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The gravity maps display, in Bipindi zone, local oval culminations of low anomalies indicative of a presence of intrusive
light
body in a subsurface but the nature, the form and the position of this body are still unknown. The analyses of established gravimetric anomaly maps, the multi-scale evaluation of maxima of gradients and the quantitative interpretation of residual anomalies by 3D modelling permit characterizing the
intrusive light body situated at Bipindi.
The multi-scale evaluation of maxima of gradients shows that
the modelling of
the
intrusive light body of Bipindi
can be done without the problem of interference of anomalies from different sources. The 3D model of Bipindi zone shows two dissymmetrical blocks of the same type of rock with a density contrast of -0.095 g
·
cm^{-3} in comparison with the density of the surrounding metamorphic rocks. The two blocks are at a distance about 3 km from one to another.
The upper surfaces of these blocks lie at a depth between 1 and 2 km. Their lower surfaces have two landings; one lies at a depth of about 8 km and another at a depth about 14 km. A consideration of the density of the modelled body, of the ranges of densities of specific rocks present in the general region indicates that the body may be composed of nepheline syenites. The intrusive body of Bipindi is situated in a senestral shear zone. The area situated between the two blocks of this intrusive body may be indicated for a detail study in the domain of mineral research.

The study area is located between latitudes 2˚50'N - 3˚28'N and longitudes 10˚20'E - 10˚45'E (

In this paper, we propose to characterize the intrusive body responsible for the negative anomalies observed in our study area by the multi-scale evaluation of maxima of the horizontal derivative of the vertical gradient of the Bouguer anomaly and by 3D modelling.

A smaller part of our study area is covered by the geological formations of Ntem Complex which is the northern portion of the Congo Craton in Cameroon. The Ntem Complex is composed mainly by Archean rocks [

The study area is constituted predominantly of formations of the Nyong Unit. This Unit corresponds to the remobilized NW edge of the Ntem Complex [

A seismological study in all the Cameroonian territory by Tokam et al. [

The gravity data used in this work were collected during gravity surveys of Central Africa, by ORSTOM and referenced in [^{3}.

The Bouguer anomaly data contains the combined effects of the deep and large basement structures and the shallow sediment layers with limited lateral extension. The polynomial fitting method is used to separate the Bouguer anomaly into its regional and residual components. This method computes the mathematical surface which gives the best fit to the gravity field within specific limits [

The Bouguer and residual anomalies maps show, in the study area (

By comparison of the geological map with the first order residual anomaly map (^{3} [^{3} because the Bouguer anomaly values were computed using a mean crustal density of 2.67 g/cm^{3 }(this means that the anomaly produced by this assembly of metamorphic formations is zero). We can see on the first order residual anomaly map (

The maxima of the horizontal gradient of the vertical derivative of Bouguer anomalies help locate contacts associated with abrupt changes in density and the multi-scale analysis of these maxima involves upward continuation of the gravity field to different heights with a view to characterize the vertical extension of anomalous structures [

· The upward continued field at different heights h. We have used the Fourpot program to calculate the upward continuation [_{o} of the gravity field [_{o} = 35 km.

· The vertical derivative at different heights. Given g_{v} to be the_{ }vertical derivative of potential field g at height h, it is calculated in the space domain using the method of finite differences proposed by Florio et al. [_{v}

where

· The horizontal gradient of the vertical derivative and its local maxima. Given g_{HV} to be the horizontal gradient of the vertical derivative at a height h; its value is calculated in the spatial domain using the formula (2):

where g_{v} is the_{ }vertical derivative of potential field g at height h as defined in relation (1). The positions of local maxima are determined by the Blakely and Simpson method [

The superposition of local maxima of horizontal gradient determined on the vertical derivative of the Bouguer anomalies upward continued at the heights 5 km, 10 km, 15 km, 25 km and 35 km help in the realization of the map shown in _{11}) corresponding to horizontal limit of the intrusive body of Bipindi. The map of maxima (_{11}) can be done without the problem of interference of anomalies from different sources.

According to Koumetio et al. [

of magenta and red maxima indicates that the depth of its bottom is between 7.5 and 12.5 km. We can also understand for a second example that the presence of blue maxima on one contact with the absence of other maxima colours indicates that the depth of its bottom is between 2.5 and 5 km.

In _{11}) shows that the NE and the SW parts of c_{11} extent until a depth lying between 12.5 km and 17.5 km (parts marked by blue, green, yellow and magenta maxima) while its centre part extent until a depth lying between 7.5 km and 12.5 km (part marked by blue, green and yellow maxima).

We used the gravity modelling program GRAV3D [_{11}) of Bipindi. In order to take into account all the constraints that are the results of Owona Angue et al. [_{11} and compare the anomaly calculated by the program GRAV3D to the experimental anomaly.

The program GRAV3D offers the possibility to construct one body using bricks which can be the rectangular parallelepipeds of different dimensions. Each brick has a constant density contrast. The dimensions of one brick must be given along two horizontal axes (Ox, Oy) and one vertical axis Oz. The coordinates (x_{o}, y_{o}, z_{o}) of the origin point O must be indicated as well as the minimal variations ∆x, ∆y and ∆z along each axis.

In the case of the intrusive body of Bipindi, we tested several values of density contrast and we selected the value of −0.095 g/cm^{3 }for each brick. The origin

The data taken in the study area allowed the program GRAV3D to generate experimental map shown on Fig- ure 5. Taking into account all the constraints mentioned above we constructed the 3D model of c_{11} brick by brick. We enter the following data in the program GRAV3D:

x_{o} = 26,400 m, y_{o} = 37,500 m, z_{o} = 0, ∆x = ∆y = 3000 m, ∆z = 1000 m

Brick 1: 35,400 ≤ x ≤ 41,400 m; 67,500 ≤ y ≤ 88,500 m; −14,000 ≤ z ≤ −2000 m

Brick 2: 41,400 ≤ x ≤ 50,400 m; 67,500 ≤ y ≤ 82,500 m; −14,000 ≤ z ≤ −2000 m

Brick 3: 50,400 ≤ x ≤ 53,400 m; 61,500 ≤ y ≤ 112,500 m; −8000 ≤ z ≤ −1000 m

Brick 4: 56,400 ≤ x ≤ 59,400 m; 58,500 ≤ y ≤ 112,500 m; −8000 ≤ z ≤ −1000 m

Brick 5: 59,400 ≤ x ≤ 62,400 m; 58,500 ≤ y ≤ 100,500 m; −8000 ≤ z ≤ −1000 m

Brick 6: 62,400 ≤ x ≤ 77,400 m; 76,500 ≤ y ≤ 112,500 m; −8000 ≤ z ≤ −1000 m

Brick 7: 77,400 ≤ x ≤ 86,400 m; 88,500 ≤ y ≤ 112,500 m; −14,000 ≤ z ≤ −2000 m

Brick 8: 77,400 ≤ x ≤ 86,400 m; 88,500 ≤ y ≤ 97,500 m; −2000 ≤ z ≤ −1000 m

The calculated map obtained (_{11}, one is at a depth of about 8 km and another is at a depth of about 14 km.

The 3D model of the intrusive body of Bipindi obtained is in accordance with the results of the multi-scale analysis of maxima of gradients. Indeed, the depth of 8 km for the base of the central portion of (c_{11}) is in the range provided by the multi-scale analysis of maxima of gradients (range of 7.5 km to 12.5 km). Similarly, the depth of 14 km for the bases of the outermost parts of (c_{11}) is in the range of 12.5 km to 17.5 km.

The disposition of the two blocks of the intrusive body of Bipindi along the axis of separation lets us think to a senestral shear zone. This is in accordance with the result of Maurizot et al. [

We assumed that the average density of the metamorphic formations in the study area is equal to 2.67 g/cm^{3} then the density of the intrusive body of Bipindi can be evaluated at about 2.57 g/cm^{3} knowing that the density contrast is −0.095 g/cm^{3}. We give in

Given all this, the intrusive body obtained by gravity modelling may be composed of nepheline syenites or granites. It is more likely that (c_{11}) is composed of nepheline syenites because Nsifa [

We know that there are more often than not transport and accumulation of mineralized substances in the deformation zones and mainly in shear zone. Then the localization of deformation belts between the two blocks of the intrusive body of Bipindi can be a guide for mineral prospection.

Intrusive rocks | Alkaline syenites | Nepheline syenites | Granites | Granodiorite | Dolerites | Tonalites | Peridotites |
---|---|---|---|---|---|---|---|

Range of density (g/cm^{3}) | 2.60 - 2.95 | 2.53 - 2.70 | 2.50 - 2.81 | 2.67 - 2.79 | 2.70 - 3.50 | 2.62 - 2.96 | 2.78 - 3.37 |

The analyses of established gravity anomaly maps, the multi-scale evaluation of maxima of gradients and the quantitative interpretation of residual anomalies by 3D modelling suggest the presence of an intrusive light body situated at Bipindi. The multi-scale evaluation of maxima of gradients shows that the modelling of the intrusive light body of Bipindi can be done without the problem of interference of anomalies from different sources. It also shows that the NE and the SW parts of this intrusive body extend until a depth lies between 12.5 km and 17.5 km while its centre part extends until a depth lies between 7.5 km and 12.5 km. The 3D model of Bipindi zone shows two dissymmetrical blocks of the same type of rock with a density contrast of −0.095 g∙cm^{−}^{3} in comparison with the density of the surrounding metamorphic rocks. The two blocks are at a distance of about 3 km from one to another. The upper surfaces of these blocks lie at a depth of between 1 and 2 km. Their lower surfaces have two landings; one lies at a depth of about 8 km and another lies at a depth of about 14 km. The 3D model of the intrusive body of Bipindi obtained is in accordance with the results of the multi-scale analysis of maxima of gradients. A consideration of the density of the modelled body, of the ranges of densities of specific rocks present in the general region indicates that the body may be composed of nepheline syenites. The intrusive body of Bipindi is situated in a senestral shear zone. The area situated between the two blocks of this intrusive body may be indicated for a detail study in the domain of mineral research.

This paper benefits from the fruitful criticism and suggestions by three anonymous reviewers, leading to an improvement of the work. They are gratefully acknowledged.