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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
e-mail: [email protected]
W. GhebreabDepartment of Biological and Physical Sciences, Columbus State Community College,
550 E Spring Street, Columbus, OH 43215, USA
e-mail: [email protected]
R.J. SternGeosciences Department, University of Texas at Dallas, Box 830688, Richardson, TX 75080, USA
e-mail: [email protected]
C.T. BarrieCTBA Geoconsulting, 12 Burnham Road, Ottawa, ON K1S 0J8, Canada
e-mail: [email protected]
R.D. HamerManafai International Trade, P.O. Box 136533, Jeddah 21313, Saudi Arabia
e-mail: [email protected]
© 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
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
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.
SOUTH AFRICAN JOURNAL OF GEOLOGY
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).
SOUTH AFRICAN JOURNAL OF GEOLOGY
64 65
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
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.
SOUTH AFRICAN JOURNAL OF GEOLOGY
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).
SOUTH AFRICAN JOURNAL OF GEOLOGY
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).
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
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
GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
SOUTH AFRICAN JOURNAL OF GEOLOGY
Table 1. Operating large-scale gold mines in the Nubian Shield.
Gold-bearing VMS and oxide gold
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
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)
Producing; figure not available
Definitive feasibility study in progress; planned production 2.4 t/y Au
(Kefi Minerals website; data
downloaded 29-4-2015)
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
Surface mine; underground 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 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).
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
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
GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
SOUTH AFRICAN JOURNAL OF GEOLOGY
Table 1. Operating large-scale gold mines in the Nubian Shield.
Gold-bearing VMS and oxide gold
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
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)
Producing; figure not available
Definitive feasibility study in progress; planned production 2.4 t/y Au
(Kefi Minerals website; data
downloaded 29-4-2015)
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
Surface mine; underground 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).
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
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.
GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
SOUTH AFRICAN JOURNAL OF GEOLOGY
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).
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
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.
GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
SOUTH AFRICAN JOURNAL OF GEOLOGY
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).
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
~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).
GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
SOUTH AFRICAN JOURNAL OF GEOLOGY
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).
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
~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).
GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
SOUTH AFRICAN JOURNAL OF GEOLOGY
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
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
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
GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
SOUTH AFRICAN JOURNAL OF GEOLOGY
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
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
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
GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
SOUTH AFRICAN JOURNAL OF GEOLOGY
74 75
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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
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
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
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in the Arabian-Nubian Shield. Journal of the Geological Society, London,
151, 767-776.
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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-
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Kusky, T. and Stern, R.J., 2011. Late Cryogenian-Ediacaran history of the
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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
P.R. JOHNSON, B.A. ZOHEIR, W. GHEBREAB, R.J. STERN, C.T. BARRIE AND R.D. HAMER
SOUTH AFRICAN JOURNAL OF GEOLOGY
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-
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GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
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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
e-mail: [email protected]
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;
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GOLD-BEARING VOLCANOGENIC MASSIVE SULFIDES AND OROGENIC-GOLD DEPOSITS IN THE NUBIAN SHIELD
SOUTH AFRICAN JOURNAL OF GEOLOGY