PETROGRAPHIC STUDIES OF SELECTED BARIUM SULPHATE DEPOSITS IN BENUE TROUGH OF NIGERIA

CHAPTER ONE
1.0 INTRODUCTION
Barytes is an orthorhombic mineral with chemical composition BaSO4. It possesses one perfect cleavage and two good cleavages, as do the isostructural minerals. The mineral has a specific gravity of approximately 4.5, and it is relatively soft, approximately 3 on Mohs scale. The colour ranges from white to yellowish, grey, pale blue or brown and a thin section is colourless. Barytes (BaSO4) are heavy spar, inert and stable. These properties make them valuable. Barytes, formerly used chiefly as filer and adulterant, is now used in glass and paints industries and the oil well drilling industries which consume 80 percent the world production. Barytes group minerals include the sulphates (SO42_) of barium (barites or barite), strontium (Celestine) and lead (anglesite). The structure of barite has sulphate group lying on a reflection plane, two oxygen of the sulphate group lie within this plane, and two oxygen and minor images across it. Each barium atom is coordinated by twelve oxygen atoms and seven separate sulphate groups. Baryte, the most common barium mineral, is abundant in moderate to low-temperature sulphide veins, being associated with fluorite. It occurs mainly as gauge minerals in metalliferous hydrothermal veins and as veins or cavity filling concretion in limestones, sandstone, shale and clay (Dunham 1984).
1.1   ORIGIN OF BARYTES
The origin of barite according to Hatch wells (1971) can be supported by the following model:
I.      Anatexis
II.     Differentiation of basaltic magma from the upper and lower crustal rock by fusion.
III.    Carbonitic origin.
Anatexis ; this is the partial melting and recrystallization of pre existing rocks which is attained as high temperature, low pressure and shallow depth (Hatch wells, 1971). The melt composition depends on phase relationships in the solid and the temperature and pressure conditions of melting. (Hatch wells, 1971). Differentiation of basaltic magma derived from the upper mantle and lower crustal rock by fusion olivine crystallizes out of the melt due to reduction in temperature.
STAGE 1; this is the early stage during which Mg/ Fe rich mineral like Olivine crystallize out of the melt, due to reduction in temperature.
STAGE 2; this is the intermediate stage when the rock forming minerals crystallize out of the melt.
STAGE 3; this is the late stage of crystallization during which the residual magma rich in water and volatiles crystallize out. The product of this stage is usually pegmatite rocks in which barite minerals are associated with. Also in this series minerals higher above, crystallize out of the melt before the ones below and the earlier formed minerals may react with magma to form mineral lower in reaction series. Iv carbonitic origin; this is divided into 3 main types;
a)      Late barite replacement carbonites which are commonly ankeritic, sideritic or maganiferous and also contain fluorite and have contained fluorite and may have RE-Th species.
b)      Veins and replacement bodies outside the complex, usually in unaltered wall rock, barites alone, barite plus fluorite or quartz or carbonates.
c)      Residual barites accumulation and supergene barites developed by weathering of carbonitic complexes.
1.1.2 CHEMISTRY OF BARYTES
Specimen of barites are generally nearly pure BaSO4. Barium (Ba) can be replaced by strontium (Sr) in a continuous solid solution series from barites to Celestine. Members of this series with a preponderance of Ba, molecule are called strontibarytes and these near the Sr end are called baryto-celestine (Heinrich and Vian, 1996). Appreciable replacement of Ba by Pb or by Ca is uncommon and it has been shown that at room temperature only about 6% CaSO4 can enter into solid solution in barites structure. The solubility of barite in water is very slight but it is increased by heating and by the presence of chlorides. When gently heated some crystal of barite deprecipate giving of H2S (Dunham, 1984).
1.2 AREA OF STUDY
The area studied is within the Guma Local Government of Benue state between latitude 07035’06’’ and 08012’56’’N and longitude 23’02’’ and 009007’52’’E, within the lower part of the middle Benue trough that contains cretaceous sediments.
1.2.1 CLIMATE
The climate is tropical with seasonal variations classified as wet and dry seasons, the middle Benue has a more prolonged wet season within early march to mid October while dry season is between late October and early march. The wet season fluctuated more but generally lies between late April to early October while dry season lies between early October and late April at upper Benue Temperatures are generally high between March and April and October to early February. However in some years when harmatan is severe the dry season last longer
1.2.2 TOPOGRAPHY
The middle Benue is generally characterized by a gently undulating topography which is punctuated by few hills. As one ascends the Benue trough through the middle to upper part of the trough the topography becomes rough. At the upper Benue, relief is exaggerated by hills like the lamudes and ligri, which rise up to 600m above the sea level.
1.2.3 VEGETATION
The vegetation of the middle Benue is of the guinea savannah and is made up the shrubs, tall grasses, scattered coconut and palm trees and a lot of mango and citrus trees
1.2.4 DRAINAGE
The Benue trough is drained by river and it tributaries meandering from north to south. Most of the river channels dry up during the dry season. The pattern is generally dendritics in the area studied.
1.2.5 GEOLOGY OF BENUE TROUGH
The Benue trough is defined as an intercontinental cretaceous basin about 1000km in length stretching in a NE-SW direction and resting unconformable upon the Precambrian basement (Barber, 1957). It is commonly subdivided into three main domains corresponding to both geological and geomorphological portion.
·         The upper Benue trough is the northeastern Y-shaped part of the basin and can  be sub divided into three domain, the Yola-Garua branch trending WNW-ESE, the Gongola branch and the Muri-Larnide domain trending NSS’’E
·         The lower Benue rough shifted south west includes two main structural units, the N 600E trending Abakaliki antclinorium flanked by the Anambra syncline trending N300E
·         The middle Benue trough is the linear part of the basin