Integrated Geochemical and Geophysical Approach to Mineral Prospecting – A Case Study on the Basement Complex of Ilesa Area, Nigeria - Advances in Data, Methods, Models and Their Applications in Geoscience - page 84

Figure 8 is the pseudosections for the four West - East profiles. The figure shows the
character of the labeled linear features earlier discussed. The linear feature labeled F1 is
observed to be dipping conductors (as are observed on the Amphiboliteat about station
1000m) in all the three traverses of Gada to Iwikun, Okeipa to Eyinta and Itagunmodi to
Aiyetoro. These are seen to have values thatrange between 15% and 70%. The occurrence of
these dipping fractures from near the surface to depth of 100m and approximately along the
N - S direction suggest that it is one and the same linear feature which cuts across the study
area. The other fractures (F2 and F3) also on the amphibolites which are almost vertical and
at between stations 3000m and 4000m occur at a depth range of 20 -70m are also one and the
same linear features which cut across along N - S direction in the Amphibolite. Vertical
conductive structures are also observed in the schist/epidiorite Complex and inthe
quartzite/quartz schist rocks. The occurrence of these linear features in the study area has
implications for mineralization, geotechnical and groundwater studies (Palacky,1988).
Minerals that are structurally controlled such as lateritic nickel, gold, talc and clay deposits
can be prospected along the identified linear features.
5.4 The magnetic data, analysis and results
Magnetism has been studied for a very long time in human history. Early Greek
philosophers knew about the attraction of iron to a magnet. The first magnets consisted of a
naturally occurring rock called lodestone, a variety of massive magnetite (almost pure iron
oxide). Magnetite is the only naturally occurring mineral with distinctly obvious magnetic
properties. Only a few other minerals have any detectable magnetism. However, extremely
sensitive magnetometers can detect trace magnetism in many different minerals. Iron,
because of its atomic structure, has the greatest tendency to become magnetized. Other
elements, such as cobalt and nickel, have fewer tendencies to become magnetic. Any mineral
or rock which contains any of these elements is likely be more magnetic.
The Earth possesses a magnetic field caused primarily by sources in the core. The form of
the field is roughly the same as would be caused by a dipole or bar magnet located near the
Earth's center and aligned sub-parallel to the geographic axis. The intensity of the Earth's
field is customarily expressed in S. I. units as nanotesla (nT) or in an older unit, gamma (γ):1
γ = 1 nT = 10 -3 μT. Except for local perturbations, the intensity of the Earth's field can vary
between about 25 and 80 μT. Many rocks and minerals are weakly magnetic or are
magnetized by induction in the Earth's field, and cause spatial perturbations or "anomalies"
in the Earth's main field. Man-made objects containing iron or steel are often highly
magnetized and locally can cause large anomalies up to several thousands of nT. Magnetic
methods are generally used to map the location and size of objects that have magnetic
properties.
In order to produce a magnetic anomaly map of a region, the data have to be corrected to
take into account the effect of latitude and, to a lesser extent, longitude (Reynolds, 1997). As
the Earth's magnetic field strength varies from 25000nT at the magnetic equator to 69000nT
at the poles, the increase in magnitude with latitude needs to be taken into account. Survey
data at any given location can be corrected by subtracting the theoretical field value F th ,
obtained from the International Geomagnetic Reference Field, from the measured value, F obs .
Regional latitudinal (φ) and longitudinal (θ) gradients can be determined for areas
concerned and tied to a base value, F o . Gradients northwards (δF/δφ) and westwards