Journal of Afrtcan Earth Sciences, Vol 9, No 3/4, pp 525-529, 1989 0899-5362/89 $3 00 + 0 00 Printed m Great Britain ~) 1990 Pergamon Press plc

Gravity and vertical magnetic gradient investigations of a localised area of the Benue Trough, Nigeria

A. G. O~mMESl and B. C. E. EGBOKA

Department of Geological Sciences, Anambra State University of Technology,

Awka-Campus. P. M.B. 5025, Awka, Nigeria

Abstract - Gravity and vertical magnetic gradient studies were carried out in Lokpanta - Lekwesi area in the Lower Benue Trough to investigate the local structures which might be associated with the Lead- Zinc mineralisation as evident m the adjacent area (Ishiagu Pb-Zn mmeralisation). For this reason 134 gravity and vertical magnetic gradient readings were taken at intervals of 250 m along major roads and track routes that connect widely separated parts of the 25 km 2 area. The instruments used are the Canadian Scintrex CG-2 gravimeter (No. 318), the American Paulin altimeter and the Proton Precession Magnetometer. The geology of the area comprises the Ezeaku Formation, and the Awgu Formation, their ages ranging from Middle to Upper Cretaceous. The study was used to delineate two igneous intrusions in the Ezeaku Formation. The intrusive bodies show high positive Bouguer anomalies which ranged from 0.5 regal to 3.3 regal in contrast to the negative Bouguer anomaly of the country rock that ranged from 0 to - 3.0 regal. The vertlcai magnetic gradient method identified the two intrusive bodies as negative anomalies having vertical magnetic gradient values ranging from -5.0 to -25.0 gammas per meter in contIast to 5.0 to 10.0 gammas per meter values of the country rock. The Profiles were interpreted using computer assisted evaluation procedure of non-linear opttrnizatlon and interactive techniques. Based on two dimensional model, the mlmmum depth computations to the upper surfaces of the anomalies A and B by gravity method are 60 m and 160 m respectively while the vertical magnetic gradient method gave a minwaum depth estimates of the anomaly A as 75 m and that of B as 180 m.

I N T R O D U C T I O N

T h e a r e a of i n v e s t i g a t i o n is b o u n d e d b y l a t i t u d e s 5 ° 52 ' N a n d 6* N, a n d l o n g i t u d e s 7 ° 27 ' E a n d 7* 29 ' E (Fig. 1). It h a s a n a r e a of a b o u t 25 k m 2. T h e E n u g u - U m u a h i a e x p r e s s r o a d m a k e s it e a s i l y a c c e s s i b l e .

T h e p h y s i o g r a p h i c f e a t u r e s of t h e a r e a a r e h i g h l y c o n t r o l l e d b y t h e u n d e r l y i n g b e d r o c k s . T h e a r e a u n d e r l a i n b y t h e E z e a k u F o r m a t i o n f o r m s a n e x t e n s i v e low- ly ing t e r r a i n of a b o u t I 0 0 to 150 m a b o v e m e a n s e a level (Fig. 2). T h e A w g u F o r m a t i o n o c c u p i e s a n a r e a w h i c h is p a r t of t h e A w g u - O k i g w e e s c a r p m e n t . E l e v a t i o n h e r e r a n g e s f r o m 3 0 0 to 3 7 0 m a b o v e m e a n s e a level. M a n y s t r e a m s d r a m t h e a r e a in a d e n d r i t i c p a t t e m .

T h e t o p o g r a p h y of t h e a r e a r e f l ec t s a l ong pe r i od of e r o s i o n t h a t r e d u c e d t h e i n c o m p e t e n t s h a l y a r e a s to f o r m t h e l o w l a n d s . T h e s a n d y s h a l e s a n d i n d u r a t e d s a n d s t o n e s r e s i s t e d e r o s i o n to f o r m t h e A w g u - O k i g w e e s c a r p m e n t . T h e s t u d y a r e a is a p a r t of t h e L o w e r B e n u e T r o u g h . T h e B e n u e T r o u g h i t se l f i s a n e l o n g a t e d t r o u g h of s u b s i d e n c e w h i c h t r e n d s n o r t h e a s t e r l y to a t t a i n a n a p p r o x i m a t e l e n g t h of 8 0 0 k m . I t h a s a w i d t h of a b o u t 130 to

150 k m a n d is fi l led w i t h C r e t a c e o u s s e d i m e n t s w h o s e a g e s r a n g e f r o m Midd l e A l b i a n to M a e s t r i c h t i a n . T h e t r o u g h is c h a r a c t e r i s e d b y s e r i e s of fo lds t h a t a r e l ong a n d n a r r o w w i t h a p r e d o m i - n a n t l y E N E - W S W ax i s a n d t h e r e is a n a r r o w z o n e of l e a d - z i n c m i n e r a l i z a t i o n r u n n i n g f r o m I s h i a g u t h r o u g h A b a k a l i k i to Z u r a k . T h e t r o u g h ax i s is f u r t h e r c h a r a c t e r i s e d b y t h e p r e s e n c e of n u m e r o u s m a f i c a n d fels ic i n t r u s i v e a n d b a s a l t i c l a v a s (Olade, 1978; AdighiJe, 1981b ; U z u a k p u n w a , 1974). N w a c h u k w u (1972) d i s c u s s e d t h e fo ld ing of A l b i a n s e d i m e n t s w i th t h e i n t e r a c t i o n of two c o n t i n e n t a l c o n v e r g e n t p l a t e s l e a d i n g to t w o p e r i o d s of folding: a p o s t - A l b i a n to p r e - T u r o n i a n e v e n t w h i c h r e s u l t - ed in t h e ea r ly P b - Z n m i n e r a l i s a t i o n wh i l e t h e S a n t o n i a n fo ld ing c a m e la te r . A b o u t 2 k i n e a s t of t h e a r e a of i n v e s t i g a t i o n is t h e P b - Z n m i n e r a l i s a - t i o n in I s h i a g u w h i c h r e s u l t e d f r o m i g n e o u s ac t iv i - ty. T h e m i n e r a l s a r e c u r r e n t l y b e i n g m i n e d b y t h e G e o m i n e x Nigeria Limi ted , a n d the O k w u o s a Mining C o m p a n y . T h e P b - Z n m i n e r a l i s a t i o n is f o u n d to o c c u r in f a u l t a n d f r a c t u r e zones .

T h e r e s u l t of t h i s s t u d y h a s c l e a r l y s h o w n the e x i s t e n c e of i n t r u s i v e i g n e o u s b o d i e s (Fig. 3 a n d 4) w h i c h m i g h t b e g e n e t i c a l l y r e l a t e d to t h e i n t r u s i v e

~S 9:M4--1 525

526 A.G. ONWUEMESl and B. C. E. EGBOKA

m +-+ 4 - . ~ 2

N

x x ~

BASEMENT COMPLEX ~ NSUKKA FOI::~TION ASU RIVER GROUP ~ IMO SHALE I:'OF~vIATION EZEAKU FORMATION ~ AME}~I FORMATION AWGU FORMATION I N BENIN FORMATION

NKPORO FORMATION ~.~_J RECENT ALLUVIUM

LOWER C.~L MF-.ASUFEIE} ~ STUDY AREA

AJALI SAND STONE 0 20 40 60M I I I I

Fig. I. Map of part of Benue Trough showlng study area

bodies found in the ad jacen t Ishiagu area t ha t were known to be assoc ia ted with Pb-Zn mineral isat ion. Addit ional geophysical invest igat ions will reveal the presence or otherwise of Pb-Zn minera l i sa t ion in the s t u d y area.

GEOLOGY OF THE AREA

Three un i t s belonging to two sed imenta ry Forma t ions under ly ing the Cross River plains of Middle to Upper Cre taceous Sys tems were mapped in the s t u d y area. These are the Ezeaku Format ion of Tu ron i an Stage and the Awgu Format ion of Coniac ian s tage (Fig. 2).

The Ezeaku S a n d s t o n e which Is the oldest m e m b e r of the Ezeaku Forma t ion outcrops in the s t u d y area. The dip direct ions of th is s ands tone range from 300 ° to 304 ° az imuth while the dip a m o u n t is be tween 20 ° and 23 °. Sieve ana lyses of the s a n d s t o n e show t h a t it is moderate ly-sor ted, negat ively-skewed and very leptokurtic.

The outcrop of Ezeaku Shale Is also seen in the area along Enugu-Okigwe express road. It is composed of shale wlth in te rbedded b a n d s of muds tone . The colour of a fresh surface of the

¢2#E

B

Fig 2. Geological and topographic map of the area

shale is grey bu t wea the r s to reddish brown. The general dip direction of the shale is conformable with the under ly ing s ands tone m e m b e r and the magn i tude of dip ranges from 18 to 20%

The Awgu Format ion which outcrops in the area consis ts of s ands tone with s a n d y shale inter- calations. Alt i tude m e a s u r e m e n t s show tha t the bed dips conformably with the under ly ing Ezeaku Format ion and the dip a m o u n t ranges from 16 ° to 19 ° .

Dolerite in t rus ions occur at Ishiagu. Lokpaukwu and Lekwesi (outside the s t u d y area) a l though tha t of Lekwesi is not well k n o w n and it is the objective of the present s t u d y to map these in t rus ives ff any by geophysical me thod in Lekwesi areas.

METHOD OF INVESTIGATION

Gravity M e t h o d

134 gravity m e a s u r e m e n t s were t aken in the area at intervals of 250 m. After effecting the necessa ry correct ions s u c h as the la t i tude cor- rection, the te r ra in correction, and the elevation correction, a simple Bouguer anoma ly da ta were obtained from the formula:

Ag = gob, + g~ + (0.3086 - 0.04193 p ) h + Tpmgal - (1) where Ag = Bouguer anomaly (mgal)

go~, = observed gravity (mgal) g~ = Latitude correction

(0.3086 - 0.04193 p ) h = Elevation correction p= density contrast (g/cm 3)

h = height of the station above or below the base station

Gravity and vertical magnetic gradient investigations of a localised area of the Benue Trough, Nigeria 527

20

~0 =

600-

w T ~ _

0-

- ~ , 0 0 .

- a ~ -

A

ANOI'I~d.Y A 7 ~ M

' COdN I A.OMA~ B N

OBSERVED

. . . . COMPUTE{ ]

- TOP~r.~ApM y

5mlM~4P5

+ . • ÷ +

÷ + • ~ + B

Fig. 3. Gravity model profile of the anomaly

5 ' 5 7

6~

21N

7~.

0 I~

Fig. 4. Vertical magnetic gradient map

--" 742~E (~ I~M 7 '2~E-" N

Fig. 5. Bouguer gravity anomaly map The Bouguer anomaly data were then plotted on

the field map and contoured to produce a Bouguer anomaly contour map (Fig. 5). The data ranges from -6.1 to 3.3 mgal. The highest Bouguer anomaly values were obtained in Ezeaku Sandstone and it decreases progressively from Ezeaku Shale towardsAwgu Sandstone. In Ezeaku Sandstone, two distinctive anomalies with high Bouguer anomaly values were isolated from the contour. Their Bouguer anomaly values range from 0.5 mgal to 2.5 regal in contrast with the 0 to - 0.3 mgaJ of the surrounding rock. The trend of the two isolated anomalies are NE-SW (Fig. 5). The regional gradient was removed from the. Bouguer anomaly from a cross section taken along AB by graphical method to obtain the residual anomaly (Figs. 5 and 6).

jo.

7s

2o-

& g t s

cmg~0 To:

A

-o5-

1 5

2o-

- 2 s

t: 1

ANOMALY A

ANOMALy B BOUGUER ANOMALY

Fig. 6. Bouguer gravity anomaly curve

The topography in the anomalous area is nearly uniform as evident from the topographic section taken along AB (Fig. 3). This shows that the isolated anomalies are not due to topographical changes.

Vertical Magnetic gradient m e t h o d The vertical magnetic gradient data were

obtained by taking magnetic measurements at vertically two different sensor positions. The reading obtained at higher sensor position was subtracted from that obtained at lower sensor position, and the value was divided by the difference in height between the sensor positions. The result obtained is the vertical magnetic gradient. The advantage of vertical magnetic gradient method is that it resolves composite or complex anomalies into their individual con- stituents and on the same basis automatically removes the regional magnetic gradient to define

528 A.G. ONWUEMESl and B. C. E. E~OKA

be t t e r t he sha l lower anomal ies a s s u m e d to be of interest .

The va lues ob ta ined b y the vertical magne t i c gradient m e t h o d in the s t u d y area ranged from - 22 g a m m a s per me te r to 14 g a m m a s pe r meter . These va lues were plot ted and con tou red (Fig. 4). The m a p s h o w s two dist inctive negative anomal ies t rending NE-SW in Ezeaku Sands tone . It w a s also observed tha t af ter the negative anomal ies of the vertical magne t ic gradient in Ezeaku S a n d s t o n e tha t the va lue inc reased progressively from Ezeaku Shale t owards Awgu Sands tone . The negative anoma ly of the causa t ive b o d y in Ezeaku S a n d s t o n e is in ag reemen t with the ear th ' s field c o m p o n e n t behaviour . The behav iou r of the ear th magnet ic field wou ld be negative over sou rce s at the magnet ic equa to r and Nigeria h a p p e n s to be in the magne t i c equator . A c ross sec t ion was t aken along AB and it c u t s ac ross the two negative vertical magne t ic gradient anomal ies (Fig. 7).

INTERPRETATION

m u m poss ib le dep ths for a good fit obta ined be t ween the observed and c o m p u t e d anomalies . Hence the m i n i m u m depth es t imate to the anoma l i e sA and B are 60 m and 160 m respect ively and also the m i n i m u m dep th es t imate to the B a s e m e n t in the immedia te ne ighbourhood of the in t rus ives ranges f rom 600 m to 800 m.

For the vertical magne t i c gradient anomaly profile a magne t i sa t ion of 1.3 A / m was a s s u m e d for the model b a s e d on the suscept ib i l i ty c o m p u t a t i o n s from dolerite s a m p l e s collected from the a d j a c e n t a rea , a n d t h e d i r ec t i on of magne t i sa t ion w a s also a s s u m e d to be horizontal which is in the direct ion of the prevailing ear th magnet ic field in the area. Figure 7 shows the bodies at thei r m i n i m u m poss ib le d e p t h s for a good fit of the observed and the c o m p u t e d anomalies . The depth es t imate to the anoma ly A is 75 m and that of the anoma ly B is 180 m. The resu l t also gives the m i n i m u m dep th es t imate to the B a s e m e n t in the immedia te ne ighbourhood of the in t rus ives as 800 m to 1.2 kin.

Both the res idua l B o u g u e r anoma ly curve and the vert ical magne t ic gradient curve were inter- pre ted in t e r m s of two-d imens iona l s t r u c t u r e s (Fig. 3 and 7) us ing c o m p u t e r a s s i s t ed evaluat ion p rocedure of non- l lnear opt imizat ion and inter- active t echn iques . For the gravity in terpre ta t ion a dens i ty con t r a s t was ob ta ined by m e a s u r i n g the dens i t ies of rock s a m p l e s collected from Ezeaku S a n d s t o n e and also the dens i t ies of the dolerite in t rus ions in the n e a r b y Ishiagu Pb-Zn mine- ral ised area. The average dens i ty of Ezeaku S a n d s t o n e is 2 .20 g / c m 3 while tha t of the dolerite in t rus ion is 2 .82 g / c m 3. Hence, a dens i ty con t ra s t of 0 .52 g / c m 3 w a s e s t ab l i shed be tween the sand- s tone hos t rock and the intrusion. Based on this con t ra s t (Fig. 3) s h o w s the bodies at their mini-

fo

5

Ao

~°

I-: - - O B S E R V E D

. . . . . . C O M P U T E D

-B

2 0 0 ,

. .

Fig. 7. Vertical magnetic gradient model profile of the

SUMMARY AND CONCLUSION

The resul t of the p re sen t s t u d y h a s clearly shown the exis tence of in t rusive igneous rocks which might be genetically re la ted to the intrusive rocks of the Pb-Zn minera l i sed a reas of Ishiagu which were k n o w n to be as soc ia ted with Pb-Zn mineral- isation. A fur ther geophysical invest igat ion that would inc lude induced polar isa t ion and magnet ic m e t h o d s will reveal the p resence or otherwise of the mineral isat ion. The resul t ha s also demon- s t ra ted the u se fu lnes s of in tegra ted geophysical m e t h o d s of investigation. In the light of the above discoveries, a r e commenda t i on w a s also made to the compan ie s tha t p roduce c r u s h e d aggregates in the a rea to extend their mining activit ies in the del ineated in t rus ion which they did and met the intrusive igneous rock (dolerite) at a dep th of 45 m for the anomaly A, as aga ins t the 60 m and 75 m c o m p u t e d by gravity and vertical magnet ic gradient m e t h o d s respectively. However, no drilling h a s been done on the anomaly B b e c a u s e It is s i tua ted in a bu i l t -up area.

REFERENCES

AdlghiJe, C. 1981b. A gravity interpretation of the Benue Trough, Nigeria. Tectonophysics, 79, 109-108.

AJakaiye, D. E. 1981. Geophysical investigations in the Benue Trough - a review. EarthEvol, Sc/1, 110-125.

AJakaiye. D. E., Hall, D. H., Millar, T. W., Verheijen P. T. J.. Arvad, M. B. and OJo, S. B. 1986. Aeromagnetic anomalies and tectonic trends in and around the BenueTrough, Nigeria. Nature 319, No. 60,582-584.

Gravity and verdcal magnetic gradient investigations of a localised area of the Benue Trough, Nigeria 529

Green, R. 1960. Remanent magnetization and Inter- pretation of magnetic anomalies. Geophysical Prospecting 8, No. 1, 98-110.

Hood, P. and McClure, D. T. 1965. Gradient Measure- ments in ground magnetic prospecting, Geophy. 80, 403-411.

Lee, T. J. 1980. Rapid computations of magnetic anomalies with demagnetization included for arbitrarily shaped magnetic bodies. Geophysics, J.R~ astr. Soc., 60, 67-75.

Nwachukwu, S. O. 1972. The tectonic evolution of the southern portion of the Benue Trough, Nigeria, Geol. Mag.. I09, 411-419.

Ofoegbu, C. O. 1985a. A review of the geology of the Benue'IYouqh of Nigeria. J. African Earth ScL, 3, 283- 292.

Ofoegbu, C. O. 1985b. Non-Linear optimization

techniques and their application to the interpretaUon of geophysical anomalies. In computers and mathematics In the modern society, edited by S.C. Fatunla 109-135. Macmillan Pub. Co. Ltd.

Olade, M. A. 1978. Early Cretaceous basalt volcanism and initial continental ~ g In Benue Trough, Nigeria. Nature Physics scL 273, 458-559.

Osazuwa,T. B., AJakalye, D. E. and VerheiJen, P. T. J. 1981. Analysis of the structure of part of the Benue Rift Valley on the basis of new geophysical data. EarthEvoL Sci., I, 126-136.

Parasnis, D. S. 1973. Methods in Geochemistry and Geophysics. 2nd revised and up-dated Ed. Elsevier Scientific Pub. Co. Amsterdam.

Uzuakpunwa, A. B. 1974. The Abakallki pyroclastics, Eastern Nigeria : new age and tectonic implication. Geol. Mag., 111, 65-70.