Transcript MINERAL INVENTORY THROUGH EXPLORATION

A Case Study In Ghana
Yakubu Iddirisu
CEO, Geological Management
Consultancy Ltd(GEOMAN CONSULT.)
The African, Caribbean and Pacific (ACP) states
constitute a significant proportion of the global
landscape – they account for approximately 19% of the
earth’s surface area.
The geology of the region is favourable and its largely
unknown potential calls for a sustainable mining
cooperation among member states.
Nearly two-thirds of the ACP states are either mining
countries or have a mining potential that if exploited
rationally would contribute to their social and
economic development. For instance mining in the subSaharan Africa excluding South Africa accounts for 8%
of the world mining production.
It generates almost 50% of the exports and 30% of the
fiscal resources of the major mining countries. 80% of
the surface area of this region has never been
represented on any detailed geological map (of a scale
of 1/200,000 or more) . Source: ACP Group Website
The need therefore for the ACP countries to take
inventory of their resources through exploration
cannot be overemphasized.
Mineral deposits in the ground are worthless unless
they are explored and tapped for the economic
development of their beneficiaries.
In 2005 Ghana was a beneficiary of an European
Funded reconnaissance exploration which covered 8
map sheets totaling 25,646 square kilometers.
The project was under the Mining Sector Support
Programme which lasted for 5years.
It included Geological Data Provision which comprised
two components:
 Geological investigation of eight 1:100,000 scale map
sheets in selected parts of the country to produce
geological data for investment promotion.
The investigation comprised geological mapping, soil
geochemistry and hydrocensuses.
The Geological Investigation was undertaken by a
Consortium comprising Council for Geoscience of
South Africa (CGS), Bureau de Recherches
Géologiques et Minières (BRGM) of France and
Geological Management Consultancy Limited
(GEOMAN) of Ghana.
Showing 8 mapped field sheets
 An airborne geophysical survey of the entire Voltaian
and Keta Basins was undertaken by Fugro Airborne
Surveys (UK) with the British Geological Survey.
The survey included the following:
 Satellite imaginary interpretation
 Magnetic and spectrometric surveys
 Gravity surveys and EM surveys
 Integrated Interpretation of Geological and
Geophysical Data
AREAS SURVEYED
The GEOTEM regional survey
included the area between the Volta
River and Keta Basins.
In addition to these regional
surveys, Fugro Airborne Surveys
conducted combined GEOTEM and
magnetic surveys in eight detailed
areas, of which six (A1-A6) are
within the Voltaian Basin, one (A7)
partly in the Keta Basin and one
(A8) over the Winneba Belt (light
red area and red polygon).
Geotech Airborne Ltd. covered four
areas with combined VTEM and
magnetic measurements (light blue
area and blue polygon).
By Courtesy of GGSD
Moreover, some mountainous areas outside the basins
like Nkwanta were covered by helicopter-borne EM
surveys undertaken by Geotech.
The Terms Of Reference (TOR)
for the mapping project were
as follows
 Review all existing topographic and geological maps
and publications concerning the map areas and their
surroundings
 Acquire, process and interpret satellite imaginery to
define structural and lithological groups
 Acquire and geologically interpret airborne
geophysical maps
 Define a lithostratigraphic classification to be used
during field mapping in accordance with
Guidelines of the Sub-Commission for
Stratigraphic Classification of IUGS Commission
for stratigraphy
 Compile all mineral occurrences located within the
map areas
 Compile data available on water boreholes
(location, depth, water level and yield)
 Prepare a GIS map to reference the outcrops and
structures identified above (synthesis map)
 Produce field base maps
Methodology
As has been pointed out earlier the investigation covered
eight map sheets at a scale 1:100,000. Because of time
constraint, it will not be possible to describe the work
carried in each map sheet. Instead Nkwanta area would
be used as the test case area. The study area is situated
between Hohoe and Yendi with the eastern limits on the
Ghana –Togo border.
Map showing the location of Nkwanta map sheet
Mapping
Each field team generally comprised a consortium
geologist and a trainee with a vehicle. Vehicles were used
mainly for transportation between camp and field work
areas and mapping was carried out mainly by foot.
Intensive coverage with a regular grid of field
observation and sampling was not possible due the
pronounced relief and logistical difficulties.
Several major traverses were carried out in the western
range and towards the Togo range at an interval of 8kms.
During the mapping process field outcrop coordinates
were systematically acquired using a GPS and stored in a
dataset.
Samples were taken and used to make thin sections, for
geochemical and geochronological analyses, and for
magnetic susceptibility measurements.
This picture depicts a typical rainforest terrain where mapping was
undertaken
An outcrop within the forest
Sampling
Due to difficult terrain in the Nkwanta area, a helicopter
was used in gaining access to predetermined sampling
points.
In total 3344 samples were collected in the field at an
average sample density of one sample per square
kilometer. Sample position coordinates were saved at the
sampling positions on GPS.
This is the helicopter used to facilitate sampling in difficult areas
South of Nkwanta
Samples were screened out in Accra and Pretoria
to<106um fractions. These size fractions were analysed
for 28 elements: Ag, Al2O3, As, Au, Ba, Bi, CaO, Cd, Co,
Cr, Cu, Fe2O3, K2O, MgO, Mn, Mo, Na2O, Ni, Pb, Sb,
SiO2, Sr, Rb, ThO2, TiO2, U3O8, V and Zn.
The helicopter lands at a pre-designed spot for a sample to be
taken
Hydrocensus
Investigation
The fieldwork consisted of geological mapping and
hydrocensus of all existing boreholes. Water samples
were collected and water levels, recovery and
permeability tests conducted on the boreholes.
Thereafter, all field data were compiled and coded
according to their lithological units into a computerized
data base from which a 1:100 000 scale hydrogeological
map was produced.
A hydrogeologist taking measurement in a borehole fitted with a
hand pump
Geology
A simplified geological map of Eastern Ghana including Nkwanta map
sheet area
The area is underlain by upper Voltaian sediments along
the western margin of the combined map sheet with the
Togo metasediments along the eastern margin on the
Ghana- Togo border.
Sandwiched between these units is the Buem running
N-S along the central parts of the mapped area. U-Pb
analysis carried out on samples from the Buem yielded
ages of 461±7Ma, a Variscan age.
From a geochronological perspective, the Buem Group
can no longer be associated with the Precambrian
history of the Volta Basin. The evidence points to an
Ordovician.
A gossan was identified in the southwestern mapping area
associated with ultra-mafics hosted in quartzites.
EM Time Domain geophysical map showing concurrence with geology
(courtesy of GSD)
Mineral Potential In
Nkwanta Map Sheet
Old map indicating the occurrence of Fe only
Current map indicating the occurrences of Ni
Nickel anomalies mostly occur in the western portion of
the Buem Group where they follow the same N-S trend
defined by other elements including Fe, Mg, Cr, Cu and
Ag.
The prominent anomaly on the Ghana –Togo border is
also repeated. High Ni probably delineates massive
sulphide type deposits within the Buem Group and
possibly ultramafic sequences in the Ghana -Togo
border.
Current map showing the distribution of Cu
Copper: The most significant copper anomalies occur
towards the western portion of the Buem Group and
follow the same general trend as silver, cadmium and
chromium. The Voltaian sediments seem to be devoid of
anomalous copper.
Current map showing the distribution of Mn
Manganese: Most of the anomalous populations occur
on the Buem Group where they define two prominent NS aligned zones in the southern parts of the map.
Environmental implications, as far as deficiencies and
toxicities of the elements in particular areas are
concerned, are also addressed. A map showing areas that
could be of concern is included.
Conclusion
The EU – funded regional mapping component of the
MSSP at a scale of 1:100,000 has improved the status of
geology and ensured a better understanding of the
subject in the eight selected areas and in particular the
Nkwanta Map Sheet.
Geochronological dating as well revealed that the Buem
group is younger than the Voltaian which has
implications for mineral exploration.
Nkwanta area that was only known to host iron and
jasper deposits had interesting and far reaching
outcomes which are presented as a lesson and a way
forward for the future.
The geochemistry mapping of soil data in particular
allowed the identification of an apparent mineralized
zones (metallogenic province) that may host specific
types of mineralization.
The provinces and anomalies that are of particular
interest are those identified for Nickel, Cobalt, Copper,
Zinc and Manganese.
Recommendation
Coming from Ghana’s experience it can be said without a
doubt that a regional mapping program when properly
executed associated with geochemistry and geophysics
opens up new possibilities for the mineral exploration
sector.
What private Exploration and Mining companies need
are good geological maps and indicators and these can
be made available by carrying out:
 Systematic Regional Geological mapping of the
country on a scale of 1:200 000 to 1:100 000 including
dating of main units.
 Conducting a regional soil sampling programme
 Airborne regional geophysical surveys: (Magnetics,
Radiometric and Electromagnetic methods) to provide
information for the integrated interpretation of the
field sheets and in helping Mining and Mineral
Exploration companies in their target generation.
 Creation of good data bases and an Information
Management System through which investors could
have access to the data so generated.
End
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