biotite and muscovite and are distinguished by large crystal of sphene.
The Younger Granites of Jos the Plateau occupy about one-third of the total surface area.The granites were first defined by Falconer (1911).He described them as cross-cutting alkali granites of reibeckite or biotite characterized by chilled margins against their country rocks.As they are known today, the rocks occur as hilly massifs sharply differentiated from the smoother topography of the surrounding basement rocks.They are petrologically distinctive series of alkali feldspar granites, associated with rhyolites and minor gabbros and syenites, which occur in subvolcanic intrusive complexes as ring dykes and related annular and cylindrical bodies (Turner, 1976).
Younger Granite complexes consist of a series of distinct intrusions often having a concentric arrangement, each intrusion having petrographic features which occur in a province.According to MacLeod et al. (1971), the rock types of the complexes include rhyolites, granites, syenites and basic rocks.There are two kinds of rhyolite on the Jos Plateau, namely Early and Late rhyolites.The Early rhyolites display a greater diversity of texture; they have porphyritic glassy and spherulitic varieties.They usually contain iron oxides, amphibole, pyroxene, sparse quartz and alkali fe1dspar.Phenocrysts in finely crystalline matrix also occur in this variety.Late rhyolites consist of very abundant phenocrysts of quartz and alkali feldspar, often fractured and aggregated.They are remarkably uniform in texture, both laterally and vertically.The granites are constant in character throughout the province although they exhibit a large variety of rock types.The granite suite comprises hornblende granite and porphyries group, biotite and reibeckite granites.The hornblende granite and porphyries group contains a wide range of colored minerals including fayalite, hedenbergitic pyroxene, hornblende, arfvedsonite and biotite.The hornblende granite sometimes appears as large plutons with granite texture, but in the ring dykes, they commonly assume a porphyritic texture, and to this facies, the term granite porphyry is applied (MacLeod and Turner, 1971).In the Jos Plateau, the early ring dykes are frequently of a coarse hornblendebiotite-granite of granite porphyry and this rock type is also abundant as plutons.The biotitegranite is the most abundant and widespread rock type on the Plateau.They form some of the largest individual intrusions (Macleod and Turner, 1971), and contain a wide variety of interesting accessory minerals which include zircon, fluorite and iron oxides, thorite, monazite and xenotime (Turner, 1976).
Syenites have a wide distribution in the Younger Granite Province but cover a comparatively small area.The most extensive occurrence is at the north-eastern corner of the province.They have both granular and porphyritic texture (MacLeod and Turner (1971).The main component is alkali feldspar but some syenites contain minor plagioclase.Common accessory minerals include ilmenite, zircon, apatite, calcite and allanite.
Basic rocks appear in many of the complexes but are always in the form of small intrusions which have-preceded the granites and have been partly obliterated.The rocks occur in the form of gabbros, dolerites and basalts.The typical gabbro is a coarse grained rock composed of basic plagioclase, hornblende, augite, occasionally olivine and accessory iron oxide; the gabbro intrusions are usually composite masses and include medium-fine grained facies (MacLeod and Turner, 1971).Basic rock in the form of dolerite and basalts occur as lava flows, dykes and semi-concordant intrusions in the rhyolites and granites of many of the complexes.Some precede the granite but, there is abundant evidence for the recurrence of basic intrusion at all stages of the acid cycle (Macleod and Turner, 1971).

Geology of the Study Area
The Gurum area and its environs lie within the Buji area.The area is underlain both by undifferentiated migmatites and Older Granite, as well as in part by Younger Granite belonging to the Buji Complex.The Buji Complex forms a part of the Northern Nigeria Younger Granite Ring Complex, which are a part of the larger igneous province extending from Afu in Nigeria to as far to the north in Niger republic.The Buji Complex is generally circular in outline covering an area of about 48km 2 with a maximum diameter of about 10km.This complex forms a high dissected Plateau in the northern part, of which Dutsen Buji (1170.74m)forms the highest point, the southern area of the Buji Complex is of more subdued topography with low rocky hills around.
The Buji Massif is composed of two (2) super imposed ring complexes, the earlier being the eastern ring complex composed almost entirely of volcanic rocks and high-level hyperbyssal intrusion, while the western ring complex is the later and is dominated by granitic rocks.However both ring complexes have a related mode of emplacement (i.e. the tectonic process that was associated with the epirogenic uplift).
The volcanic activities are marked by two (2) stages; the patterns of both are controlled by essentially the same tectonic processes that control the emplacement of the granite plutons and ring-dykes at greater depth.The earlier stage being the development of vent along the line of ring fault, the later being marked by segmental fracturing and differential subsidence, containing mainly the Early and Late Rhyolites Macleod et al. (1971) Quartz-hedenbergite porphyry has been introduced into the Late Rhyolite and also a ring dyke of quartz feldspar-porphyry has been emplaced in the complex.A comparatively simple pattern granite ring-structure occupies the western half of the complex, the emplacements of which was preceded by the intrusion of cone-sheets which cuts the rhyolites and the Basement Complex on the eastern and the northern sides of the granite.A discontinuous ring-dyke of aegirine-micro-granite has followed the cone sheets, which in turn has been partly obliterated by the main intrusion of biotite granite.At the western side of the complex albite-riebeckite granite are also found.
This study essentially involved the use of geophysics to determine the aquifer geologic characteristics in relation to the water yielding potentials.To be aquiferous, the subsurface rocks must be sufficiently weathered or fractured for improved porosity.
Groundwater tends to be highly limited in extent in basement and other crystalline rock terrains (Olorunfemi et al., 1999;Offodile, 2002).Thereby a subsurface method of finding water is important.There are many important geophysical techniques used in groundwater exploration, like gravity, magnetic, seismic refraction, and geo-electrical prospecting methods.Out of this, electrical resistivity method helps in solving groundwater problems through its highest resolving power and economical viability.Electrical resistivity methods are used to investigate the different lithological formations, bed rock dispositions, the depth to water table or zone of saturated formations, thickness of weathered zones, detection of fissures, fractures, and fault zones, establishment of their depths, thickness and lateral extent of aquifer.

Principles of Electrical Resistivity Survey
Electrical resistivity survey was employed in this research by passing a known electric current into the ground by means of two current electrodes and the potential differences between the other two potential electrodes is measured.The potential variations may be change due to size, shape and conducting capacity of the material in the subsurface and from the quantities of potential differences and the current applied where the resistance is calculated.Electrical resistivity can be termed in ohm-meter recorded as a standard unit.Electrical Resistivity Surveys could find out a good electrical resistivity difference between the water bearing formations and the surrounding rocks can be achieved using software (Zohdy et. al., 1974).In this process a known value of electric current (I) is passed into the ground by two outer metals stakes (C1 and C2) that are buried in the ground.The potential variation (V) is measured between two inner electrodes termed potential electrodes (P1and P2).
The ratio of V/I provides the resistance (R) and by multiplying R with the geo-electrical factor (K) of the electrode separation, the resistivity 'ρ', and it is inverse of conductivity of the ground may be described.
The potential variation passes because of the externally pressing current between various electrodes, the apparent resistivity of the elements established in the given geologic formation which does not match the average resistivity and it may be lower than the lowest and higher than the highest resistivity within the subsurface to which it pertains.The apparent potential value of 'ρ' corresponds to the true resistivity, if the ground is homogeneous and isotropic when it is obtained from the measurements over a layered or heterogeneous ground, then it is only an apparent resistivity and is signed by 'ρa', the quality being used in the interpretation of electrical methods.
The resistivity of geological subsurface formations differs very broadly not only from formation to formation but also within one lithological unit and is related to

•
Size and shape of the aquifer materials, density, porosity, pore size of the material

•
Quality of water, size, shape, pore space and density of the aquifer horizons

•
Distribution of water in the rocks due to the structural and textural characteristics and

•
The temperature of the subsurface of the water environment.
Many different electrode arrangements have been proposed and used for resistivity exploration.Wenner Schlumberger and Axial Dipole-Dipole arrangements are widely used because the interpretation tools are well developed and they will be adequate for application to groundwater and shallow geologic problems.The Schlumberger arrangement is widely used for quantitative interpretation in Vertical Electrical Sounding compared with Wenner.It offers the important advantage of being less sensitive to unknown lateral homogeneities because the potential electrode (M & N) remains in fixed position during a large number of sensitive measurements.
Data is collected using standard electrode configuration of schlumberger configuration using the principal that the current electrodes vary along a straight line in both directions.However, the potential electrodes remain constant and moved when better results of subsurface strata is needed in case of weak signals.Four electrodes are placed along a straight line on the Earth's surface in the same order, AMNB, as in the Wenner array, but with AB ≥ 5 MN.
Field geophysical measurements were carried out using the vertical electrical sounding (VES) technique and using the Allied Ohmega Terrameter as the field operating equipment as shown in Plate 1.The maximum electrical spread was (AB/2) was 125meters.
With a Schlumberger array the various resistance values obtained from the Terrametre for the various AB/2 points were multiplied by the different K-factors to obtain various electrode spacing AB/2 values were then plotted against the apparent resistivity values using the Zohdy geophysical software which produced the geophysical curves as well as the processed data.The sounding sites were selected across the area especially within the various settlements.In all, ten (10) vertical electrical soundings VES points were made within the study area.

Results
The electrode space (AB/2) in metres, resistance and apparent resistivity data obtained in the field, as well as the processed data obtained using the Zohdy geophysical software and are presented in Tables 3 (a&b) to 12 (a&b).Tables 3b to 12b are the processed data from which the depth to various lithostratas as well as the geophysical nature of the rock materials below each VES where obtained.Details of the interpretation of the results are presented.If the ground is composed of three layers of resistivities ρ1, ρ2, ρ3 and thicknesses h1, h2, and h3 = ∞, the geo-electric section is described according to the relation between the values of ρ1, ρ2, ρ3.
There are four possible combinations between the values of ρ1, ρ2, ρ3.These are; 1) ρ1 > ρ2 < ρ3 …… H-type curve (minimum type) 2) ρ1 < ρ2 < ρ3 …… A-type curve (ascending type) 3) ρ1 < ρ2 > ρ3 …... K-type curve (maximum type) 4) ρ1 > ρ2 > ρ3 …… Q-type curve (descending type) Types H and K curves have a definite minimum and maximum, indicating a bed or beds of anomalously low or high resistivity respectively at intermediate depth.Types A and Q curves show fairly uniform change in resistivity, the first increasing the second decreasing with depth.
A-type curves are obtained in the hard rock environments with conductive topsoil.In this case, the resistivity of the layers will be continuously increasing (ρ1<ρ2<ρ3).Sounding curves which have maximum peak and are occupied by low resistivity values (ρ1<ρ2>ρ3) are termed K-type curves and such curves result from different environment.A sounding curve with continuously decreasing resistivity (ρ1>ρ2>ρ3) is said to be Q-type curve and this type of curves are commonly obtained in coastal region because of the saline water.The sounding curves with a central minimum (ρ1>ρ2<ρ3) are called H-type curves.This type of sounding curves are normally found in hard rock formations and it consists of a dry top soil of high resistivity as the first layer, water saturated weathered layer of minimum resistivity as the second layer and compact hard rock unit with a very high resistivity as the third layer.
If the ground is composed of more than three horizontal layers of resistivities ρ1, ρ2, ρ3, ….. ρn and thicknesses h1, h2, h3, …. hn = ∞, the geo-electric section is described in terms of the relationship between the resistivities of the layers, and the letters H, A, K and Q are used in combination to indicate the variation of resistivity with depth.Geophysical investigation carried out here revealed that the area has good groundwater potential.The aquifers are relatively thick and the apparent resistivity values are low, meaning that conductivity is high, suggesting the possibility of good groundwater yield in most places.It can be concluded therefore that the aquifer in this area are good and can support either shallow hand pump boreholes or motorized boreholes in most places.

Figure 1 .
Figure 1.Aerial photograph of Location map of the study area Gurum, Bassa, Plateau State Source: Google Earth Dr Mahadev Unde, Department of Electrical Engineering, Zeal College of Engineering and Research Observatory, Department of Geography and Planning, University of Jos.

Figure 3 .
Figure 3. Mean Monthly Temperature(C°) Chart for Station in Jos, Plateau State from 2010-2015 Source: Observatory, Department of Geography and Planning, University of Jos.
below is a geological map of Nigeria showing the location of the Younger Granite rocks.

Figure
Figure 4.A simplified geologic map of Nigeria Source: Obaje, 2009

Figure
Figure 5.The Schlumberger Array

Figure
Figure 6.Geophysical Curve for VES 1 Water bearing to dry 10, geo-electric section CC' along VES 1 to VES 6.

Figure 27 .
Figure 27.Aquifer thickness map of the study area