Ministry of Mines and Energy

Ethiopian Institute of Geological Surveys

Regional Geology Department

GEOLOGY OF THE AGERE MARYAM AREA

Compiled by

NASIR HASSEN, TESFAYE YEMANE AND WOLDEGABRIEL GENZEBU

1991
Addis Ababa

CONTENTS
 Page

ABSTRACT
The Agere Maryam map sheet is covered by widely distributed

precambrian basement rocks of both stratified and intrusive

derivatives, Cenozoic volcanic sequences and Quaternary covers.

The extensively exposed stratified high grade gneisses of

broad composition exhibit early granitization and migmatization

at varying intensities. They generally display metamorphism of

principally mid-upper amphibolite facies, with pockets of

granulitic rocks in the western part. Weakly developed E-W

folding followed by a major and prominent sub-meridional folding

represent the early structural history of the rocks. The latter

is believed to be part of the Mozambique orogeny.

A narrow longitudinally trending Adola low grade belt,

mainly constituted by weakly metamorphosed volcanosedimentary

rocks, occurs in the eastern part of the map sheet. This belt is

tectonically bounded with the gneissic country rocks. These low

grade rocks were deformed N-S and they are presumably correlable

to similar Upper Proterozoic rocks of the Arabian-Nubian shield.

A broad range of plutonic rocks are exposed in various parts

of the map sheet. The emplacement of these rocks is attributed

to the different tectono-thermal events that prevailed in the

area during the Precambrian time.

 Both brittle and ductle tectonic activities occurred in the
map area during the Precambrian. Different sets of faults and

shear zones, of both sinistral and dextral sense, have been

recognized. The different sets of brittle faults were

reactivated during the Cenozoic, particularly in the western part

of the map area.

Cenozoic volcanic rocks appear widespread in the northern

and northwestern parts of the map sheet. They are mainly

basaltic flows, ignimbrites, tuffs and trachytes. Based on the

major tectonic event of rifting, the volcanic rocks are

subdivided into pre-rift, rift and post-rift volcanics. Volcanic

activity has been present in the region since mid-Eocene (45 m.y)

and important rifting took place during mid-Late miocene,

resulting in the formation of the Amaro horst, and the lake

Chamo, Segen and Gelana basins.

Quaternary sediments of eluvial, alluvial and

undifferentiated sediments, including lacustrine deposits, are

fairly clustered in the western part where Tertiary faulting and

subsequent rifting were significant.

The Lega Dembi primary gold deposit, and a number of placer

gold occurrences, are the most significant mineralizations in the

map area. Base metals like chromium, copper, titanium and

manganese, and showings of industrial minerals (silica, kaoline,

& garnet), including other non-metals (mainly talc, graphite,

mica & asbestos), are also known to occur in the area of

interest. Most of these mineralizations appear to be restricted
to the Adola low grade belt.

INTRODUCTION 1

1.1 General
Regional geological mapping of Agere Maryam map sheet (NB

37-10) was carried out by the Regional Geology Department of

the Ethiopian Institute of Geological Surveys (EIGS) from 1985 to

1989. The Adola gold fields lie in the eastern part of this map

sheet. The geology of this part of the map sheet, bounded by

longitudes 38° 36' and 39° 00' E and latitudes 5° 09' and 6° 00'

N, was taken from the 1:100,000 scale map accompanying the report

of Kozyrev et al.. (1985). The geological map of the area

bounded by longitudes 38° 35' and 38° 53' E and latitudes 5° 00'

and 5° 09' N was obtained from the Bulbul-Agere Maryam Mineral

Exploration Project (BAMEP) of EIGS. The rest of the map area

(about 14,000 sq km) was mapped by the department.

1.2 Location
The map sheet is bounded by longitudes 37° 30' and 39° 00' E

and latitudes 5 and 6 (Fig. 1). The western and southern parts of

Agere Maryam map sheet fall in North Omo and Borena

administrative regions, respectively. The northern, eastern and

southeastern parts of the map sheet are included in Sidamo

administrative region. The towns of Agere Maryam, in the center,
and Kibre Mengist, in the northeast, are the major ones in the

map sheet. They fall within Borena and Sidamo administrative

regions, respectively.

1.3 Access
The Addis Ababa-Awassa-Moyale asphalted road traverses the

central part of the map area, passing through Agere Maryam. In

the central part, dry weather roads motorable by four-wheel-drive

vehicles link Agere Maryam with Melka Soda, Gerba with Kercha and

Finchaa with Galeba. Finchaa village is connected with Melka

Soda by a rough road passing through Baya Gundi village which is

also linked with Agere Maryam.

An all-weather road connecting the town of Arba Minch with

Konso and Brinder villages serves as the most useful route to

approach the western part of the map area. Rough roads

negotiable by four-wheel-drive vehicle in this part of the area

include the Arba Minch-Debre Menena road, the Konso-Soyoma-Agere

Maryam road, the Yabello-Soyoma-Amaro Kele road, and the

Chelelektu-Amaro Kele road.

The towns of Kibre Mengist and Shakisso in the northeastern

part of the area can be reached by the all-weather road from

Awassa via Aleta Wendo. Dry weather roads that serve the eastern

part of the area include those connecting Shakisso and Dila

towns, the Shakisso-Kenticha-Dermi Dama road and the Shakisso-

Megado-Digati-Galeba road. Rough roads negotiable only by four-
wheel-drive vehicle connect Megado with Allona and Hayadima with

Dermi Dama. A twice weekly flight is operated by Ethiopian Air

Lines from the capital Addis Ababa to Shakisso during the dry

season.

Throughout the map area there is a good network of foot

trails connecting the various settlements and villages; these are

very useful for geological traversing.

1.4 Climate and vegetation

The northern region of the map area is characterized by a

relatively wet climate; here the rainy season lasts from April to

July. A little precipitation may occur, particularly in the

northeast, during the months of August to October, while the

region remains generally dry for the rest of the year. The

southern part of the map area has a semi-arid climate, the rainy

season falling between March and mid June. The southwestern part

of the map area is the most arid, with a little precipitation

occurring in April

and May.

Vegetation in the map area appears to vary with the climate.

In the northern region, where the climate is relatively wet,

vegetation is dense and deciduous trees dominate, making

geological traversing difficult. The southern region, however,
is largely characterized by scarce bushy trees, so that access

and rock exposure are good.

1.5 Physiography
The north-central part of the map area is a highland plateau

of mostly volcanic cover. Drainage is poorly developed, the

region mainly serving as a starting point for many south and

southeasterly draining rivers. In the rest of the map area,

three fairly dis- tinct physiographic regions can be identified:

the western, the central and the eastern regions ( Enclosure 1).

In the western region, the influence of the Main Ethiopian

Rift is pronounced. This area is characterized by narrow to

moderately broad submeridionally trending ridges and valleys. It

also exhibits a marked contrast in elevation, the highest point

being 3232 m above sea level on top of the Amaro horst, and the

lowest about 850 m in the Segen valley. The Amaro horst, that

rises majestically above its surroundings, is an uplifted block

that extends from the northern edge of the sheet southwards to

the Segen river, gradually decreasing in elevation. To the west

it is bounded by the Chamo and Segen grabens, to the east by the

Gelana graben. The main perennial stream is the Gelana river that

drains to the north. The southwesterly draining Segen river and

many other small streams in this area are seasonal.

 The central region is a broad relatively elevated block that
has a longitudinal trend and exhibits a gentle southerly slope.

Most of the streams in this area are intermittent and drain

south. Their courses are basically controlled by the Precambrian

fracture system in the region.

The eastern region consists of a gently southeasterly

sloping block on the west, while its eastern part is dominated by

rugged topography with narrow ridges and valleys of submeridional

trend. This region is highly dissected by perennial rivers,

among which the Awata, Kojoa, Mormora, Dawa and Aflata are

notable. These rivers are largely controlled by the Precambrian

structure of the area, and generally flow south or southeasterly.

1.6 Population
In the map area the population density is low. The

inhabitants are mainly of the Gujee, Gedeo, Burji, Konso, Gerba,

Borena and Koyra ethnic groups. The Oromo language is widely

spoken, being the language of the Gujee, Borena and Gerba tribes.

The Gujees are broadly distributed throughout the map area,

exercising a semi-nomadic mode of life. The Gedeo, Konso, Burji

and Koyra basically depend on farming; except for the Gedeo who

inhabit the northern highlands, the others are confined to the

western part of the map area. A nomadic mode of life is

practiced by the Borena and Gerba tribes, who inhabit the south-

central and northwestern parts of the map area, respectively.

1.7 Previous work
Parts of the map area and of southern Ethiopia have been

studied by various workers. Among the earliest geological

studies is that of Lebling (1940), who described the various

gneisses and granites that occur along the Yabello-Negele road

south of the map sheet.

Jelenc (1966) described the geology of the eastern part of

the map area and the exposures along the Agere Maryam-Yabello

road. In the former case he indicated the occurrence of N-S

trending low grade "green rocks" overlying the supposedly older

"Gariboro Series" of rocks; a possible unconformable relationship

between the two was suggested.

With the exception of the northwestern part, Agere Maryam

map sheet was mapped by Hunting Geology and Geophysics Limited

(1969) during a photogeological survey. A photogeological map at

1:100,000 scale and an accompanying report were prepared for the

area of interest. In this work two groups of metasediments

separated by an unconformity were recognized. Furthermore two

folding events, an earlier E-W one and a later N-S one, were

identified. The occurrence of nickel and gold mineralization was

also noted.

Mohr (1962) briefly discussed the geology of parts of the

Agere Maryam area in his book "Geology of Ethiopia". In the

second edition of the same book (Mohr, 1975) he attempted to

broadly classify the Precambrian rocks of southern Ethiopia into
three zones, and he indicated the occurrence of rocks of the

older two zones in this map sheet.

Regional geological studies were conducted in the eastern

part of Agere Maryam map sheet by Gilboy (1970) and Chater (1971)

for their respective Ph.D. theses. Systematic geologic mapping

at 1:100,000 scale, structural and metamorphic studies and some

geo- chronological work were carried out. On the basis mainly of

lithologic characteristics, they classified the rocks in the area

into the Lower, Middle and Upper Groups. Chater (1971) assumed

that all three groups of rocks equally underwent the various

tectonothermal events being buried at different levels in the

crust but having no break between them. Gilboy (1970) suggested

a major unconformity between the Middle and Upper Groups.

Kazmin (1972) in his work "The Geology of Ethiopia"

distinguished three complexes in the Precambrian rocks of the

country: the Lower, the Middle and the Upper Complexes. He

believed that these complexes represent a stratigraphic sequence

and that they are separated from each other by marked

unconformities. According to him rocks of all three complexes

occur in the map area.

In the western part of the map sheet, Levitte et al.. (1974)

carried out a reconnaissance survey of the Amaro horst. They

suggested that normal faulting had raised the horst to

topographic elevations higher than the plateau outside the rift.

The evolution of the Chencha escarpment and the Ganjuli graben,
north west of the Amaro horst, were studied by Zanettin et al..

(1978). Five major tectonostratigraphic zones

were identified in the Precambrian rocks of Ethiopia by Chewaka

and Dewit (1981). According to them the map area falls in Zone

4 where rifting of the continental crust, represented by the

Lower Complex rocks, was followed by basic volcanism and clastic

Sedimentation constituting the Middle Complex. They took the

Adola Group of rocks to represent an ophiolitic sequence,

indicating sea floor spreading and format- ion of oceanic crust.

Potential types of mineralization in the eastern part of the map

sheets were also predicted by these workers.

Davidson (1983) compiled a report and geological map at

1:500,000 scale of the Omo region west of the map area. He des-

cribed the lithology, structure and metamorphism of the

crystalline basement rocks and also the broad stratigraphy of the

volcanic rocks and the rift structures in the Omo area.

Systematic regional mapping and mineral exploration work was

conducted by the Adola Gold Exploration Project of the Ethiopian

Mineral Resources Development Corporation in the eastern part of

the map area from 1979 to 1981. A geological map at 1:100,000

Scale and an accompanying report were produced (Kozyrev et al..

1985). A number of primary and placer gold targets, as well as

other mineral occurrences were identified. According to this

work two groups of Precambrian rocks, the Middle and Upper

Complexes separated by a structural and metamorphic break, are
exposed in the area.

Recent studies have been conducted in the eastern part of

Agere Maryam map sheet by the Training for Mineral Exploration

Project of the Institute. Beraki et al.. (1989) and others (in

unpublished reports) appeared to be in favor of the idea that the

ultramafic rocks of the Adola low grade belt are related to an

ophiolitic sequence. They also proposed that the associated

metavolcanic and metasedimentary rocks represent an immature

island arc. Five deformational events (including four folding

events) and two major prograde metamorphic episodes were

identified by these workers in this part of the Agere Maryam

area.

The Bulbul-Agere Maryam Mineral Exploration Project recently

conducted exploration work in the central and southeastern parts

of the Agere Maryam area; this work is still proceeding.

1.8 Method of mapping

Prior to field work aerial photographs of about 1:60,000

scale were studied, photogeological interpretation carried out,

and 1:50,000 scale photogeological maps produced. Traverse

routes were planned on the basis of these maps.

Field mapping and data collection were conducted using the

aerial photographs. Field observations were indicated directly

on the aerial photographs. The data were then transferred onto

1:50,000 topographic maps and field geological maps prepared for
each subsheet (15' by 15'). Representative rock samples from

each map unit were collected during the course of the field work,

later being thin-sectioned and petrographically studied. Final

geological maps at 1:50,000 scale and the accompanying reports

were prepared. These maps were then reduced to 1:250,000 scale.

Geological field mapping in Agere Maryam sheet was conducted

by members of the Regional Geology Department as follows (see Fig

1 for subsheet layout):

a) subsheets A, B and western half of C by Tesfaye Yemane and

Tadesse Yehunie

b) subsheets D, eastern half of C and western half of E by

Tadesse Alemu, Nasir Hassen, Kiros Mehari, and Mandefro

Belayneh.

c) subsheets G and H by Gebriel Tamiru

d) subsheets J and K by Getahun Seyid, Nasir Hassen and Melaku

Tesfaye

e) subsheets N and O by Getahun Seyid and Alula Habtegiorgis

f) subsheets P and Q by Samuel Gichile, Kiros Mehari, Solomon

Berhane and Tadesse Alemu

g) subsheet X and western halves of L and R by Woldegabriel

Genzebu and Gabriel Tamiru

h) subsheets T and U by Tadesse Alemu

i) subsheets V and W by Kiros Mehari and Kinetibeb Yifa.

 The following members of the department also participated in
the field work: Amenti Abraham, Ashebir Woldegiorgis, Berhanu

Wondaferew and R. K. Kiessling.

Final checking of the geological map in the field and compi-

lation of the final map and report was carried out as follows:

a) subsheets A, B, C, D, G, H, J, N and T by Tesfaye Yemane

b) subsheets K, O, P, Q, U, V, W and southern half of X by

Woldegabriel Genzebu
c) subsheets E, F, L, M, R, S, Y and northern half of X by Nasir

Hassen

2 REGIONAL GEOLOGY

The Mozambique belt, first named by Holmes (1951), consists

of high grade gneisses and migmatites with infolded schists,

marbles and amphibolites intruded by granites and pegmatites

(Almond, 1983). This orogenic belt extends from Mozambique in
the south to Ethiopa in the north (Cahen et al. 1984; Berhe

1990). It is characterized by a meridional trend with a

similarly oriented structural fabric and generally consists of

rocks having ages between 1300 and 480 Ma (Cahen et al. 1984).

In east and northeast Africa important tectonothermal events

occurred between 950 and 900 Ma.

The highly metamorphosed and deformed gneissic rocks of

southern Ethiopia are thought to be part of the Mozambique belt

(Gilboy 1970; Chater 1971; Kazmin 1978a). These rocks,

presumably of middle-Proterozoic age, make up about 70% of the

Agere Maryamarea. The metamorphic grade generally falls between

middle and upper amphibolite facies, reaching granulite facies

along the western margin of the map area and further west

(Davidson, 1983).

Greenschist facies sedimentary rocks, mafic-ultramafic comp-

lexes and intrusives occur extensively in Saudi Arabia. Similar

rocks of island arc affinity have been observed in the Eastern

Desert of Egypt and the Red Sea Hills of Sudan (Kroner, 1985).

These occurrences in Saudi Arabia and northeast Africa,

comprising the Arabian-Nubian Shield, are generally believed to

be of Pan-African age, i.e. 1100-500 Ma (Kroner 1979; Gass 1981).

 The mafic-ultramafic rocks of the Arabian-Nubian Shield are
believed to continue southwards into Ethiopia and Kenya (Berhe

and Rothery 1986; Kazmin et al. 1978; Kazmin 1978a). In southern

Ethiopia they are partly represented by the metavolcanic and

meta- sedimentary rocks and the ultramafic of the Adola belt.

These rocks, exposed in the eastern part of the map area and

constituting about 5% of it, are the possible northward

continuation of the Moyale low grade belt (Berhe and Rothery,

1986). Similar rocks are exposed in western and northern

Ethiopia (Davidson 1983; Kazmin 1972).

Little is known as to the relationship between the

Mozambique belt and the Arabian-Nubian Shield. Almond (1983)

suggested that a major time break existed between stabilization

of the Mozambique belt and onset of sedimentation and volcanism

in the Shield.

Volcanic rocks of Cenozoic age are widespread in various

parts of Ethiopia. Those occurring in the map area are

associated with the Main Ethiopian Rift and the Southern Plateau
flanking it to the east. They cover about 15% of the area.

3 CRYSTALLINE BASEMENT ROCKS

3.1 Introduction
The crystalline basement rocks of the Agere Maryam region

have a wide range of compositions, varying from melanocratic

amphibolite to leucocratic quartzofeldspathic gneiss.

Migmatization, pegmatization and granitization of these rocks

are highly variable. The highly metamorphosed stratified rocks

of volcanic and sedimentary origin that crop out in the map area

exhibit structural fabrics due to various deformational episodes

with varying clarity. Because of the complex geology, the

stratigraphic relationships between these high grade rocks could

not be established with certainty at the scale of the present

study. Where geologic and structural data were obtained the

relative stratigraphic positions of the rock units have been

suggested. The order of description of the stratified high grade

rocks in this chapter does not necessarily imply a stratigraphic

sequence. For the low grade rocks, in which primary features are

preserved, the geological and structural evidence has been found

adequate to establish their relative ages with fair confidence.

The relative ages of groups of similar metaintrusives in the

map area is deduced on the basis of their metamorphic grade,

degree of migmatization and state of deformation. Absolute age

data available for a few plutons have also been utilized in
establishing the stratigraphy.

A number of rocks from the map sheet have been

radiometrically dated for the Institute at the University of

British Columbia, Canada by Teklewold Ayalew and R.L. Armstrong

using the U/Pb method on zircons. All absolute ages for basement

rocks mentioned in the text refer to this work unless otherwise

specified.

Igneous and metamorphic nomenclature in this report follow

Streckeisen (1975 and 1979) and Winkler (1979) respectively.

3.2 Stratified high grade rocks

3.2.1 Hornblende-hypersthene-quartz-diopside-labradorite

granulite

(Pdfg)

Outcrops of this granulite are well exposed in the south-

western part of the map area at the Segen bridge and south of

Mount Germikelo. This unit is an extension of the granulite

facies rocks of the Omo River Project area (Davidson, 1983), also

referred to as the Konso gneiss by Kazmin (1972).

The granulite is medium to coarse grained, mesocratic to

melanocratic, weakly to well foliated, slightly migmatized and

intimately interlayered in various ratios with mafic hornblende

gneiss. Light pink, slightly foliated quartzofeldspathic gneiss

and calc-silicate gneiss are present as thin layers locally,
particularly along the Brinder-Teltele road and south of Mount

Germikelo.

In thin section the granulite is composed of 50%

labradorite, 20% diopside, 15% quartz, 8% hypersthene, and

7% hornblende. Both Hypersthene and hornblende may appear as the

dominant mafic mineral, whereas garnet, kyanite and augite are

present in small amounts locally. Magnetite and apatite may

occur as traces. At places, gneissosity is defined by

melanocratic bands consisting of prismatic hornblende crystals

together with diopside.

3.2.2 Hornblende-biotite-quartz-feldspar gneiss intercalated with

biotite-quartz-feldspar gneiss (Phbg)

This unit underlies the western part of the map sheet

extending north-south along the Amaro horst. It is best

exposed along the Segen river and along small streams draining

the Amaro horst. In general this unit is heterogeneous and

migmatitic, migmatization being more pronounced in its western

and southern parts. It has a gradational contact with unit Pmfg

and is intruded by unit Pghg.

The hornblende-biotite gneiss is the predominant rock type

in this map unit. It is medium to fine grained, dark grey, well

banded and commonly migmatitic. The intensity of migmatization
varies from place to place. Biotite generally is more abundant

than hornblende.

Thin sections show the typical rock to be composed of 35%

plagioclase, 25% quartz, 20% microcline, 10% biotite, 5%

hornblende, and minorgarnet and magnetite. Textures are

dominantly lepidoblastic, sometimes granoblastic.

Intercalations of quartzofeldspathic gneiss occur with the

main rock type mentioned above. This rock is in general pinkish

grey to light grey, fine grained and weakly gneissose. The rock

consists of 35% microcline, 30% plagioclase, 30% quartz, 3-5%

biotite and minor magnetite.

This map unit has been referred to as the Burji gneiss of

the Lower Complex by Kazmin (1972).

3.2.3 Magnetite-quartz-feldspar gneiss (Pmfg)

This unit occurs in two large areas covering over 1500 sq

km. Various rock types were grouped under this unit, as it was

difficult to subdivide into separate mappable units. On its

western margin the unit appears to have a gradational contact

with unit Phbg. To the east it has a sharp structural contact

with unit Pmcg. The main rock type making up this unit is

composed predominantly of quartz, feldspar and magnetite with

little or no biotite. Outcrops around Finchaa village are

generally medium grained, greyish white, leucocratic and rich in

magnetite. Thin sections of the gneiss in this area show a
xenoblastic, fine to medium grained rock exhibiting strong

alignment of minerals. Microcline constitutes 60%, quartz 20%,

plagioclase 10%, biotite 3% and magnetite upto 5%of the rock.

Muscovite or sillimanite occur in minor amounts.

In the Soyoma area this unit is generally ridge forming.

Here the gneiss is medium to coarse grained, pinkish grey and

composed predominantly of pink feldspar. Its composition is

similar to that of Finchaa, consisting of 50% microcline, 20%

quartz, 15% plagioclase, 5% biotite, 5% magnetite and 3%

muscovite. Magnetite is present either as disseminated clots or

forming thin bands parallel to the foliation.

Sillimanite bearing biotite-quartz-plagioclase gneiss and

layers of biotite and amphibole gneisses occur as subordinate

intercalations in unit Pmfg. Small lenticular bodies of granite

are also encountered.

A leucocratic gneiss with biotite as the main mafic mineral,

the remaining minerals being plagioclase, quartz and a little

magnetite, occurs on the southwestern margin of unit Pmfg in

subsheet P. Although it can be mapped separately, it was found

close to unit Pmfg in its overall features, and so has been

grouped under this unit.

Gneisses of the same composition as those of unit Pmfg are

also exposed closely interlayered with other rock units in the

Bari dome. Here they form part of the layered gneisses making up
the dome, being cut by pegmatitic dykes and small bodies of
granite.

Unit Pmfg is regarded as of sedimentary origin. Its hetero-

geneity and the magnetite banding parallel to the layering

support this idea. This unit was regarded as part of the Yabello

gneiss of the Middle Complex by Kazmin (1970).

3.2.4 Hornblende-quartz-feldspar gneiss (Phqg)

This unit is one of the main components of the elliptical

Bari dome in subsheet U, consisting of thin layers of biotite-

hornblende and hornblende gneisses. Having an areal coverage of

not more than 10 sq km, it forms prominent resistant ridges in

the outer part of this feature.

In hand specimen the predominant rock type is a light grey

to pinkish grey, medium grained rock with fine magnetite rich

bands. Thin sections show the gneiss to be granoblastic

inequigranular and composed of 30% quartz, 30% microcline, 20%

oligoclase, 10% hornblende, 5% magnetite and minor biotite and

sphene. Hornblende has partially altered to sphene. The rock also

shows perthitic texture.

3.2.5 Biotite hornblende-quartz-feldspar gneiss (Pmqg)

Unit Pmqg occurs in limited areas at Berka and Bari, where

it makes up part of the elliptical bodies at these places. Its

extent is not more than 30 sq km. It contains more biotite than
unit Phqg, to which it is similar.

This rock type is poorly gneissose, medium grained and light

grey.

A thin section of a sample from to this unit showed 30%

quartz, 25% K-feldspar, 20% oligoclase, 15% hornblende, 5%

biotite and minor garnet and magnetite. Myrmekite is common.

The texture is inequigranular granoblastic.

3.2.6 Interlayered quartz-feldspar gneiss and hornblende gneiss

(Pbag)

This unit also occurs in the Bari dome, underlying its upper

part. Two rock typesare present, biotite bearing quartz-

microcline gneiss and hornblende gneiss. The former is pinkish

grey, fine grained and poorly gneissose, and is the main rock

type making up unit Pbag. The hornblende gneiss, which is

subordinate, is a darkgrey, medium grained, gneissose rock. Minor

biotte gneiss is associated with it.

3.2.7 Metaquartzite (Pmqz)

This unit occur as narrow, elongate ridges trending NNW in

subsheets P and W.
 The metaquartzite is fine to medium grained, brown to pink
to light grey, ferruginous and typically banded. The banding is

defined by layers of differing grain size and by variable iron

exide content. The rock is composed predominantly of quartz,

with a little magnetite and garnet.

This unit is associated with ultramafic rocks which occur as

small elongate bodies within and adjacent to the quartzite. Its

narrow elongate shape and its association with ultramafic rocks

has been taken to point to its probable tectonic origin along

fault zones.

3.2.8 Oligoclase-hornblende-biotite-quartz gneiss, calc-silicate,

biotite and biotite-hornblende gneisses (Pfbg)

Rocks belonging to this unit are found exposed over a

limited area east of Finchaa village. This unit is flanked to

east and west by units Pmcg and Pmfg respectively.

Oligoclase-hornblende-biotite-quartz gneiss is the pre-

dominant rock type in the unit. It is fine to medium grained,

mesocratic and layered. Layers of calc-silicate, biotite and

biotite-hornblende gneisses occur interlayered. There are a few

small occurrences of sillimanite-muscovite, muscovite-feldspar-

quartz and garnet bearing schists, the latter being closely

associated with fine grained amphibolite and amphibole-biotite
schist.

The occurrence of calc-silicates and sillimanite bearing

gneiss together with the well developed layering exhibited by the

rocks indicate the sedimentary origin of the unit. The same rock

types occur within unit Pmcg, suggesting its sedimentary origin

as well.

3.2.9 Augen biotite-quartz-feldspar gneiss, strongly granitized

(Pmcg)

Unit Pmcg forms large north-south running belt that widens

southwards east of Finchaa. The Rocks of this unit are exposed

as discontinuous ridge forming bodies of granitoid rocks, and as

well developed gneisses outcropping mainly along river beds. The

eastern contact of the unit is marked by a shear zone. The unit

seems to occupy the core of an antiform.

Unit Pmcg is comprised of biotite-oligoclase-quartz-

microcline gneiss of granitic appearance, hornblende-biotite and

biotite gneisses. Intense pegmatization and migmatization of the

gneisses is apparent. The quartz-microcline gneiss is a

leucocratic, light grey to pink rock made up of pink feldspars

which are commonly augen, plagioclase, quartz, biotite and

magnetite with or without hornblende. It is gneissose to weakly

foliated, and resembles an intrusive plutonic rock. The

gneissosity seems to be more developed towards the margins of the
unit.

In thin section, this gneiss is granolepidoblastic to grano-

blastic in texture, having an average composition of 40% micro-

cline, 25% quartz, 20% oligoclase, 10% biotite, 1% opaque

minerals and 1% sphene with trace amounts of hornblende and

zircon.

Although the unit widens southwards, there is a scarcity of

outcrops in this area. Hornblende-biotite gneiss here outcrops

widely, intercalated mostly with quartzofeldspathic and biotite

gneisses. This gneiss is a medium to fine grained, well layered,

dark grey rock consisting of plagioclase, quartz, hornblende and

biotite. Calc-silicate rocks occur as small lenses. A layer of

light greyish green anthophyllite fels has been reported.

The heterogeneity of this unit, and the composition of the

various rock types suggest a sedimentary origin.

3.2.10 Biotite-quartz-feldspar gneiss, strongly migmatized (Pbfg)

This unit is exposed in the northeastern part of the map

sheet, covering an area of about 50 sq km. It is by and large

homogeneous, with rare interbeds of biotite-hornblende and

quartzofeldspathic gneisses.

The main rock type is medium to coarse grained, mesocratic,

and shows well pronounced compositional banding defined by

alternating layers of dark and light minerals. The effects of
strong migmatization are characteristically exhibited. The

migmatites are thinly laminated, showing stromatic structure.

Thin sections show lepidogranoblastic texture, the constituent

minerals being 40-45% microcline, 25-30% quartz, 20-25%

oligoclase and 5-15% biotite. Apatite and sphene are

subordinate, there being traces of chlorite.

3.2.11 Oligoclase-quartz-microcline gneiss (Pqfg)

This map unit was previously named the Wadera Group by

Kazmin (1972). Kozyrev et al.. (1985) called it the Zembaba

Formation and believed it to be correlative with the Yabello

gneiss of Kazmin (1972). It is widely distributed in the eastern

part of the map area. In the southeastern part it occupies the

limbs of the Gariboro and Burjiji antiforms. Based on structural

relationships this unit is thought to be older than units Pfqg

and Pbmg (see also Kozyrev et al.. 1985). It is by and large

homogeneous, with rare, thin interbeds of quartzite, amphibolite

and muscovite or biotite bearing schists and gneisses.

The dominant oligoclase-quartz-microcline gneiss is a

leucocratic, fine to medium grained rock that is mostly friable.

A coarse grained hard variety which often forms ridges is not

uncommon. Foliation is more pronounced in the fine grained type,

the coarse grained type generally appearing massive.

 Petrographically the rock is granoblastic, often with
inequigranular texture and poorly discernible schistosity defined

by biotite crystals. Compositionally it consists of 45-55%

microcline, 20-30% quartz, and 10-20% oligoclase. Magnetite is

characteristically present (upto 6%). Muscovite and biotite may

also occur (upto 4% each).

3.2.12 Biotite-quartz-oligoclase gneiss, medium grained

amphibolite and minor oligoclase-quartz-microcline gneiss (Pfqg)

Widely distributed in the eastern part of the map area, this

unit was termed the Aflata Formation by Kozyrev et al.. (1985).

It is presumably older than unit Pbmg, which occupies the limbs

of major F2 antiforms whose cores are formed by unit Pfqg (see

also Kozyrev et al.. 1985). At places the unit grades to

biotite-hornblende gneiss containing upto 10% hornblende. Inter-

calated are bands of medium grained amphibolite and quartzofeld-

spathic gneiss, as well as rare mica schist, marble, graphitic

schist and kyanite-staurolite bearing gneiss.

The dominant rock type, biotite-quartz-oligoclase gneiss, is

leucocratic and mostly fine grained, with a subordinate coarse

grained variety. It is often schistose, but also exhibits large

scale compositional layering. Some migmatization is indicated by

moderately developed thin lenticular banding. This rock type

displays a lepidogranoblastic texture, at places being inequi-

igranular. 50-70% Oligoclase, 20-35% quartz and 10-20% biotite

are the major constituents. Muscovite and microcline may occur
(upto 5% each), with minor apatite, opaque minerals and epidote.

Some sections display syntectonic porphyro- blasts of garnet with

poikiloblastic texture.

The second most important rock type in unit Pfqg is fine to

medium grained amphibolite which occurs mainly as bands. It is a

melanocratic and often schistose rock exhibiting well developed

mineral lineation defined by hornblende crystals. In thin

section it shows nematogranoblastic texture and consists of upto

55% hornblende, 25-30% andesine/oligoclase, and 10-15% quartz.

Upto 10% Garnet may also occur, in some cases exhibiting

poikiloblastic and porphyroblastic snowball textures with upto

540 degrees of rotation. Minor epidote, sphene and opaque

minerals are present.

The third main rock type is a medium grained, leucocratic

rock with ill defined gneissosity. It consists of 50%

microcline, 25-30% quartz, upto 15% oligoclase, and minor biotite

and magnetite. It is generally equigranular, but may also

exhibit porphyroblastic feldspars.

3.2.13 Biotite-microcline-quartz gneiss, medium grained

amphibolite, garnet-staurolite gneiss, graphitic schist and

marble (Pbmg)
 This map unit is exposed in the eastern part of the map
sheet and was termed the Kenticha Formation by Kozyrev et al..

(1985). It generally forms submeridionally trending belts

occupying the limbs of major F2 antiforms. Unlike unit Pfqg, the

interlayered graphitic schist and marble are a significant

component. The amphi- bolite horizons are characteristically

similar in both map units.

The dominant rock type, biotite-microcline-quartz gneiss, is

a medium grained, leucocratic rock with a mostly schistose, but

at places banded, fabric. It is generally hard and ridge

forming. The effects of migmatization and granitization are by

and large poorly discernible. This rock type consists of 50%

quartz, upto 25% microcline and up to 10% biotite, with lesser

muscovite and oligoclase. Porphyroblastic varieties contain

augen of either quartz or feldspar or both, the rock otherwise

being generally equigranular.

Frequent horizons of garnet-staurolite gneiss occur in unit

Pbmg. This gneiss is a medium grained, leucocratic rock with

well developed compositional banding. The rock is composed of

45% feldspar (mostly oligoclase), 25% quartz, 10-20% biotite,

upto 13% staurolite and 5-8% garnet. Lesser minerals include

opaques, muscovite, apatite and chlorite. The staurolite

porphyroblasts exhibit post-deformational (D2) crystallization

and are poikilo- blastic.

 The graphitic schist is dark grey to black and invariably
fine grained, with a laminated, schistose fabric. Graphite rich

laminae 0.2 mm thick alternate with chlorite and sericite rich

ones. Quartz and feldspar make up 50 to 60% of the rock. 15-20%

Graphite, 10-30% sericite and upto 10% chlorite are present.

Graphite mainly occurs as idiomorphic grains in lenticular pods

parallel to schistosity.

The marble intercalated with the above rock types is a

generally massive rock, but with ill defined schistosity at

places. It is mostly white, in some cases with a brownish tint.

Where it contains graphitic admixtures, it exhibits a mottled

appearance. In thin section it is granoblastic with a laminated

fabric due to alternating fine and medium grained bands. It

consists of 80-95% carbonates (mainly calcite), upto 10% quartz

and minor muscovite, graphite, feldspar and chlorite.

3.3 Stratified low grade rocks

3.3.1 General

Low grade metavolcanic and metasedimentary rocks are exposed

in the eastern part of the map area, forming the submeridionally

trending Adola low grade belt, 5-10 km wide and over 100 km long.

These low grade rocks are tightly folded and appear to have

tectonic contacts with the adjacent gneisses. Kazmin (1972)

suggested a thrust plane between these rocks and the gneisses.

The rocks forming this belt are confined to a large scale F4 fold
(the Megado syncline); metavolcanics occupy the outer limbs of
this syncline, while the core is dominated by metasediments. In

general the metasediments show primary sedimentary structures.

Unit Pcas and Ppss were together termed the Adola Group by Kazmin

(1972) and also by Kozyrev et al. (1985).

3.3.2 Fine to medium grained amphibolite and plagioclase-

chlorite-actinolite schist (Pcas)
This map unit is exposed on the outer limbs of the Megado

syncline. Greenschists with minor amphibolites are principally

represented on the eastern limb, amphibolites on the western

limb. Repeated linear horizons of this unit are observed,

possibly due to tight folding (F4). Intercalated rock types are

minor graphitic metaquartzite, phyllite and quartz-mica schist.

The amphibolite is a light to dark grey, generally fine

grained and fissile rock, exposed mainly in the hilly Megado

region. The medium to coarse grained variety that occurs at

places is characteristically massive and hard. The rock is

composed of upto 70% amphibole, both actinolite and hornblende,

upto 35% plagioclase, 5-10% chlorite and 5-8% quartz.

Clinozoisite may reach upto 30% in a few cases. The texture is

mostly nematogranoblastic; porphyroblastic texture is rarely

displayed by the coarse grained variety.

It has been established that the amphibolites (metamorphosed

lavas) are widespread in the Adola area and occupy the base of
the low grade belt (Kozyrev et al. 1985). Chater (1971)
described exposures of small elongate bodies of weakly cleaved

amphibolite in the Chochobo river which, according to him,

represent deformed pillow structures. If so, this suggests that

some of these rocks were laid down in a subaqueous environment.

Based on the outcrop pattern the low grade rocks display, it is

envisaged that the major axis of the depositional basin was

parallel to the low grade belt.

The second major rock type, plagioclase-chlorite-actinolite

schist, is a dark greenish grey to black rock, mostly fine

grained and with a strongly schistose fabric. It is mainly

exposed on the eastern limb of the Megado syncline.

Petrographically it displays fibroblastic, nematogranoblastic or

porphyroblastic textures. Micro-bedding is also exhibited by

disrupted laminae 2-3 mm thick, mainly of actinolite. The rock is composed of 40-80% actinolite, upto 20%

chlorite, 20-25% quartz and feldspar, 5-10% magnetite and upto 5% carbonate in a few cases.

3.3.3 Phyllite, metasiltstone and metasandstone (Ppss)

These metasedimentary rocks make up the core of the Adola

low grade belt. They are absent at its northern and southern

ends. The metasandstone is similar to that described under unit

Pmsc, where it is more significant.

 The phyllite is grey to black, fine grained and fissile,
with poorly developed foliation. Bedding is defined by laminae

enriched in graphite that are oriented transverse to the

schistosity. The graphite imparts the dark color, giving rise in

places to impure graphitic schist. Schistosity is fairly

pronounced in graphitic schist, and in associated sericite and

biotite schists. Primary sedimentary structures such as current

ripple marks, graded bedding and slump structures are well

preserved.

Thin sections show the phyllite to consist of 55-60% quartz

and feldspar, 15-30% sericite, 10-12% graphite and 5-15% biotite.

The rock contains porphyroblasts, mainly rounded or elongate

biotites about 0.5 cm in diameter on average, the groundmass

being lepidogranoblastic.

The rather weakly fissile metasiltstone is dark grey, fine

grained and shows well preserved primary sedimentary bedding.

Thin sections exhibit relict detrital fragments of quartz and

feldspars in a finer matrix. Quartz and feldspar together make up

70-75% of the rock, biotite upto 15%, sericite 10-15%, magnetite

2-5%, traces of zircon being present.

3.3.4 Metasandstone and metaconglomerate (Pmsc)

The metasandstone and metaconglomerate are closely

associated, mostly being intercalated and at places showing a

gradational relationship. These rocks are of restricted

occurrence, being found mainly in the central part of the Adola
low grade belt and often forming ridges. They were termed the

Kajimiti Beds by Kazmin (1972) and Kozyrev et al. (1985).

Metasandstone occurs as a light to dark grey, medium

grained, hard rock with weak foliation. Schistosity is more

pronounced in mica rich varieties. The rock may be massive or

flaggy, bedding being sharply defined by alternation of laminae

differentiated by grain size of biotite. Cross-bedding and

graded bedding are also well exhibited (for example about 0.5 km

north of Hayadima village). Based on the displayed primary

sedimentary structures, Kozyrev et al. (1985) suggested deltaic

deposition of this rock type.

In thin section quartz porphyroblasts are common. 35-45%

Quartz, 15-20% biotite, and upto 10% feldspar are the major

constituents. The matrix is chiefly a fine grained aggregate of

biotite with minor calcite, muscovite, epidote and opaque

minerals.

The rather ill-sorted and poorly foliated metaconglomerate

occurs as horizons within the metasandstone. The clasts are

mostly equant or elliptical pebbles; in some cases cobbles and

even boulders have been observed. The long axes of the

elliptical clasts appear mostly concordant with the foliation in

the matrix. These commonly rounded or subrounded clasts are

predominantly vein quartz; gneisses of various composition,

amphibolite and meta- quartzite occur at places in varying

proportions. The matrix, principally composed of sandy material
that has similar constitu-

ents to the metasandstone, makes up 60-80% of the whole rock by

volume.

3.4 Intrusive rocks

3.4.1 Gneissose hornblende-biotite metagranite (Pghg)

Rocks belonging to this unit occupy a large area west of

Agere Maryam town. The general trend of the rock body is north-

south, although exposures of the granite are discontinuous and

patchy. It is intruded by unit Phhg and may possibly represent

the granitized lower part of unit Pmfg.

The granite is leucocratic, fine to medium grained and well

foliated to gneissose, with thin bands of light and dark

minerals. It has a color index of 20. Magnetite is common,

followed by biotite, being relatively rare. The unit is rich in

hornblende towards the south-central part of subsheet P.

Thin sections of rock samples belonging to this unit show a

granoblastic texture and an average composition of 40-60% micro-

perthitic microcline, 10-40% oligoclase, 15-30% quartz, 5%

magnetite, 4% biotite and 2% sphene with traces of zircon. upto

8% Hornblende occurs at places. Garnet and clinopyroxene also

occur at places in minor amounts.

3.4.2 Foliated biotite metagranite (Pgbg)

 This unit is exposed in the southeast corner of the map
area, forming ridges arranged en echelon that rise majestically

above the surrounding area and that core major F2 antiforms.

The granite is generally coarse grained, leucocratic with

colour index upto 10 and homogeneous. It is foliated, at places

gneissose. Minor migmatization is apparent. Intense pegmatite

veining occurs mainly along the western margin of the Gariboro

mass. On aerial photographs a northwesterly trending structural

grain, presumably a fracture system, is also very well displayed

by this mass.

The rock exhibits hypidiomorphic granular to granoblastic

textures in thin section. It contains upto 50% microcline, 20-

25% oligoclase, 20% quartz, 5-8% biotite and accessory opaque

minerals and epidote, with traces of apatite.

This unit appears to intrude the gneissic country rocks even

though its contact with the adjacent unit Pqfg is mostly

gradational. Gilboy (1970) reported the occurrence of xenoliths

of unit Pfqg in this granite and assumed it to be among the

earliest intrusives in the region.

3.4.3 Hornblende-biotite-quartz-plagioclase gneiss (Pbqg)

Outcrops belonging to this unit occur in the east-central

part of the map sheet, occupying a large area. The unit appears

to have structural contacts with the adjacent map units to east

and west.
It underlies various topographic features, outcropping in the
valley of the Sebetto river, on the plains around Baya Gundi

village and forming small ridges here and there.

The typical rock forming this unit is a medium to coarse

grained, grey weathering, mesocratic, homogeneous, commonly

biotite rich gneiss of tonalitic composition and appearance. At

places, it contains hornblende with or without biotite and is

poor in quartz, becoming closer to quartz diorite in composition.

Occasional dyke-like bodies similar to the host rock also occur

in this unit.

On average the rock has a color index of 20 and consists of

about 55% oligoclase/andesine, 20% quartz, upto 10% biotite, 7%

hornblende, 5% K-feldspar and a little clinopyroxene. Its

textural and mineralogical homogeneity, the occasionally

preserved plutonic texture and the presence of xenoliths of

adjacent rocks indicate its igneous origin.

Towards the north the unit is poorly exposed; Pyrite

bearing diorite andhornblende gneiss have been observed as small

patches below the Tertiary basalt cover, but their relationship

to unit Pbqg is not known. In the Aflata river valley there are

good outcrops where this unit is represented by melanocratic

migmatitic hornblende-biotite gneiss similar in composition to

the tonalitic gneiss. The continuity of this unit to the east is

not clear as the exposure is very poor.

 U/Pb dating indicated an age of 765 ± 3 Ma for the
emplacement of the intrusion.

3.4.4 Biotite metagranite (Pmbg)

This unit includes a pluton consisting of three oval north-

south elongated ridges south of Finchaa village in the south-

central part of the map sheet. Its contacts with the adjacent

rocks are not clear. A U/Pb age of 708 ± 5 Ma has been obtained

from this body. This granite is medium grained, equigranular,

pink poorly foliated to gneissose and at places well lineated.

Thin sections of rocks from this body show 33-50%

microcline, 20-35% plagioclase, 20-25% quartz, 3-5% biotite and

upto 2% hornblende. Perthitic and myrmekitic intergrowths are

common and feldspars are in some cases sericitized.

Granitic bodies similar to the one above, but more

leucocratic and richer in plagioclase have been observed

southeast of Galeba village. Another pluton grouped under unit

Pmbg is foundk in subsheet A.

3.4.5 Hornblende-biotite metagranite (Phmg)

This unit outcrops as a crescent shaped body in subsheets U

and V. The outcrops making up this body consist of small

granitic ridges exposed here and there, separated from each other

by the biotite and biotite-amphibole gneisses of unit Phbg which

have been intruded.

 The granite is leucocratic with colour index 20, coarse
grained and poorly to well foliated. Foliation is best developed

towards the northern margin of this unit. Where foliation is

well developed, the feldspars are commonly augened. At places

the rock exhibits a strong lineation consisting of both mafic and

felsic minerals elongated in the downdip direction.

Thin sections show an inequigranular granoblastic texture.

The rock consists of 40% microcline, 20% quartz, 15% plagioclase,

10% biotite and 5% hornblende, opaques, zircon and secondary

minerals making up the rest. Some of the microcline is

microperthitic and some myrmekite is present.

3.4.6. Quartz syenite (Pqsy)

Two oval quartz syenite plutons, intrusive into the adjacent

gneisses, are present in the southwestern part of the map sheet.

These bodies are massive to weakly foliated and homogeneous.

The rock is medium to coarse grained, dark grey on fresh

surfaces, weathering to dark brown. The average mineralogocal

composition is 35% microcline, 12% oligoclase, 30% hornblende,

12% biotite, 5% quartz, with minor magnetite, sphene and

sericite.
3.4.7. Metaquartz diorite (Pmdt)
This unit consists of two large plutons and associated

smaller bodies upto 1.5 km across, all found in teh eastern part

of the map sheet. The southern Galeba pluton is fairly well

exposed, outcrops in the northern Kojaa pluton being much

sparser. This triangle shaped pluton has an area of about 40 sq

km. These plutons are weakly foliated parallel to their contacts

at their margins, becoming massive towards their centres.

Inclusions of foliated amphibolite and of various gneisses were

observed in the Galeba pluton.

The Galeba pluton consists of quartz diorite, the Kojoa

pluton containing gabbronorite as well. In the latter the

relationship between the two rock types is gradational.

The quartz diorites are grey, medium to coarse grained,

massive rocks. Thin sections show a subhedral granular texture.

These rocks consist of 30-65% plagioclase (An 27-38), 5-35%

hornblende, 5-20% biotite, 5-20% quartz and upto 5% magnetite.

Minor apatite, lencoxene and sulfides are present.

The gabbronorites are massive, grey, medium-grained,

inequigranular rocks. Thin sections show igneous textures.

These rocks consist of 35-40% plagioclase (An 42-72), 15-30%

augite, 5-25% hypersthene, 5-9% quartz, upto 20% biotite, upto

15% hornblende and upto 3% magnetite. Apatite and leucoxene are

occasionally present.
3.4.6 Talc-tremolite schist (Ptts)
This rock type is mainly exposed in the eastern part of the

map sheet, forming prominent ridges along two meridional belts

each 750 m wide on average. The western belt extends N-S across

the whole map area, marking the eastern boundary of the Adola low

grade belt. It reaches 3.75 km in width southwest of Mount

Burjiji. The eastern belt occurs east of the Gariboro pluton,

occupying the core of a poorly defined northerly plunging

synform. In the area northeast of Megado village the unit

appears to cut across units Pcas and Ppss.

The rock is light green and medium to coarse grained, with a

schistose fabric. Adjacent to serpentinite bodies the rock is

dominantly composed of tremolite, while elsewhere it is mainly

talc rich. Chlorite also occur in fair amount in some cases.

Prismatic crystals of tremolite and clustered grains of opaque

minerals are observable in hand specimen. In thin section the

unit shows lepidonematoblastic or porphyroblastic textures, with

mainly tremolite porphyroblasts. Talc (upto 40%), tremolite

(upto 65%) and opaque minerals (3%) are the main constituents,

with a few sections containing upto 15% chlorite.

Gilboy (1970) believed that the western belt was emplaced

along a zone of extensive movement, while Kozyrev et al. (1985)

suggested emplacement along deep seated faults for both belts.

3.4.9 Undifferentiated metaultramafics (Pumf)

 Various minor ultramafic bodies are grouped in this unit.
They include highly weathered and altered undifferentiated rocks

as well as metapyroxenites and birbirites.

The ultramafic bodies exposed close to the Agere Maryam-

Yabello road in the south-central part of the map area are the

main constituents of this unit. These rocks occur as elongate

bodies, roughly aligned in a north-south direction. The

metapyroxenite bodies north of Sirupa village are made up of

pyroxene partly altered to talc, amphibole and minor chlorite.

They look weakly deformed in outcrop. Their relationship to the

adjacent rocks is not clear as the contacts are not exposed.

However these bodies lie close to the contact of unit Pmcg and

Pmfg.

In thin section, one of the metapyroxenite bodies north of

Sirupa is composed of 30-50% actinolite, 20-45% diopside, upto

25% plagioclase and about 3% opaques and other minerals. It has a

color index of about 80.

All five bodies of ultramafic rock occurring south of Sirupa

consist of birbirite. This is a yellowish grey, fine grained,

massive, silicified variety of ultramafic rock. In thin section

birbirite samples show 50% quartz, 40% chalcedony and 10% iron

oxide. Such rocks have formed by weathering of ultramafics in

the Yubdo area in western Ethiopia (Kazmin, 1972).

Northwest of Finchaa ultramafic rocks occur as small

elongate lenses within unit Pmqz. These are strongly deformed

dark, fine grained gabbroid to pyroxenitic rocks as well as
lighter talc-tremolite schists. Near Galeba, the ultramafics are

represented mainly by talc-tremolite-actinolite schists, serpen-

tinites and altered rocks.

3.4.10 Hornblendite (Phbt)

This unit occurs as small bodies of hornblendite northeast

of Galeba village. The typical rock belonging to this unit is

olive to dark grey, massive, coarse grained and composed of

hornblende with little or no feldspar. It is sometimes weakly

foliated and is found associated with talcose and amphibolitic

rocks which make up about 30% of the unit.

Chater (1971) reported that contact relationships indicates

that the hornblendite was formed after most tectonism had cuased.

The occurrence of this unit is probably related to the metaquartz

diorite (Pmdt).

3.4.11 Serpentinite (Pspt)

Serpentinite is exposed as small patches, often hill

forming, in the eastern part of the map area. Individual bodies

may be subcircular or elongate. The largest body is the one

forming Katawicha ridge in the southeast. It has a width of about

half a kilometer. and is 4 km in length. This unit invariably

occurs in close spatial association with unit Ptts.

The serpentinite is fine grained, massive, light to dark green,

often mottled and bears chalcedony stringers. At places it also
exhibits honeycomb structure filled with iron oxide. The rock

consists chiefly of serpentine with upto 5% chromite. Talc,

chlorite, carbonate, anthophyllite and tremolite may be present

in varying proportions. The mottled texture is produced by

irregularly shaped clots of serpentine. Porphyroblasts of

serpentine, talc, chlorite, anthophyllite or tremolite occur.

3.4.12 Subvolcanic amphibolite (Psva)

This unit occurs as a chain of elongate bodies mainly

restricted to the western edge of the Adola low grade belt, and

extending N-S for about 50 km. In the north-central part of the

Adola belt these rocks intrude units Pcas and Ppss. Towards the

south they are found in the high grade gneisses. Individual

bodies are upto 8 km long and 1 to 2 km wide.

The amphibolite is massive to poorly foliated, fine to

medium grained and equigranular or porphyroblastic, with a green

to dark grey color. In thin section the rock is

nematogranoblastic, or porphyroblastic with a nematogranoblastic

groundmass. It consists chiefly of 60-65% amphibole (mainly

ctinolite), 25-30% plagioclase, 4-5% clinozoisite, upto 5%

chlorite and 1% opaques. The plagioclase occurs as granoblastic

aggregates ranging in composition between albite and oligoclase.

The amphibolite bodies can be easily delineated on air

photographs due to their uniformly massive appearance against teh

background of the layered rocks of the Adola low grade belt.
These rocks are generally more massive and coarser grained than

the layered amphibolites of unit Pcas, and re thought to be of

subvolcanic intrusive origin. Their emplacement along the

western boundary of the Adola belt possibly indicates that this

boundary is a deep seated fault (Kozyrev et. al., 1985).

3.4.13 Metagabbro (Pmgb)

These weakly foliated gabbroic plutons are found southwest

of Shakisso town, mainly lying in the Adola low grade belt. They

occur as a cluster of bodies which are concordant to the country

rock. Individual bodies are characteri- istically elongate,

being upto 5 km in length and half a kilometer in width. Kozyrev

et al. (1985) indicated that these rocks cut across unit Ptts,

suggesting a younger age.

These plutons consist of fine to medium grained melanocratic

rock with relict gabbroic texture. Thin sections show 50-70%

hornblende, 25-55% plagioclase (mainly andesine) and minor

opaques, biotite and chlorite. Some of these metgabbros are

albite-clino- zoisite-actionolite rocks.

3.4.14 Biotite granite (Pbgt)

These granites occur mostly in the eastern part of the map

sheet, the largest body however occuring in the southwest.

Several of the eastern bodies are in fair spatial association
with N-S, NE and NW striking fractures (Kozyrev et al. ,1985).

These granites are circular to elliptical and upto 5 km across.

They character- istically show sharp contacts with the

surrounding country rocks and invariably exhibit a homogeneous

appearance.

This rock type is hard, compact coarse grained, generally

pink and characteristically massive in hand specimen. Color index

ranges upto 5. Thin sections exhibit mostly equigranular, rarely

porphy- ritic textures, phenocrysts commonly being quartz, less

frequently K-feldspar crystals. The constituent minerals are 35-

50% K-feldspar, 25-35% quartz, 20-25% plagioclase, 1-5% biotite

and minor muscovite. Sphene, rutile, apatite and opaque mineral

appear as accessories. Alteration effects are generally absent.

These granites intrude the Adola low grade rocks as well as

the high grade gneisses. A U/Pb age of 554 ± 23 Ma was obtained

for the Robelie granite which lies just east of the map sheet at

5° 48'N, (39° 02'E).

3.4.15 Hypersthene-hornblende granite (Phhg)

This unit is comprised of intrusive bodies well exposed on

the Berguda ridges, in the Chebi Arboro area and in the western

part of the map sheet north of the Segen river.

 The typical rock belonging to this unit is a medium to
coarse grained, weakly foliated rock of color index about 15. In

fresh outcrops it is dark greenish grey; weathered varieties are

pinkish grey in color. Finer grained varieties are found closely

associated with the coarser ones.

Thin sections of these rocks exhibit strong myrmekitic,

antiperthitic and perthitic features. The texture is inequi-

granular. The constituent minerals are 35% K-feldspar, 30%

plagioclase, 20% quartz, 5% biotite, 10% hornblende, 5%

hypersthene and minor biotite and opaques. The pyroxene does not

appear to be of metamorphic origin, but rather is a primary

igneous mineral. These rocks appear to be of intrusive origin,

having been later subjected to weak deformation.

The contact relationships between unit Phhg and adjacent

units are not clear as there are no good exposures. Based on

evidence gathered at a few localities, unit Phhg seems to intrude

units Pmfg and Pghg. The Berguda pluton yielded a concordant

U/Pb age of 529 ± 2Ma.

4 PRECAMBRIAN STRUCTURES

4.1 Introduction
Basement rocks exposed in Agere Maryam map sheet exhibit

many structures varying in size from large scale to microscopic

which indicate that these rocks have been subjected to polyphase

deformation. Preliminary work involving field description of

the structures and stereographic plotting was carried out by

Gilboy (1970) and Chater (1971) for the eastern part of the map

sheet. Since then no regional work on the nature of the

structures and their significance has been carried out, though

Kazmin (1970) subdivided the Precambrian rocks of the country

into three complexes on the basis of litho logy and structural

style.

Recently some work was carried out by Beraki et al. (1989)

in the eastern part of the map area, and further ideas on the

structure of the Precambrian basement here were put forward.

This work has applied some of the new concepts that have evolved

in recent years in the field of structural geology.

The present compilation has taken into consideration the

work carried out so far in the light of the data available and an

attempt has been made to classify the structures into six

deformational events (see Enclosure 3).

4.2 Phases of deformation
4.2.1 First deformational phase (D1)

The earliest deformational phase recognized is poorly

represented due to the superimposition of later deformation and

metamorphism. The earliest recognizable planar feature (S1) is

defined by a well developed layering in the high grade gneisses,

formed by segregation of minerals. Intense migmatization and

granitization has developed along the layering. Apart from S1

gneissic layering, identification of D1 structural elements has

proven difficult. No major structure belonging to this

deformational phase has been distinguished.

Folds and lineations belonging to this phase of deformation

are very scarce. The earliest recognized folds, denoted F1, are

mesoscopic, isoclinal, intrafolial folds probably coeval with S1.

At the hinges of F2 folds, F1 folds are seen to be E-W trending

and recumbent, whereas on F2 fold limbs they are reclined folds

with fold hinges parallel to the dip direction of the folded S1

layering.

There is little data on lineations belonging to the first

deformation, mainly because of the fact that their separation

from younger lineations is difficult. However, at a few

localities (for example, west of Soyoma village) such lineations
have been distinguished, having been folded by F2. In the area

west of Melka Soda village, gently east plunging lineations

parallel to F1 fold hinges were observed in a minor

quartzofeldspathic unit.

4.2.2 Second deformational phase (D2)

Structures belonging to the second deformational phase are

well developed in the map area. Prominent large scale features

as well as mesoscopic to microscopic structures related to this

event occur widely. The planar structures, folds and lineations

of this phase are restricted to the high grade gneisses. This

phase is regarded as the second most prominent event after D1 in

the high grade rocks, controlling the outcrop pattern and

producing N-S structural trends.

Axial planar mineral growth associated with this phase of

deformation is common. However distinguishing the S2 surfaces

developed during this event is difficult as the foliation

encountered is mostly a composite S1/S2 structure. S2 axial

planar surfaces are distinctly observable. At the hinges of F2

folds. These surfaces are defined by new mineral growth with

preferred orientation parallel to them. The trend of the S2

surfaces is dominantly north-south with minor deviations; at a

few places the effects of later deformation have produced

different trends. Dips are moderate to steep to east or west,

 Mesoscopic folds belonging to this phase are generally
symmetric to asymmetric, tight to isoclinal and fold S1 gneissic

layering of the earlier. These folds have well developed axial

planar cleavage and axes plunging gently to moderately

northwards. Fold hinges are generally angular but rounded hinges

are not uncommon and wavelengths vary from tens of centimeters to

several hundred meters.

Large north-south running antiforms and synforms form major

D2 structural features in the Gariboro-Burjiji area, in the

southeastern part of the map sheet (Enclosure 3). Such folds,

though not common, have also been recorded south of Kibre Mengist

town (Gilboy, 1970).

In the central part of the map area, southeast of Agere

Maryam town, large open folds with sub-horizontal to gently

plunging north-south axes which extend for over 15 km have been

recorded. Here the folds are parallel to the general lithologic

trend. In the western part of the map sheet, where shearing is

well developed, large folds are not prominent.

Well developed lineations associated with the second phase

of deformation are widespread throughout the area. The lineations

are represented by mineral aggregates, intersection of S1 and S2

surfaces and grooves all parallel to F2 fold axes. Lineations

defined by boudin axes and elongate feldspars are also observed

at many places. Generally the plunge of the lineations ranges

between 10° and 40° to north or south.

 The second deformational event is believed to correspond to
the Mozambiquian orogeny, which is characterized by north-south

folding.

4.2.3 Third deformational phase (D3)

Structures grouped under this deformational event are mainly

wide spread large to small shear zones and associated asymmetric

folds. The general structural trend of the shear zones is NNW,

NNE and N-S, with steep dips to east or west. Large-scale shear

zones are observed in the Amaro, Galeba, Burjiji, Katawicha,

Sirupa and other areas of the map sheet, being restricted to the

high grade rocks (Enclosure 3). All have been grouped into a

single phase of deformation despite differences in the sense of

movement and uncertainty regarding their ages.

At places poorly developed mylonitic foliation has been

noted, for example north of Galeba village. Crenulation

cleavage, representing local shear planes cutting D2 structures,

is common in unit Pbmg west of Galeba village. Folds developed

during this deformational phase are large to small asymmetric

folds character- ized by steeply plunging axes, steep NNE or NNW

trending axial planes and their association with D3 shears. F3

folds have at places developed axial planar foliation (S3), but

this is not common. Where present, this foliation is steeply

dipping to east or west and trends NNE or NNW, rarely N-S.

 A large ductile shear zone 100 km long and over 16 km wide
is present along the Amaro horst. It trends NNE and dips steeply

west. Here the rocks are migmatitic and are characterized by

zones of fine grained, thinly banded gneisses with intensely

developed shear fabric alternatomg with zones of coarse grained

gneisses exhibiting tight, asymmetric, dominantly S-shaped folds.

Although there are features indicating dextral movement, overall

the shear zone appears to be a sinistral strike-slip one.

The D3 Galeba shear zone is a NNW trending ductile shear

over 60 km long lying west of Galeba village. The outcrop

pattern in the area between Soyoma and Chebi Arboro suggests the

presence of a large scale S-fold, probably formed in association

with the Amaro shear zone. Its western limb appears to run N-S

in western subsheet O. The central limb runs E-W for about 30 km

along the boundary of subsheets O and U. This limb appears to

have been cut at its eastern end by the large northeasterly fault

passing through subsheet U. The continuation of the central limb

east of this fault is probably the E-W trending western part of

unit Phmg, which then curves southwards forming the eastern limb

of the above large S-fold. It was noted in section 3.4.12 that

unit Phmg consists of high grade gneisses enclosing small

scattered bodies of granite. It is thought that these granites

formed coevally with the S-fold. The two Pqsy plutons were also

probably intruded synchronously with the formation of this

structure. The shear zone is distinct on air photographs,

becoming an indistinct feature with north-south trend in subsheet
Q. Along the shear zone reliable indicators of sense of movement

are of very limited occurrence. Asymmetric folds, most of which

are S-shaped, are present. However it is thought that this is a

dextral strike-slip shear zone, based on the fact that on the

macroscopic scale the N-S trending foliation to the northeast of

this shear curves westward into it.

The Burjiji and Katawicha shear zones display a meridional trend

and bound the Gariboro massif to west and east respectively.

The Burjiji shear zone is distinct on Landsat images.

Asymmetric, steeply plunging S-folds are the dominant associated

structures. The Katawicha shear zone exhibits quartz-rich

mylonites, highly friable muscovite schist, and boudined and

sheared quartz veins of sigmoidal shape. The general sense of

movement in this area is sinistral strike-slip. However, there

are a few zones where asymmetric feldspar porphyroblasts indicate

dextral movement.

A large shear zone marked by strong foliation and numerous

pegmatites aligned north-south occurs a few kilometres east of

Sirupa village. Pegmatites are much less abundant in adjacent

areas. Tight to close, steeply plunging, Z-shaped minor folds

indicating a dextral strike-slip sense of movement have been

observed near this village.

4.2.4 Fourth deformational phase (D4)

and
 Structures of this generation are similar to D2 structures

are seen mostly in the Adola low grade belt. The structural

trend is north-south, with steep dips to east or west. This is

the first tectonic event to affect the rocks of the Adola low

grade belt.

Folds associated with this deformation are tight to close,

with generally upright, north-south trending axial planes. Fold

hinges are generally rounded, in some cases angular. Axes are

subhorizontal in the northern and central parts of the Adola

belt. To the south, however, axes plunge moderately south,

becoming much steeper (upto 75°) in the area between Burjiji and

the Karadissa river. In the regions to east and west of the low

grade belt, F4 folds are close and subhorizontal or plunge gently

to north or south. Hook-type interference patterns developed

through the superposition of F4 on F2 folds have been observed at

a few localities (northeast of Kibre Mengist town and southeast

of Digati village). In the western part of the map area, major

D4 structures have not been distinguished. However, on minor

scales, close to open N-S trending folds have been observed at

some places refolding D3 and D2 structures.

The main foliation which occurs in the rocks of the Adola

belt was generated during this event. Many folds in the

lithologic layering have an axial planar foliation defined by

preferred orientation of minerals such as amphibole, mica and

chlorite. Crenulation cleavage is also common in the rocks of
the low grade belt. The general trend observed for all the

planar surfaces associated with the low grade belt is north-

south, with minor deviation to east or west.

Mineral lineations (L4) formed during this event are

widespread, but are easily identifiable mainly in the low grade

rocks. Elsewhere it is difficult to distinguish these lineations

from earlier ones. The lineations are defined by oriented

amphiboles and micas, and also by the elongation direction of

pebbles. They are generally subhorizontal with N-S trend in the

northern and central parts of the Adola area, having steeper

(upto 80°) southerly plunges in the Digati area. Lineations with

steep southerly plunges observed in phyllite along the Chochobo

river are also considered to be L4 lineations. In general L4

lineations are subparallel to F4 fold axes, indicating their

close genetic relationship.

4.2.5 Fifth deformational phase (D5)

Generally north-south trending shear zones and associated

asymmetric folds deform D4 structures and belong to the fifth

deformational event. Such features are abundant in the low grade

rocks of the Adola belt.

The shear zones are dominantly sinistral strike-slip. Within

the chlorite and talc-chlorite schists of the low grade rocks, a

well developed shear fabric is observed cutting across the

earlier D4 foliation. S-shaped folds, which are the predominant
asymmetric folds in the Adola rocks, are believed to be related

to these shear zones.

Mineral lineations in the Karadissa river in the eastern

part of the map area, which are assumed to be related to D5

sinistral shearing, plunge moderately to the north.

4.2.6 Sixth deformational phase (D6)

Expressions of the last deformational phase are seen at many

spots in the surveyed area. The deformation is weak and is

repre- sented mainly by mesoscopic, open, symmetrical folds with

small amplitudes (a few centimeters to 100 m). The folds have

east-west trending axial planes and gently to steeply plunging

fold axes. Some E-W folds of this phase are macroscopic, with

wavelengths of 15-20 km. These are found mainly between Kibre

Mengist and Gariboro. They are the cause of a large scale wavy

outcrop pattern in the rocks of the north-south running lowgrade

belt.

Brittle and brittle-ductile faults upto 30 m in length

cutting all earlier deformational features, were noted in the

sourtheastern part of the map sheet. Such faults are dominantly

dextral strike-slip and of northerly trend.

4.3 Lineaments
 All lineaments on air photographs 2.5 km or longer as well
as some lineaments from Landsat imagery are shown on Enclosure 4.

These lineaments are mostly fractures (both joints and faults),

but include stream segments, topographic depressions, ridges and

various linear tonal variations on air photographs. Lineaments

are best developed in the western and eastern parts of the map

area. In the contral part, where the topography is subdued, they

are less conspicuous (see Enclosure 3).

Preferred orientations can be distinguished from. Fig. 2 N-

S trending lineaments (0°-010°) are the dominant ones in the

area. Such lineaments upto 20 km long are common, and are parti-

cularly abundant in the eastern and western parts of the map

sheet.

NW trending lineasments (300°-330°) are the second most

common group, being especially widespread in the central and

western parts of the area.

NNE trending lineaments (010°-030°) are also widespread and

generally display lengths varying from 5 to 10 km. A few

prominent NE trending lineaments are present in the eastern part

of the map area.

The Aflata river follows an 80 km long lineament running

NNW. Latitudinally trending lineaments appear to be very rare.

4.4 Faults
 In the Adola low grade belt N-S faults form contacts between
lithologic units. In many cases mafic and ultramafic rocks are

aligned along them. These faults extend for long distances as

continuous or discontinuous features, lengths over 100 km having

been recorded by Kozyrev et al. (1985). Striations and

stretching lineations observed along the contact between units

Ptts and Pbmg, northeast of Ula Ulo and Digati villages indicate

dip-slip movement. In this part of the map sheet such faults are

thought to be normal faults related to the formation of the

depositional basin containing the low grade rocks. Some of these

faults have a strike-slip sense of movement, dominantly

sinistral.

NE and NW trending sets of faults possibly conjugate are

also present in the eastern part of the map area. Kozyrev et al.

(1985) noted that the intrusion sites of post-tectonic granites

(Pbgt) tend to occur at the intersections of these sets of

faults. Lateral movement along these faults reaches half a

kilometer.
N-S faults occurring in the western part of the map sheet

are associated with the Main Ethiopian Rift. They are mostly

normal faults connected with the formation of graben and horsts

that developed during the Cenozoic (see chapter 8). NW faults

are also common here.

4.5 Igneous activity in relation to episodes of deformation

 Based on the gradational contacts displayed between
units Pghg and Pmfg it is suggested that the former is a partial

melt of the latter. The S1 foliation seen in unit Pmfg is

present also in unit Pghg. Thus unit Pghg is pretectonic or

syntectonic to the first deformational event (D1).

The foliated biotite metagranite (Pgbg) lacks the character-

istic migmatization and granitization effects of D1. S2

foliation is poorly displayed by this granite, but its occurrence

appears to be confined to the cores of major F2 antiforms. These

observations and the D3 shearing displayed on its margins

indicate that emplacement of the granite took place towards the

end of the D2 event.

The biotite-oligoclase-quartz gneiss (Pbqg) and biotite

metagranite (Pmbg) were intruded at 765 Ma and 708 Ma,

respectively. The gneissosity exhibited by unit Pbqg is not as

intense as the typical S2 foliation and D2 structures are not

seen. It appears that this body as well as unit Pmbg escaped the

D2 deformation. Moreover the occurrence of asymmetric folds and

minor ductile shears in unit Pbqg indicates emplacement before or

during the third deformational phase.

The Phmg and Pqsy plutons in the western part of the map

area are closely associated with D3 structures and are thought to

be of this age. The Galeba metadiorite (Pmdt) appears to have

been affected by the D3 Galeba shear zone. Thus this and the

Kojoa pluton may be syntectonic to D3. However the small Pmdt

pluton east of Galeba intrudes the Adola low grade rocks.
Possibly then the Pmdt plutons were emplaced in the time between

the D3 and D4 events.

Units Ptts and Pspt are in most cases spatially associated,

occurring along structural contacts between the Adola rocks and

the high grade gneisses. Some of these rocks show the effects of

the D4 deformational event, and they were possibly emplaced along

deep seated fractures before D4.

Unit Psva intrudes the Adola low grade rocks, exhibits

poorly discernible S4 foliation and is therefore thought to be

pretectonic to syntectonic to D4.

The metagabbros (Pmgb) of the central part of the Adola belt

the low grade rocks, including unit Ptts (Kozyrev et al. 1985).

These rocks are generally massive, but their elongate shapes

possibly indicate that they were emplaced syntectonically to D4.

The Robelie granite (Pbgt) and the Berguda hypersthene-

hornblende granite (Phhg) gave emplacement ages of 554 Ma and 529

Ma respectively.
These plutons and the others grouped in these units are

mostly massive, often cut across the adjacent lithologies and are

late to post-tectonic.

5 METAMORPHISM

5.1 Introduction
The polymetamorphic history of the Agere Maryam area in
general and that of the eastern part in particular was obvious to

most of the early workers in the area. Gilboy (1970) suggested

two regional prograde metamorphic events and a later

retrogressive process in the Gariboro region, whereas Chater

(1971) indicated three phases of prograde metamorphism in the

Megado region. Recent work by Beraki et al. (1989) revealed a

metamorphic history similar to that of Gilboy (1970) for the

eastern part of the map area. According to Gilboy (1970) only

the high grade gneisses in the area underwent the first

metamorphic event, the second event exhibiting an increase in

grade from greenschist facies in the low grade rocks to the

sillimanite zone of the amphibolite facies in the gneisses to the

east. Chater (1971) stated that his three metamorphic episodes

affected both the high and the low grade rocks. A gradational

relationship was suggested to explain the variation in

metamorphism in these rocks. Davidson (1983) noted prograde

metamorphism of middle amphibolite to granulite facies in the

high grade terrain west of the map sheet. A later retrograde

event, locally manifested, was also observed.

5.2 Phases of Metamorphism
Based on field investigations, structural correlation and

petrographic study of various rock units in the map area, at

least three distinct prograde metamorphic events and a later

retrograde overprint are recognized in the present work. The

metamorphic classification used here is according to Winkler

(1979).

The earliest recognizable thermal event (M1) in the area is

represented by large scale migmatization, pegmatization and

granitization. These features are very well displayed in some of

the high grade stratified gneisses, particularly in units Pbfg,

Pmcg and Phbg, and in most cases are parallel to the gneissic

layering (S1). This episode presumably accompanied the formation

of the widespread gneissic layering and the emplacement of the

earliest intrusive (Pghg). No metamorphic index minerals

believed to belong to this event have been observed; such

minerals were either totally destroyed or mimetically

recrystallized during later tectonothermal events.

The second metamorphic event (M2) is displayed by the

stratified gneissic rocks in the region and the early intrusive

unit Pghg. This event is regional in character as it is

represent- ed by widespread metamorphic mineral assemblages given
below for some of the units.

1. In unit Pbfg:

a) quartz + biotite ± plagioclase ± microcline

b) quartz + biotite ± sillimanite ± garnet ± muscovite

2. In unit Pfqg:

a) hornblende + garnet + quartz ± epidote ± oligoclase

b) biotite + garnet + quartz

c) diopside + hornblende + andesine/labradorite + quartz

d) diopside + hornblende + labradorite + scapolite + quartz

e) kyanite + sillimanite + plagioclase +quartz ± K-feldspar

3. In unit Pbmg:

a) staurolite + garnet + quartz ± biotite ± muscovite

b) staurolite + muscovite + biotite ± quartz

c) garnet + biotite + muscovite ± quartz

4. In unit Pmfg:

a) sillimanite + muscovite + K-feldspar + plagioclase + quartz

b) diopside + epidote + hornblende + andesine + quartz

c) K-feldspar + oligoclase/andesine + quartz ± hornblende

± magnetite

5. In unit Pfbg:

a) hornblende + epidote + plagioclase + quartz

b) sillimanite + K-feldspar + muscovite + quartz

6. In unit Pmcg:

a) diopside + scapolite + titanite ± quartz ± garnet ± calcite

b) andesine + quartz ± diopside ± titanite ± epidote ± carbonate
c) microcline + quartz + plagioclase ± muscovite ± garnet

d) hornblende + diopside + labradorite ± epidote

7. In unit Phbg:

a) K-feldspar + oligoclase + biotite + magnetite

b) K-feldspar + oligoclase + biotite + hornblende + quartz

c) microcline + hornblende + quartz ± muscovite ± epidote

d) hornblende + quartz + clinopyroxene ± plagioclase

Assemblages 1b, 2a, 2b and 7c are suggestive of the

transition from greenschist to lower amphibolite facies

conditions. Assemblages 2a, 3a, 3b, 4b, 5b, 6c and 6d indicate

middle amphibolite facies metamorphism. The rest of the

assemblages (including 6a and 6b for carbonate rocks) are

suggestive of equilibrium conditions in the upper amphibolite

facies and possibly in the transition to the granulite facies.

The following M2 minerals were noted in unit Pdfg:

a) hypersthene + quartz + garnet + hornblende + biotite

b) quartz + plagioclase + orthoclase + garnet + sillimanite

c) labradorite + diopside + hornblende + hypersthene + pyrope +

quartz

These assemblages are characteristic of metamorphism under

granulite facies conditions.

It is therefore apparent that the second metamorphic event

is generally of middle to upper amphibolite facies throughout the

map area, except in the western part where there are islands of
granulite facies rocks.

Following the M2 regional metamorphism a lower grade

metamorphic event (M3) took place. This event appears to have

been generally restricted to the Adola low grade belt in the

eastern part of the map sheet. The mineral assemblages noted in

some of the rocks from this belt are given as follows.

1. In unit Pmsc:

a) biotite + oligoclase + epidote

2. In unit Ppss:
a) garnet + biotite + quartz

b) chlorite + quartz ± biotite ± muscovite ± garnet

c) biotite + muscovite + quartz

3. In unit Pcas:

a) chlorite + talc + quartz ± plagioclase

b) biotite + chlorite + quartz ± plagioclase

The above assemblages are suggestive of middle greenschist

facies metamorphism. A few thin sections from the fine to medium

grained amphibolite in unit Pcas have revealed the assemblage

hornblende + quartz + chlorite ± opaques, suggestive of a

transition to the amphibolite facies.

The retrograde metamorphic event (M4) appears to have

affected both the high grade gneisses and the low grade rocks.

It was local in nature, as all thin sections do not exhibit this

effect. It is represented by breakdown of hornblende to epidote
and chlorite, garnet to biotite, and replacement of biotite by

chlorite. Epidotization, which mostly occurs along fracture

zones in the area, is also believed to be part of this process.

5.3 Relationship between metamorphism and deformation

Field observations at a few places in the central part of

the map area have revealed that certain pegmatites and migmatites

were affected by the earliest E-W folds (F1). At these sites the

penetrative S1 gneissic foliation (thought to be coeval with M1)

is parallel to the axial planar to these folds. It thus appears

that the earliest thermal event (M1) was initiated before D1 but

continued throughout this phase.

The M2 metamorphic minerals throughout the stratified

gneisses in the map area define the submeridionally striking S2

regional foliation that often parallels the S1 surface.

Petrographic study of most of the thin sections from these rocks

has further revealed that these minerals also appear transverse

to the composite foliation. Idioblasts of staurolite in unit

hornblende in units Pbmg and Pfqg and garnet in units Pbmg and

Pfqg also occur. These observations collectively suggest that

the second metamorphic phase was initiated accompanying the

second deformational event, reaching its peak after the D2 phase

had ceased.
 The characteristic M3 metamorphic minerals define the main
foliation surface (S4) in the low grade rocks. Only in rare

instances were these minerals observed discordant to the S4

foliation. The third metamorphic event, therefore, took place

accompanying the D4 event that produced the main N-S folds in the

low grade rocks of the Adola belt.

The local M4 retrograde metamorphism is not believed to be

associated with any distinct deformational event. The dominantly

dextral shearing (D3), the later mostly sinistral shearing (D5)

and the rather weakly developed E-W open folds (D6) may well have

been accompanied by metamorphism, but the effects of any such

episodes are not easily discernible.

6 PRECAMBRIAN GEOLOGIC HISTORY

The high grade stratified gneisses, mainly of sedimentary

(and volcanic) origin, represent the oldest rocks in the map

area. The regional gneissic layering S1 and the D1 highly

appressed F1 folds of latitudinal trend are the earliest reco-

gnizable structural elements, developed during D1in these rocks.

The accompanying M1 metamorphic event was obviously of high

grade. Emplacement of the earliest pluton, Pghg, along with

pegmatization, migmatization and granitization of some of the

gneissic rocks occurred during this phase.

 The D2 deformational event led to the development of
generally upright, frequently open folds in S1 with by and large

penetrative north-south trending axial planar foliation

(Mozambiquian). Regional metamorphism of middle to upper amphi-

bolite facies, with pockets of granulite facies rocks developing

in the far west, was associated with this deformation.

The Gariboro foliated granite (Pgbg) was emplaced in the

core of a major F2 antiform towards the end of D2, being

associated with the M2 thermal event. The region remained under

high temperature conditions for some time after the D2 phase was

over.

The region was then subjected to the third deformational

phase marked by ductile strike-slip faulting that produced major

shear zones. Steeply plunging, tight, mesoscopic S- and Z-folds,

the latter predominating over the former, were generated. These

folds generally have submeridional axial traces. Emplacement of

units Pbqg and Pbmg (756 Ma), Pmbg (780 Ma), Pqsy and Phmy

probably occurred during this tectonic event.

Deep seated N-S trending faults, possibly normal, formed in

the eastern part of the map sheet. Consequently a graben-like

submeridional trough developed along the site of the present

Adola low grade belt. The extrusion of a thick pile of mafic

volcanics (Pcas) within this trough was followed by deposition of

mostly muds and silts (Ppss), followed by sands and minor

conglomerate (Pmsc) in an aqueous environment.

 Deposition of units Pcas, Ppss and Pmsc was succeeded by the
emplacement of ultramafics (Pspt, Ptts, Phbt, Pumf) and mafic

plutons (Pmdt, Psva, Pmgb), mainly along the Adola belt. This

was apparently facilitated by the presence of the faults formed.

The depositional process was terminated by D4 which produced

tight upright, N-S folds and resulted in the closure of the

elongate Adola basin. The M3 metamorphism was by and large

restricted to this basin and was limited to greenschist facies

conditions.

N-S trending shear zones, mainly sinistral strike-slip, cut

the basement during D5.

E-W trending open folds, mostly mesoscopic, and NE and NW

faults, possibly conjugate, formed during D6. N-S dextral

strike-slip faults foundk in the southeast apparently are the

youngest structures in the area.

Emplacement of the post-tectonic plutons Pbgt and Phhg, at 554

Ma and 529 Ma respectively, appear to be the last events in the

history of the basement rocks of the Agere Maryam area.

9.1 Introduction

9 MINERAL OCCURRENCES
The Agere Maryam area has been of interest to many workers,

particularly because of its economic gold deposits. Some of the

mineral occurrences in the map area, their reserve estimates and

genesis were noted by Jelenc (1966). The exploration work

conducted by the Adola Gold Exploration Project (1979-81) has

enabled mineralized areas to be more clearly delineated in the
Adola area in the eastern part of the map sheet. Various

anomalous zones for gold and other minerals have been identified

by this project (Kozyrev et al., 1985). Only a brief account is

given in this chapter of significant mineral occurrences in this

map sheet. Sites of current mining for gold are shown on the

geological map.

9.2 Gold
9.2.1 Placer gold

Placer gold has been produced in the Adola region, in the

eastern part of the map area, since the mid 1930's, with about 31

tons of gold produced in the years 1942-81 (Kozyrev et al.,

1985). According to these workers the placer gold occurrences

and deposits in the region define a zone that extends from the

northern margin of the map sheet down to the Dawa valley in the

south. (Placer deposits were defined as those with gold values

averaging 0.1 g or more per cubic meter of gravel and with gold

reserves of 30 kg or more). They also recognized that the zone

of auriferous placers displays a good spatial association with

the submeridionally trending Adola low grade belt. The placer

gold occurrences and deposits in the map area are genetically of

eluvial, eluvial-alluvial and alluvial types. The grades of

individual gold occurrences and deposits, including reserve

estimates for alluvial auriferous placers, is given in Kozyrev et
al. (1985).

9.2.2 Primary gold

Prior to the year 1965 only a couple of primary gold

showings were known in the Adola region. These were considered

uneconomical due to low gold content and thick overburden

(Jelenc, 1966). However, later on the work of the Adola Gold

Exploration Project resulted in the discovery of the Lega Dembi

deposit and a number of other promising targets. Among those

considered significant, and currently undergoing detailed

exploration, is the Sakaro occurrence. The Kumudu, Serdoshet,

Korkoro, Wollena and Chamola occurrences, earlier thought to be

promising, are presently abandoned sites. All the above sites

are confined to the Adola low grade belt.

The Lega Dembi gold deposit, located 7 km southeast of

Shakisso town, is currently being exploited. The gold is hosted

by quartz veins within quartz-sericite schist (of unit Pcas)

close to the eastern tectonic contact between the low grade rocks

and the Pbmg gneiss. According to Kozyrev et al. (1985) the

auriferous veins and stringers of this deposit altogether make up

a zone whose width is 85-165 m with a traceable length of about 2

km. The northern section of the Lega Dembi deposit is estimated

to contain reserves of 15-20 tons of gold to a depth of 120 m,

with an average value of 10 g of gold per ton.

9.3 Other metals
9.3.1 Nickel

In the early 1960's prospecting work for nickel was carried

out in the Adola area by Jelenc (1966). He identified a number

of nickel occurrences, among which Tula, Ula Ulo and Kilta fall

in Agere Maryam map sheet. The nickel mineralization is strictly

associated with serpentinite bodies. The Kilta serpentinite body

is the only one unmapped at the scale of the present work.

Fresh serpentinites in the area gave an average of 0.5% Ni,

with enriched layers containing over 2% Ni, whereas 0.3% Ni was

obtained for peridotites (Jelenc, 1966). Details of the geology,

grade and reserve estimates of Tula, Ula Ulo and Kilta nickel

occurrences are given by Vlaicu and Yehwalawork (1967).

Reference to these occurrences was also made by Levitte and Kent

(1968) who suggested that the nickel deposits in the region could

be developed for commercial purposes.

A number of anomalous areas for nickel, including the above

targets, have been identified in the Adola region by Kozyrev et

al. (1985). These workers recommended detailed studies at some

of the targets.

9.3.2 Chromium
 The occurrence of residual chromite in the Adola area was
indicated by Jelenc (1966). Kozyrev et al. (1985) also

recognized chromite occurrences in the region. Those at Dermi

Dama and Katawicha fall in the map area.

The Dermi Dama occurrence, like the others in the region, is

associated with a serpentinite body about 50x100 m. Lenses,

veinlets and nests 5-15 cm by 30-70 cm occur within serpentinite

bedrock. Only chromite float was observed in the Katawicha area.

9.3.3 Copper

Malachite and chalcopyrite showings were reported by Jelenc

(1966) about 5 km northeast of Digati.

Disseminated chalcopyrite was observed during the present

work in amphibolite of unit Pfqg about 5 km southwest of Digati,

on the road to Galeba. The chalcopyrite concentration was

visually estimated to be about 0.4%, the size of the occurrence

not being determined due to lack of exposure. Associated

sulphides included pyrite and pyrrhotite, together constituting

about 0.4% of the rock.

9.3.4 Titanium
According to Jelenc (1966) rutile and ilmenite were recorded

by Antolini (1958) in the east-central part of the map area along

the Dawa river and on the western slope of the Aflata river

valley in subsheet R. The rutile is found as either eluvial or

alluvial occurrences. Details of their grade and distribution

are given in Jelenc (1966). Primary rutile mineralization in
quartz veins hosted by units Pqfg and Pfqg was noted in the same

area by Kozyrev et al. (1985), who suggested a metamorphic origin

for both the quartz veins and rutile.

9.3.5 Manganese
Manganese showings were reported 6.5 km east of the Sirupa-

Finchaa road in the south-central part of the map area by Kazmin

(1970). He indicated that the manganese is associated with

metaquartzite (Pmqz) and that the mineralized outcrops range

between 30 and 100 m in width over 1.5 km. The occurrence was

economically insignificant due to its low Mn content.

Manganese showings were also noted in the above area by the

Regional Geology Department. The manganese distribution in the

host unit (Pmqz) appears to be variable. An analyzed manganese

bearing sample of Pmqz revealed over 1% Mn content.

9.4 Radioactive minerals

Radioactive mineralization was reported by Du Bois and

Bekele (1975) in the central part of the map area 5° 15'N and 38°

15'E. According to these workers the radioactivity recorded was

twenty five times that of the background. U, Th and Y were

identified in a sampled mineral of the euxenite-polycrase series.

9.5 Industrial minerals
Talc, graphite, mica and asbestos are known to occur in the

map area. Talc mineralization, often occurring as lenses, sheets

and cross-cutting veins, is associated with units Ptts and Pspt

in the eastern part of the map sheet (Jelenc, 1966 and Kozyrev et

al., 1985). Good quality talc occurs 3 km east of Allona village

and about a kilometer northeast of the new Mormora bridge.

Jelenc (1966) reported graphite occurrences near Shakisso

airport, near Kibre Mengist and around the power station on the

Mormora river east of Megado. The best developed graphitic

horizon is found in unit Pbmg, extending from Dermi Dama

northwards for 70 km. This horizon, which is a graphite schist,

lies mostly just east of 39°E and contains upto 20% graphite

(Kozyrev et al., 1985).

Jelenc (1966) reported a 20-25 m thick layer of compact mica

intercalated with schists east of Agere Maryam town. Micaceous

pegmatites have been reported by Kozyrev et al. (1985) in the

southeastern part of the map sheet, east of the Gariboro pluton.

The pegmatite veins are discordant to the gneissic country rock,

the micas mainly occurring as flakes and sheets.

Asbestos is known to occur about 4.5 km southeast of Agere

Maryam town (Jelenc, 1966). According to the available data the

asbestos is of cross-fibre type and is hosted by small lenses of

talc schist.
9.6 Other non-metals
Occurrences of quartz, kaolin and garnet have been observed

in the study area. Jelenc (1966) noted that in the Adola area

quartz gravels make up about 65% of the tailings obtained after

extraction of gold from placers. He envisaged a possible use of

the quartz for glass manufacturing based on its low iron content.
The different types of quartz and their proportions in the

tailings are given by Jelenc. A few high quality quartz

occurrences were also reported by Kozyrev et al. (1985)

A clay occurrence was reported by Jelenc (1966) near

Shakisso airport, towards the Mormora river. He indicated that

the occurrence was rich in kaolinite, but of limited quantity.

Garnet appears to be common in the amphibolite of unit Pbmg,

particularly along the Aflata river northeast of Galeba village,

where it constitutes about 1-2% of the rock as estimated

visually.
 ACKNOWLEDGEMENTS

Ato Amenti Abraham, head of the Regional Geology Department,

was the principal coordinator and supervisor of the field work

(1985-89). He rendered assistance in thoroughly discussing the

geology and structure of the region, also patiently carried out

the technical editing of the geological report.

Dr. C. J. Ebinger of the University of Leeds, facilitated

U.K, the acquisition of K/Ar ages for six Cenozoic volcanic rock

samples from the area, the dating being done at the Los Alamos

National Laboratory (U.S.A.) by Dr. Giday WoldeGabriel. We are

indebted to both.

Finally we would like to extend our appreciation to all the

members of the Regional Geology Department who participated in

the field work and contributed valuable ideas on the geology of

the region.
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