ASSESMENT FOR GOLD MINERALISATION POTENTIAL OVER ANKA SCHIST BELTS NW NIGERIA, USING AEROMAGNETIC DATA

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INTRODUCTION
Naturally, gold is a relatively common mineral that usually exist in quartz veins, alteration and oxidized zones of many sulphides and hydrothermal deposits, and in streams/ or rivers in most parts of the world (Augie and Ridwan, 2021). Hydrothermal gold deposits occur in orogenic belts that range in age from Precambrian to Late Tertiary (Garba, 2000). They are commonly found associated with stocks, batholiths and other igneous intrusions of intermediate to acid composition (Garba, 2003). The gold orebodies mainly occur in quartz veins, veinlets and tiny stringers. It's can be found associated with; phyllites, schists and quartzites (Augie and Sani, 2020). They have a sharp vein-wall rock contacts and the gold occurs in its native form as inclusions in minerals like chalcopyrite and galena (Sani et al., 2017). The veins do not always have a welldeveloped wall rock alteration pattern, are marked by silicification, sericitization and carbonation. At the old openpit mines, the miners often mined the soft (altered) wall rocks which must have been gold bearing, leaving behind the main vein of a massive hard quartz body (Sani et al., 2019). The earth's magnetic field originated from the core and effectively magnetized susceptible rocks usually in the earth's crust. The processes by which this can happen are the phenomena of magnetic induction, and this play most important/accountable for the great majority of magnetic variation observed by aeromagnetic surveys (Reeves, 2005). Usually, earth's magnetic field behaves remarkably like a bar magnet located in the earth core. It is dipolar, having north pole in the Arctic and a south pole in the Antarctic, these magnetic poles do not coincide with the geographic poles, and they are not stationary. The strength and orientation of the magnetic field vary relatively smoothly across the globe reaching minimum strength and having a horizontal orientation in equatorial regions (Isles and Rankin, 2013). The magnetic field definitions of north, south, upward and downward are scientific conventions; the key point for the aeromagnetic interpreter is that magnetic anomalies due to rock bodies which vary according to their location on the globe because the earth's field is the principal cause of magnetization in the crust (Augie and Ologe, 2020). Bonde et al. (2019) used aeromagnetic data for structural mapping associated with solid mineral potential zones in some part of Anka schist belt. Similarly, Sani et al. (2019) adopted the same method for the analysis of gold mineral potentials in Anka schist belt. Study area falls under the low latitude area and none of the Authors applied reduction to equation technique. However, at low latitudes, a separate amplitude needs to be corrected to prevent North-South signal in the data from having abnormal noise in the results according to Authors; Holden et al. (2008), Core et al. (2009) and Augie et al. (2022). This study used aeromagnetic data with the view to delineating gold bearing alteration zones in the study area. This was achieved by employing reduction to magnetic equator (RTE), first and second vertical derivatives (FVD & SVD), geologic contact, analytic signal (AS), centre for exploration targeting (CET) and source parameter imaging FUDMA Journal of Sciences (FJS) ISSN online: 2616-1370ISSN print: 2645-2944Vol. 5 No. 4,December, 2021, pp 235 -242 DOI: https://doi.org/10.33003/fjs-2021

METHODOLOGY Location and Geological Setting of the Study Area
The study area lies between latitudes 11°0′0″N and 12°0′0″N, and longitudes 5°30′0″E and 6°0′0″E. These covers the following areas; southern part of Bukkuyum, Anka, Maru, Wasagu/Danko, northern part of Sakaba and Mariga (see Figure 1.  2.). Furthermore, Danbatta (2008) discussed that the dacites/rhyolites are overlain and intrude the basement gneisses, metasediments and granitic rocks of the Anka-Yauri schist part Kebbi. A brittle fault zone cuts the area consisting of sub-parallel phyllites and crushed and uncrushed quartzites, and forms part of the mapped Anka transcurrent fault which is interpreted as a possible Pan-African crustal suture (Danbatta 2005).

Data Acquisition
This study used aeromagnetic data acquired from Nigeria Geological Survey Agency (NGSA This method measures basically admixture of the earth's core field and the field due to magnetized susceptible rocks in the earth's crust combine with the remanence field of the rocks (Augie el., 2022). For this, these data were further corrected by removing geomagnetic gradient using the International Geomagnetic Reference Field (IGRF). Acquired grid data were assembled and merged in order to have the composite map of the study area. These was further converted in digitized form of; X (longitude measured in metre), Y (latitude measured in metre) and Z (magnetic intensity measured in nano Tesla). The corrected aeromagnetic dataset was subjected to the minimum curvature gridding method to produce the Total Magnetic Intensity (TMI) map of the study area ( Figure 3A).

DATA PROCESSING
Processing techniques like reduction of magnetic equator (RTE), first and second vertical derivative (FVD & SVD), geologic contact, centre for exploration target (CET), analytic signal (AS) and source parameter imaging (SPI) were used to process this data with aid of Geosoft (Oasis Montaj) and Surfer software. The TMI value acquired from NGSA was short-up by 33,000 nT for the convenience of contouring or imaging, the value (33,000 nT) was added back to give the TMI grids for the area. The generated core fields (IGRF for the epoch period) are subtracted from the grid values of TMI so as to give the magnetic anomaly as shown in Figure 3A.

Reduction to Magnetic Equator (RTE)
The study area falls within magnetic equatorial zones of low latitudes/inclination. In this area, the North to South bodies have undetectable induced magnetic anomaly at zero geomagnetic inclination. Consequently, the TMI map ( Figure  3A) were subjected to magnetic equator to produce anomalies of magnetized body that usually depend on the inclination, declination, local earth's field and orientation of the body with respect to the magnetic north. The resultant composite map of the reduced-to-equator residual magnetic anomalies are given in Figure 3B. High and low magnetic zones were characterized by difference in rock formations and these lead to difference in the magnetic susceptibility of the rocks within the area (see Figure 3B). Susceptible rocks usually occur at depths shallower than the curie points isotherm.

First and Second Vertical Derivatives (FVD & SVD)
FVD and SVD techniques were applied to TMI anomaly (RTE) in order to quantify the spatial rate of changes of the magnetic field in horizontal, or vertical directions. These techniques are usually essential in enhancing the shallow features (high frequency anomalies) as compare to deep features (low frequencies anomalies) and also used to sharpen the edges of anomalies. Observing Figure 4A closely, most of the structures delineated are found within northern parts of Bukkuyum, Anka, Maru, Sakaba and Mariga, and southern part of Wasagu/Danko areas.
SVD map ( Figure 4B) is more effective and resolving power than FVD map and it provided much more detailed structures and also enhances the high frequency anomalies as measured in RTE map. Figure 4B has intensified the major delicate anomalies thereby enhanced the boundaries of anomaly in near surface which effects the characterized edges of the causative bodies. These structures found in FVD and SVD maps are the architecture of a mineralized body associated with gold mineral and these zones were corresponded to the aforementioned areas.

Centre for Exploration Target (CET) And Geologic Contact
CET and Geologic contact techniques were also applied to RTE grid data anomaly for structural features (fractures, faults/ or shears zones), delineating the boundaries within schist belts, and detect the position of the outcrops and intrusive bodies in the area. Figure 5A reveal the regions with lineament and these zones were trending to South-East of the study area. The zones with linear structures have falls under the following areas; northern parts of Bukkuyum, Anka, Maru, Sakaba and Mariga, and southern part of Wasagu/Danko. These structures were also falls within the basement complex in area underlain by the following earth materials; quartz, mica schist, diorite, migmatite, biotite-homblende, granite, medium coarse grained, sandstones, ironstones and laterites as compared with the geological setting of the area. These zones have also correlated well with structures found in FVD and SVD maps of the area. Most of linear structure found within these areas, are trending in the NE directions. Regions having these shears zones (lineaments) were represent as vein of mineralization as compared with the geology of the area (Figure 2). The rocks forming in the area usually play an important role in gold mineralization exploration since most gold deposits in Nigeria are found in quartz veins. Figure 5B, has generates a heat map called geologic contact map that highlights high density of structural contacts combined with junctions and intersections of different structures and locations where structures have significant orientation changes. These zones were located at; northern parts of Bukkuyum, Anka, Maru, Sakaba, Mariga, and southern part of Wasagu/Danko, and these regions were well agreed with the results found in CET, FVD and SVD maps.

RESULTS AND DISCUSSION
RTE, FVD, SVD, CET, Geologic contact, and SPI processing techniques shows that the regions were made of different magnetic zones and also exhibit substantial positive magnetic susceptibility values that are more of ferromagnetic minerals such as gold mineralization potential. The structures found in FVD, SVD and CET maps play important role in delineation of gold mineralization within the study area. The results found from aforementioned techniques have delineated the possible pathways for gold exploration and exploitation as describes in northern parts of Bukkuyum, Anka, Maru, Sakaba, Mariga, and southern part of Wasagu/Danko areas. These regions were occupied with quartz, mica schist, diorite, migmatite, biotite-homblende, granite, medium coarse grained, biotite gneiss and migmatite as compared with the geological setting of the area. The estimated depth to the magnetic sources found within the regions was found to be from 90 m to 122.8 m ( Figure 6B). The aforementioned rocks type found within the regions usually contained Fe-bearing minerals (gold mineral) and the distribution of the crystallized gold species indicated in these areas increasingly with oxidized of Fe-bearing minerals from mafic to felsic rocks. There are also regions of sedimentary rocks as observed ( Figure 6B) called Zone B. These regions coincide with the following rocks formation; sandstones, ironstones and laterites. These zones are strongly controlled by carbonates content and species that usually dependent on both sedimentary facies and sediment provenance, and the area corresponded to northern part of Wasagu/Danko.

CONCLUSION
The result from this geophysical study has revealed the regions that might host the gold mineralization as shown in Figures 3 to 6 and these regions were corresponded to the following local government areas; in northern parts of Bukkuyum, Anka, Maru, Sakaba and Mariga, and the southern part of Wasagu/Danko. The study revealed the structures (faults, fractures/ or shears zones) associated the vein of mineralisation with the area. These structures play an important role in determine gold and other minerals. SPI depth map specified the depth of the boundaries of causative bodies with the aforementioned area, and its was found to be from 90 m to 122.8 m.