Ground magnetic survey was carried out to investigation Lead-Zinc ore presence in Edor area of Cross River State, Nigeria. Magnetic-intensity profiles were made along seven profile lines over the target area. All the magnetic lows match in position with aeromagnetic lows which have been associated with Pb-Zn mineralisations in the area. It was therefore concluded that the Pb-Zn mineralisation is depicted by magnetic lows

Atuchukwu, Adaora, G.

C. L, Eze,

English

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Advances in Physics Theories and Applications                                                                                                  www.iiste.org ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.70, 2018          1  Ground Magnetic Survey over Lead-Zinc Mineralization in Edor, Cross River State, Nigeria  Eze, C. L.       Adaora, G. Atuchukwu Institute of Geosciences and Space Technology, Rivers State University, Port Harcourt, Nigeria  Abstract Ground magnetic survey was carried out to investigation Lead-Zinc ore presence in Edor area of Cross River State, Nigeria. Magnetic-intensity profiles were made along seven profile lines over the target area. All the magnetic lows match in position with aeromagnetic lows which have been associated with Pb-Zn mineralisations in the area. It was therefore concluded that the Pb-Zn mineralisation is depicted by magnetic lows.   Introduction Magnetic survey is one of the tools used by exploration geophysicists in their search for mineral-bearing ore bodies. Through magnetic Survey, the Earth's magnetic field and the spatial variation in it can be analyzed. The object of the survey was a reconnaissance of the area and magnetic-intensity profiles were made along seven profile lines over the target area. Lead-Zinc ores usually occur together. They are often associated with copper and silver. They occur in commercial quantities in the northeast and central region of Nigeria (Victor, Onwuemesi, Aniwetalu and Emmanuel, 2015).   Geology of the Deposit The epigenetic Pb-Zn deposits of the southern Benue Valley (Nigeria) are localized within Cretaceous sediments of an intracontinental rift basin. Fluid inclusion studies of vein minerals from the Abakaliki and Ishiagu orebodies show that sphalerite and quartz were deposited at relatively low temperatures (102–175 °C), with ore-fluid salinity mostly in the range of 17–25 equiv. wt% NaCl. Trace-element contents of sphalerite and galena are also consistent with the low temperature of formation and epigenetic origin. On the basis of the geotectonic setting, the mode of occurrence and fluid-inclusion characteristics, mineralization is attributed to connate brines set into motion by a high geothermal gradient accompanying continental rifting. Mineral deposition was caused principally by rapid cooling due to either reaction with wall rocks or mixing with meteoric or descending water of low salinity.  Review of previous work Many workers have carried out some studies of the lead-zinc mineralization of the cretaceous sediments of southern Nigeria (Olade and Morton, 1985; Ofoegbu, 1985; Fatoye, Ibitomi and Omada 2014, Victor et al., 2015, Ogundipe and Obasi, 2016). According to Victor et al., (2015), The Pb-Zn deposits in Ishiagu area appear to be the southern limit of mineralization in the Benue Trough and the Pb-Zn mineralized zone extends over a distance of 500 km in a narrow belt from Ishiagu in the lower Benue Trough to Zurak in the upper Benue Trough, likewise the extent of igneous intrusions in the Benue Trough. Nwachukwu (1972) and Wright (1976) have shown that the Asu River group of sediments deposited during the Transgressive phase of the middle Albian have undergone a minor folding during the Cenomanian time. These sediments were later refolded during the Santonian tectonic episode associated with igneous activities (Short and Stauble, 1967, Nwachukwu, 1972). Associated with these igneous activities are the lead-zinc mineralization in the area (Orajaka, 1975) and the mineralization coincides with the main belt of igneous rocks.  Okezie (1957) mapped the igneous and structural geology of Ezekwe, Wanikanda, Oju and Nkpume-Akwaokutu districts of Ogoja, Benue and Abakaliki provinces and was able to locate igneous intrusions of different structural compositions ranging from gabbro, Augite-diorite, Micro-diorite and lead-zinc mineralization have been established on some of the Augite-diorite intrusions. Ezepue (1992) noted that Pb-Zn deposits of Ishiagu are associated with the shales of the Asu-River Group and that they occur along the NW-SE trending fractures. He found the deposits to occur on the Abakaliki anticlinorium and remarked that there was a close correlation between the Pb-Zn mineralization and the structures.  Farrington (1952) showed the Pb-Zn mineralization to be commonly associated with dolerite and albitized dolerites in the anticlinal structures and suggested a hydrothermal origin for the mineralization. Grant (1971) is rather of the opinion that a more likely source is deep circulating hot brines which leached metals from the sediments and underlying crystalline basement. The igneous bodies acted both as source of heat for the circulating fluid and sites for emplacement of the mineral veins. Wright (1981) attributed their altered condition to the passage hydrothermal solution through them and to some extent, metasomatism resulting from intrusion into the wet sediments.  Advances in Physics Theories and Applications                                                                                                  www.iiste.org ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.70, 2018          2  Survey Method Seven magnetic traverses were run. Measurements were taken at 90m. The base station was reoccupied after every three hours to determine the diurnal effects and to ascertain the drift of the instrument if any. The maximum difference recorded at the base station was 1gammas in nine hours and the average change per reoccupation of the base station was about 2 gammas. The insignificance of this change made the corrections for the above factors unnecessary. Moreover in mineral prospecting and all other cases where the anomaly of interest is as large as 500gammas such corrections are not required (Telford et al 1976). This survey was carried out on a local scale and in addition the site is a fairly flat area whose maximum elevation difference is 11m. It is known that in sedimentary formation magnetic susceptibility variations are not very erratic and as such terrain corrections were not necessary. A total of 359 interval vertical magnetic intensity readings were taken from the seven lines and these were summoned up from which an average of 1445 gammas was obtained. This average was regarded as the regional value which was subsequently subtracted from each of the station reading to obtain the residual magnetic anomaly.  Results and Discussions  The instrument readings as well as the residual values in line 1 are shown on Table 1. This is typical of other lines. The residual values (vertical magnetic intensity) were plotted against the station distance (Figure 1-4). The plots of line 1 exhibit changes in vertical magnetic intensity values and in anomaly polarity throughout the entire 4km length. These changes in intensity and polarity reflect surface magnetic inhomogeneity. The residual value of line 2 a magnetic low centered at 720m. The magnet low has a lateral extent of 2.3km and a peak value of -600gammas.   Advances in Physics Theories and Applications                                                                                                  www.iiste.org ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.70, 2018          3  Table 1: Magnetic Readings Line 1 OB.PT. (m) INST.Readings SENS Value IN  Polarity Time Residual 0 200 3 2016 + 8.50 571 90 240 2 2419 + 9.50 974 180 240 3 2419 + 10.07 974 470 120 3 1210 + 10.30 -236 360 180 2 1814 + 10.35 369 450 80 4 806 + 10.45 -639 540 120 3 1210 + 10.51 -236 630 520 2 5242 + 11.01 3797 720 60 3 605 + 11.11 -840 810 80 4 806 + 11.20 -639 900 70 4 706 + 11.30 -739 990 200 3 2016 + 11.40 571 1080 500 2 5040 + 11.50 3595 1170 80 3 806 - 12.02 -639 1260 80 3 806 - 12.10 -639 1350 40 4 403 + 12.17 -1042 1440 330 1 3326 + 12.27 1881 1530 110 2 1109 - 12.35 -336 1620 200 2 2016 - 12.45 571 1710 320 2 3226 - 12.55 1781 0 220 3 2218 + 1.30 773 1800 200 2 2016 + 2.00 571 1890 180 2 1814 + 2.10 369 1980 200 2 2016 + 2.20 571 2070 200 2 2218 + 2.32 571 2160 220 2 1008 + 2.40 773 2250 100 3 1008 + 2.50 -437 2340 120 3 1210 - 2.50 -236 2430 240 3 2419 - 3.06 974 2520 185 3 1865 - 3.16 420 2610 640 2 6451 + 3.20 5006 2700 500 2 5040 + 3.30 3595 2790 240 2 2419 + 3.41 974 2880 300 2 3024 + 3.50 1579 2970 320 2 3226 + 4.00 1781 3060 120 3 1210 + 4.08 -236 3150 360 2 3629 + 4.20 2184 3240 80 4 806 + 4.30 -639 3330 240 2 2419 + 4.40 974 3420 240 2 2419 + 4.49 974 3510 190 3 1915 + 5.00 470 3600 200 3 2016 + 5.07 571 3690 200 3 2016 + 5.10 571 3780 220 3 2218 + 5.15 773 3870 180 3 1814 + 5.20 369  Advances in Physics Theories and Applications                                                                                                  www.iiste.org ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.70, 2018          4   Figure 1: Line 1 Profile   Figure 2: Line 2 Profile   Figure 3: Line 3 Profile   Figure 4: Line 4 Profile Five kilometers of line 3 was covered by magnetic. The graph of the residual vertical magnetic intensity -100001000200030000 1000 2000 3000 4000 5000 6000Vertical magnetic intensity (g) Distance (m)-200002000400060000 1000 2000 3000 4000 5000Vertical Magnetic Intensity (g) Distance (m)-2000-100001000200030004000500060000 1000 2000 3000 4000 5000 6000Vertical magnetic intensity (g) Distance (m)Advances in Physics Theories and Applications                                                                                                  www.iiste.org ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.70, 2018          5  values plotted against the stations. Line 3 does not show any significant anomaly anywhere along the line. Rather the graph goes from positive to negative over very short distance in response to near surface magnetic in homogeneities. This picture agrees with the EM survey profile over the same area according to Mama and Eze (1985) which also only reflects near surface inhomogeneity in conductivity and showed no anomaly that could be associated with an ore body.  Sixty one vertical intensity magnetic readings were taken along line 4 over the distance of 5km. The plot of residual values against the stations shows a negative anomaly which lies between 630m and 2790m along the line from the origin of the profile. The anomaly reaches its negative peak at a distance of 1.5km with a value of about -850grammas.  All the magnetic lows match in position (with little displacement in some cases) with aeromagnetic lows which have been associated with Pb-Zn mineralisations in the area. It could be therefore concluded that the Pb-Zn mineralisation is depicted by EM and magnetic laws. The position of Mhxx corresponds to the point where Okezie (1957) mapped a diorite intrusion as an igneous rock likely to be associated with mineralisation. This result was in contrary to the suggestion of mineralization of the diorite.  Conclusions and Recommendation A rigourous interpretation of magnetic data over a lead sulphide field serves as an effective tool for delineating probable mineralization areas. This survey was able to identify the NE-SW trending fractures. Magnetic survey must, however, only be used as a complementary exploration tool for lead sulphide.   Reference Ezepue, M.C. (1992). Petrography and petrochemistry of igneous sills in the Ishiagu lead-zinc sulphide district, Eastern Nigeria. Journal of Mining and Geology, 28(1): 345 - 351. Fatoye F.B., Ibitomi M. A. and Omada J.I. (2014). Lead-Zinc-Barite mineralization in the Benue Trough, Nigeria: Their geology, occurrences and economic prospective. Advances in Applied Science Research, 5(2): 86-92 Farrington, J.L. (1952). A preliminary description of the Nigerian lead-zinc field. Economic Geology, 47: 583–608  Grant N. K.  (1971). “The South Atlantic, Benue Trough and Gulf of Guinea Cretaceous Triple Junction”. Bulletin of the Geological Society of America, 82: 2295-2298 Mbah V. O., Onwuemesi A.G., Aniwetalu Emmanuel U. (2015). Exploration of Lead-Zinc (Pb-Zn) Mineralization Using Very Low Frequency Electromanetic (VLF-EM) in Ishiagu, Ebonyi State. Journal of Geology & Geoscience 4:214.  Nwachukwu S.O. (1972), “The Tectonic Evolution of the Southern Portion of the Benue Trough, Nigeria”. Geological Magazine, 109: 411-419. Olade, M.A. and Morton, R.D. (1985). Origin of lead-zinc mineralization in the Southern Benue Trough, Nigeria: Fluid Inclusion and Trace element studies. Mineral. Deposita; 20(2): 76–80  Okezie, C .N. (1957). The igneous rocks of the Izekwe, Ominyi and Nkpumeakwaokuku districts of Ogoja Province. Interim Report, No. 2 on the mapping of the Cretaceous igneous districts of Southeastern Nigeria. Geological Survey of Nigeria Report No. 1380. Ofoegbu C.O. (1985). A review of the geology of the Benue Trough, Nigeria. Journal of African Earth Sciences. 3(3): 283-291 Ogundipe I. E and Obasi R.A. (2016). Geology and Mineralization in the Albian Sediments of the Benue Trough. Nigeria. British Journal of Earth Sciences Research. 4(3): 1-15 Short K. C. and Stauble A. J. (1967). “Outline of Geology of Niger Delta”. Bulletin of American Association of Petroleum Geologists, 54(5): 76-79  Strangway D. W., McMahon B. E., Honea R. M. (1967). Stable Magnetic Remanence in Anti-Ferromagnetic Goethite: Science, 158 (3802): 785-787.  Telford , W.M., Geldart, L.P., Sherif R.E., Keys D.A. (1976), Applied Geophysics, Cambridge University Press, N.Y. Wright, J.B. (1981) Review of the origin and evolution of the Benue Trough in Nigeria. Earth Evolution Science 1:98–103   

Ground Magnetic Survey over Lead-Zinc Mineralization in Edor, Cross River State, Nigeria

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Ground Magnetic Survey over Lead-Zinc Mineralization in Edor, Cross River State, Nigeria

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