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1.6 LAND USE .................................................................................................................................................... 5
CHAPTER 2. MAPPING EQUIPMENT AND PROCEDURE ....................................................................................... 6
2.1 EQUIPMENT AND TOOLS USED .................................................................................................................. 6
CHAPTER 3. MAPS AND SECTION ......................................................................................................................... 8
3.1 BASE MAP ................................................................................................................................................... 8
2.1 EQUIPMENT AND TOOLS USED The equipment and tools used during the field mapping exercise include;
base map for the mapping exercise. This had prominent features such as roads, footpaths, schools,
churches, etc. which aided in the determining of our positions on the field. The traverse, structural,
outcrop and geological maps were all drawn on the base map.
Compass and clinometer for measuring attitudes (dip and strike) of structures on the field.
hammer for breaking the rocks as samples and also to expose fresh surfaces.
Hand lens to examine the mineral grains of the rocks more closely.
Field notebook for recording information and observations, such as, strike and dip values, outcrop
description, sample names, etc.
Markers for the marking and naming of the samples.
Field bag for holding samples.
Camera for taking photographs of the outcrops and other significant features.
Fig. 1. Some of the equipment used during the mapping exercise.(measuring tape, compass, hand lens, geological hammer, cutlass, field notebook, from left)
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2.2 MAPPING PROCEDURE
The following procedure was used during the field mapping exercise;
along roads and footpaths to find outcrops and/or exposures. We would traverse
the nearby bushes if outcrop or exposures are rare to find along the roads.
etermine the location coordinates of the outcrop or other feature using the GPS.
easily
recognisable minerals, and suggest a field name for the outcrop in our field notebooks.
attitude measurements of the structures in the outcrop, noting which structures they
are.
ure to aid future analysis.
We took fresh samples of the outcrop or exposure for further analysis to confirm the field name or
otherwise.
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CHAPTER 3. MAPS AND SECTION
3.1 BASE MAP
A base map of the area was provided for the exercise. It simply shows geographic features of interest
and relief of areas, using contours. It is the base on which the other maps were developed.
3.2 TRAVERSE MAP
This map was developed to show the route of traverse in the field. The paths of each day were
differentiated using different colours.
Fig 2 Traverse map
3.4 OUTCROP MAP
It shows the locations of outcrops seen in the field. The different types of outcrops were also
differentiated using colours. (Yellow for metasediments, pink for fine grain granitoids and purple for
coarse grain granitoids)
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Fig 3 Outcrop map
3.5 STRUCTURAL MAP
A map showing the structural measurements recorded on the field. Since these structures are usually
taken on outcrops, their locations should coincide with the locations of outcrops on the outcrop map.
Fig 4 Structural map
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3.6 GEOLOGICAL MAP
The primary aim of the mapping exercise is to produce a geological map of the area and write a report.
The geological map shows the rock units underlying the area. It is essentially a combination of the
outcrop map and the structural map. The similar rock outcrops are connected and boundaries of
different rock units are shown or inferred where they were not seen on the field.
Fig 5 Geological map
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3.7 CROSS SECTION
We took a cross section along 6 ̊ 47’ from “A” to “B” as shown on the geological map. The cross
section shows the progression of rock units in moving from “A” to “B”.
Fig 6 Cross section at 6047'
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CHAPTER 4. GEOLOGICAL SETTING
4.1 REGIONAL GEOLOGY
Regionally, the area is in the Ashanti uplands. The geology is that of the lower Birimian series. The
lower Birimian forms part of the Birimian system. The Birimian system is one of the units forming the
West African cratonic region.
The Birimian terrains in the West African craton are a mixture of highly metamorphosed volcanic and
plutonic rocks with low grade metavolcanic and metasediments. The Birimian is divided into the older
“lower Birimian”, consisting of predominantly metasedimentary rocks, and a younger “upper
Birimian”, consisting chiefly of metavolcanic rocks.
Our area falls within the Lower series which consist of metasedimentary rocks mainly phyllites,
greywackes. Also associated are intrusives of felsic (granitic and granodiorite) and mafic (dolerite)
compositions.
4.2 ECONOMIC GEOLOGY:
The granitoids of the area are suitable for use as aggregates for construction. There is a quarry,
operating on the Achinsi classification range.
The phyllite can also be used for decoration purposes.
4.3 STRATIGRAPHY:
The major rock units encountered were the birimian granitoids and metasediments. From the principle
of original horizontality, sediments would be laid horizontally or almost horizontally. The fact that the
sedimentary rocks have been metamorphosed and also inclined provide evidence of deformation. The
metamorphism was most likely due to heat from a magmatic intrusion. The intrusion could also be the
cause of the dipping nature of the metasediments. Combining these, we can infer that sedimentary
rocks were laid, they got intruded by the granitoids. The force of the intrusion disturbed the sedimentary
beds, causing them to incline. The heat from the magma metamorphosed the sedimentary rocks. There
are two types of granitoids, differentiated on the basis of grain size into coarse grain and fine grain
types.
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4.4 STRUCTURES:
The major geological structures encountered were joints. Some of the joints have provided space for
vein intrusions. Some of the veins were quartz and others were pegmatite veins. The joints strike in the
North – East direction, generally. The foliations of the metasediments strike in the North – East
direction and dip generally in the North – West.
Stereo plot of foliation strike showing a general direction of NE-SW.
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Rose diagram showing general strike direction of quartz veins (NE-SW)
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Rose diagram showing general strike of pegmatite veins (almost N-S).
4.5 IGNEOUS ACTIVITY:
The most prominent igneous bodies are the granitoids. There were quartz and pegmatitic vein and
veinlet intrusions in both the fine and coarse grained granitoids.
Fig.2. Granitoid with quartz and pegmatite intrusions at Asuofua.
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4.6 METAMORPHISM:
The presence of phyllite in and around Dabaa, Kapro and Kumi is an evidence of a medium grade
metamorphism. Again there were areas in Ntiribuoho and Nkukua where the minerals in the granitoids
have started aligning themselves. Thus, the area is moderately metamorphosed.
Fig.2. Outcrop of phyllite seen near Dabaa.
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CONCLUSION
The area is underlain by three major rock types, thus, fine grain granitoids, coarse grain granitoids, and
metasedimentary rocks (phyllite). The granitoids intruded original sedimentary rocks, metamorphosing
them. Therefore, even though the area appears dormant and stable at present, it must have undergone
remarkable igneous activity in the past.
RECCOMMENDATION 1. The pre-mapping training should done over a longer duration to allow students to really grasp
the concepts of mapping, especially determining attitudes of structures.
REFERENCES
1. GEOLOGY AND MINERAL DEPOSITS. Ghana Ministry of lands forestry and lands (Mines
section). Retrieved 2009-03-17.
2. “Geological evolution and metallogeny through the Birimian”. (PDF). Prospectors and
Developers Association Canada.2009. Retrieved 2009-03-17.
3. J.P. Miles, P. Ledru, P. Ankrah, V. Johan, E. Marcoux and Ch. Vinchon. “The metallogenic
relationship between Birimian and Tarkwaian gold deposits in Ghana”. SpringerLink.
Retrieved 2009-03-16.
4. www.ghanadistricts.gov.gh (website), accessed on 8th April, 2015.