Gold-bearing volcanogenic massive sulﬁdes and orogenic-gold deposits …rjstern/pdfs/Johnson et al... · 2018-07-27 · Ÿ Manganese Deposits of Africa ... sought gold ore. At Bisha

62 63

P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER

P.R. Johnson6016 SW Haines Street, Portland, OR 97219, USA

e-mail: [email protected]

B.A. ZoheirDepartment of Geology, Faculty of Science, Benha University, 13518 Benha, Egypt


W. GhebreabDepartment of Biological and Physical Sciences, Columbus State Community College,

550 E Spring Street, Columbus, OH 43215, USA


R.J. SternGeosciences Department, University of Texas at Dallas, Box 830688, Richardson, TX 75080, USA


C.T. BarrieCTBA Geoconsulting, 12 Burnham Road, Ottawa, ON K1S 0J8, Canada


R.D. HamerManafai International Trade, P.O. Box 136533, Jeddah 21313, Saudi Arabia


© 2017 March Geological Society of South Africa

Gold-bearing volcanogenic massive sulfides and orogenic-gold deposits in the Nubian Shield

SOUTH AFRICAN JOURNAL OF GEOLOGY 2017 • VOLUME 120.1 PAGE 63-76 • doi:10.2113/gssajg.120.1.63

Abstract

The Nubian Shield is a large region of juvenile Neoproterozoic rocks that, together with its counterpart in the

Arabian Shield, is part of a major accretionary orogen. It originated as late Tonian-Cryogenian island arcs on the site

of the Mozambique Ocean formed by breakup of Rodinia. Arc collisions, subsequent magmatism, volcanism,

sedimentation, and orogeny, associated with Cryogenian-Ediacaran convergence of cratonic blocks during the

assembly of Gondwana, converted the region into the East African Orogen. The Nubian Shield region also has

important Paleozoic-Neogene strata, including significant flood basalt, that conceal large areas of the basement.

The Shield contains hundreds of gold occurrences and evidence of a 5,500-year history of gold mining. The main

deposit types are orogenic gold and gold associated with volcanogenic massive sulfides (VMS). The juvenile

subduction-related origin of the Shield rocks and the pervasiveness of shearing and greenschist-to-amphibolite

facies metamorphism associated with late Neoproterozoic orogeny are geologic features highly favorable for the

development of these types of deposits, and make the combined Arabian-Nubian Shield the Earth's largest

Neoproterozoic gold resource. Gold-bearing VMS deposits are currently mined at Bisha (Eritrea) and are soon to be

Full list of papers dealing with The Great Mineral Fields of Africa.

The SAJG thematic set:Ÿ Gold-bearing volcanogenic massive sulfides and orogenic-gold deposits in the Nubian Shield

P.R. Johnson, B.A. Zoheir, W. Ghebreab, R.J. Stern, C.T. Barrie and R.D. Hamer

Ÿ Tantalum-(niobium-tin) mineralisation in pegmatites and rare-metal granites of Africa

F. Melcher, T. Graupner, T. Oberthür and P. Schütte

Ÿ Gold on the Kaapvaal Craton outside the Witwatersrand Basin, South Africa

T. Pearton and M. Viljoen

Ÿ The coastal heavy mineral sand deposits of Africa

A. Rozendaal, C. Philander and R. Heyn

Ÿ Mesoproterozoic base metal sulphide deposits in the Namaqua Sector of the Namaqua-Natal

Metamorphic Province, South Africa: a review

A. Rozendaal, T-K. Rudnick and R. Heyn

The Episodes Special Issue:Ÿ Introduction

S. Frost-Killian, S. Master, R.P. Viljoen and M.G.C. Wilson

Ÿ A Review of the Witwatersrand Basin - The World's Greatest Goldfield

R.F. Tucker, R.P. Viljoen and M.J. Viljoen

Ÿ Lake Victoria Goldfields

J. Henckel, K.H. Poulsen, T. Sharp and P. Spora

Ÿ West African Goldfields

M. Robertson and L. Peters

Ÿ A Review of the Birimian Supergroup- and Tarkwaian Group-hosted Gold Deposits of Ghana

A.J.B. Smith, G. Henry and S. Frost-Killian

Ÿ Overview of Diamond Resources in Africa

Mike de Wit, Z. Bhebhe, J. Davidson, S.E. Haggerty, P. Hundt, J. Jacob, M. Lynn, T.R. Marshall,

C. Skinner, K. Smithson, J. Stiefenhofer, M. Robert, A. Revitt, R. (Spaggs) Spaggiari and J. Ward

Ÿ The Bushveld Complex – Host to the World's Largest Platinum, Chromium and Vanadium Resources

M. Viljoen

Ÿ Palaeoproterozoic banded iron formation-hosted high-grade hematite iron ore deposits of the

Transvaal Supergroup, South Africa

A.J B. Smith and N.J. Beukes

Ÿ Manganese Deposits of Africa

N.J. Beukes, E.P.W. Swindell and H. Wabo

Ÿ An overview of nickel mineralisation in Africa with emphasis on the Mesoproterozoic East African

Nickel Belt (EANB)

D.M. Evans, J.P.P.M. Hunt and J.R. Simmonds

Ÿ Uranium in Africa

J.A. Kinnaird and P.A.M. Nex

Ÿ Tin in Africa

J.A. Kinnaird, P.A.M. Nex and L. Milani

Ÿ Rare Earth Deposits of Africa

R.E. Harmer and P.A.M. Nex

Ÿ The Coalfields of South-Central Africa: A Current Perspective

P.J. Hancox

Ÿ The Oil and Gas Basins of Africa

R.C. Selley and D. van der Spuy

Ÿ Potash Deposits in Africa

A. Pedley, J. Neubert and S. van der Klauw

A limited number of the Episodes special issue, (volume 39-2) and the South African Journal of Geology,

Volume 120-1, are available for purchase from the offices of the Geological Society of South Africa

through [email protected].

THE GREAT MINERAL FIELDS OF AFRICA

SOUTH AFRICAN JOURNAL OF GEOLOGY

64 65


Figure 1. Gold occurrences and operating mines in the Nubian Shield and its counterpart in the Arabian Peninsula, the Arabian Shield. The occurrences

are predominantly late Cryogenian-Ediacaran orogenic gold in a variety of structural and lithologic settings, but include Tonian-Cryogenian gold-

bearing VMS. After Botros, 2002 (for the Eastern Desert, Egypt); Klemm et al., 2001 (for northern Sudan); Tadesse et al., 2003 (for Ethiopia); Jelenc, 1966

(for Eritrea), and the Saudi Arabian Geological Survey (for the Arabian Shield). Selected deposits are named; operating and planned mines shown by

crossed hammers. The inset shows tectonic domains characterized by differing orogenic styles (after Fritz et al., 2013). Boxes show the locations of

Figures 3, 4, 5, and 6.


Introduction

Gold has been extracted from the Nubian Shield of northeast

Africa for over 5,500 years (Klemm et al., 2001; Klemm and

Klemm, 2013) from hundreds of ancient mines and, today, from

extensive artisanal workings and eleven modern, operating or

planned, large-scale mines ( ). Gold occurs in Figure 1; Table 1

alluvium, altered ultramafic rocks (listwaenite), banded-iron

formation, and Sn ± W ± Mo-bearing granites, but the dominant

deposit types are:

Ÿ gold-bearing quartz vein systems or orogenic gold (Böhlke,

1982; Groves et al., 1998; Robert et al., 2007; Dubé and

Gosselin, 2007),

Ÿ gold-bearing VMS (e.g., Dubé et al., 2007) and

Ÿ oxide gold in weathered zones above gold-bearing VMS

deposits (Cottard et al., 1986) ( ).Figure 2

Gold-enriched weathered caps (oxide gold) extending to depths

of 100 m above primary sulfides are distinctive features of the

central part of the Nubian Shield. Weathering involved oxidation

and supergene processes that produced gold-rich gossans and

quartz-kaolinite-barite rock (SBR) (Figure 2). Gold enrichment in

weathered zones above VMS deposits is, of course, a worldwide

phenomenon, but the degree of gold enrichment in the Nubian

Shield appears to be unique and makes occurrences of

Nubian Shield oxide gold an especially valuable and intently

sought gold ore. At Bisha (Eritrea), the oxide zone contains as

much as ~7 g/t Au, representing a ten-fold increase in gold grade

from the primary sulfides (0.76 g/t Au) (Barrie et al., 2007).

Grades in the oxide zone in the Ariab Mineral District (Sudan)

reach 5 to 10 g/t Au, whereas the primary sulfides have grades of

~1 to 1.5 g/t Au (Bosc et al., 2012).

Artisanal gold mining is important in Sudan and Ethiopia, but

details are not available and artisanal mining is not further

discussed. Sukari, Hamash, Lega Dembi, and Sakaro mines are

working orogenic-gold deposits; Bisha mine and the Ariab group

of mines are working gold-bearing VMS and oxide gold. The

purpose of this paper is to illustrate the scope and style of gold

mineralization in the Nubian Shield by reference to selected

occurrences in different parts of the region and to summarize

their structural and tectonic settings.

Geologic Setting

2The Nubian Shield (~1.9 million km ) is part of an accretionary

orogen at the northern end of the East African Orogen (EAO). It

extends from Sinai to Kenya (Figure 1) and comprises juvenile,

late Tonian-Cryogenian suprasubduction ophiolites and island-

arc rocks that formed in the Mozambique Ocean during and after

Rodinia breakup, Ediacaran volcano-sedimentary basins, and

syn-, late-, and post-tectonic Tonian-Ediacaran mafic to felsic

intrusions (here using the Tonian and Cryogenian age ranges as

currently defined; Cohen et al., 2013 updated). Deformation,

metamorphism, and accretion occurred between ~850 Ma and

550 Ma, but peak metamorphism and pervasive shearing mostly

occurred between 650 Ma and 500 Ma, concurrent with final

~620 Ma assembly of the Nubian Shield (see reviews by Johnson

et al., 2011; Fritz et al., 2013). The Nubian Shield and coeval rocks

of the Arabian Shield, in the western part of the Arabian

Peninsula, represent a vast tract of juvenile Neoproterozoic

rocks that were embedded in end-Neoproterozoic Gondwana

and together constitute the world's largest resource of

Neoproterozoic gold.

Variations in the interplay of structural styles during

Ediacaran assembly of the Nubian Shield resulted in a tripartite

tectonic division (Figure 1, inset; Fritz et al., 2013). Pure shear-

dominated transpression and lateral extension in the southern

Nubian Shield resulted in a pervasive north-south structural

trend represented by north-trending shear zones and greenstone

belts. The central Nubian Shield experienced oblique and

orthogonal east-west compression and north-south stretching,

leading to the development of the Oko and Hamisana shear

zones and contemporaneous Keraf suture (Miller and Dixon,

1992; Abdelsalam, 1994; Abdelsalam and Stern, 1996;

Abdelsalam et al., 1998) (Figure 4). The northern Nubian Shield

underwent shearing and northwest-directed thrusting,

extension, and tectonic escape, which resulted in a dominant

northwesterly structural trend and extension into the Nubian

Shield of the Najd fault system found in the Arabian Shield.

Basement rocks in the Eastern Desert of Egypt include:

~750 to 730 Ma ophiolites and arc-assemblages composed of

greenschist- to lower-amphibolite-facies basalt, andesite, tuffs,

tuffaceous metasedimentary rocks, and local banded-iron

formation (BIF) (Ali et al., 2010). The arc rocks host gold-bearing

VMS, whereas steeply dipping northwest-trending shear

zones and thrusts controlled the emplacement of orogenic gold

(e.g., Helmy et al., 2004; Zoheir, 2012a). Some orogenic gold may

be reworked from older VMS deposits.

Volcanic-arc rocks and arc-related intrusions in the central

Nubian Shield range in age from ~900 Ma to 720 Ma. The Hamisana

shear zone overprinted and displaced the Allaqi-Heiani-Onib-Sol

Hamed suture at the southern end of the Eastern Desert terrane

(Figure 4) by as much as 50 km (Stern et al., 1990). The Oko shear

zone overprinted and sinistrally offset the Nakasib suture by as

much as 10 km. The main controls on gold mineralization in this

part of the Nubian Shield are the Hamisana and Oko shear zones,

the Keraf suture zone, and volcanic assemblages along the

Nakasib, Allaqi-Heiani-Onib-Sol Hamed, and Keraf sutures.

GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD

mined at Hassai (Sudan). Orogenic gold is mined at Sukari and Hamash (Egypt), Qbgbih and Kamoeb (Sudan),

Koka (Eritrea), and Lega Dembi and Sakaro (Ethiopia), and mining is due to start at Gupo (Eritrea) and Tulu Kapi

(Ethiopia) in the near future. More than twenty companies are actively exploring for gold in the region and

potentially important deposits are known in Egypt (Hamama-VMS), in northern Sudan (orogenic gold and VMS),

along strike from Bisha and in the Asmara area, Eritrea (VMS and orogenic gold), in northern Ethiopia (VMS), and in

western and southern Ethiopia (orogenic gold and sparse VMS).


64 65


Figure 1. Gold occurrences and operating mines in the Nubian Shield and its counterpart in the Arabian Peninsula, the Arabian Shield. The occurrences

are predominantly late Cryogenian-Ediacaran orogenic gold in a variety of structural and lithologic settings, but include Tonian-Cryogenian gold-

bearing VMS. After Botros, 2002 (for the Eastern Desert, Egypt); Klemm et al., 2001 (for northern Sudan); Tadesse et al., 2003 (for Ethiopia); Jelenc, 1966

(for Eritrea), and the Saudi Arabian Geological Survey (for the Arabian Shield). Selected deposits are named; operating and planned mines shown by

crossed hammers. The inset shows tectonic domains characterized by differing orogenic styles (after Fritz et al., 2013). Boxes show the locations of

Figures 3, 4, 5, and 6.


Introduction

Gold has been extracted from the Nubian Shield of northeast

Africa for over 5,500 years (Klemm et al., 2001; Klemm and

Klemm, 2013) from hundreds of ancient mines and, today, from

extensive artisanal workings and eleven modern, operating or

planned, large-scale mines ( ). Gold occurs in Figure 1; Table 1

alluvium, altered ultramafic rocks (listwaenite), banded-iron

formation, and Sn ± W ± Mo-bearing granites, but the dominant

deposit types are:

Ÿ gold-bearing quartz vein systems or orogenic gold (Böhlke,

1982; Groves et al., 1998; Robert et al., 2007; Dubé and

Gosselin, 2007),

Ÿ gold-bearing VMS (e.g., Dubé et al., 2007) and

Ÿ oxide gold in weathered zones above gold-bearing VMS

deposits (Cottard et al., 1986) ( ).Figure 2

Gold-enriched weathered caps (oxide gold) extending to depths

of 100 m above primary sulfides are distinctive features of the

central part of the Nubian Shield. Weathering involved oxidation

and supergene processes that produced gold-rich gossans and

quartz-kaolinite-barite rock (SBR) (Figure 2). Gold enrichment in

weathered zones above VMS deposits is, of course, a worldwide

phenomenon, but the degree of gold enrichment in the Nubian

Shield appears to be unique and makes occurrences of

Nubian Shield oxide gold an especially valuable and intently

sought gold ore. At Bisha (Eritrea), the oxide zone contains as

much as ~7 g/t Au, representing a ten-fold increase in gold grade

from the primary sulfides (0.76 g/t Au) (Barrie et al., 2007).

Grades in the oxide zone in the Ariab Mineral District (Sudan)

reach 5 to 10 g/t Au, whereas the primary sulfides have grades of

~1 to 1.5 g/t Au (Bosc et al., 2012).

Artisanal gold mining is important in Sudan and Ethiopia, but

details are not available and artisanal mining is not further

discussed. Sukari, Hamash, Lega Dembi, and Sakaro mines are

working orogenic-gold deposits; Bisha mine and the Ariab group

of mines are working gold-bearing VMS and oxide gold. The

purpose of this paper is to illustrate the scope and style of gold

mineralization in the Nubian Shield by reference to selected

occurrences in different parts of the region and to summarize

their structural and tectonic settings.

Geologic Setting

2The Nubian Shield (~1.9 million km ) is part of an accretionary

orogen at the northern end of the East African Orogen (EAO). It

extends from Sinai to Kenya (Figure 1) and comprises juvenile,

late Tonian-Cryogenian suprasubduction ophiolites and island-

arc rocks that formed in the Mozambique Ocean during and after

Rodinia breakup, Ediacaran volcano-sedimentary basins, and

syn-, late-, and post-tectonic Tonian-Ediacaran mafic to felsic

intrusions (here using the Tonian and Cryogenian age ranges as

currently defined; Cohen et al., 2013 updated). Deformation,

metamorphism, and accretion occurred between ~850 Ma and

550 Ma, but peak metamorphism and pervasive shearing mostly

occurred between 650 Ma and 500 Ma, concurrent with final

~620 Ma assembly of the Nubian Shield (see reviews by Johnson

et al., 2011; Fritz et al., 2013). The Nubian Shield and coeval rocks

of the Arabian Shield, in the western part of the Arabian

Peninsula, represent a vast tract of juvenile Neoproterozoic

rocks that were embedded in end-Neoproterozoic Gondwana

and together constitute the world's largest resource of

Neoproterozoic gold.

Variations in the interplay of structural styles during

Ediacaran assembly of the Nubian Shield resulted in a tripartite

tectonic division (Figure 1, inset; Fritz et al., 2013). Pure shear-

dominated transpression and lateral extension in the southern

Nubian Shield resulted in a pervasive north-south structural

trend represented by north-trending shear zones and greenstone

belts. The central Nubian Shield experienced oblique and

orthogonal east-west compression and north-south stretching,

leading to the development of the Oko and Hamisana shear

zones and contemporaneous Keraf suture (Miller and Dixon,

1992; Abdelsalam, 1994; Abdelsalam and Stern, 1996;

Abdelsalam et al., 1998) (Figure 4). The northern Nubian Shield

underwent shearing and northwest-directed thrusting,

extension, and tectonic escape, which resulted in a dominant

northwesterly structural trend and extension into the Nubian

Shield of the Najd fault system found in the Arabian Shield.

Basement rocks in the Eastern Desert of Egypt include:

~750 to 730 Ma ophiolites and arc-assemblages composed of

greenschist- to lower-amphibolite-facies basalt, andesite, tuffs,

tuffaceous metasedimentary rocks, and local banded-iron

formation (BIF) (Ali et al., 2010). The arc rocks host gold-bearing

VMS, whereas steeply dipping northwest-trending shear

zones and thrusts controlled the emplacement of orogenic gold

(e.g., Helmy et al., 2004; Zoheir, 2012a). Some orogenic gold may

be reworked from older VMS deposits.

Volcanic-arc rocks and arc-related intrusions in the central

Nubian Shield range in age from ~900 Ma to 720 Ma. The Hamisana

shear zone overprinted and displaced the Allaqi-Heiani-Onib-Sol

Hamed suture at the southern end of the Eastern Desert terrane

(Figure 4) by as much as 50 km (Stern et al., 1990). The Oko shear

zone overprinted and sinistrally offset the Nakasib suture by as

much as 10 km. The main controls on gold mineralization in this

part of the Nubian Shield are the Hamisana and Oko shear zones,

the Keraf suture zone, and volcanic assemblages along the

Nakasib, Allaqi-Heiani-Onib-Sol Hamed, and Keraf sutures.


mined at Hassai (Sudan). Orogenic gold is mined at Sukari and Hamash (Egypt), Qbgbih and Kamoeb (Sudan),

Koka (Eritrea), and Lega Dembi and Sakaro (Ethiopia), and mining is due to start at Gupo (Eritrea) and Tulu Kapi

(Ethiopia) in the near future. More than twenty companies are actively exploring for gold in the region and

potentially important deposits are known in Egypt (Hamama-VMS), in northern Sudan (orogenic gold and VMS),

along strike from Bisha and in the Asmara area, Eritrea (VMS and orogenic gold), in northern Ethiopia (VMS), and in

western and southern Ethiopia (orogenic gold and sparse VMS).


66 67

intermittently over a strike length of about 75 km, centered on

the Bisha VMS deposit (Barrie et al., 2007); gold occurrences in

the Asmara-Nakfa belt and its continuation in northern Ethiopia

extend over a strike length of >200 km. Both belts are prime

targets for VMS and orogenic gold and are extensively covered

by mining and exploration licenses (Eritrean Mining Journal,

2013). Gold occurrences in western and southern Ethiopia,

dominantly orogenic gold, are in greenstone belts of the WES

and SES, particularly in or close to north-trending shear zones

(Figure 6).

Gold-bearing VMS and oxide gold

Gold-bearing VMS deposits in the Nubian Shield are known at

Hamama, in the Eastern Desert of Egypt, in the Ariab Mineral

District along the Nakasib suture in northeastern Sudan, and at

Figure 2. Models of gold mineralization types in the Nubian Shield. (A) Orogenic gold, typically associated with low-grade, sheared volcanic,

volcaniclastic, and epiclastic rocks (after Groves et al., 2003). (B) Primary VMS deposit showing a stratiform lens of massive sulfide overlying a discordant

stringer sulfide zone within an envelope of altered rock (alteration pipe) (py = pyrite, cp = chalcopyrite, po = pyrrhotite, sp = sphalerite, and gn = galena)

(after Gibson et al., 2007). (C) Oxide-gold mineralization in weathered profiles developed above Cu-Zn-Au VMS mineralization in the Ariab Mineral

District (Sudan), showing zones of gold-rich gossans and quartz-kaolinite-barite “SBR” rock (after Cottard et al., 1986).



The southern tectonic domain in the Nubian Shield

comprises north-trending, Tonian arc-assemblage greenstone

belts, and flanking domains of gneiss and schist. The arc

assemblages range in age from 890 to 840 Ma in the Southern

Ethiopian Shield (SES) (Tsige 2006; Woldehaimanot and

Behrmann, 1995), to ~875 Ma in the Western Ethiopian Shield

(WES) (Grenne et al., 2003), and ~850 to 800 Ma in northernmost

Ethiopia and Eritrea (Teklay et al., 2002a, b). They include

tholeiitic to calc-alkaline basalt, andesite, dacite and rhyolite

together with variable amounts of volcaniclastic wacke, pelite,

graphitic schist, quartzite, and marble (Grenne et al., 2003;

Woldehaimanot and Behrmann, 1995). The flanking gneiss

domains include quartzofeldspathic gneiss, amphibolite,

sillimanite-kyanite-bearing schist, diorite, granite, and pegmatite

(Worku and Yifa, 1992; Woldehaimanot and Behrmann, 1995;

Tsige, 2006; Abdelsalam et al., 2008; Stern et al., 2012). Gold

occurrences in Eritrea and northern Ethiopia are concentrated in

the two north-trending, curvilinear greenstone belts of

transpressional deformation shown in Figure 5 (Ghebreab et al.,

2009). Gold in the western Augaro-Adobha belt occurs



Table 1. Operating large-scale gold mines in the Nubian Shield.


Orogenic gold

Mine Mine type Country Size Production

2.3 Mt ore in 2014

(Nevsun press release, 3-2-2015)

~1.4 t Au in 2013(La Mancha, 2014)

~11 t/yr Au(Centamin, 2014)

Produced ~0.5 t Au in 2010; recent figures not available

(Taib, 2012)

Producing; figure not available


Planned milling rate of 660,000 t/yr

Production envisioned for 2017

~3.5 t/yr Au (4.5 t gold-silver

doré; 78% Au, 21‰ Ag)(Midroc Gold Mine website,

downloaded 28-4-2015)


Definitive feasibility study in progress; planned production 2.4 t/y Au

(Kefi Minerals website; data


As of February, 2015: Resources (inclusive of reserves):

Measured and indicated 26 Mt containing 18 t Au; Inferred 3.5 Mt containing 3 t Au (Nevsun press release, 3-2-2015)

As of 12-31-11: Indicated VMS resources 81 Mt @ 1.26 g/t

Au; Inferred VMS resource 37 Mt @1.17 g/t Au. Figures for Hassai S., Hadal Awatib E., and Hadayamet (Bosc et al., 2012)

Surface mine resources (inclusive of reserves) 198 Mt grading 1.31 g/t Au (cut off 0.5 g/t Au). Underground measured and indicated resources (inclusive of reserves)

2.9 Mt grading 5.2 g/t Au(Centamin Annual Report, 2014)

Figures not available

~66 t Au(B. Zoheir, written communication 2014)

Resources 2.7 Mt @2.9 g/t Au, containing 6 t Au (Bosc et al., 2012)

Probable reserve 4.6 Mt @ 5.1 g/t gold, containing 21 t Au (Carville et al., 2010; Eritrean Mining Journal, 2014).

Resource 2.8 Mt @ 1.7 g/t Au, containing 4 t Au

(Ross and Martin, 2012)

Surface mine reserves ~66 t @ 3.7 g/t Au. Underground reserves estimated ~11 t @3.6 g/t Au

(MBendi's email mining news, downloaded 3-6-2014)

~20 t Au

(Midroc newsletter 4-6-2014)

Indicated resources 18.8 Mt @ 2.67 g/t Au containing 46 t AuInferred resources 5.33 Mt @ 2.3 g/t Au containing 11 t Au

(Kefi Minerals website; data downloaded 29-4-2015)

Eritrea

Sudan

Egypt

Egypt

Sudan

Sudan

Eritrea

Eritrea

Ethiopia

Ethiopia

Ethiopia

Surface mine

Surface mine; underground

mine

Surface mine; underground mine

Surface mine

Surface mine inaugurated 2013

Surface mine

Surface mine; in production since

end-2015

Surface mine


Underground mine inaugurated 2014

Open-pit mining planned for 2015

Bisha

Ariab Mineral

District

Sukari

Hamash

Qbgbih

Kamoeb

Koka

Gupo

Lega Dembi

Sakaro

Tulu Kapi

66 67

intermittently over a strike length of about 75 km, centered on

the Bisha VMS deposit (Barrie et al., 2007); gold occurrences in

the Asmara-Nakfa belt and its continuation in northern Ethiopia

extend over a strike length of >200 km. Both belts are prime

targets for VMS and orogenic gold and are extensively covered

by mining and exploration licenses (Eritrean Mining Journal,

2013). Gold occurrences in western and southern Ethiopia,

dominantly orogenic gold, are in greenstone belts of the WES

and SES, particularly in or close to north-trending shear zones

(Figure 6).


Gold-bearing VMS deposits in the Nubian Shield are known at

Hamama, in the Eastern Desert of Egypt, in the Ariab Mineral

District along the Nakasib suture in northeastern Sudan, and at

Figure 2. Models of gold mineralization types in the Nubian Shield. (A) Orogenic gold, typically associated with low-grade, sheared volcanic,

volcaniclastic, and epiclastic rocks (after Groves et al., 2003). (B) Primary VMS deposit showing a stratiform lens of massive sulfide overlying a discordant

stringer sulfide zone within an envelope of altered rock (alteration pipe) (py = pyrite, cp = chalcopyrite, po = pyrrhotite, sp = sphalerite, and gn = galena)

(after Gibson et al., 2007). (C) Oxide-gold mineralization in weathered profiles developed above Cu-Zn-Au VMS mineralization in the Ariab Mineral

District (Sudan), showing zones of gold-rich gossans and quartz-kaolinite-barite “SBR” rock (after Cottard et al., 1986).



The southern tectonic domain in the Nubian Shield

comprises north-trending, Tonian arc-assemblage greenstone

belts, and flanking domains of gneiss and schist. The arc

assemblages range in age from 890 to 840 Ma in the Southern

Ethiopian Shield (SES) (Tsige 2006; Woldehaimanot and

Behrmann, 1995), to ~875 Ma in the Western Ethiopian Shield

(WES) (Grenne et al., 2003), and ~850 to 800 Ma in northernmost

Ethiopia and Eritrea (Teklay et al., 2002a, b). They include

tholeiitic to calc-alkaline basalt, andesite, dacite and rhyolite

together with variable amounts of volcaniclastic wacke, pelite,

graphitic schist, quartzite, and marble (Grenne et al., 2003;

Woldehaimanot and Behrmann, 1995). The flanking gneiss

domains include quartzofeldspathic gneiss, amphibolite,

sillimanite-kyanite-bearing schist, diorite, granite, and pegmatite

(Worku and Yifa, 1992; Woldehaimanot and Behrmann, 1995;

Tsige, 2006; Abdelsalam et al., 2008; Stern et al., 2012). Gold

occurrences in Eritrea and northern Ethiopia are concentrated in

the two north-trending, curvilinear greenstone belts of

transpressional deformation shown in Figure 5 (Ghebreab et al.,

2009). Gold in the western Augaro-Adobha belt occurs



Table 1. Operating large-scale gold mines in the Nubian Shield.


Orogenic gold

Mine Mine type Country Size Production

2.3 Mt ore in 2014

(Nevsun press release, 3-2-2015)

~1.4 t Au in 2013(La Mancha, 2014)

~11 t/yr Au(Centamin, 2014)

Produced ~0.5 t Au in 2010; recent figures not available

(Taib, 2012)



Planned milling rate of 660,000 t/yr

Production envisioned for 2017

~3.5 t/yr Au (4.5 t gold-silver

doré; 78% Au, 21‰ Ag)(Midroc Gold Mine website,



Definitive feasibility study in progress; planned production 2.4 t/y Au

(Kefi Minerals website; data


As of February, 2015: Resources (inclusive of reserves):

Measured and indicated 26 Mt containing 18 t Au; Inferred 3.5 Mt containing 3 t Au (Nevsun press release, 3-2-2015)

As of 12-31-11: Indicated VMS resources 81 Mt @ 1.26 g/t

Au; Inferred VMS resource 37 Mt @1.17 g/t Au. Figures for Hassai S., Hadal Awatib E., and Hadayamet (Bosc et al., 2012)

Surface mine resources (inclusive of reserves) 198 Mt grading 1.31 g/t Au (cut off 0.5 g/t Au). Underground measured and indicated resources (inclusive of reserves)

2.9 Mt grading 5.2 g/t Au(Centamin Annual Report, 2014)

Figures not available

~66 t Au(B. Zoheir, written communication 2014)

Resources 2.7 Mt @2.9 g/t Au, containing 6 t Au (Bosc et al., 2012)

Probable reserve 4.6 Mt @ 5.1 g/t gold, containing 21 t Au (Carville et al., 2010; Eritrean Mining Journal, 2014).

Resource 2.8 Mt @ 1.7 g/t Au, containing 4 t Au

(Ross and Martin, 2012)

Surface mine reserves ~66 t @ 3.7 g/t Au. Underground reserves estimated ~11 t @3.6 g/t Au

(MBendi's email mining news, downloaded 3-6-2014)

~20 t Au

(Midroc newsletter 4-6-2014)

Indicated resources 18.8 Mt @ 2.67 g/t Au containing 46 t AuInferred resources 5.33 Mt @ 2.3 g/t Au containing 11 t Au

(Kefi Minerals website; data downloaded 29-4-2015)

Eritrea

Sudan

Egypt

Egypt

Sudan

Sudan

Eritrea

Eritrea

Ethiopia

Ethiopia

Ethiopia

Surface mine

Surface mine; underground

mine


Surface mine

Surface mine inaugurated 2013

Surface mine

Surface mine; in production since

end-2015

Surface mine


Underground mine inaugurated 2014

Open-pit mining planned for 2015

Bisha

Ariab Mineral

District

Sukari

Hamash

Qbgbih

Kamoeb

Koka

Gupo

Lega Dembi

Sakaro

Tulu Kapi

68 69

Bisha, Debarwa, and elsewhere in Eritrea (Figures 3, 4, 5). Recent

discoveries of hydrothermal alteration zones and gossans

indicate potential VMS mineralization at Tanashieb, along the

Keraf suture ( Johnson, 2014), and at Kata, Abetselo, and Azale

south of the Dish Mountains in the WES (Tadesse et al., 2003).

The Hamama deposit, currently under exploration

(Alexander Nubia, 2015), comprises disseminated and semi-

massive Au- and Ag-bearing Zn sulfides capped by a gold-rich

oxide zone ~35 m thick, and locally by a supergene silica-barite

rock similar to that known in the Ariab Mineral District. The

deposit is hosted by altered and bleached metavolcanic rocks,

underlain by argillaceous metasedimentary rocks and BIF, and

overlain by basaltic andesite, all belonging to the late Tonian-

Cryogenian island-arc assemblage found in this part of

the Nubian Shield. The mineralized rocks dip moderately to the

north and extend as much as 3 km along strike in a westerly

direction. The oxidized cap averages 41 m in width, with grades

of 1.2 to 3 g/t Au (Alexander Nubia, 2013; 2015). The sulfide zone

is as much as 75 m wide and is open at depth. Examples of drill-

hole intersections include 8 to 13 m of semi-massive sulfide

grading ~5% Zn, 0.25 to 0.4% Cu, 0.30 to 1.18 g/t Au, and

~37 to 69 g/t Ag, and 38 to 43 m of semi-massive sulfide grading

1.1 to 1.2 g/t Au and 25 to 65 g/t Ag, with grades increasing with

depth (Alexander Nubia, 2013; 2015).

Mining has been conducted in the Ariab Mineral District

(Figure 4) since open-pit operations in oxide gold ore began in

1991. By 2012, 2.3 Moz Au had been mined from the upper gold-

rich oxidized cap rock of multiple deposits (Bosc et al., 2012), but

most oxide gold in the district is now depleted. The district

contains as many as 13 VMS deposits, 12 of which are exposed in

the floor of open pits in the oxide zone. Ariab VMS deposits cluster

in discrete areas, each containing several small sulfide bodies.

They are tabular and mostly 0.3 to 25 m thick although,

exceptionally, may be as much 200 m thick because of structural

repetition. Ore bodies extend up to 2,500 m along strike and 400 m

down dip (Plyley et al., 2009). The deposits are stratabound in a

discontinuous stratigraphic unit of altered felsic tuffs 10 to 100 m

thick that is part of a regional sequence of intermediate to mafic

calc-alkaline and tholeiitic lavas, tuffs, and volcaniclastic

sedimentary rocks assigned to the Ariab Series and dated

Figure 4. Selected gold occurrences in the Nubian Shield in Sudan and part of southern Egypt, after Klemm et al. (2001), Gaskell (1985), Lissan and

Bakheit (2010), and La Mancha Resources Inc. and other company websites. Structural trends after Abdelsalam et al. (1998), and Kusky and Ramadan

(2002). Inset diagrammatically shows the style of shearing and loci of orogenic gold mineralization at the intersection of the Allaqi-Heiani and Onib-Sol

Hamed sutures and the Hamisana shear zone (after unpublished map by Basem Zoheir).



Figure 3. Gold occurrences in the Eastern Desert, Egypt, compilation by Basem Zoheir (this study) with additions from Botros (2004) and Alexander

Nubia International Inc. (www.alexandernubia.com). Sukari and Hamash are producing mines; Hamama is a promising exploration project. Orogenic-

gold in BIF and listwaenite and intrusion-related gold are shown for completeness, but are not further discussed. Inset, after Helmy et al. (2004), illustrates

the structural control of orogenic gold in the central Eastern Desert.



68 69

Bisha, Debarwa, and elsewhere in Eritrea (Figures 3, 4, 5). Recent

discoveries of hydrothermal alteration zones and gossans

indicate potential VMS mineralization at Tanashieb, along the

Keraf suture ( Johnson, 2014), and at Kata, Abetselo, and Azale

south of the Dish Mountains in the WES (Tadesse et al., 2003).

The Hamama deposit, currently under exploration

(Alexander Nubia, 2015), comprises disseminated and semi-

massive Au- and Ag-bearing Zn sulfides capped by a gold-rich

oxide zone ~35 m thick, and locally by a supergene silica-barite

rock similar to that known in the Ariab Mineral District. The

deposit is hosted by altered and bleached metavolcanic rocks,

underlain by argillaceous metasedimentary rocks and BIF, and

overlain by basaltic andesite, all belonging to the late Tonian-

Cryogenian island-arc assemblage found in this part of

the Nubian Shield. The mineralized rocks dip moderately to the

north and extend as much as 3 km along strike in a westerly

direction. The oxidized cap averages 41 m in width, with grades

of 1.2 to 3 g/t Au (Alexander Nubia, 2013; 2015). The sulfide zone

is as much as 75 m wide and is open at depth. Examples of drill-

hole intersections include 8 to 13 m of semi-massive sulfide

grading ~5% Zn, 0.25 to 0.4% Cu, 0.30 to 1.18 g/t Au, and

~37 to 69 g/t Ag, and 38 to 43 m of semi-massive sulfide grading

1.1 to 1.2 g/t Au and 25 to 65 g/t Ag, with grades increasing with

depth (Alexander Nubia, 2013; 2015).

Mining has been conducted in the Ariab Mineral District

(Figure 4) since open-pit operations in oxide gold ore began in

1991. By 2012, 2.3 Moz Au had been mined from the upper gold-

rich oxidized cap rock of multiple deposits (Bosc et al., 2012), but

most oxide gold in the district is now depleted. The district

contains as many as 13 VMS deposits, 12 of which are exposed in

the floor of open pits in the oxide zone. Ariab VMS deposits cluster

in discrete areas, each containing several small sulfide bodies.

They are tabular and mostly 0.3 to 25 m thick although,

exceptionally, may be as much 200 m thick because of structural

repetition. Ore bodies extend up to 2,500 m along strike and 400 m

down dip (Plyley et al., 2009). The deposits are stratabound in a

discontinuous stratigraphic unit of altered felsic tuffs 10 to 100 m

thick that is part of a regional sequence of intermediate to mafic

calc-alkaline and tholeiitic lavas, tuffs, and volcaniclastic

sedimentary rocks assigned to the Ariab Series and dated

Figure 4. Selected gold occurrences in the Nubian Shield in Sudan and part of southern Egypt, after Klemm et al. (2001), Gaskell (1985), Lissan and

Bakheit (2010), and La Mancha Resources Inc. and other company websites. Structural trends after Abdelsalam et al. (1998), and Kusky and Ramadan

(2002). Inset diagrammatically shows the style of shearing and loci of orogenic gold mineralization at the intersection of the Allaqi-Heiani and Onib-Sol

Hamed sutures and the Hamisana shear zone (after unpublished map by Basem Zoheir).



Figure 3. Gold occurrences in the Eastern Desert, Egypt, compilation by Basem Zoheir (this study) with additions from Botros (2004) and Alexander

Nubia International Inc. (www.alexandernubia.com). Sukari and Hamash are producing mines; Hamama is a promising exploration project. Orogenic-

gold in BIF and listwaenite and intrusion-related gold are shown for completeness, but are not further discussed. Inset, after Helmy et al. (2004), illustrates

the structural control of orogenic gold in the central Eastern Desert.



70 71

deposit is classified as an example of bimodal-siliciclastic VMS

(Barrie et al., 2007). The host rocks are not directly dated, but are

similar to volcanic rocks elsewhere in Eritrea dated ~850 Ma

(Teklay et al., 2003). The Bisha sulfide body was metamorphosed

and folded together with its host rocks, and the deposit is

modeled as lying in a north-trending, steep-limbed antiform. The

primary ore, grading ~1 g/t Au, comprises fine- to medium-

grained pyrite, sphalerite, and chalcopyrite, lesser galena and

pyrrhotite, and accessory arsenopyrite, tetrahedrite, tannantite,

and enargite (Barrie et al., 2007). Gangue minerals include

quartz, chlorite, sericite-muscovite, clay minerals, siderite, and

ferroan carbonate.

Orogenic gold

Of the hundreds of ancient gold mines in the Nubian Shield

(Klemm and Klemm, 2013; Klemm et al., 2001), most consist of

adits and trenches on quartz veins, and it is evident that ancient

miners were systematically working orogenic gold. The deposits

occur as gold-bearing quartz vein systems concentrated in

dilatant-extensional en-echelon fractures in sheared and strongly

altered volcanic and volcaniclastic rocks and/or small felsic

intrusions. The quartz-vein arrays may exhibit both reverse and

normal senses of shear and, in places, cut through or structurally

overprint VMS deposits (Ghebreab et al., 2009).

Figure 6. Gold occurrences and producing mines plotted on a simplified map of Precambrian geology in western and southern Ethiopia. Gold

occurrences after Tadesse et al. (2003); geology after “Geology of Ethiopia”, Ethiopian Ministry of Mines, Geological Survey of Ethiopia. Subdivisions of the

Western Ethiopian Shield after Woldemichael et al. (2009): WMGT=western migmatitic high-grade gneissic terrane, CVST=central volcano-sedimentary

terrane, EMGT=eastern migmatitic high-grade gneissic terrane. Subdivision of the Southern Ethiopian Shield after Tsige and Abdelsalam (2005). Inset

schematically shows the structural setting of gold occurrences in the Southern Ethiopian shield, with a concentration of mineralization between sinistral

and dextral shear zones (after Worku and Schandelmeier; 1996).



~880 Ma (Cottard et al., 1986; Deschamps et al., 2004; Abu Fatima,

2006). The sulfides are mostly massive, fine grained, layered, and

locally brecciated. They include pyrite, sphalerite, and chalcopyrite

with lesser pyrrhotite, galena, tetrahedrite-freibergite, and

arsenopyrite, and contain ~1 to 1.5 g/t Au (Plyley et al., 2009; Bosc

et al., 2012). Wall-rock alteration includes proximal silicification

and more distant chloritization, sericitization, and local

carbonatization (Abu Fatima, 2006). As shown in Figure 2, as much

as 10 g/t Au has been found in the oxide zone.

The Bisha deposit (Figure 5) is well documented as a world-

class gold-bearing VMS deposit (Barrie et al., 2007) and is,

currently, the largest such deposit known in the Nubian Shield

(see Table 1). The deposit was discovered in 1998. Mining of the

copper-rich supergene ore commenced in 2013, and open-pit

mining of the primary Zn-Cu ore is planned for 2016 (Nevsun

Resources Ltd news release http://www.nevsun.com/projects/

bisha-main downloaded 4/28/2015). The deposit comprises an

oxide zone or near-surface gold-enriched oxide cap 35 m thick

(8.5 g/t Au; 206 g/t Ag); a supergene zone containing a copper-

enriched massive sulfide horizon (3.7% Cu, 0.03% Zn, 0.6 g/t Au,

23 g/t Ag); and a primary massive sulfide zone containing high

grade zinc and appreciable gold and silver (1% Cu, 5.7% Zn, 0.7

g/t Au, 45 g/t Ag). This zone is open at depth to below 450 m. The

Bisha deposit is hosted by felsic ash and lapilli ash tuffs in the

middle to upper part of a regional sequence more than 3000 m

thick containing crystal tuffs, minor rhyolitic flows and flow

breccias, basalt and volcaniclastic sedimentary rocks (Barrie

et al., 2007; Gribble et al., 2013). Based on this lithology, the

Figure 5. Selected gold occurrences in the Nubian Shield in Eritrea and northern Ethiopia, showing the prevalence of north to north-northeast structural

trends. Transpressional belts in Eritrea after Ghebreab et al. (2009). Extension of the Asmara-Nakfa belt into Ethiopia is inferred from geologic maps and

project reports (e.g., Archibald et al., 2014).



70 71

deposit is classified as an example of bimodal-siliciclastic VMS

(Barrie et al., 2007). The host rocks are not directly dated, but are

similar to volcanic rocks elsewhere in Eritrea dated ~850 Ma

(Teklay et al., 2003). The Bisha sulfide body was metamorphosed

and folded together with its host rocks, and the deposit is

modeled as lying in a north-trending, steep-limbed antiform. The

primary ore, grading ~1 g/t Au, comprises fine- to medium-

grained pyrite, sphalerite, and chalcopyrite, lesser galena and

pyrrhotite, and accessory arsenopyrite, tetrahedrite, tannantite,

and enargite (Barrie et al., 2007). Gangue minerals include

quartz, chlorite, sericite-muscovite, clay minerals, siderite, and

ferroan carbonate.

Orogenic gold

Of the hundreds of ancient gold mines in the Nubian Shield

(Klemm and Klemm, 2013; Klemm et al., 2001), most consist of

adits and trenches on quartz veins, and it is evident that ancient

miners were systematically working orogenic gold. The deposits

occur as gold-bearing quartz vein systems concentrated in

dilatant-extensional en-echelon fractures in sheared and strongly

altered volcanic and volcaniclastic rocks and/or small felsic

intrusions. The quartz-vein arrays may exhibit both reverse and

normal senses of shear and, in places, cut through or structurally

overprint VMS deposits (Ghebreab et al., 2009).

Figure 6. Gold occurrences and producing mines plotted on a simplified map of Precambrian geology in western and southern Ethiopia. Gold

occurrences after Tadesse et al. (2003); geology after “Geology of Ethiopia”, Ethiopian Ministry of Mines, Geological Survey of Ethiopia. Subdivisions of the

Western Ethiopian Shield after Woldemichael et al. (2009): WMGT=western migmatitic high-grade gneissic terrane, CVST=central volcano-sedimentary

terrane, EMGT=eastern migmatitic high-grade gneissic terrane. Subdivision of the Southern Ethiopian Shield after Tsige and Abdelsalam (2005). Inset

schematically shows the structural setting of gold occurrences in the Southern Ethiopian shield, with a concentration of mineralization between sinistral

and dextral shear zones (after Worku and Schandelmeier; 1996).



~880 Ma (Cottard et al., 1986; Deschamps et al., 2004; Abu Fatima,

2006). The sulfides are mostly massive, fine grained, layered, and

locally brecciated. They include pyrite, sphalerite, and chalcopyrite

with lesser pyrrhotite, galena, tetrahedrite-freibergite, and

arsenopyrite, and contain ~1 to 1.5 g/t Au (Plyley et al., 2009; Bosc

et al., 2012). Wall-rock alteration includes proximal silicification

and more distant chloritization, sericitization, and local

carbonatization (Abu Fatima, 2006). As shown in Figure 2, as much

as 10 g/t Au has been found in the oxide zone.

The Bisha deposit (Figure 5) is well documented as a world-

class gold-bearing VMS deposit (Barrie et al., 2007) and is,

currently, the largest such deposit known in the Nubian Shield

(see Table 1). The deposit was discovered in 1998. Mining of the

copper-rich supergene ore commenced in 2013, and open-pit

mining of the primary Zn-Cu ore is planned for 2016 (Nevsun

Resources Ltd news release http://www.nevsun.com/projects/

bisha-main downloaded 4/28/2015). The deposit comprises an

oxide zone or near-surface gold-enriched oxide cap 35 m thick

(8.5 g/t Au; 206 g/t Ag); a supergene zone containing a copper-

enriched massive sulfide horizon (3.7% Cu, 0.03% Zn, 0.6 g/t Au,

23 g/t Ag); and a primary massive sulfide zone containing high

grade zinc and appreciable gold and silver (1% Cu, 5.7% Zn, 0.7

g/t Au, 45 g/t Ag). This zone is open at depth to below 450 m. The

Bisha deposit is hosted by felsic ash and lapilli ash tuffs in the

middle to upper part of a regional sequence more than 3000 m

thick containing crystal tuffs, minor rhyolitic flows and flow

breccias, basalt and volcaniclastic sedimentary rocks (Barrie

et al., 2007; Gribble et al., 2013). Based on this lithology, the

Figure 5. Selected gold occurrences in the Nubian Shield in Eritrea and northern Ethiopia, showing the prevalence of north to north-northeast structural

trends. Transpressional belts in Eritrea after Ghebreab et al. (2009). Extension of the Asmara-Nakfa belt into Ethiopia is inferred from geologic maps and

project reports (e.g., Archibald et al., 2014).



72 73

lens-shaped bodies of quartz pods and quartz veins, and

abundant quartz stringers, stockworks, and quartz breccia in a

matrix of silicified and sulfidized host rock (Tadesse, 2004).

Wallrocks include quartz-feldspar-mica schist, quartz-feldspathic

gneiss, and massive to foliated metagabbro (Tadesse, 2004). The

deposit extends over an area of about 1000m by 100m, and

individual veins, a few centimeters to 3 m in thickness, extend for

hundreds of meters along strike and down dip (Tadesse, 2004).

Overall, the quartz vein system grades between 3 and 4 g/t Au;

highest gold grades are in quartz veins, stringers, and stockwork;

lower grades are in sheared and altered wall rocks. The ore

minerals include gold, sulfides (pyrrhotite, chalcopyrite, galena,

pyrite, arsenopyrite), tellurides (altaite, hessite, petzite),

sulfosalts, and lesser amounts of cubanite, tetrahedrite,

mackinawite (Tadesse, 2004).

Discussion

The gold-bearing VMS deposits in the Nubian Shield are hosted

by juvenile, submarine, arc-related volcanic, volcaniclastic, and

sedimentary rocks. The deposits are enclosed by alteration zones

and exhibit some degree of metal zoning. They comprise lenses

of massive to semi-massive sulfides of iron, copper, zinc, and

lead and have variable amounts of telluride. The primary sulfides

grade ~1 to 1.5 g/t Au. Gold enrichment occurs in the oxide and

supergene zones and is particularly notable in the central part of

the Nubian Shield with grades of 5 to 10 g/t Au. A feature of the

distribution of known occurrences of gold-bearing VMS deposits

in the Nubian Shield, is their apparent confinement to arc

assemblages in the northern half of the Shield, from northern

Ethiopia northward. The greenstone belts of western and

southern Ethiopia contain Neoproterozoic juvenile, submarine

volcanic rocks, but have only sparse indications of VMS

mineralization. The reason for this is uncertain, but the

distribution raises interesting questions about whether it reflects

an original difference in geologic history between the south and

the north, so that VMS deposits never developed in the south, or

whether it is an effect of preferential preservation of VMS

deposits in the north because of less intense exhumation and

erosion of the volcanic-arc host rocks.

Direct dating of volcanic assemblages hosting gold-bearing

VMS deposits in the shield is not widely available. Zircon-207 206

evaporation Pb/ Pb dating of granitoids in the western part of

the Nakfa terrane of Eritrea at 849 ± 20 Ma suggests a major

intrusive event at ~850 Ma (Teklay et al., 2003), compatible with

indications elsewhere in the region of magmatism at 850 to 800 Ma

(Teklay, 2006; Teklay et al., 2002a,b; 2003). Galena, cerrusite, and

anglesite lead-isotope analysis from the Bisha district give a mean

model age of ~780 Ma. Galena from the Adi Nefas deposit in the

Asmara-Nakfa belt, ~170 km to the east of Bisha, yields a lead

model age of 720 Ma (Barrie et al., 2007), broadly similar to detrital

zircons in Neoproterozoic diamictites and Ordovician

siliciclastic rocks in northern Ethiopia that yield U-Pb SHRIMP

ages of ~740 Ma (Avigad et al., 2007). Perhaps the VMS volcanic

host rocks in Eritrea are actually younger than currently

interpreted; alternatively, the discrepancy may be resolved by

dating both host rocks and mineralization with greater precision.

Stratiform VMS and barite mineralization in the Ariab Mineral

district have a galena, cerrusite and anglesite mineral lead model

age of 702 ± 15 Ma (C.T. Barrie, unpublished data). In contrast, a

probable basement granite intrusion structurally adjacent to gold

mineralization at the Kamoeb gold mine has a single zircon 207 206

Pb/ Pb -evaporation age of 888 ± 4 Ma (Deschamps et al.,

2004). No age dating has been reported for the Hamama VMS

deposit, but volcanic-arc rocks in the Eastern Desert have a

general age of ~750 to 730 Ma (Ali et al., 2009) suggesting that

Hamama mineralization may be late Tonian-Cryogenian.

Orogenic-gold occurrences are widespread in the Nubian

Shield. They are characterized by native gold or gold-sulfide-

telluride assemblages disseminated in silicified, brittle-ductile

shear zones. The ore mineralogy includes pyrite ± arsenopyrite

± chalcopyrite ± sphalerite ± tellurides in a matrix of quartz ±

carbonate. The hosting shear zones belong to the episode of

transpressional deformation, strike-slip shearing, and shortening

that characterized late Cryogenian-Ediacaran, tectonic escape,

extension, and orogenic collapse in the Shield. The shear zones

and quartz veins occur within granitoid plutons, and in adjacent

zones of sheared and strongly altered volcano-sedimentary

and/or ultramafic rocks. In the northern Nubian Shield, shear

structures are mainly those of the northwest-trending Najd fault

system; farther south, they include the north-trending Hamisana

and Oko shortening zones and Keraf suture, and in Eritrea and

Ethiopia they include north-trending sinistral and dextral shear

zones in, and at the margins of, greenstone belts.

The ages of orogenic gold formation in the Nubian Shield are

not well established. Broad inferences can be made by

correlating gold mineralization with shearing and adjacent

magmatic events, but the results are not sufficiently robust to be

satisfactory from a metallogenic point of view. Arsenopyrite in

quartz veins at Fawakhir, which is interpreted as the most likely

age of mineralization, yields a Re-Os age of 601 ± 17 Ma (Zoheir

et al., 2014). This age is consistent with the inferred age of

shearing at the deposit, and also overlaps with the 598 ± 3 Ma U-

Pb zircon age of the host Fawakhir pluton (U-Pb TIMS on zircon;

Andresen et al., 2009). However, the Re-Os age has a large error

and is not definitive evidence for a genetic relationship between

mineralization, shearing, and/or granite magmatism. Rubidium-

strontium analysis of sericite concentrates obtained from gold-

bearing quartz veins at Lega Dembi gives an age of 484 ± 67 Ma

(Billay et al., 1997). But the result is an errorchron (MSWD=32)

and is not compatible with the result of structural analysis, which

suggest that gold mineralization occurred during active shearing,

concurrent with 660 Ma amphibolite-greenschist facies

metamorphism (Ghebreab et al., 1992). Galena at the Koka

deposit yields an anomalously young lead-model age of 495 ±

30 Ma, and lead data for one sample from the Kamoeb gold-

quartz vein system at Ariab give an anomalously old age of

881 Ma (C.T. Barrie, unpublished data).

Conclusions

The Nubian Shield evolved as an accretionary orogen through a

~300 million year process of arc-ophiolite-TTG magmatism, arc-

arc collision and terrane amalgamation, greenschist-amphibolite



The Sukari deposit in the Eastern Desert of Egypt is at the

southern end of the major belt of Najd shearing (the “Najd fault

corridor”) that extends obliquely across, and controls much of,

the orogenic gold in the region (Figure 3, inset; Helmy et al.,

2004). Other deposits along this belt include El Sid and Fawakhir

(Zoheir et al., 2014). Modern mining began at Sukari in 2009. The

deposit comprises an array of quartz veins in the Sukari pluton,

an intrusion of granodiorite and tonalite weakly constrained to

630 to 580 Ma (Harraz, 1991). The deposit is elongated north-

northeast - south-southwest; it is ~2.3 km long, 100 to 600 m

wide, and extends to a depth of at least 1200 m. The pluton is

located in a local flower structure developed in brittle-ductile

shear zones within a stack of north-northwest-verging thrusts on

the northeastern flank of the Hafafit gneiss dome (Helmy et al.,

2004). The ore mineralogy in the veins includes pyrite,

arsenopyrite and, less common, sphalerite, chalcopyrite, galena

and gold grading as much as ~5 g/t Au. Grades of 2.5 g/t Au are

reported from the alteration zones that surround the quartz veins

(Khalaf and Oweiss, 1993).

The Fawakhir deposit is a quartz-vein system in the Fawakhir

pluton and adjacent sheared ophiolitic rocks (Zoheir et al., 2014).

The pluton and ophiolitic rocks are in the belt of northwest-

trending strike-slip shears and thrusts belonging to the Najd fault

corridor, mentioned above. The mineralization consists of

subparallel, easterly trending and south-dipping, sulfide and

gold-bearing quartz veins within a ~40 m wide zone of shearing

and hydrothermal alteration. The veins, as much as 400 m long

and 1.3 m wide, consist of massive and laminated quartz with

subordinate calcite and sericite. They have a mineral assemblage

of pyrite-arsenopyrite-pyrrhotite-sphalerite-galena-chalcopyrite-

electrum ± Ag-Au-Bi tellurides, and have grades of 1.5 to ~23 g/t

Au (Gabra, 1986).

Orogenic gold farther south is concentrated along the

Hamisana shear zone, and the Keraf, Allaqi-Heiani-Onib-Sol

Hamed, and Nakasib sutures (Figure 4). Mining is reported to

have started at the Qbgbih deposit along the Keraf suture but

little information is available. Exploration is underway elsewhere

along the Keraf suture at the Galat Sufa, Negeim, Kimaweit,

Artoli, and Umtrambiesh prospects, and along the Nakasib

suture at Kamoeb. The Romite deposit (Zoheir, 2012b) is an

example of orogenic gold in the structurally complex area east of

the intersection of the Allaqi-Heinai-Onib-Sol Hamed suture by

the Hamisana shear zone (Figure 4). It is located in a north-

northeast-trending sinistral shear zone that splays off the

Hamisana shear zone. Gold, grading as much as 7 g/t Au, occurs

as free grains in north-northeast-trending quartz and quartz-

carbonate veins in quartz diorite and as disseminations in iron-

stained, carbonate-altered and silicified wall rock. Sulfides in the

veins include pyrite, arsenopyrite, and lesser chalcopyrite and

pyrrhotite (Zoheir, 2012b). The morphology, pinch and swell

structure, and quartz texture of the veins suggest vein formation

was synkinematic with north-northeast-shearing (Zoheir, 2012b)

and it appears that quartz precipitated in spaces created at

releasing bends in the shears.

At Galat Sufar, currently the best known example of orogenic

gold along the Keraf suture ( Johnson, 2014), mapping and

drilling reveal mesozonal, shear-zone-hosted orogenic gold over

a strike length of >10 km. The mineralization comprises a system

of quartz veins in tension gashes and shear zones, and swarms of

veinlets in well-foliated and folded, schistose volcano-

sedimentary rocks ( Johnson, 2014). Examples of drill-hole

intersections include 23 m grading 2.34 g/t Au and 49 m grading

1.90 g/t Au ( Johnson, 2014).

As mentioned above, orogenic-gold occurrences in Eritrea

and northern Ethiopia (Figure 5) are concentrated in belts of

transpressional deformation, the same belts that contain VMS

deposits. The Koka deposit comprises an elongate stockwork

>600m long and 20 to 30 m wide in microgranite in the Augaro-

Adobha belt (Chalice, 2010; Eritrean Mining Journal, 2014). The

sulfide mineralogy includes pyrite, sphalerite, galena and

chalcopyrite, and gold is present as tiny blebs within the galena.

The Konate prospect is an area of artisanal workings on quartz

stockwork in silica-sericite altered microgranite, ~6 km south

of and virtually along strike from Koka. The Gupo deposit

comprises gold and sulfides in quartz veins along a shear zone in

the Asmara-Nakfa belt (Ross and Martin, 2012). Adi Goshu and

Lihamat, along the same belt in northern Ethiopia, are close to

the contact between a northeast-trending elongate body of

quartz-feldspar porphyry and sericite-altered mafic and felsic

volcanic and volcaniclastic rocks, and local BIF (Archibald

et al, 2014).

Orogenic-gold deposits in the Western and Southern

Ethiopian Shields are exemplified by Tulu Kapi and Lega Dembi

(Figure 6). The Tulu Kapi deposit is a historic gold mine and a

definitive feasibility study is in progress in preparation for

renewed surface mining and planned processing of 1.2 Mt/a ore

at a grade of 2.4 g/t Au for an estimated 2.4 t Au per year

(Kefi Minerals, 2014a, b). The Tulu Kapi deposit is hosted by

syenite emplaced in strongly sheared metavolcanic and

metasedimentary rocks belonging to the central volcano-

sedimentary terrane greenstone belt (CVST) (Figure 6).

Mineralization is known over an area of 1500 m by 400 m, and

extends for >400 m beneath the surface. It consists of gold-

bearing quartz-carbonate veins, veinlets, and stockwork in a

series of stacked subhorizontal lodes (or lenses). The upper

lodes are characterized by gold, pyrite, and minor sphalerite and

galena, whereas the lower lodes contain significant amounts of

sphalerite, galena and minor arsenopyrite and chalcopyrite as

well as gold (Nyota Minerals, 2014). Based on its setting in a

strongly sheared greenstone belt close to the Tulu Dimtu-Baruda

shear zone, Tulu Kapi has the character of a typical orogenic-

gold deposit. However, its occurrence in syenite raises the

possibility that the deposit may belong to the class of intrusion-

related gold. This possibility requires further study and work is

underway to locate an intrusion that may have acted as the metal

source (Nyota Minerals, 2014).

Orogenic-gold occurrences in the Southern Ethiopian Shield

are in greenstone belts and sheared gneiss and schist in a

structural scissor between north- and northwest-trending shear

zones (Figure 6, inset). They include Lega Dembi, a surface and

underground mine, and Sakaro, an underground mine. The Lega

Dembi deposit is located in strongly sheared rocks at the eastern

margin of the Megado greenstone belt (Billay et al., 1997;

Ghebreab et al., 1992, 2009). The deposit is related to tabular and



72 73

lens-shaped bodies of quartz pods and quartz veins, and

abundant quartz stringers, stockworks, and quartz breccia in a

matrix of silicified and sulfidized host rock (Tadesse, 2004).

Wallrocks include quartz-feldspar-mica schist, quartz-feldspathic

gneiss, and massive to foliated metagabbro (Tadesse, 2004). The

deposit extends over an area of about 1000m by 100m, and

individual veins, a few centimeters to 3 m in thickness, extend for

hundreds of meters along strike and down dip (Tadesse, 2004).

Overall, the quartz vein system grades between 3 and 4 g/t Au;

highest gold grades are in quartz veins, stringers, and stockwork;

lower grades are in sheared and altered wall rocks. The ore

minerals include gold, sulfides (pyrrhotite, chalcopyrite, galena,

pyrite, arsenopyrite), tellurides (altaite, hessite, petzite),

sulfosalts, and lesser amounts of cubanite, tetrahedrite,

mackinawite (Tadesse, 2004).

Discussion

The gold-bearing VMS deposits in the Nubian Shield are hosted

by juvenile, submarine, arc-related volcanic, volcaniclastic, and

sedimentary rocks. The deposits are enclosed by alteration zones

and exhibit some degree of metal zoning. They comprise lenses

of massive to semi-massive sulfides of iron, copper, zinc, and

lead and have variable amounts of telluride. The primary sulfides

grade ~1 to 1.5 g/t Au. Gold enrichment occurs in the oxide and

supergene zones and is particularly notable in the central part of

the Nubian Shield with grades of 5 to 10 g/t Au. A feature of the

distribution of known occurrences of gold-bearing VMS deposits

in the Nubian Shield, is their apparent confinement to arc

assemblages in the northern half of the Shield, from northern

Ethiopia northward. The greenstone belts of western and

southern Ethiopia contain Neoproterozoic juvenile, submarine

volcanic rocks, but have only sparse indications of VMS

mineralization. The reason for this is uncertain, but the

distribution raises interesting questions about whether it reflects

an original difference in geologic history between the south and

the north, so that VMS deposits never developed in the south, or

whether it is an effect of preferential preservation of VMS

deposits in the north because of less intense exhumation and

erosion of the volcanic-arc host rocks.

Direct dating of volcanic assemblages hosting gold-bearing

VMS deposits in the shield is not widely available. Zircon-207 206

evaporation Pb/ Pb dating of granitoids in the western part of

the Nakfa terrane of Eritrea at 849 ± 20 Ma suggests a major

intrusive event at ~850 Ma (Teklay et al., 2003), compatible with

indications elsewhere in the region of magmatism at 850 to 800 Ma

(Teklay, 2006; Teklay et al., 2002a,b; 2003). Galena, cerrusite, and

anglesite lead-isotope analysis from the Bisha district give a mean

model age of ~780 Ma. Galena from the Adi Nefas deposit in the

Asmara-Nakfa belt, ~170 km to the east of Bisha, yields a lead

model age of 720 Ma (Barrie et al., 2007), broadly similar to detrital

zircons in Neoproterozoic diamictites and Ordovician

siliciclastic rocks in northern Ethiopia that yield U-Pb SHRIMP

ages of ~740 Ma (Avigad et al., 2007). Perhaps the VMS volcanic

host rocks in Eritrea are actually younger than currently

interpreted; alternatively, the discrepancy may be resolved by

dating both host rocks and mineralization with greater precision.

Stratiform VMS and barite mineralization in the Ariab Mineral

district have a galena, cerrusite and anglesite mineral lead model

age of 702 ± 15 Ma (C.T. Barrie, unpublished data). In contrast, a

probable basement granite intrusion structurally adjacent to gold

mineralization at the Kamoeb gold mine has a single zircon 207 206

Pb/ Pb -evaporation age of 888 ± 4 Ma (Deschamps et al.,

2004). No age dating has been reported for the Hamama VMS

deposit, but volcanic-arc rocks in the Eastern Desert have a

general age of ~750 to 730 Ma (Ali et al., 2009) suggesting that

Hamama mineralization may be late Tonian-Cryogenian.

Orogenic-gold occurrences are widespread in the Nubian

Shield. They are characterized by native gold or gold-sulfide-

telluride assemblages disseminated in silicified, brittle-ductile

shear zones. The ore mineralogy includes pyrite ± arsenopyrite

± chalcopyrite ± sphalerite ± tellurides in a matrix of quartz ±

carbonate. The hosting shear zones belong to the episode of

transpressional deformation, strike-slip shearing, and shortening

that characterized late Cryogenian-Ediacaran, tectonic escape,

extension, and orogenic collapse in the Shield. The shear zones

and quartz veins occur within granitoid plutons, and in adjacent

zones of sheared and strongly altered volcano-sedimentary

and/or ultramafic rocks. In the northern Nubian Shield, shear

structures are mainly those of the northwest-trending Najd fault

system; farther south, they include the north-trending Hamisana

and Oko shortening zones and Keraf suture, and in Eritrea and

Ethiopia they include north-trending sinistral and dextral shear

zones in, and at the margins of, greenstone belts.

The ages of orogenic gold formation in the Nubian Shield are

not well established. Broad inferences can be made by

correlating gold mineralization with shearing and adjacent

magmatic events, but the results are not sufficiently robust to be

satisfactory from a metallogenic point of view. Arsenopyrite in

quartz veins at Fawakhir, which is interpreted as the most likely

age of mineralization, yields a Re-Os age of 601 ± 17 Ma (Zoheir

et al., 2014). This age is consistent with the inferred age of

shearing at the deposit, and also overlaps with the 598 ± 3 Ma U-

Pb zircon age of the host Fawakhir pluton (U-Pb TIMS on zircon;

Andresen et al., 2009). However, the Re-Os age has a large error

and is not definitive evidence for a genetic relationship between

mineralization, shearing, and/or granite magmatism. Rubidium-

strontium analysis of sericite concentrates obtained from gold-

bearing quartz veins at Lega Dembi gives an age of 484 ± 67 Ma

(Billay et al., 1997). But the result is an errorchron (MSWD=32)

and is not compatible with the result of structural analysis, which

suggest that gold mineralization occurred during active shearing,

concurrent with 660 Ma amphibolite-greenschist facies

metamorphism (Ghebreab et al., 1992). Galena at the Koka

deposit yields an anomalously young lead-model age of 495 ±

30 Ma, and lead data for one sample from the Kamoeb gold-

quartz vein system at Ariab give an anomalously old age of

881 Ma (C.T. Barrie, unpublished data).

Conclusions

The Nubian Shield evolved as an accretionary orogen through a

~300 million year process of arc-ophiolite-TTG magmatism, arc-

arc collision and terrane amalgamation, greenschist-amphibolite



The Sukari deposit in the Eastern Desert of Egypt is at the

southern end of the major belt of Najd shearing (the “Najd fault

corridor”) that extends obliquely across, and controls much of,

the orogenic gold in the region (Figure 3, inset; Helmy et al.,

2004). Other deposits along this belt include El Sid and Fawakhir

(Zoheir et al., 2014). Modern mining began at Sukari in 2009. The

deposit comprises an array of quartz veins in the Sukari pluton,

an intrusion of granodiorite and tonalite weakly constrained to

630 to 580 Ma (Harraz, 1991). The deposit is elongated north-

northeast - south-southwest; it is ~2.3 km long, 100 to 600 m

wide, and extends to a depth of at least 1200 m. The pluton is

located in a local flower structure developed in brittle-ductile

shear zones within a stack of north-northwest-verging thrusts on

the northeastern flank of the Hafafit gneiss dome (Helmy et al.,

2004). The ore mineralogy in the veins includes pyrite,

arsenopyrite and, less common, sphalerite, chalcopyrite, galena

and gold grading as much as ~5 g/t Au. Grades of 2.5 g/t Au are

reported from the alteration zones that surround the quartz veins

(Khalaf and Oweiss, 1993).

The Fawakhir deposit is a quartz-vein system in the Fawakhir

pluton and adjacent sheared ophiolitic rocks (Zoheir et al., 2014).

The pluton and ophiolitic rocks are in the belt of northwest-

trending strike-slip shears and thrusts belonging to the Najd fault

corridor, mentioned above. The mineralization consists of

subparallel, easterly trending and south-dipping, sulfide and

gold-bearing quartz veins within a ~40 m wide zone of shearing

and hydrothermal alteration. The veins, as much as 400 m long

and 1.3 m wide, consist of massive and laminated quartz with

subordinate calcite and sericite. They have a mineral assemblage

of pyrite-arsenopyrite-pyrrhotite-sphalerite-galena-chalcopyrite-

electrum ± Ag-Au-Bi tellurides, and have grades of 1.5 to ~23 g/t

Au (Gabra, 1986).

Orogenic gold farther south is concentrated along the

Hamisana shear zone, and the Keraf, Allaqi-Heiani-Onib-Sol

Hamed, and Nakasib sutures (Figure 4). Mining is reported to

have started at the Qbgbih deposit along the Keraf suture but

little information is available. Exploration is underway elsewhere

along the Keraf suture at the Galat Sufa, Negeim, Kimaweit,

Artoli, and Umtrambiesh prospects, and along the Nakasib

suture at Kamoeb. The Romite deposit (Zoheir, 2012b) is an

example of orogenic gold in the structurally complex area east of

the intersection of the Allaqi-Heinai-Onib-Sol Hamed suture by

the Hamisana shear zone (Figure 4). It is located in a north-

northeast-trending sinistral shear zone that splays off the

Hamisana shear zone. Gold, grading as much as 7 g/t Au, occurs

as free grains in north-northeast-trending quartz and quartz-

carbonate veins in quartz diorite and as disseminations in iron-

stained, carbonate-altered and silicified wall rock. Sulfides in the

veins include pyrite, arsenopyrite, and lesser chalcopyrite and

pyrrhotite (Zoheir, 2012b). The morphology, pinch and swell

structure, and quartz texture of the veins suggest vein formation

was synkinematic with north-northeast-shearing (Zoheir, 2012b)

and it appears that quartz precipitated in spaces created at

releasing bends in the shears.

At Galat Sufar, currently the best known example of orogenic

gold along the Keraf suture ( Johnson, 2014), mapping and

drilling reveal mesozonal, shear-zone-hosted orogenic gold over

a strike length of >10 km. The mineralization comprises a system

of quartz veins in tension gashes and shear zones, and swarms of

veinlets in well-foliated and folded, schistose volcano-

sedimentary rocks ( Johnson, 2014). Examples of drill-hole

intersections include 23 m grading 2.34 g/t Au and 49 m grading

1.90 g/t Au ( Johnson, 2014).

As mentioned above, orogenic-gold occurrences in Eritrea

and northern Ethiopia (Figure 5) are concentrated in belts of

transpressional deformation, the same belts that contain VMS

deposits. The Koka deposit comprises an elongate stockwork

>600m long and 20 to 30 m wide in microgranite in the Augaro-

Adobha belt (Chalice, 2010; Eritrean Mining Journal, 2014). The

sulfide mineralogy includes pyrite, sphalerite, galena and

chalcopyrite, and gold is present as tiny blebs within the galena.

The Konate prospect is an area of artisanal workings on quartz

stockwork in silica-sericite altered microgranite, ~6 km south

of and virtually along strike from Koka. The Gupo deposit

comprises gold and sulfides in quartz veins along a shear zone in

the Asmara-Nakfa belt (Ross and Martin, 2012). Adi Goshu and

Lihamat, along the same belt in northern Ethiopia, are close to

the contact between a northeast-trending elongate body of

quartz-feldspar porphyry and sericite-altered mafic and felsic

volcanic and volcaniclastic rocks, and local BIF (Archibald

et al, 2014).

Orogenic-gold deposits in the Western and Southern

Ethiopian Shields are exemplified by Tulu Kapi and Lega Dembi

(Figure 6). The Tulu Kapi deposit is a historic gold mine and a

definitive feasibility study is in progress in preparation for

renewed surface mining and planned processing of 1.2 Mt/a ore

at a grade of 2.4 g/t Au for an estimated 2.4 t Au per year

(Kefi Minerals, 2014a, b). The Tulu Kapi deposit is hosted by

syenite emplaced in strongly sheared metavolcanic and

metasedimentary rocks belonging to the central volcano-

sedimentary terrane greenstone belt (CVST) (Figure 6).

Mineralization is known over an area of 1500 m by 400 m, and

extends for >400 m beneath the surface. It consists of gold-

bearing quartz-carbonate veins, veinlets, and stockwork in a

series of stacked subhorizontal lodes (or lenses). The upper

lodes are characterized by gold, pyrite, and minor sphalerite and

galena, whereas the lower lodes contain significant amounts of

sphalerite, galena and minor arsenopyrite and chalcopyrite as

well as gold (Nyota Minerals, 2014). Based on its setting in a

strongly sheared greenstone belt close to the Tulu Dimtu-Baruda

shear zone, Tulu Kapi has the character of a typical orogenic-

gold deposit. However, its occurrence in syenite raises the

possibility that the deposit may belong to the class of intrusion-

related gold. This possibility requires further study and work is

underway to locate an intrusion that may have acted as the metal

source (Nyota Minerals, 2014).

Orogenic-gold occurrences in the Southern Ethiopian Shield

are in greenstone belts and sheared gneiss and schist in a

structural scissor between north- and northwest-trending shear

zones (Figure 6, inset). They include Lega Dembi, a surface and

underground mine, and Sakaro, an underground mine. The Lega

Dembi deposit is located in strongly sheared rocks at the eastern

margin of the Megado greenstone belt (Billay et al., 1997;

Ghebreab et al., 1992, 2009). The deposit is related to tabular and



74 75

Carville, D., Lee, D. and Gordon, D., 2010. Technical report on the Koka Gold

Project, Eritrea, Chalice Gold Mines Limited: 120p.

Centamin, 2014. Value driven growth: Annual Report for 2014. Centamin plc,

152p.

Chalice, 2010. Building an African gold mining powerhouse. Chalice

Gold Mines PowerPoint presentation, September, 2010, 21p

(www.chalicegold.com).

Cohen, K.M., Finney, S.C., Gibbard, P.L. and Fan, J.-X., 2013 (updated). The ICS

international chronostratigraphic chart. Episodes, 36, 199-204.

Cottard, F., Deschamps, Y., Bernadet, G. and El Samani, Y., 1986. Gold deposits

of Ariab area. BRGM Rep. No. 86 SDN 110, Khartoum, 55p.

Deschamps, Y., Lescuyer, J. L., Guerrot, C. and Osman, A. A., 2004. Lower

Neoproterozoic age of the Ariab volcanogenic massive sulphide

mineralization, Red Sea Hills, NE Sudan. 20th Colloquium of African

Geology, BRGM, Orléans, France, 2-7 June 2004, 133p.

Dubé, B. and Gosselin, P., 2007. Greenstone-hosted quartz-carbonate vein

deposits. In: W.D. Goodfellow (Editor). Mineral Deposits of Canada:

A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution

of Geological Provinces, and Exploration Methods. Geological Association

of Canada, Mineral Deposits Division, Special Publication No. 5, 49-73.

Dubé, B., Gosselin, P., Hannington, M. and Galley, A., 2007. Gold-rich

volcanogenic massive sulpfide deposits. In: W.D. (Editor). Mineral Deposits

of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the

Evolution of Geological Provinces, and Exploration Methods. Geological

Association of Canada, Mineral Deposits Division, Special Publication No. 5,

75-94.

Eritrean Mining Journal 2013. Report produced for the Asmara Mining

Conference 2013. Eritrean Ministry of Energy and Mines, 22p.



Fritz, H., Abdelsalam, M., Ali, K.A., Bingen, B., Collins, A., Fowler, A.R.,

Ghebreab, W., Hauzenberger, C.A., Johnson, P.R., Kusky, T.M., Macey, P.,

Muhongo, S., Stern, R.J. and Viola, V., 2013. Orogen styles in the East African

Orogen: a review of the Neoproterozoic to Cambrian tectonic evolution.

Journal of African Earth Sciences, 86, 65-106.

Gabra, Sh. Z., 1986. Gold in Egypt: a commodity package, Minerals, petroleum

and groundwater assessment program. USAID project 363-0105, Geological

Survey of Egypt, 86p.

Gaskell, J.L., 1985. Reappraisal of Gebeit gold mines, northeast Sudan: a case

history. In: Prospecting in Areas of Desert Terrain. London, Institution of

Mining and Metallurgy, 49-58.

Ghebreab, W., Greiling, R.O. and Solomon, S., 2009. Structural setting of

Neoproterozoic mineralization, Asmara district, Eritrea. Journal of African

Earth Sciences, 55, 219-235.

Ghebreab, W., Yohannes, E. and Woldegiorgis, L., 1992. The Lega Dembi gold

mine: an example of shear zone-hosted mineralization in the Adola

greenstone belt, Southern Ethiopia. Journal of African Earth Sciences, 15,

489-500.

Gibson, H. L., Allen, R. L., Riverin, G. and Lane, T. E., 2007. The VMS Model:

Advances and Application to Exploration Targeting. In: B. Milkereit (Editor).

Proceedings of Exploration 07: Fifth Decennial International Conference on

Mineral Exploration, 713-730.

Grenne, T., Pedersen, R.B., Bjerkgård, T., Braathen, A., Selassie, M.G. and

Worku, T., 2003. Neoproterozoic evolution of Western Ethiopia: igneous

geochemistry, isotope systematics and U-Pb ages. Geological Magazine,

140, 373-395.

Gribble, P., Melnyk, J. and Munro, P., 2013. Bisha Mine, Eritrea, Africa. NI 43-101

Techncial Report prepared for Nevsun Resources Ltd., 328p.

Groves, D.I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G. and Robert, F.,

1998. Orogenic gold deposits: A proposed classification in the context of

their crustal distribution and relationship to other gold deposit types. Ore

Geology Reviews, 13, 7-27.

Groves , D.I., Goldfarb, R.J., Robert, R. and Hart, C.J., 2003. Gold deposits in

metamorphic belts: Overview of current understanding, outstanding

problems, future research, and exploration significance. Economic

Geology, 98, 1-29.

Harraz, H.Z., 1991. Lithochemical prospecting and genesis of gold deposits in

El Sukari gold mine, Eastern Desert, Egypt. PhD Thesis (unpublished),

Tanta University, 494pp.

Helmy, H.M., Kaindl, R., Fritz, H. and Jürgen Loizenbauer., 2004. The Sukari

gold mine, Eastern Desert-Egypt: structural setting, mineralogy and fluid

inclusion study. Mineralium Deposita, 39, 495-511.

Jelenc, D., 1966. Mineral Occurrences of Ethiopia. Addis Ababa, Ministry of

Mines, 720p.

Johnson, N., 2014. NI 43-101 Independent Technical Report Block 14 project,

Republic of the Sudan, prepared for Orca Gold Inc., 137p.

Johnson, P.R., Andresen, A., Collins, A.S., Fowler, A.R., Fritz, H., Ghebreab, W.,

Kusky, T. and Stern, R.J., 2011. Late Cryogenian-Ediacaran history of the

Arabian-Nubian Shield: a review of deposition, plutonic, structural, and

tectonic events in the closing stages of the northern East African Orogen.


Khalaf, I.M. and Oweiss, K.A.,1993. Gold prospection in the environs of Sukari

gold mine, central Eastern Desert, Egypt. Annals of the Geological Survey of

Egypt, 19, 97-108.

Kefi Minerals, 2014a. Resources and Reserves. Kefi Mineral plc website;

downloaded October 3, 2014, 1p.

Kefi Minerals, 2014b. Independently reviewed cost estimates for the Tulu Kapi

open pit. Kefi Minerals Plc, press announcement 1 October 2014, 4p.

Klemm, R. and Klemm, D., 2013. Gold and Gold Mining in Ancient Egypt and

Nubia, Natural Science in Archaeology, 341, DOI 10.1007/978-3-642-22508-

6_6, Springer-Verlag, Berlin and Heidelberg.

Klemm, D., Klemm, R. and Murr, A., 2001. Gold of the Pharaohs - 6000 years of

gold mining in Egypt and Nubia. Journal of African Earth Sciences, 33,

643-659.

Kusky, M. and Ramadan, T.M., 2002. Structural controls on Neoproterozoic

mineralization in the south Eastern Desert, Egypt: an integrated field,

Landsat TM, and SIR-C/X SAR approach. Journal of African Earth Sciences,

35, 107-12.

La Mancha; 2014. Corporate Presentation, October 2014, 33p.

Lissan, N.H. and Bakheit, A.K., 2010. Nature and characteristics of gold

mineralization at Al Abediya Area, Berber Province, northern Sudan.

Research Journal of Applied Sciences, 5, 285-302.

Miller, M.M. and Dixon, T.H., 1992. Late Proterozoic evolution of the northern

part of the Hamisana zone, northeast Sudan: constraints on Pan-African

accretionary tectonics. Journal of the Geological Society, London 149,

743-750.

Nyota Minerals, 2014. Tulu Kapi Project Summary; Nyota Minerals Limited web

site; download June 12, 2014, 5p.

Plyley, B., Kachrillo, J.-J., Bennett, M., Bosc, R. and Barrie, T., 2009. Hassai

South Cu-Au VMS deposit, Sudan, resource estimate, NI 43-101 Technical

Report prepared for La Mancha Resources, Inc., 112p.

Robert, F., Brommecker, R., Bourne, B.T., Dobak, P.J., McEwan, C.J., Rowe, R.R.

and Zhou, X., 2007. Models and exploration methods for major gold deposit

types. In: B. Milkereit (Editor). Proceedings of Exploration 07: Fifth

Decennial International Conference on Mineral Exploration, 691-711.

Ross, A.F. and Martin, C.J., 2012. Gupo Gold Mineral Resource Estimate Update,

Adi Nefas Property, Eritrea. NI-43-101 Technical Report prepared for

Sunridge Gold Corp., 97p.

Stern, R.J., Ali, K.A., Abdelsalam, M.G., Wilde, S.A. and Zhou, Q., 2012. U-Pb

zircon geochronology of the eastern part of the Southern Ethiopian shield.

Precambrian Research, 206-207, 159-67.

Stern, R.J., Nielsen, K.C., Best, E., Sultan, M., Arvidson, R.E. and Kröner, A.,

1990. Orientation of late Precambrian sutures in the Arabian-Nubian shield.

Geology, 18, 1103-1106.

Tadesse, S., 2004. Genesis of the shear-zone-related gold vein mineralization of

the Lega Dembi gold deposit, Adola gold field, Southern Ethiopia.

Gondwana Research, 7, 481-488.

Tadesse, S., Milesi, J.P., Deschamps, Y., 2003. Geology and mineral potential of

Ethiopia: a note on geology and mineral map of Ethiopia. Journal of African

Earth Sciences 36, 273-313.

Taib, M., 2012. The mineral industry of Egypt. U.S. Geological Survey 2012

Mineral Yearbook. 14.1-14.12.

Teklay, M., 2006. Neoproterozoic arc-back-arc system analog to modern arc-

back-arc systems: evidence from tholeiite-boninite association, serpentinite

mudflows, and across-arc geochemical trends in Eritrea, southern Arabian-

Nubian shield. Precambrian Research, 145, 81-92.

Teklay, M., Berhe, K., Reimold, W.U., Armstrong, R., Asmerom, Y. and Watson,

J., 2002a. Geochemistry and geochronology of a Neoproterozoic low-K



metamorphism, deposition of younger sedimentary-volcanic

basins, emplacement of vast amounts of late- to post-tectonic

granitoids, and collisional orogeny, shearing, and orogenic

collapse. Together with the formerly contiguous Arabian Shield,

the Nubian Shield forms the largest tract of juvenile

Neoproterozoic crust on Earth and hosts the largest expanse of

Neoproterozoic gold mineralization.

Orogenic gold and gold-bearing VMS are the most abundant

gold-deposit types in the Nubian Shield. However, their genesis

and relationships to lithology, stratigraphy, and structure in the

Shield are not yet well constrained. Furthermore, because of

prolonged deformation and metamorphism, VMS deposits are

locally overprinted by orogenic gold so that a given gold deposit

may exhibit features of both types.

At the present time, three main districts of syn-arc VMS

mineralization are recognized in the Nubian Shield:

Ÿ the Eastern Desert of Egypt,

Ÿ the Nakasib suture zone in Sudan, and

Ÿ north-south greenstone belts in Eritrea. The districts in Sudan

and Eritrea contain world-class gold-sulfide deposits.

Orogenic gold is widespread in the Shield. Deposits of orogenic

gold have been worked at hundreds of ancient mines for more

than 5,500 years and are currently worked by modern mines at

Sukari, Lega Dembi, and Sakaro.

More than twenty exploration and mining companies are

active in the region. Exploration for gold-bearing VMS deposits is

underway in the northern Eastern Desert, northern Sudan,

Eritrea, and northern Ethiopia as well as in the vicinity of existing

mines at Bisha and the Ariab Mineral District. Exploration for

orogenic gold deposits is underway along the Keraf and Nakasib

sutures in Sudan, along north-trending shear zones in plutons

and greenstone belts in Eritrea and Ethiopia, and along

northwest-trending shear zones in Egypt.

From the perspective of this paper, 850 to 800 Ma arc rocks in

Eritrea-Northern Ethiopia; ~890 Ma arc rocks in Sudan; and ~750 to

730 arc rocks in the Eastern Desert of Egypt are the most important

host rocks for gold-bearing VMS deposits. Such late Tonian-

Cryogenian rocks host gold-bearing VMS deposits at Bisha, Hassai,

and Hamama. Ediacaran brittle-ductile shearing in greenstone-

ophiolite belts and plutonic rocks was the main control on Nubian

Shield orogenic gold deposits.

Using radiometric ages and regional structural

considerations to constrain the ages of host rocks, shearing,

and mineralization is a useful approach to dating both VMS and

orogenic-gold mineralization, but greater precision and scope

are required. The importance of metallic mineralization to the

economies of the countries covered by the Nubian Shield

warrants a focused geochronologic effort on dating the deposits.

Appropriate techniques include U-Pb zircon dating of host rocks,

Ar-Ar dating of metamorphic events and the formation of sericite

and fuchsite, Re-Os dating of sulfide minerals, and U-Pb-Th

dating of hydrothermal processes. This would be helpful for

linking the interests of academic geoscientists seeking to

understand the crustal evolution of the region with

explorationists seeking to better understand the location of

economic gold targets.

Acknowledgements

We would like to thank the editors for inviting this contribution

and we gratefully acknowledge the work of many colleagues

whose previous studies of gold in the Nubian Shield make our

contribution possible. We are also especially thankful for two

reviews of an earlier version of this paper that caused us to make

significant changes in scope and presentation.

References

Abdelsalam, M.G., 1994. The Oko shear zone; post-accretionary deformations

in the Arabian-Nubian Shield. Journal of the Geological Society, London,

151, 767-776.

Abdelsalam, M.G. and Stern, R.J., 1996. Structures and shear zones in the

Arabian-Nubian Shield. Journal of African Earth Sciences, 23, 298-310.

Abdelsalam, M.G., Stern, R.J., Copeland, P., Elfak, E.M., Elhur, B. and Ibrahim,

F.M., 1998. The Neoproterozoic Keraf suture in NE Sudan: sinistral

transpression along the eastern margin of West Gondwana. Journal of

Geology, 106, 133-147.

Abdelsalam, M.G., Tsige, L., Yihunie, T. and Hussien, B., 2008. Terrane rotation

during the East African Orogeny: evidence from the Bulbul shear zone,

southern Ethiopia. Gondwana Res. 14, 497-508.

Abu Fatima, M., 2006. Metallogenic genesis and geotectonic evolution of the

Polymetallic massive sulphides and the associated gold deposits at Ariab -

Arbaat Belt, Red Sea Hills, Sudan. Unpublished Ph.D. thesis, Henri Poincare

University, Nancy, France, 340p.

Alexander Nubia; 2013. Delineate, Focus, Grow: Follow the Pharaohs' golden

path. Alexander Nubia International Inc. PDF presentation downloaded

August 2014, www.alexandernubia.com

Alexander Nubia, 2015. Prime locations, intelligent exploration, thoughtful

leadership. Alexander Nubia International Inc. PDF presentation

downloaded May 2015, www.alexandernubia.com

Ali, K.A., Azer, M.K., Gahlan, H.A., Wilde, S.A., Samuel, M.D. and Stern, R.J.,

2010. Age constraints on the formation and emplacement of

Neoproterozoic ophiolites along the Allaqi-Heiani Suture, Southeastern

Desert of Egypt. Gondwana Research, 18, 583-595.

Andresen, A., Abu El-Rus, M.A., Myhre, P.I., Boghdady, G.Y. and Corfu, F., 2009.

U-Pb TIMS age constraints on the evolution of the Neoproterozoic Meatiq

Gneiss Dome, Eastern Desert, Egypt. International Journal of Earth

Sciences, DOI 10.1007/s00531-007-0276-x

Archibald, S.M., Martin, C. and Thomas, D.G., 2014. NI 43-101 Technical Report

on a Mineral Resource Estimate at the Terakimti Prospect, Harvest Property

(centred at 38°21'E, 14°19'N), Tigray National Region, Ethiopia. Prepared for

Tigray Resources Inc., 207p.

Avigad, D., Stern, R.J., Beyth, M., Miller, N. and McWilliams, M., 2007. Detrital

zircon U-Pb geochronology of Cryogenian diamictites and lower Paleozoic

sandstone in Ethiopia (Tigrai): age constraints on Neoproterozoic glaciation

and crustal evolution of the southern Arabian-Nubian Shield. Precambrian

Research, 154, 88-106.

Barrie, C.T., Nielsen, F.W. and Aussant, C.H., 2007. The Bisha volcanic-

associated massive sulfide deposit, western Nakfa terrane, Eritrea.

Economic Geology, 102, 717-738.

Billay, A.Y., Kisters, A.F.M., Meyer, F.M. and Schneider, J., 1997. The geology of

the Lega Dembi gold deposit, southern Ethiopia: implications for Pan-

African gold exploration. Mineralium Deposita, 32, 491-504.

Böhlke, J.K., 1982, Orogenic (metamorphic-hosted) gold-quartz veins. In: R.L.

Erickson (Editor). Characteristics of Mineral Deposit Occurrences. U.S.

Geological Survey Open-File Report 82-795, 70-76p.

Bosc, R., Tamlyn, N. and Kachrillo, J.-J., 2012. The Hassai Mine project VMS

resources update Red Sea State, Sudan. NI 43-101 Technical Report

prepared for La Mancha Resources Inc., 227p.

Botros, N.S., 2002. Metallogeny of gold in relation to the evolution of the

Nubian Shield in Egypt. Ore Geology Reviews, 19, 137-164.

Botros, N.S., 2004. A new classification of the gold deposits of Egypt. Ore




74 75

Carville, D., Lee, D. and Gordon, D., 2010. Technical report on the Koka Gold

Project, Eritrea, Chalice Gold Mines Limited: 120p.

Centamin, 2014. Value driven growth: Annual Report for 2014. Centamin plc,

152p.

Chalice, 2010. Building an African gold mining powerhouse. Chalice

Gold Mines PowerPoint presentation, September, 2010, 21p

(www.chalicegold.com).

Cohen, K.M., Finney, S.C., Gibbard, P.L. and Fan, J.-X., 2013 (updated). The ICS

international chronostratigraphic chart. Episodes, 36, 199-204.

Cottard, F., Deschamps, Y., Bernadet, G. and El Samani, Y., 1986. Gold deposits

of Ariab area. BRGM Rep. No. 86 SDN 110, Khartoum, 55p.

Deschamps, Y., Lescuyer, J. L., Guerrot, C. and Osman, A. A., 2004. Lower

Neoproterozoic age of the Ariab volcanogenic massive sulphide

mineralization, Red Sea Hills, NE Sudan. 20th Colloquium of African

Geology, BRGM, Orléans, France, 2-7 June 2004, 133p.

Dubé, B. and Gosselin, P., 2007. Greenstone-hosted quartz-carbonate vein

deposits. In: W.D. Goodfellow (Editor). Mineral Deposits of Canada:

A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution

of Geological Provinces, and Exploration Methods. Geological Association

of Canada, Mineral Deposits Division, Special Publication No. 5, 49-73.

Dubé, B., Gosselin, P., Hannington, M. and Galley, A., 2007. Gold-rich

volcanogenic massive sulpfide deposits. In: W.D. (Editor). Mineral Deposits

of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the

Evolution of Geological Provinces, and Exploration Methods. Geological

Association of Canada, Mineral Deposits Division, Special Publication No. 5,

75-94.





Fritz, H., Abdelsalam, M., Ali, K.A., Bingen, B., Collins, A., Fowler, A.R.,

Ghebreab, W., Hauzenberger, C.A., Johnson, P.R., Kusky, T.M., Macey, P.,

Muhongo, S., Stern, R.J. and Viola, V., 2013. Orogen styles in the East African

Orogen: a review of the Neoproterozoic to Cambrian tectonic evolution.


Gabra, Sh. Z., 1986. Gold in Egypt: a commodity package, Minerals, petroleum

and groundwater assessment program. USAID project 363-0105, Geological

Survey of Egypt, 86p.

Gaskell, J.L., 1985. Reappraisal of Gebeit gold mines, northeast Sudan: a case

history. In: Prospecting in Areas of Desert Terrain. London, Institution of

Mining and Metallurgy, 49-58.

Ghebreab, W., Greiling, R.O. and Solomon, S., 2009. Structural setting of

Neoproterozoic mineralization, Asmara district, Eritrea. Journal of African


Ghebreab, W., Yohannes, E. and Woldegiorgis, L., 1992. The Lega Dembi gold

mine: an example of shear zone-hosted mineralization in the Adola

greenstone belt, Southern Ethiopia. Journal of African Earth Sciences, 15,

489-500.

Gibson, H. L., Allen, R. L., Riverin, G. and Lane, T. E., 2007. The VMS Model:

Advances and Application to Exploration Targeting. In: B. Milkereit (Editor).

Proceedings of Exploration 07: Fifth Decennial International Conference on

Mineral Exploration, 713-730.

Grenne, T., Pedersen, R.B., Bjerkgård, T., Braathen, A., Selassie, M.G. and

Worku, T., 2003. Neoproterozoic evolution of Western Ethiopia: igneous

geochemistry, isotope systematics and U-Pb ages. Geological Magazine,

140, 373-395.

Gribble, P., Melnyk, J. and Munro, P., 2013. Bisha Mine, Eritrea, Africa. NI 43-101

Techncial Report prepared for Nevsun Resources Ltd., 328p.

Groves, D.I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G. and Robert, F.,

1998. Orogenic gold deposits: A proposed classification in the context of

their crustal distribution and relationship to other gold deposit types. Ore


Groves , D.I., Goldfarb, R.J., Robert, R. and Hart, C.J., 2003. Gold deposits in

metamorphic belts: Overview of current understanding, outstanding

problems, future research, and exploration significance. Economic

Geology, 98, 1-29.

Harraz, H.Z., 1991. Lithochemical prospecting and genesis of gold deposits in

El Sukari gold mine, Eastern Desert, Egypt. PhD Thesis (unpublished),

Tanta University, 494pp.

Helmy, H.M., Kaindl, R., Fritz, H. and Jürgen Loizenbauer., 2004. The Sukari

gold mine, Eastern Desert-Egypt: structural setting, mineralogy and fluid

inclusion study. Mineralium Deposita, 39, 495-511.

Jelenc, D., 1966. Mineral Occurrences of Ethiopia. Addis Ababa, Ministry of

Mines, 720p.

Johnson, N., 2014. NI 43-101 Independent Technical Report Block 14 project,

Republic of the Sudan, prepared for Orca Gold Inc., 137p.

Johnson, P.R., Andresen, A., Collins, A.S., Fowler, A.R., Fritz, H., Ghebreab, W.,

Kusky, T. and Stern, R.J., 2011. Late Cryogenian-Ediacaran history of the

Arabian-Nubian Shield: a review of deposition, plutonic, structural, and

tectonic events in the closing stages of the northern East African Orogen.


Khalaf, I.M. and Oweiss, K.A.,1993. Gold prospection in the environs of Sukari

gold mine, central Eastern Desert, Egypt. Annals of the Geological Survey of

Egypt, 19, 97-108.

Kefi Minerals, 2014a. Resources and Reserves. Kefi Mineral plc website;

downloaded October 3, 2014, 1p.

Kefi Minerals, 2014b. Independently reviewed cost estimates for the Tulu Kapi

open pit. Kefi Minerals Plc, press announcement 1 October 2014, 4p.

Klemm, R. and Klemm, D., 2013. Gold and Gold Mining in Ancient Egypt and

Nubia, Natural Science in Archaeology, 341, DOI 10.1007/978-3-642-22508-

6_6, Springer-Verlag, Berlin and Heidelberg.

Klemm, D., Klemm, R. and Murr, A., 2001. Gold of the Pharaohs - 6000 years of

gold mining in Egypt and Nubia. Journal of African Earth Sciences, 33,

643-659.

Kusky, M. and Ramadan, T.M., 2002. Structural controls on Neoproterozoic

mineralization in the south Eastern Desert, Egypt: an integrated field,

Landsat TM, and SIR-C/X SAR approach. Journal of African Earth Sciences,

35, 107-12.

La Mancha; 2014. Corporate Presentation, October 2014, 33p.

Lissan, N.H. and Bakheit, A.K., 2010. Nature and characteristics of gold

mineralization at Al Abediya Area, Berber Province, northern Sudan.

Research Journal of Applied Sciences, 5, 285-302.

Miller, M.M. and Dixon, T.H., 1992. Late Proterozoic evolution of the northern

part of the Hamisana zone, northeast Sudan: constraints on Pan-African

accretionary tectonics. Journal of the Geological Society, London 149,

743-750.

Nyota Minerals, 2014. Tulu Kapi Project Summary; Nyota Minerals Limited web

site; download June 12, 2014, 5p.

Plyley, B., Kachrillo, J.-J., Bennett, M., Bosc, R. and Barrie, T., 2009. Hassai

South Cu-Au VMS deposit, Sudan, resource estimate, NI 43-101 Technical

Report prepared for La Mancha Resources, Inc., 112p.

Robert, F., Brommecker, R., Bourne, B.T., Dobak, P.J., McEwan, C.J., Rowe, R.R.

and Zhou, X., 2007. Models and exploration methods for major gold deposit

types. In: B. Milkereit (Editor). Proceedings of Exploration 07: Fifth

Decennial International Conference on Mineral Exploration, 691-711.

Ross, A.F. and Martin, C.J., 2012. Gupo Gold Mineral Resource Estimate Update,

Adi Nefas Property, Eritrea. NI-43-101 Technical Report prepared for

Sunridge Gold Corp., 97p.

Stern, R.J., Ali, K.A., Abdelsalam, M.G., Wilde, S.A. and Zhou, Q., 2012. U-Pb

zircon geochronology of the eastern part of the Southern Ethiopian shield.

Precambrian Research, 206-207, 159-67.

Stern, R.J., Nielsen, K.C., Best, E., Sultan, M., Arvidson, R.E. and Kröner, A.,

1990. Orientation of late Precambrian sutures in the Arabian-Nubian shield.

Geology, 18, 1103-1106.

Tadesse, S., 2004. Genesis of the shear-zone-related gold vein mineralization of

the Lega Dembi gold deposit, Adola gold field, Southern Ethiopia.

Gondwana Research, 7, 481-488.

Tadesse, S., Milesi, J.P., Deschamps, Y., 2003. Geology and mineral potential of

Ethiopia: a note on geology and mineral map of Ethiopia. Journal of African

Earth Sciences 36, 273-313.

Taib, M., 2012. The mineral industry of Egypt. U.S. Geological Survey 2012

Mineral Yearbook. 14.1-14.12.

Teklay, M., 2006. Neoproterozoic arc-back-arc system analog to modern arc-

back-arc systems: evidence from tholeiite-boninite association, serpentinite

mudflows, and across-arc geochemical trends in Eritrea, southern Arabian-

Nubian shield. Precambrian Research, 145, 81-92.

Teklay, M., Berhe, K., Reimold, W.U., Armstrong, R., Asmerom, Y. and Watson,

J., 2002a. Geochemistry and geochronology of a Neoproterozoic low-K



metamorphism, deposition of younger sedimentary-volcanic

basins, emplacement of vast amounts of late- to post-tectonic

granitoids, and collisional orogeny, shearing, and orogenic

collapse. Together with the formerly contiguous Arabian Shield,

the Nubian Shield forms the largest tract of juvenile

Neoproterozoic crust on Earth and hosts the largest expanse of

Neoproterozoic gold mineralization.

Orogenic gold and gold-bearing VMS are the most abundant

gold-deposit types in the Nubian Shield. However, their genesis

and relationships to lithology, stratigraphy, and structure in the

Shield are not yet well constrained. Furthermore, because of

prolonged deformation and metamorphism, VMS deposits are

locally overprinted by orogenic gold so that a given gold deposit

may exhibit features of both types.

At the present time, three main districts of syn-arc VMS

mineralization are recognized in the Nubian Shield:

Ÿ the Eastern Desert of Egypt,

Ÿ the Nakasib suture zone in Sudan, and

Ÿ north-south greenstone belts in Eritrea. The districts in Sudan

and Eritrea contain world-class gold-sulfide deposits.

Orogenic gold is widespread in the Shield. Deposits of orogenic

gold have been worked at hundreds of ancient mines for more

than 5,500 years and are currently worked by modern mines at

Sukari, Lega Dembi, and Sakaro.

More than twenty exploration and mining companies are

active in the region. Exploration for gold-bearing VMS deposits is

underway in the northern Eastern Desert, northern Sudan,

Eritrea, and northern Ethiopia as well as in the vicinity of existing

mines at Bisha and the Ariab Mineral District. Exploration for

orogenic gold deposits is underway along the Keraf and Nakasib

sutures in Sudan, along north-trending shear zones in plutons

and greenstone belts in Eritrea and Ethiopia, and along

northwest-trending shear zones in Egypt.

From the perspective of this paper, 850 to 800 Ma arc rocks in

Eritrea-Northern Ethiopia; ~890 Ma arc rocks in Sudan; and ~750 to

730 arc rocks in the Eastern Desert of Egypt are the most important

host rocks for gold-bearing VMS deposits. Such late Tonian-

Cryogenian rocks host gold-bearing VMS deposits at Bisha, Hassai,

and Hamama. Ediacaran brittle-ductile shearing in greenstone-

ophiolite belts and plutonic rocks was the main control on Nubian

Shield orogenic gold deposits.

Using radiometric ages and regional structural

considerations to constrain the ages of host rocks, shearing,

and mineralization is a useful approach to dating both VMS and

orogenic-gold mineralization, but greater precision and scope

are required. The importance of metallic mineralization to the

economies of the countries covered by the Nubian Shield

warrants a focused geochronologic effort on dating the deposits.

Appropriate techniques include U-Pb zircon dating of host rocks,

Ar-Ar dating of metamorphic events and the formation of sericite

and fuchsite, Re-Os dating of sulfide minerals, and U-Pb-Th

dating of hydrothermal processes. This would be helpful for

linking the interests of academic geoscientists seeking to

understand the crustal evolution of the region with

explorationists seeking to better understand the location of

economic gold targets.

Acknowledgements

We would like to thank the editors for inviting this contribution

and we gratefully acknowledge the work of many colleagues

whose previous studies of gold in the Nubian Shield make our

contribution possible. We are also especially thankful for two

reviews of an earlier version of this paper that caused us to make

significant changes in scope and presentation.

References

Abdelsalam, M.G., 1994. The Oko shear zone; post-accretionary deformations

in the Arabian-Nubian Shield. Journal of the Geological Society, London,

151, 767-776.

Abdelsalam, M.G. and Stern, R.J., 1996. Structures and shear zones in the

Arabian-Nubian Shield. Journal of African Earth Sciences, 23, 298-310.

Abdelsalam, M.G., Stern, R.J., Copeland, P., Elfak, E.M., Elhur, B. and Ibrahim,

F.M., 1998. The Neoproterozoic Keraf suture in NE Sudan: sinistral

transpression along the eastern margin of West Gondwana. Journal of

Geology, 106, 133-147.

Abdelsalam, M.G., Tsige, L., Yihunie, T. and Hussien, B., 2008. Terrane rotation

during the East African Orogeny: evidence from the Bulbul shear zone,

southern Ethiopia. Gondwana Res. 14, 497-508.

Abu Fatima, M., 2006. Metallogenic genesis and geotectonic evolution of the

Polymetallic massive sulphides and the associated gold deposits at Ariab -

Arbaat Belt, Red Sea Hills, Sudan. Unpublished Ph.D. thesis, Henri Poincare

University, Nancy, France, 340p.

Alexander Nubia; 2013. Delineate, Focus, Grow: Follow the Pharaohs' golden

path. Alexander Nubia International Inc. PDF presentation downloaded

August 2014, www.alexandernubia.com

Alexander Nubia, 2015. Prime locations, intelligent exploration, thoughtful

leadership. Alexander Nubia International Inc. PDF presentation

downloaded May 2015, www.alexandernubia.com

Ali, K.A., Azer, M.K., Gahlan, H.A., Wilde, S.A., Samuel, M.D. and Stern, R.J.,

2010. Age constraints on the formation and emplacement of

Neoproterozoic ophiolites along the Allaqi-Heiani Suture, Southeastern

Desert of Egypt. Gondwana Research, 18, 583-595.

Andresen, A., Abu El-Rus, M.A., Myhre, P.I., Boghdady, G.Y. and Corfu, F., 2009.

U-Pb TIMS age constraints on the evolution of the Neoproterozoic Meatiq

Gneiss Dome, Eastern Desert, Egypt. International Journal of Earth

Sciences, DOI 10.1007/s00531-007-0276-x

Archibald, S.M., Martin, C. and Thomas, D.G., 2014. NI 43-101 Technical Report

on a Mineral Resource Estimate at the Terakimti Prospect, Harvest Property

(centred at 38°21'E, 14°19'N), Tigray National Region, Ethiopia. Prepared for

Tigray Resources Inc., 207p.

Avigad, D., Stern, R.J., Beyth, M., Miller, N. and McWilliams, M., 2007. Detrital

zircon U-Pb geochronology of Cryogenian diamictites and lower Paleozoic

sandstone in Ethiopia (Tigrai): age constraints on Neoproterozoic glaciation

and crustal evolution of the southern Arabian-Nubian Shield. Precambrian

Research, 154, 88-106.

Barrie, C.T., Nielsen, F.W. and Aussant, C.H., 2007. The Bisha volcanic-

associated massive sulfide deposit, western Nakfa terrane, Eritrea.

Economic Geology, 102, 717-738.

Billay, A.Y., Kisters, A.F.M., Meyer, F.M. and Schneider, J., 1997. The geology of

the Lega Dembi gold deposit, southern Ethiopia: implications for Pan-

African gold exploration. Mineralium Deposita, 32, 491-504.

Böhlke, J.K., 1982, Orogenic (metamorphic-hosted) gold-quartz veins. In: R.L.

Erickson (Editor). Characteristics of Mineral Deposit Occurrences. U.S.

Geological Survey Open-File Report 82-795, 70-76p.

Bosc, R., Tamlyn, N. and Kachrillo, J.-J., 2012. The Hassai Mine project VMS

resources update Red Sea State, Sudan. NI 43-101 Technical Report

prepared for La Mancha Resources Inc., 227p.

Botros, N.S., 2002. Metallogeny of gold in relation to the evolution of the

Nubian Shield in Egypt. Ore Geology Reviews, 19, 137-164.

Botros, N.S., 2004. A new classification of the gold deposits of Egypt. Ore




76 77

F. MELCHER, T. GRAUPNER, T. OBERTHÜR AND P. SCHÜTTE

F. MelcherChair of Geology and Economic Geology, University of Leoben, Peter-Tunnerstraße 5, 8700 Leoben, Austria


T. Graupner, T. Oberthür and P. SchütteFederal Institute for Geosciences and Natural Resources, Stilleweg 2, D-30655 Hannover, Germany

e-mail: [email protected]; [email protected]; [email protected]

© 2017 March Geological Society of South Africa

Tantalum-(niobium-tin) mineralisation in pegmatites and rare-metal granites of Africa

SOUTH AFRICAN JOURNAL OF GEOLOGY 2017 • VOLUME 120.1 PAGE 77-100 • doi:10.2113/gssajg.120.1.77

Abstract

Tantalum is an important metal for high-technology applications of the modern world. It is recovered from oxide

minerals that are present as minor constituents in rare-metal granites (RMG) and granitic rare-element pegmatites

(REP). Significant potential exists as a by-product of spodumene mining from pegmatites, and from Nb-Zr-rare earth

element (REE) mining from carbonatites and peralkaline igneous rocks. Columbite-group minerals (CGM) account

for the majority of the current Ta production; other Ta-Nb oxides such as tapiolite, wodginite, ixiolite, rutile and

pyrochlore-supergroup minerals are of minor importance. The estimated Ta resources of Africa are >50,000 tons of

contained Ta O , representing 16% of the world resources. Currently, the economically most important mining 2 5

districts in Africa are located within the Kibara Belt (mainly Rwanda and the DR Congo), around the Kenticha mine

(Ethiopia) and in the Alto Ligonha Province (Mozambique). Production from pegmatites and related placer deposits

is mostly from hundreds of artisanal and small-scale workings with only a handful of semi-industrial Ta mines.

Collectively, African operations are currently responsible for more than half of the world's primary Ta.

Rare-metal granite and REP districts in Africa are part of Archaean, Palaeoproterozoic, Neoproterozoic,

Palaeozoic and Mesozoic provinces. Each period of Ta ore formation is characterised by specific mineralogical and

geochemical features. Compositions of CGM are variable:

Ÿ Fe-rich types predominate in the Man Shield (Sierra Leone), the Congo Craton (DR Congo), the Kamativi Belt

(Zimbabwe) and the Jos Plateau (Nigeria);

Ÿ Mn-rich columbite-tantalite is typical of the Alto Ligonha Province (Mozambique), the Arabian-Nubian Shield

(Egypt, Ethiopia) and the Tantalite Valley pegmatites (southern Namibia) and

Ÿ Large compositional variations through Fe-Mn fractionation, followed by Nb-Ta fractionation are typical for

pegmatites of the Kibara Belt of Central Africa, pegmatites associated with the Older Granites of Nigeria and

many pegmatites in the Damara Belt of Namibia. Trace element concentrations in CGM and other Ta-Nb oxides

are highly variable, with Ti, W, Sn, Zr, Hf, Y, Sc and REE displaying significant regional differences that may be

used to discriminate Ta provinces.

Introduction

The African continent contains a large number of rare-element

pegmatites (REP), rare-metal granites (RMG), alkaline granites

and carbonatites (Schneiderhöhn, 1961; von Knorring, 1970; von

Knorring and Condliffe, 1987; von Knorring and Fadipe, 1981;

Fetherston, 2004; Melcher et al., 2015). Central-East African

countries have an almost 100-year-long history of Sn and

Ta production. A map and database compiling more than

1,500 locations of known deposits of Ta and Nb in Africa was

tholeiite-boninite association in Central Eritrea. Gondwana Research, 5,

597-611.

Teklay, M., Kröner, A. and Mezger, K., 2002b. Enrichment from plume

interaction in the generation of Neoproterozoic arc rocks in northern

Eritrea: implications for crustal accretion in the southern Arabian-Nubian

Shield. Chemical Geology, 184, 167-184.

Teklay, M., Haile, T., Kröner, A., Asmerom, Y. and Watson, J., 2003. A back-arc

paleotectonic setting for the Augaro Neoproterozoic magmatic rocks of

western Eritrea. Gondwana Research, 6, 629-640.

Tsige, L., 2006. Metamorphism and gold mineralization of the Kenticha-

Katawicha area; Adola belt, southern Ethiopia. Journal of African Earth

Sciences, 45, 16-33.

Tsige, L. and Abdelsalam, M.G., 2005. Neoproterozoic-Early Paleozoic

gravitational tectonic collapse in the southern part of the Arabian-Nubian

Shield: the Bulbul belt of southern Ethiopia. Precambrian Research, 138,

297-318.

Woldehaimanot, B. and Behrmann, J.H., 1995. A study of metabasite and

metagranite chemistry in the Adola region (south Ethiopia): implications for

the evolution of the East African orogen. Journal of African Earth Sciences,

21, 459-476.

Woldemichael, B.W., Kimura, J.I., Dunkley, D.J., Tani, K. and Ohira, H., 2009.

SHRIMP U-Pb zircon geochronology and Sr-Nd isotopic systematic of the

Neoproterozoic Ghimbi-Nedjo mafic to intermediate intrusions of Western

Ethiopia: a record of passive margin magmatism at 855 Ma? International

Journal of Earth Sciences, 99, 1773-1790.

Worku, H. and Yifa, K., 1992. The tectonic evolution of the Precambrian

metamorphic rocks of the Adola belt, southern Ethiopia. Journal of African


Worku, H. and Schandelmeier, H., 1996. Tectonic evolution of the

Neoproterozoic Adola Belt of southern Ethiopia: evidence for a Wilson

cycle process and implications for oblique plate collision. Precambrian

Research, 77, 179-210.

Zoheir, B.A., 2012a. Lode-gold mineralization in convergent wrench structures:

Examples from South Eastern Desert, Egypt. Journal of Geochemical

Exploration, 114, 822-97.

Zoheir B.A., 2012b. Controls on lode gold mineralization, Romite deposit,

South Eastern Desert, Egypt. Geoscience Frontiers, 3, 571-585.

Zoheir, B.A., Creaser, R. and Lehmann, B., 2014. Re-Os geochronology of gold

mineralization in the Fawakhir area, Eastern Desert, Egypt. International

Geology Review, DOI: 10.1080/00206814.2014.935964.

Editorial Handling: S. McCourt.



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