Distribution of Heavy Minerals in Sediments of Osun River Basin Southwestern Nigeria

Transcription

1 Research Journal of Earth Sciences 1 (2): 74-80, 2009 ISSN IDOSI Publications, 2009 Distribution of Heavy Minerals in Sediments of Osun River Basin Southwestern Nigeria 1 2 E.O. Joshua and O.A. Oyebanjo 1 Department of Physics, University of Ibadan, Nigeria 2 Department of Physics and Mathematical Sciences, Tai Solarin University of Education Ijebu-Ode, Nigeria Abstract: Heavy minerals assemblage of sediments in Osun River basin have been studied in five states in Southwest Nigeria to determine their concentrations and provenance. Twenty-five sediment samples were analysed for heavy mineral assemblage determination. The separation of these minerals was carried out using bromoform (Specific gravity 2.85) and slide examination under the petrographic microscope. The heavy minerals assemblages of Zircon, Tourmaline, Rutile, Silimatate, Garnet, Epidote and Staurolite indicate its derivation from mixed sources of acid igneous rock to medium and high grade metamorphic rocks. The calculated ZTR % values varied between 21.1% in Ekiti to 56.7% in Lagos state and the mean ZTR (Zircon, Tourmaline, Rutile) percentage indices was 36.4%. The two most abundant are Staurolite and Rutile with 43.92% and 11.95% respectively. We found that their concentrations and sizes increase from Ekiti state in the upper section of Osun basin to Lagos in the southern part of the basin according to coastal drift whereas the mineralogical component remains constant. Key words:heavy minerals % River sediments % Activity concentration % Provenance % Mineralogical component % Rock % River-Osun % Southwest nigeria INTRODUCTION River sediments originate from the near surface, exposed igneous, volcanic and sedimentary rocks. Some of these are easily eroded, whereas others, especially the crystalline and metamorphic rocks, are affected by streams only when altered in surface layers. Additional sources of river sediments are soils which inherited their mineral content (with some alteration) from bedrock or which in the tropic may consist completely of newly formed minerals [1]. The drainage system of Osun River rises in Oke-Mesi ridge, about 5 km North of Effon Alaiye on the border between Oyo and Ondo States of Nigeria and flows North 1 through the Itawure gap to latitude 7 53 before winding its way Westwards through Osogbo and Ede and Southwards to enter Lagos lagoon about 8 km east of Epe (Fig. 2) is underlain by metamorphic rocks of Precambrian basement complex, the great majority of which are very ancient in age. These basement complex show great variations in grain size and in mineral composition. The rocks are quartz geniuses and schist consisting essential of quartz with small amounts of white micaceous minerals [2]. According to Akande [3] in grain size and structure, the rocks vary from coarse grained pegmatite to medium grained geniuses. The rocks are strongly foliated and they occur as outcrops especially in Efon alaiye and Ikere Ekiti area. Sedimentary rocks of cretaceous and latter deposits are found in the southern sections of the Osun basin. The remaining sections are composed of crystalline rocks of the basement complex, consisting mainly of folded gneiss, schist and quartzite complexes, which belong to the older intrusive series. Although in many places outcrops are plainly visible, large areas are overlain by layers of laterite soil formed by weathering and decomposition of the prevent rock material. Along the river basin the provenance of the minerals have been dealt with based on heavy minerals studies. However, characterization of opaque is not yet attempted from the present study region. The present work highlights the distribution of non-opaque minerals with emphasis of possible source and concentration for exploitation. Corresponding Author: O.A. Oyebanjo, Department of Physics and Mathematical Sciences, Tai Solarin University of Education. Ijagun Ijebu-Ode, Nigeria 74

2 Fig. 1: Showing Apoje Labour Campijebu-Igbo Where Sample 17 were Collected Fig. 2: Map of South Western Nigeria Showing state along river Osun basin of sample collection MATERIALS AND METHODS Field Measurement: One hundred and six (106) samples of River Sediments were collected at twenty five (25) different locations. As shown in Figure 3, the locations span the length of the river. In each location, samples were collected at a minimum of four different sports. The number of locations per state is partly due to accessibility, the distance transverse by the River in each state and largely the level of human activities along the river basin. At each samples location, about 200g of the samples were collected. Each sample was packed in cellophane bags tied and labeled accordingly and taken to the laboratory for further investigation. Both the Granulometric and the Heavy mineral separation analyses were carried out at the Geology Department Laboratory, University of Ibadan. 75

3 Fig. 3: Map showing the sampling points in five states along river Osun basin in southwestern Nigeria 7% 3% 11% 12% 43% Z T R G St Ep Sill 11% 13% Fig. 4: Overall percentage (%) of each heavy mineral Experimental: 25 bedload representatives samples have The mixture was vigorously stirred and left for about been selected between the months of February and 5 minutes. The heavy minerals present in he sample with March 2006 across the South western Nigeria taking 4 specific gravity > 2.85g/cm3 were allowed to settle to the samples each in Ekiti and Osun states, 3 samples each in bottom of the separating funnel or after which the filtrate Oyo and Lagos and 11 samples in Ogun states (heavy mineral) were thoroughly washed with acetone to respectively ranging form North to South. Samples were move any trace of bromoform and also dry. The heavy sieved with a Ro-tap machine and statistical parameters minerals were mounted on slides with the aid of Canada were obtained [4]. balsam [5] and the opaque mineral was identified based on 5g of each dried samples was released into a [6] colour guide as shown in (Fig 5). The non-opaque separating funnel using the dense-liquid technique, with minerals were studied under transmitted light microscope 3 bromoform (Specific gravity 2.8g/cm ). Samples were [7]. The results obtained are shown in Table 2. boiled in 10% Hydrochloric acid for two minutes in order The ZTR index, which is a quantitative definition of to disintegrate the samples. Boiled samples were rinsed mineral assemblage, was calculated using the percentage with water to remove the Hydrochloric acid and dried in of combined Zircon, Tourmaline and Rutile grains for each an oven in order to separate the fine sand fraction. sample. 76

4 100% 80% 60% 40% Sil l E p St G R T Z 20% 0% SAMPLES Fig. 5: Proportion of non-opaque heavy mineral in the Sediment Samples RESULTS AND DISCUSSION Result of grain size statistics (Table 1) indicates the average mean is 1.14 and the average (standard deviation) sorting is 0.84 which shows that it is moderately sorted and therefore indicative of submature sediments. Statistical parameters such as average grain size and sorting normally reflect the energetic condition of the environment [8]. The sediments have an average skewness of which indicates an appreciable energy environment. The average kurtosis is 1.69 (very leptokurtic) which shows that the sediments could be from a single source. The sediments characteristics are medium to fine grained, moderately to well sorted ( ) and negatively to symmetrically skewed. The heavy mineral analysis carried out on the twenty five (25) representatives samples revealed the presence of Zircon, Tourmaline, Rutile, Silimanite, Steurolite, Epidote, garnet and opaque minerals (Table 2). Percentage of opaque minerals in all samples are generally very high relative to the heavy minerals they are generally referred to as iron-stained minerals and because of the coating; illumination in a petrographic microscope cannot penetrate them and hence their study will require chemical analytical approaches. Table 2 shows the proportion of each mineral in each sample. The Zircon, Table 1: Calculated grain size parameters for Osun river sediments State Sample code Mean Sorting Kurtosis Skewness HEK HEK Ekiti HEK HEK HEK HOS HOS Osun HOS HOS HOS HOY HOY Oyo HOY HOY HOY HOG HOG Ogun HOG HOG HOG HLA HLA Lagos HLA HLA HLA AVERAGE

5 Table 2: Proportion of Heavy Minerals Concentrations in the sediment samples CODE SERIAL NO Z T R G St Ep Sill Op HEK HEK HEK HEK HOS HOS HOS HOS HOY HOY HOY HOG HOG HOG HOG HOG HOG HOG HOG HOG HOG HOG HLA HLA HLA Table 3: ZTR Maturity index and individual percentage mineral of the river oshun sediments samples SERIAL NO Z T R G St Ep Sill Total Z+T+R ZTR%index Total % Individual mineral % abundance Average ZTR% index= 36.4%. Opaque = 3402 Non-opaque =

6 Tourmaline, Rutile (ZTR) percentage index calculated for igneous rock source of the sediments. The possibility each sample is included in Table 3. of the source rock being igneous rock is very low Staurolite which has the highest individual mineral because augite, diopside, hypersthenes or olivine are percentage of % is a red brown to black, mostly largely absent from the heavy mineral assemblage [11]. opaque, neosilitate minerals with a white spreak. It has a Instead Staurolite, rutile and garnet occur in relatively complex chemical formula (Fe, Mg, Zn) 2 Alg (Si, Al) 4 O22 fairly large quantities with respect to epidote and (OH) 2. The iron magnesium and zinc usually occur in silimanite, which are indicative of dynamo thermal variable ratios. Steurolite is one of the index minerals metamorphic rock source. that are used to estimate the temperature, depth and Hubert [12] stated that the non-opaque or pressure of which a rock undergoes metamorphism transparent non-micaceous heavy mineral assemblage Staurolite is a regional metamorphic mineral of of the quartz heavy mineral assemblage of the quartz intermediate high grade. arenites are predominantly zircon, tourmaline and The highest proportion of the Staurolite obtained rutile and these grains are ultimately concentrated in recorded in all the states shows regionally sandstones by prolonged abrasion. The chemically metamorphosed rocks such as iron-rich peliters, which stable minerals are normally provided insufficient may also have high Fe, Fe ratios at medium grades of quantities by most granitic and; low rank metamorphic metamorphisms. The mineral often occur alongside garnet source interdependent. Thus the ZTR index is of and kyanite. modification or maturity of entire heavy minerals Rutile (13.1%) is next to Staurolite and relatively assemblage of river sediments. abundant in most samples, only the red colored variety The highest value of ZTR (56.7%) is recorded in was recognized. Epidote (7.13%) and silimanite (2.52%) Lagos state and of the river while the lowest value are equally in small proportion. Rutile is a common (21.1%) is found in Ekiti state where the river originates. accessory mineral in high temperature and high This shows that maturity increases with the distance pressure metamorphic and igneous rocks. According to traveled by the sediments. Pellant and Phillips [9] Rutile is a non-silicate mineral occurring as an accessory constituent of igneous rocks CONCLUSION and many granites, diorites and their metamorphic derivatives such as gneisses and amphibolites. Rutile is Heavy mineral assemblages indicate the presence of used as a source of titanium. opaque and non-opaque mineral. The non-opaque Tourmaline comes next with 11.95% abundance. minerals include Zircon, Tourmaline, Rutile, Silimanite, Tourmaline occurs on granite pegmatites. It is usually Garnet, Epidote and Staurolite while opaque minerals brown in colour (sometimes greenish) or brownish which accounted for 52% dominated the assemblage. yellow. Its shape is commonly euhedral. It is a common Heavy mineral studies that Osun river sediment is detrital heavy mineral in sedimentary rocks. So varieties mineralogical immature. of tourmaline are used gemstones. From the heavy mineral analysis the presence of The quantity of zircon (same proportion as garnet) minerals such as zircon, tourmaline and rutile (ZTR), being the next abundant non-opaque heavy minerals which are mainly igneous and metamorphic minerals to tourmaline in the sediment shows that its shapes are coupled with dominant element of Staurolite have shown only fairly mechanically altered, (commonly euhedral to that the sediments of Osun river basin in southwestern subherdal) due to their stability and lack of good Nigeria could have been derived principally from acid cleavage. They appear commonly colorless. Figure 4 igneous rock with significant contribution from medium to shows the individual mineral percentage abundance high grade metamorphic rock. in all samples put together. The ZIR percentage index calculated range from 21.1 to 56.7%. This is also depicted RECOMMENDATION in the stacked column bar chart in Figure 5 to show the proportion of the non-opaque minerals in sediment 40 Owing the higher occurrence of K non-series samples of Osun river basin. elements and its significant contribution to the river According to modification of heavy mineral sediment samples throughout the five geological regions, association and provenance by Feo-Codecido [10] the the presence of potassium elements in Osun river basin presence of zircon, rutile and tourmaline an acid presents a formidable challenge both technically and 79

7 economically. A comprehensive study of sediments in the 4. Folk, R., L. and W.C. Ward, Brazos River bar: lower section of the river basin may bring out the A study in the significance of grain size parameters. enrichment of minerals southward which could be J. Sediment Petrology, 27: exploited. Additional study is needed to link these suites 5. Krynine, P.D., Petrology and genesis of the to the environments of deposition. However, the present third bradford sand. Bull. Pennysylvania state coll. data provide useful information to investigate future Min. Ind. Exp. Sta., 29: occurrence of heavy minerals and elemental composition 6. Mange, A., A. Maria and F.W. Heinz-Maurer, of the river basin in the region. Heavy minerals in colour. Chapmam and hall. ACKNOWLEDGEMENT 7. London, pp: 230. Taisuke, S., Heavy minerals composition of the recent sediments in three different environments in The Authors are grateful to the Senate Research geological survey of Japan part 1. Committee, Tai Solarin University of Education Ijebu-Ode, 8. Anfuso, G., M. Achab, G. Cultrone and F. Lopez- Ogun state for their financial support and Department of Aguayo, Heavy mineral distribution in the Geology University of Ibadan for access to the littoral between Sanlucar and Rota. Boletin Instituto Laboratory. Espanol De Oceanogafia, 15(1-4): Pellant, C. and R. Phillips, Rocks, Minerals and REFERENCES Fossils of the World. Boston: Little, Brown and Company. 1. Irion, G., Sedimentologish-mineralogische 10. Feo-Codecido, G., Heavy Minerals techniques untersuchuungen an den sedimenten des and their application to Venezuela stratigraphy meerfelder maares. N: Irion, G. and negeendank, J. AAPG, 40: Das meerfelder maar, cour. Forsch. Inst. 11. Akinmosin, A., O.T. Olabode and C.E. Bassey, Senckenberg, 65: Provenance study of bituminous sands in Eastern 2. Tahal consultant, (Nigeria) Master plan for Dahomey Basin SW Nigeria of river sediments in development resources in oshun-ona river basins. Ondo state. Ife J. Sci., 7(1): Akande, A., Radiological Hazard Index 12. Hubert, A., The geology of part of assessment of River Osun sediment. A B.Sc. Project Southwestern Nigeria. Geological Survey of Nigeria submitted to the Department of Physics, Tai Solarin Bulletin, 3: 87. University of Education, Ijagun, Ijebu-Ode. Ogun state. 80