GEOL 400

DEPARTMENT OF EARTH SCIENCE

UNIVERSITY OF GHANA, LEGON

GEOL 400: FIELD EXERCISE: VISIT TO ROCK OUTCROPS OF THE BIRIMIAN AND SOME MINING CENTRES OF GHANA

NAME: AMOABENG BRIGHT

STUDENT ID: 10274787

DATE: 28TH MARCH, 2012

SUPERVISORS: DR. EBENEZER HAYFORD

DR.MARK SANDOW YIDANA

REFERENCE

GEOL 400

TABLE OF CONTENT

CHAPTER PAGE

CHAPTER ONE 1.0. INTRODUCTION 2

CHAPTER TWO SUMMARY 3

CHAPTER THREEE THE BIRIMIAN EXPOSURES 4

3.0. EWUTU BRAKWA 4

MAPPING 4

3.1. WINNEBA 5

3.2 . MAAKOADZE 5

3.3. BUTRE 6

3.4. DIXCOVE 6

3.5 AXIM,ALONG THE SHORE 7

3.6. AXIM,CASTLE BEACH HOTEL 8

3.7 CAPE THREE POINT 8

3.8 AROUND UMAT 8

3.9 BONSA 8

CHAPTER FOUR GOLDFIELDS GHANA LTD, DAMANG 9

CHAPTER FIVE ARTEESINAL MINING(GALAMSEY) 13

CHAPTER SIX DISCUSSION 15

CHAPTER SEVEN CONCLUSION 16

CHAPTER EIGHT FIGURES AND PICTURES17

CHAPTER NINE REFERENCES 19

REFERENCE

GEOL 400

CHAPTER ONE

1.0 INTRODUCTION

This is a report of field exercises carried out by final year students of the Earth Science

department, University of Ghana between the 18th and 22nd of January, 2012.

Geology 400, Geological field exercise is a one credit, geological field-based course comprising a

two part field trip to some Birimian exposures and associated Eburnean granitoids along the

southern end of the Ashanti belt.The second part of the trip covers a visit to Goldfields Ghana

Limited, Damang.

The places we visited include Tarkwa, Mankoadze, Butre, Axim and Cape three point and their

environs and we used equipment such as GPS, compass, hand lens, cameras and geological

hammer.

AIMS OF THE TRIP

The aim of the trip is as follows:

1. To study the geology of the Birimian, its history and origin and classify them.

2. To do a petrographic study and undertake detailed mapping of the rocks in the various

units.

3. To enable students know and infer the geological processes that lead to the formation

and deformation of various the individual lithologies and structures of this area.

To help students practice and take structural attitudinal measurements and rock sampling data

collection.

REFERENCE

GEOL 400

CHAPTER TWO

2.0. SUMMARY

Ghana falls mostly within the West African Craton which stabilized in the early Proterozoic

(2000Ma) during theEburnean Orogeny. These early Proterozoic event comprise rocks of

extensive belts of metamorphosed volcanic and sedimentary rocks exposed in Ghana and other

parts of West Africa.

These early Proterozoic terrane known as the Birimian System hosts most of the country's

mineral deposits especially gold and diamond and occupies the southern,western and

northernmost part of the country.

The Birimian consists of metamorphosed volcanic and sedimentary rocks which form five sub

parallel belts of volcanic rocks separated by broad “basins” of sedimentary rocks. The places we

visited mainly form within the Ashanti belts.

The Birimian is made up of two varieties. They are the tensional belt which alternate with

compressional basin. The compressional basin is further made up of upper arenacious and

lower argillaceous rocks. The tensional belt is also further divided into acid volcanic subseries,

basic volcanic subseries and sedimentary volcanic subseries. There are intrusive materials

within the basin and are called Cape Coast Granitoids and intrusive materials within the belt are

called dixcove granitoids.

Mineralization is mostly found in the greenstone belt within the contact between the

metavolcanics and metasedimentary rocks. This is where most of the mining companies mine.

Goldfields Ghana Limited, Damang operates an open pit mine within the Tarkwaian .The ore

occurs as disseminated plaeo-placer type deposit locked up in the matrix of the banket

conglomerates and the hydrothermal type deposit. The company recovers the ore chemically

by heap leaching with cyanide and has environmental policies in place to ensure environmental

restoration after mine closure. The various department of the company are the exploration

department, mine geology department, plant or pit department and the metallurgy

department.

REFERENCE

GEOL 400

CHAPTER THREE

BIRIMIAN EXPOSURES

3.0 EWUTU BRAKWA AROUND TONISCO GROUP GHANA LTD (STATION ONE)

The first station was at Ewutu Brakwa around a company called Tonisco Group Ghana Limited.

We travelled South-West from Accra and left the Togo and Buem formation. Rocks here form

part of the Birimian supergroup within the age of Precambrian. It stretches from South-west

towards north-east.

The Birimian is made up of two varieties. They are the tensional belt which alternate with

compressional basin. The compressional basin is further made up of upper arenacious and

lower argillaceous rocks. The tensional belt is also further divided into acid volcanic subseries,

basic volcanic subseries and sedimentary volcanic subseries.

There are intrusive materials in both the belt and the basin. Intrusives within the basin are

called Cape Coast Granitoids and intrusive within the belt are called Dixcove Granitoids. Cape

Coast granitoid are mica type granitoid and are the dominant rocks here.

MAPPING OF THE AREA

I was part of group one and we undertook a detailed mapping of this area.We found out that

the outcrops are in the south-western part of the country. The outcrops are exposed about

150meters from the main street. There are boulders as well as different insitu rocks here. The

rocks here have heterogeneous colours with black and white colours dominating. The rocks are

medium grained and granular. They are intrusive volcanic igneous rocks their mineralogical

compositions are quartz and muscovite. Some of the rocks here have undergone weathering

and experienced exfoliation.

We traversed 50meters and encountered outcrops with dark and grey colours. They contain

flow structures which indicate sub aqueous eruption under water and eye like structures called

boudinage (fig 1.7). The rock contain heterogeneous grain size with minerals like muscovite,

plagioclase, feldspar and biotite. rocks are also have homogeneous with light grey coloour. The

REFERENCE

GEOL 400

extend of the outcrop is 20m-30m. The outcrop is coarse grained and is massive and hence no

attitudinal measurements were taken. The rock is a volcanic igneous rock. There are green

minerals in some of the rocks indicating chlorotization as a result of reaction between chlorine

and mafic minerals in the rock.

Trend of outcrop;

3.1 WINNEBA (FIRST DAY, STATION TWO)

Here we saw other host rocks belonging to the tensional belt and compressional basin. There is

also a contact between the two types of rocks (fig 1.1).Mineralizations are located within these

contacts. The granitoids here are part of the compressionl basin.

The rocks here have different grain sizes and are meta- sedimentary alternating with meta-

volcanic. There are also greywackes here. The mineralogical composition of the rocks are

feldspar and quartz with inhomogeneous grain sizes. Quartz veins are also present here. There

are also metavolcanics rocks within the tensional belt here. There are various contact between

the metavolcanics and metasedimentary(pegmatite) (fig1.1) here. The rocks here are light and

dark coloured. There are pegmatite alternating with metavolcanics. There are coarse and fine

grained rocks here as well.

3.2 MAAKOADZE (FIRST DAY, STATION THREE)

The rocks here belong to the basic volcanic sub series. There are pyroclastic rocks with mafic

color and containing minerals like amphibolites, greenschist and actinolite. Some part of the

rocks here appears in the dixcove.

REFERENCE

GEOL 400

There are massive joints in the rocks and the rocks are medium grained and are

metamorphosed. There are also yellowish greywacke here. There are vesicular textures in the

rocks indicating they are igneous rocks. There are also quartz veins in some of the rocks.

Originally it was perceived that the metasediment are older than the metavolcanics but

detailed studies have proved that the two types of rocks are contemporaneous. This is very

evident at places where the types of rocks are lying adjacent to each other. At some places one

lie on top of the other which is as a result of reverse fault and complex folding.

3.3 BUTRE (DAY TWO, STATION ONE)

Butre is located in the Ashanti belt and is found in the lowermost part of the sequence followed

by Cape three point in the Axim.

Rocks here are basically metavolcanics and are fine grained rocks with homogeneous dark

green colour and also granular in nature.The rocks are basalt and contain vesicles (fig 1.4).

There are pillow lavas (fig 1.3) here indicating subaqeous eruption of the rocks. The

mineralogical compositions of the rocks are hornblende, quartz, pyrite. The pyrite is a

pathfinder for gold which indicate presence of gold in the rocks here.

There are different types of rocks here ranging from greenstones, argillaceous rocks, arenacious

rocks and meta-greywacke.

We took GPS of the outcrops here and the following figures were obtained:

N 04⁰ 49.432 W 001⁰ Elevation (4+/-3) m

3.4 DIXCOVE (DAY TWO, STATION TWO)

Belt type granitoid are the dominant rocks here and are called dixcove granitoid (fig 1.2). These

belt type granitoid separate Birimian rocks found in Axim, Butre and Cape three points.

REFERENCE

GEOL 400

The rocks are not foliated and are coarse grained with homogeneous grey colour. The rocks are

massive as well and contain quartz veins. The mineralogical composition of the rocks are

hornblende, sodium feldspar, muscovite and quartz. The rocks have joints which become very

prominent moving down the sea.

3.5 AXIM BEHIND THE CASTLE ALONG THE SHORE (DAY THREE, STATION 1)

We are within the Ashanti belt were we have metasediments and metavolcanics. Belts are

100km wide and basins are 100km broad. This place falls within the southern branch of the

Ashanti belts.

There are different types of rocks here ranging from coarse grained rocks to medium grained

rocks. We have medium grained rocks here which are have homogeneous grains and dark

coloured. They contain minerals like biotite, muscovite, hornblende and pyrite. They have joints

and are granular. They contain pyrite (fig 1.0) indicating gold mineralization. The gold here are

termed lode gold as compare to theplacer gold in Takoradi. There are quartz veins here as well.

The grain size of the rocks become coarser as we traversed westwards along the sea and they

contain quartz (fig 1.0) and feldspar.

The grain size of the rocks becomes small moving eastwards along the sea. They are basalts and

the coarse grain rocks are gabbros. The basalt contains minerals like pyroxene and olivine.

Behind the Axim castle along the shores three questions were asked. The questions and

answers are as follows:

1)Write the names of all the rocks here.

Answer: Meta-greywacke, Thioleiitic basalt, Meta-gabbro, Quartzite, Diorite

2) What are the structures here?

Answer: Joints, Quartz veins, Vesicles

3) What are some of the minerals here?

REFERENCE

GEOL 400

Hornblende, Biotite, Pyrite,Feldspar, Quartz, Muscovite

3.6AXIM CASTLE BEACH HOTEL (DAY THREE, STATION TWO)

We have meta-volcanic and meta-sedimentary rocks here. Most of the rocks here are medium

grained rocks and have light green colour.

There are structures like quartz veins and joints. We took attitudinal measurements of the

joints and the following results were obtained:

Strike: 006⁰Dip : 38⁰Dip direction: 96⁰ South east

We have quartz porphyry here as well and a contact (fig 1.1) between the meta-sediments and

meta-volcanic and encountered meta-volcanic as we moved eastwards. The quartz porphyry is

euhedral to subhedral.

3.7 CAPE THREE POINT (DAY THREE, STATION THREE)

This is a town along the coast and co-ordinate from three towns, Takoradi, Tarkwa and Axim,

hence the name. Rocks here are situated on the Ashanti belt.

Rocks here have homogeneous mafic coloour and are coarse grained. The rocks contain

plagioclase in a fine matrix therefore the rocks are plagioclase porphyry (fig 1.6) as compared to

the quartz porphyry encountered earlier. The mineralogical compositions of the rocks are

plagioclase, quartz and hornblende.The third type of meta-volcanic that is the sedimentary

volcanic subseries can be found here. There are joints here. The surface of some the rocks are

brown due to iron stains. There are no pyrite here.

REFERENCE

GEOL 400

3.8 AROUND UMAT (FOURTH DAY, STATION ONE)

Rocks here are phyllite. They are fine and contain homogenous grains with extensive foliations

(fig 1.5). The Rocks which are adjacent the phyllite are of different materials and contain clays.

The phyllite here is called Tarkwa phyllite (fig 1.5). We took attitudinal measurement and the

readings are as follows: Dip=33⁰ Strike= 040⁰ Dip direction= 320⁰

3.9BONSA RIVER (FOURTH DAY, SECOND STATION)

Rocks here are called Kawere conglomerate and are exposed around the Bonsa river. The major

minerals they contain are migmatite. They also contain pebbles which are not rounded. The

Kawere does not contain gold. They have inhomogeneous grain sizes. The heavy minerals in the

Banket series arehematite which are more oxidized than the migmatite. This is why the Banket

contains gold but the Kawere does not.

CHAPTER FOUR

GOLDFIELDS GHANA LIMITED, DAMANG

This is a gold mining company and the rocks carrying gold undergo various processes before

gold is finally obtained from them. They have various departments which perform perculiar

functions at various stages of gold exploration and gold extraction. The main departments are:

Exploration department

Mine geology

Plant/pit

Metallurgy(Processing plant)

The roles of the various departments at various stages of gold exploration and gold extraction

are as follows;

EXPLORATION DEPARTMENT

This presentation was done by Humphrey

REFERENCE

GEOL 400

The main task of the exploration department is explore and locate mineralized zones. The

lithological history in increasing order of age of the area is as follows:

Huni sandstone

Tarkwa Phyllite

Banket series

Kawere group

Birimian basement

These lithologies were then affected by folding and faulting which distorted the lithology of the

area.

Huni sandstone which is the uppermost Tarkwaian unit is fine grained monotonous sandstone.

Tarkwa phyllite is fine grained and laminated and contains phyllite and chloritoid-bearing

phyllitewith subsidiary arenaceous beds. Tarkwa phyllite contains reefs which hold paleoplacer

mineralization. The reefs are quartzite pebbles and are divided into three sub-series namely:

Golder, Morter and Star. Star is 1.5m thick and mostly quartz pebbles. Star is the best in terms

of economic value followed by morter and subsequently Golder.

The Banket series contain quartzites, grits, breccias and banket conglomerates. Club reefs are

located in the banket conglomerate. It also contain quartz pebbles. Hydrothermal gold

moneralisation are gotten from the club reefs.

The Kawere group contains sandstones, quartzite, grit, breccias and conglomerate. The Kawere

conglomerate here is somehow useless because contains no quartz pebbles hence no gold

mineralization.

The Kawere is overlain by, and is in marked contrast with,the Banket Series which consists of

mature, clean, quartzite, grit, breccia and conglomeratecomposed in part of well sorted quartz

pebble conglomerate beds.

The Tarkwaian are directly on the Birimian basement lithologies. There is no mineralizations in

the birimian here.

Damang sit on an anticline and Damang Goldfields limited mine both hydrothermal and

paleoplacer golds.Placer gold refers to alluvial or eluvial deposit of gravel containing gold. As

REFERENCE

GEOL 400

such placer gold is gold that has weathered from the host rock where it was formed and been

placed either on hillside(eluvial placer), stream bed or alluvial fans by the action of water,

glaciers or other geological forces.

Hydrothermal deposit form from hot, metal-rich fluids that are left during the late stages of the

movement and cooling of magma. Smoky quartz veins are preferred veins for hydrothermal

mineralization

Features that help them find gold here are:

Intersecting structures of two contrasting orientations(high angle intersection or better)

Antiform closures

Late porphyry intrusions

Fault irregularity or jog

Evidence of mineralization(Galamsey)

Evidence of anomalism and soil geochemistry

Magmatic signatures

MINE GEOLOGY

They come in after exploration geologist are done with their work. They rely on the detailed

exploration work by the exploration department.

Damang is on Ashanti belt which strike North East- South West. Mineralisation in Damang occur

in the Banket series and upwards but there is no mineralization in the kawere group.

Intrusives such as dolerite sill was injected into the stratigraphy of the Tarkwaian followed by

subsequent anticline folding.

The mine geology department explore on the two types of mineralization which is

hydrothermal and paleoplacer golda. The pathfinder for gold here are pyrite and quartz veins.

The paleoplacer golds are hosted by various conglomeratic horizonsnin the Banket series. Gold

occur in the free grains associated with heavy minerals like hematite.

The various activities of the mine geology department are;

Grade controlled drilling at operational pits

Mining supervision of operational excavations

REFERENCE

GEOL 400

Geological mapping or ore markups

Optimizing ore extraction and minimizing ore dilution at all operational areas.

Monitoring, recording and reporting ore waste movement.

Undertake resource extensional drilling to delineate additional ore.

Integral of production meetings leading to achieving production targets.

Drill hole logging, sedimentological analyses leading to three dimensional modeling.

Hydrothermal is structurally controlled and grid controlled drilling whiles paleoplacer is strata

bound and chanelled sampling.

PLANT/PIT DEPARTMENT

Blasting of rocks is done here. They also locate waste and ore point. Their work involves three

main operations, namely

Drill and blast

Hold and haul

Mill

A picture of a pit at Goldfields Ghana Ltd, Damang is shown in fig 1.8

METALLURGY PROCESSING PLANT

This is the final stage of gold production and where gold is extracted from the solid rock and

processed.

The first stage here is to break down rocks into smaller particles by blending and crushing with

crushing circuit(primary crusher). The rock particles are further broken down with secondary

crusher which crushes rocks of 150mm to 40mm into smaller particles. These broken down

rocks are then stock piled in a storage area.

Grinding is the second stage of extraction where rocks are pulverized. Oversized trommel(more

than 80% of 60mm) is recycled by undergoing a further size reduction using the pebble crusher

machine. The pebble crusher products are then returned to CV3 by means of conveyor and fed

to the SAG mill.

REFERENCE

GEOL 400

Limestones are then added to the materials to convert them into basic medium since it is not

safe to work in the acidic medium. Cyanide which is a carrier of gold is then added to the

grinded material to leach out gold. The leaching or adsorption equation is:

4Au+8CN+O2+2H2O→ 4Au(CN)2+4OH

Carbon is then added to convert gold from the liquid phase to the solid phase. The carbon is

charcoal made from coconut husk and absorbs gold.

The waste after removing the gold is pumped into a tailing storage. Carbon absorb other

impurities apart from gold. Some of these impurities are magnesium and calcium. The carbon is

then soaked in hydrochloric acid which burns all other impurities and leaves the gold on the

carbon. The gold is then stripped from the carbon using caustic soda and heating the carbon to

125⁰C.

The next stage is the elution or desorption process which is done by the elution circuit. Here the

gold is reduced to solution and themselves to cathode. The gold is then stripped from from the

cathode and heated to liquid. The pure gold sit at the at the bottom since it is more dense with

glassy substance at the top. The glass is broken and the pure gold bar obtained. Damang

goldfields limited mine at a grade of 1.2g/t to 1.3g/t.

CHAPTER FIVE

ARTESINAL MINING (GALAMSEY)

Artesinal mining also known as Galamsey in Ghana refers to small scale gold mining operations

that mine and process ore with basic tools and equipments. One Galamsey operation facility at

Nsuaem in Tarkwa was visited by students.

REFERENCE

GEOL 400

Mining and Ore recovery

Artesinal mining involves no prior exploration. Most of these operators locate and develop

mining pits near already established mining companies. They dig narrow pits up to 100 meters

deep into the ground. From the narrow pits they scoop out rocks which they believe are rich in

ore and transport it to the surface using buckets tied with ropes. The rocks are then crushed

into smaller pieces using a hammer. The samples are then milled in corn mills (Fig.1.9) into finer

sizes. The milled samples are then washed with fresh water on an inclined wooden sluice lined

with a thick blanket (Fig. 2.0). Due to the specific gravity of gold, it is trapped with other heavy

materials onto the blanket.

The blanket containing the gold and other heavy minerals is then removed and washed in fresh

water contained in a basin. This process removes the gold along with other dense materials

into the basin of water. The sample is then panned in a circular piece of rubber cut from a car

tire tube to further remove sand. Mercury is then added to the dense residue and the mixture

is thoroughly mixed with the bare hands. This process traps the gold to form gold –mercury

amalgam. The amalgam (Fig. 2.1) is finally heated over fire to vaporize the mercury leaving the

gold.

The gold recovered is not purified but sold in that form to middlemen, individuals, goldsmiths

and small jewelry companies.

REFERENCE

GEOL 400

CHAPTER SIX

DISCUSSION

The rocks exposures observed at Butre, Cape Three Point and Axim are all metavolcanic rocks and

members of the Upper Birimian metavolcanics. All these metavolcanic rocks were previously andesitic

and basaltic lava now metamorphosed to greenstone. These rocks have a metamorphic grade in the

greenschist facies. The metavolcanic rocks overlie the lower Birimian metasedimentary rocks and both

form the supracrustal rocks of the West African Craton.

The rocks observed at Butre and Axim are all basalts which are strongly deformed and metamorphosed

to the greenschist facies. The pillow lavas observed in these basalts indicates that the basaltic magma

was injected into to subaqueous marine environment.

The porphyritic basalts observed at Cape Three Points possibly had two difference environments or

rates of cooling. There was an initial phase of slow cooling at depth during which the plagioclase

phenocrysts formed and a second phase of rapid cooling during which the groundmass formed as the

magma was moved to shallower crustal levels.

The metasedimentary rocks which were observed at in contact with the metabasalts at Butre and also as

subordinate outcrops at Axim are members of the Lower Birimian metasedimentary rocks

The granitoids observed at Dixcove represent the granitoids which were formed during the Eburnean

event. They are associated with and intrude the Birimian rocks. The mafic inclusions observed in the

granitoids are possibly xenocrysts. The xenocrysts were probably picked up from the walls of the magma

REFERENCE

GEOL 400

chamber or surrounding rocks as the magma upwelled. The granitoids are thus younger than the

xenocryst which appears mafic and is possibly basalt.

Stratigraphically the basalts at Butre are the lowest and form the base of the Upper Birimian

Metavolcanics and they are overlain by the porphyritic basalts of Cape Three Points which are then

overlain by the basalts observed at Axim. All the metavolcanics observed overlie the Lower Birimian

metasediments which form the base of the supracrustal rocks.

CHAPTER SEVEN

CONCLUSION

The trip was successful as every intended place was visited and every aim achieved. The various

outcrop that make up the Birimian exposures were intensively learnt. Students acquired the

ability to observe and identify various rocks, their textures, colour and mineralogical

composition under the Birimian exposure.

The various mining techniques used by Goldfields Ghana Limited, Damang and the impact of

their activities on the environment as well as post mining activities done to reinstate the natural

state of the environment were learnt by students.

REFERENCE

GEOL 400

FIGURES

Fig.1.0

Pyrite in quartz veinz QuartzShowing Pyrite in quartz veins

Fig. 1.1Contact between metasediment and metavolcanics

Metavolcanics Metasediment

Fig. 1.2 ShowingDixcove granitoidFig1.3 ShowingPillow basalt (Basalt of Butre)

Fig.1.4 ShowingBasalt with Vesicles (Basalt of butre)Fig 1.5 Showing Joints and foliations in Tarkwa Phyllite

REFERENCE

GEOL 400

Plagioclase and PhenocrystFig. 1.6 Showing Granitoids of Cape 3 points (Plagioclase Porphyry) Fig. 1.7 Showing Bouedin at mapped area

Fig. 1.8 Showing mining Pit of Goldfields Ghana Limited, Damang Fig. 1.9 Galamsey operator on the mill

Fig. 2.0 Washing the milled samples on the sluice box.

Fig. 2.1 Gold-Mercury amalgam in pan

REFERENCE

GEOL 400

REFERENCES

Compton, R.R. (1962) Manual of Field Geology, John Wiley & Sons, Inc,

New York, 378pp.

Kesse, G.O. (1985) The Mineral and Rock Resources of Ghana, A.A Balkema, Rotterdam, 660pp.

Lahee, F.H. (1941) Field Geology, Mc Graw-Hill Book Co. New York, 853pp.

www.google.com accessed on 27th March, 2012

Wikipedia encyclopedia accessed on 27th March, 2012

REFERENCE