GRAVITY AND MAGNETIC STUDIES OF THE SUB-BASALT CASSITERITE DEPOSIT IN PLATEAU STATE, NIGERIA
GRAVITY AND MAGNETIC STUDIES OF THE SUB-BASALT CASSITERITE DEPOSIT IN PLATEAU STATE, NIGERIA
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Date
1984-10
Authors
AVJAJIOKAN, ENOCH IKPOKONTE
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Abstract
A detailed gravity and magnetic survey carried out
on the N'Gall basalt area of Jos Plateau region, was
aimed at studying which analytical technigue is practically
suitable to the exploration of sub-basalt deposits, with
a view to estimating the basalt thickness and removing
its masking effect from the acguired data in problem
areas such as the sub-basalt cassiterite fields.
It was thought that this approach will further
reveal a knowledge of the relative variation of the
bedrock surfaces, tne v/eathered/v/eak zones of the
bedrock, and the ancient river channel along which
tin-bearing sands are believed to be deposited and later
covered by sediments overlain by thick hard basalt flow.
Results show that the area of investigation includes
a possible downstream extension of the ancient N'gell
river flowing to the NE-SW direction into a basement low
surface, resulting into a thick pile of sediments, later
covered by basalt flow and compacted. Dopth estimates
from the potential field sources are comparable with
those done on an available control borehole section and
are in aggreement with the borehole results. The basaltic
layer thicknesses obtained from gravity data ranges from
10 to 29 m„, and from magnetics a range of 11 to 40 m.
Statistical correlation of both results is fair (0.44
to 0.99). The basalt overburden depth from gravity ranges
from 9 - 36 m. with an average of 16 m. while estimate
from magnetics ranges from 14 - 21 m. with average of
17 m. B' ::'-!t overburden, depths was then fairly
obtained from total field magnetic data and compared
with those from gravity. The gravimetric depths to
basement ranges from 60 - 80 m. with an average of 65 m.
and a range of 47 - 69 m„ was obtained from magnetic
data. An estimate done on gravity gradients in the
area yielded a depth range (to source) of 32.4 - 44„5 m»
with an average of 38.0 m. which is indicative of the
approximate depth to the base of basaltic layer and
is in aggreement with the: borehole results.
From these results, it was then possible to remove
the basaltic masking effect from gravity data observed
on the basalt, by a stripping technigue and this enabled
a fair estimation of approximate depths to sub-basalt
bedrock -and'interpretation of the sub-basalt basin outline.
From the stripped gravity data then, bedrock depths
ranges from (49 - 65) m„ in the north - western part of
the study area; (52 - 100) m. with bedrock dipping
towards south-west in the middle part, and a range of
(45 - 70) m. in the south-eastern part of the study
area. The sub-basalt sedimentary thickness ranges from
10 - 30 m. in the north-western part, 30 - 80 m. in the
middle part, with average of 70 m. and 3 - 30 m. with
Description
A THESIS SUBMITTED TO THE POSTGRADUATE SCHOOL,
AHMADU BELLO UNIVERSITY, IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER
OF SCIENCE, APPLIED GEOPHYSICS.
DEPARTMENT OF PHYSICS,
AHMADU BELLO UNIVERSITY,
ZARIA-NIGERIA.
OCTOBER 1984
Keywords
GRAVITY, MAGNETIC, STUDIES, SUB-BASALT, CASSITERITE, DEPOSIT, PLATEAU, NIGERIA