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Arabian Nubian Shield Overview |
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Saudi Arabia |
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Au Cu Zn Ag
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Super Porphyry Cu and Au
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The Arabian-Nubian Shield covers a large area in Saudi Arabia and parts of Yemen, Egypt, Sudan, Eritrea, Ethiopia and Kenya. It evolved between ~870 and 550 Ma and is one of the largest tracts of juvenile Neoproterozoic crust on Earth. It extends over a north-south interval of 3500 km and at its widest is >1500 km east-west. It is dominated by intra-oceanic arc volcanism, widespread batholithic granitoid intrusions and late molassic volcano-sedimentary sequences. Gold and/or base metal deposit are found throughout the shield, including porphyry Cu-Au (e.g., Jebel Ohier in Sudan); volcanic hosted massive sulphide (VHMS) Cu-Zn, Cu-Au and Au deposits (e.g., Jabal Sayid in Saudi Arabia; Bisha-Hambok cluster in Eritrea; Hassai/Ariab in Sudan); epithermal gold, or epithermal overprint on VHMS mineralisation, (e.g., Mahd Ad'Dhahab and Al Amar in Saudi Arabia); intrusive related orogenic gold hosted in suture zones by carbonate altered ophiolites (e.g., Hamdah, Mansourah, Masarrah in Saudi Arabia) or in competent rocks, including intermediate to felsic intrusions and/or metamorphosed volcanic rocks (e.g., Sukari in Egypt; Al Sukhaybarat, Bulghah in Saudi Arabia). Historically important sediment hosted copper mineralisation was exploited in antiquity in the districts surrounding the Feinan deposits in western Jordan, Timna Valley in southern Israel and the Bir Nasib District in southwestern Sinai Peninsula, Egypt. A more complete list of the deposits of the Shield and their setting is included below.
Tectonic and Structural Setting
The largely Neoproterozoic Arabian-Nubian Shield is split between the current Arabian and African plates, to the east and west respectively of the NNW-SSE elongated Cenozoic Red Sea rift basin. The Arabian-Nubian Shield is also elongated NNW-SSE, with maximum dimensions of ~2100 x 1400 km, and extends from Egypt in the NW, the Sinai Peninsula in the north, Saudi Arabia in the NE, and to Yemen and Kenya in the SE and SW, respectively. It comprises juvenile, late Tonian to Cryogenian supra-subduction ophiolites and island arc rocks which formed in the Mozambique Ocean that opened as a result of the Rodinia breakup. It also includes Ediacaran volcano-sedimentary basins, and syn-, late- and post-tectonic Tonian-Ediacaran mafic to felsic intrusions (Johnson et al., 2017). The current outline of the the Arabian-Nubian Shield is an artifact of an early broad scale stage of doming, a post-Jurassic extensional feature, that culminated in the opening of the Red Sea Rift.
Deformation, metamorphism and accretion of a series of terranes occurred between ~850 and 550 Ma, although peak metamorphism and pervasive shearing was mostly from 650 to 500 Ma, concurrent with final assembly of the Shield at ~620 Ma.
The Arabian-Nubian Shield records three stages of an orogenic cycle that began with the initiation of the Rodinia Supercontinent breakup and extension between 870 and 800 Ma. This breakup progressed from intracontinental rift basin development with associated bimodal magmatism, into the opening of the Mozambique Ocean, the western coast of which was occupied, from north to south, by the Saharan Metacraton, and the Congo-Tanzanian and Kalahari cratons. Stern and Johnson (2010) interpret the oceanic spreading phase to have occurred between 800 and 670 Ma, with expansion being accommodated by both i). widening of the ocean; and ii). intraoceanic subduction. Intraoceanic subduction and related suprasubduction island arcs are interpreted to have occurred in a number of spatially and/or temporally separated zones and were progressively amalgamated through arc-arc collision, while spreading continued at the main and subordinate mid-ocean ridges.
The initial episode of overall extension was followed by a period of oblique transpressional to transtensional convergence and tectonic reorganisation which was terminated by the NW-SE directed approach and collision of East and West Gondwana between 650 and 600 Ma (e.g., Johnson et al., 2011, and references therein). The eastern margin of West Gondwana was defined by the cratonic blocks on the western coast of the Mozambique Ocean as listed above, whilst the western margin of East Gondwana included, from north to south, the poorly defined elongate Azania Block, Madagascar, India and Antarctica. Peak M1 metamorphism (characterised by garnet, cordierite, sillimanite at >800°C) followed by M2 amphibolite-facies (>640°C, 6 to 8.5 kbar) apparently accompanied collision and was pre 640 Ma (Neumayr et al., 1996). This was followed and/or accompanied by orogenic collapse in areas of thicker crust between 640 and 630 Ma (Johnson et al., 2011).
The same authors suggest this was succeeded from ~600 to 550 Ma by a period of post-collisional crustal and lithospheric reworking which involved continued shortening and thickening, gravitational collapse and deposition, as well as both extrusive and intrusive magmatism and late tectonic relaxation. Gravitational collapse largely took place prior to 605 Ma in areas of high relief, predominantly in the western part of the shield. This took the form of erosion, gravity slumping, listric faulting, nappe development and extension, with deposition in terrestrial intermontane molasse basins in the west and marine foreland basins to the east, as well as in strike-slip pull-apart basins. While this shallow extension occurred in areas of high relief, contraction and shortening continued in the underlying shield resulting in crustal thickening, accompanied by lateral and vertical escape tectonics, tectonic reworking and magmatism. This shortening took the form of NW-SE directed thrusting and folding about east-west axes to 605 Ma, followed by bulk east-west shortening and transtension along NW-SE sinistral strike-slip faults, known as the Najd Fault System, to 590 Ma. This was followed by a period of tectonic relaxation and rebound from 587 to 579 Ma with NW-SE to north-south directed extension accompanied by exhumation of mid-crustal level gneisses in the centres of metamorphic core complex domes and along major suture and shallow extensional fault zones. This extension is also reflected by extensive 600 to 565 Ma post-tectonic mafic, felsic and composite mafic–felsic dyke swarms (Johnson et al., 2011).
Variations in the geometry of the advancing West and East Gondwana during Ediacaran assembly of the Shield resulted in a three part tectonic division (Fritz et al., 2013). To the south, in the southern Nubian section of the Shield in Ethiopia and Kenya, pure shear-dominated transpression and lateral extension produced a pervasive north-south structural trend represented by north-trending shear zones and 'greenstone belts'. The central part of the Nubian and southwestern Arabian sections of the Shield was subjected to oblique and orthogonal east-west compression accompanied by north-south stretching, leading to the development of the Oko and Hamisana shear zones and coeval Keraf suture (Miller and Dixon, 1992; Abdelsalam, 1994; Abdelsalam and Stern, 1996; Abdelsalam et al., 1998). NOTE: The Oko Shear zone, not shown on the accompanying image, strikes NNW-SSE and offsets the Nakasib Fault in a sinistral sense by ~10 km, near the Hassai/Ariab deposits (Abdel Salam, 1994). In the northern Nubian and the northern and eastern Arabian sections of the Shield, shearing and northwest-directed thrusting, extension and tectonic escape continued, with a dominant northwesterly structural trend and extension, producing the Najd Fault System, as described above (Johnson et al., 2017).
Metamorphism in the Arabian-Nubian Shield ranges from greenschist facies to the north and east, increasing to amphibolite and granulite facies to the south and west, persisting in the exposures further to the south into Uganda. The Arabian-Nubian Shield is interpreted to represent the northern continuation of the higher grade and more strongly deformed (amphibolite- and granulite-facies) rocks of the Mozambique Belt in East Africa and Antarctica. The latter, however, was more long-lived, persisting into the Lower Cambrian. In contrast to the broad swathe of juvenile Neoproterozoic rocks preserved in the Arabian-Nubian Shield, the Mozambique Belt is predominantly composed of Archaean to Mesoproterozoic rocks reworked during Neoproterozoic metamorphism and anatexis. The Arabian-Nubian Shield and Mozambique Belt, both of which are sandwiched between East and West Gondwana, together constitute the East African Orogen.
Whilst the Mozambique Belt is bounded by well defined pre-Neoproterozoic cratonic blocks, the western, northern and northeastern margins of the Arabian-Nubian Shield are less well defined or known, being masked by cover sequences. The western margin of the shield is taken to be the up to 50 km wide Keraf Suture separating it from the Sahara 'Metacraton', an ~5 million km2 pre-Neoproterozoic cratonic block including rocks dated from 3.1 to 0.5 Ga. This block, which remained a coherent rheological entity, was variably remobilised and reworked during Neoproterozoic time, coeval with development of the Arabian-Nubian Shield (Blades et al., 2017; Johnson 2014), and was dismembered into at least three smaller remnant cratonic blocks (Liégeois et al.. These blocks were separated by zones of shearing accompanied by deformation, metamorphism ranging from low-grade metasediments to high-grade gneiss and significant volumes of high temperature Neoproterozoic migmatitic gneiss and intrusion. The latter included granites derived from melting of Mesoproterozoic crust at ~665 to 654 Ma and abundant 600 to 560 Ma magmatism throughout its northeastern sections (Blades et al., 2017). The 'decratonisation' of the the Saharan 'Metacraton' has been attributed to Neoproterozoic collision to the west (Trans-Saharan Belt), south (Oubanguides) and east (Arabian-Nubian Shield) producing tectonic thickening that led to mantle delamination and detachment which, in turn, resulted in anomalously thin lithosphere, at least in the outer 'few hundred' kilometres of the 'metacraton' (Abdelsalam et al., 2011). During the Neoproterozoic and Lower Palaeozoic, the northern edges of the West African Craton, Saharan Metacraton and Arabian-Nubian Shield most likely represented the northern passive margin of Gondwana, although possible Mesoproterozoic basement is indicated north of the Red Sea in Sinai (Johnson, 2014).
To the SE, in coastal Kenya, Somalia, Yemen to central Saudi Arabia, the Arabian-Nubian Shield is bounded by the poorly defined, and sparsely exposed NE-SW elongated Archaean to Palaeoproterozoic crustal 'Azania' block. However, the margin of the shield has not been established in northeastern Saudi Arabia. Juvenile Neoproterozoic rocks of the Arabian-Nubian Shield continue east and north of the northern tip of 'Azania' block crustal exposure to the farthest exposed edge of the shield where they are unconformably overlain by >10 km of Phanerozoic passive margin strata. The easternmost exposed terranes of the shield (Ad Dawadimi and Ar Rayn) are composed of >689 to ~625 Ma volcanosedimentary rocks and arc-related 689 to 616 Ma tonalite-trondhjemite-granodiorite (TTG) intrusions, cut by 607 to 583 Ma syn- to post-tectonic alkali granite. These are the youngest arc rocks known in the Arabian section of the shield, similar in age to an arc proposed at the opposite western edge of the shield in Egypt. This is taken to imply either collision of a continental crust sliver well to the east, or the accretion of a late Cryogenian-Ediacaran intra-oceanic arc near the eastern most limit of the currently exposed shield. Following this collision, the NE margin of the Arabian-Nubian Shield remained a passive margin until the Oligocene-Eocene collision with Eurasia in the Zagros Mountains of southwestern Iran.
The entire East African Orogen remained intact from the Ediacaran until the dispersal of Pangea from the Early to Middle Jurassic. The exposure of the Neoproterozoic sequences has been masked by Palaeozoic cover sequences and younger successions associated with the opening of the ~30 Ma Red Sea-East African rift and the deposition of recent volcaniclastic rocks and lava flows covering large areas of Saudi Arabia, Yemen and Ethiopia (Stern and Johnson, 2019).
Geologic Setting
The Arabian-Nubian Shield is a broad expanse of predominantly Neoproterozoic juvenile oceanic, oceanic island, intraoceanic island arc, suprasubduction ophiolite, volcano-sedimentary basin and microcontinental suites (e.g., Johnson and Woldehaimanot, 2003; Stern and Johnson, 2010).
Three distinct igneous events have been recognised in the northern Arabian-Nubian Shield of Egypt where more geological data is available. These events occurred from 850 to 800 Ma; 760 to 710 Ma; and 630 to 610 Ma (Hassan and Hashad,1990, El-Bialy and Omar, 2015). Similarly, further south in the West Ethiopian section of the shield, three ages of magmatism are recognised, from ~850 to 810 Ma, 780 to 700 Ma and 620 50 55 Ma (e.g., Kebede et al., 2001, Blades, M.L., et al., 2015).
The volcano-sedimentary sequences deposited during these intervals were sutured and intruded by numerous subduction-related calc-alkaline I-type granitoids, granodiorites and gabbrodiorite complexes between 850 and 610 Ma and again from 600 to 525 Ma by Pan-African calc-alkaline A-type granite and rhyolite, syenite and monzogranite, and by mafic to felsic dykes (Stern, 1994; Osman and El Kalioubi, 2014; El-Bialy and Omar, 2015; Barrie et al., 2016). This Cryogenian to Ediacaran magmatism in the shield is interpreted to have evolved from arc-related tholeiite and calc-alkaline TTG assemblages, to collisional-related calc-alkaline TTG and granite assemblages, to post-collisional 'within-plate' A-type granitoids that formed in extensional regimes
during orogenic collapse ((Stern and Hedge, 1985; Beyth et al., 1994; Moghazi et al., 1998; Garfunkel, 1999; Jarrar et al., 2003;
Mushkin et al., 2003; Moussa et al., 2008). However, Lundmark et al. (2011), has shown this trend, although overall correct, to be simplistic.
Rocks within the shield aged between ~870 and 650 Ma were predominantly deposited from Late Tonian to Middle Cryogenian oceanic spreading centres and in suprasubduction settings, either intra-oceanic, or between oceanic crust and rifted blocks of the former Rodinia Supercontinent within the Mozambique Ocean (Stoeser and Camp, 1985; Genna et al., 2002; Johnson and Woldehaimanot, 2003; Stoeser and Frost, 2006). On the basis of the closeness of their crystallisation and Nd model ages, these rocks can be shown to have constituted juvenile additions to the crust (Stern, 2002).
The Neoproterozoic rocks of the Arabian-Nubian Shield have been divided into a series of terranes, most of which are bounded and defined by high-strain shear zones commonly containing dismembered ophiolites (Berhe, 1990) and refolded recumbent folds. Whilst the shear zones are generally interpreted to represent sutures formed during terrane amalgamation (although challenged by Church, 1991), some are younger strike-slip shear structures which modified or reworked original sutures (e.g., Kusky and Matsah, 2003). The major sutures and shear zones are nor clearly defined structures, but broad zones that are often kilometres to tens of kilometres in width of intensely deformed schist and gneisses, frequently containing rafts of ophiolitic rocks.
These terranes converged and were amalgamated through a succession of subduction-driven arc–arc and ultimately arc–continent collisions.
Terranes on the northeastern, and to a lesser degree on the southwestern margin of the Shield tend to have trends parallel to the NNW-SSE elongation of the Shield and the younger Red Sea rifting (which opened at ~25 Ma; Bosworth, 2015). However, the bulk of the terranes in the longitudinal centre of the Shield form a 'core group' composed of 3 main correlated pairs of sub-terranes on opposite sides of the Red Sea, defining three NE-SW elongated terranes normal to the long axis of the Arabian-Nubian Shield. These pairs are, from SSE to NNW, the i). Asir-Nakfa; ii). Jiddah-Haya; and iii). Hijaz-Gebeit (and Gabgaga) terranes (the first named of each pair being to the NE of the Red Sea in Arabia, and the second to the SW, in Africa). The oldest Neoproterozoic crust (870 to 680 Ma) occurs in these three terrane pairs which converged and amalgamated during the middle Cryogenian (780 to 750 Ma), separated by two main sutures, the Barka Shear-Ad Damm Fault zone to the SE, and the Nakasib Shear-Bir'Umq Suture to the NE, defining the boundaries between the central Jiddah-Haya and the bounding terranes.
The geology of each of these terranes is described in summaries for deposits within them, e.g., for those of the Arabian section of the Shield see the Central Arabian Gold and Base Metal Region record; the
Gebeit Terrane - the Jebel Ohier record; and the Haya and Nakfa terranes - the Bisha-Hambok record.
Two further terranes are recognised in Eritrea separating the Nakfa and Haya terranes, and are also part of the 'core group'. These are the wedge shaped Hager/Tokar Terrane to the NE, bounded by the Barka and Adobha sutures to the NW and SE respectively, and the overlapping Barka Terrane in the SW. The Barka Terrane is on the opposite, northwestern side of the Barka Suture, and has a poorly defined boundary with the Haya Terrane on its NW margin. The Barka Terrane is composed of upper amphibolite to hornblende-granulite facies orthogneiss, amphibolite, marble, pelitic schist and orthoquartzite, intruded by a swarm of east-west felsic dykes and has been interpreted to represent a 'far travelled' exotic terrane of unknown age (de Souza and Drury, 1998). However, it is more likely to represents the margin of the Haya Terrane that was deeply underthrust below the Nakfa Terrane and subsequently exposed during post-orogenic rebound and relaxation. The Hager/Tokar Terrane, in contrast, is composed ultramafics, and olistostrome sediments within an 870 to 840 Ma volcano-sedimentary layered sequence of mid-greenschist facies mafic and felsic meta-volcanic rocks, intruded by gabbro, pyroxenite, diorite, tonalite and granodiorite. It would seem to be a broader section of the Barka-Adobha Suture complex. See the Bisha-Hambok record for more detail.
By 700 Ma a fourth subterrane pair, the Midyan-Eastern Desert composite terrane had collided and was welded onto the NNW margin of the 'core group' across the composite Yanbu-Onib-Sol Hamid-Allaqi-Heiani Suture Zone. Together these formed what is known as the 'Western Arc/Oceanic Terranes' of the Arabian Nubian Shield (e.g., Stoeser and Frost, 2006).
The adjacent 'Eastern Arc/Oceanic Terranes' incorporates those NNW-SSE trending terranes along the northeastern margin of the Shield and is predominantly composed of the Afif Terrane and a number of smaller blocks. The Eastern Arc Terranes includes both i). the Khida Terrane, where Late Palaeoproterozoic granitoids are exposed and older rocks reworked during the Neoproterozoic contain Palaeoproterozoic zircons, and ii). three Neoproterozoic arc assemblages of about 840 to 820, 750 to 720 and 700 to 680 Ma in age. A suturing event occurred along the eastern margin of the Afif Terrane (and Eastern Arc Terranes) at ~680 to 670 Ma, marked by the Halaban Ophiolite (Al-Saleh et al., 1998) probably marking accretion of either a younger intraoceanic arc of an older Rodinian break-up sliver of cratonic rock. The entire eastern margin of the shield is masked by Phanerozoic cover.
Isotopic and geochemical data suggest the Western Arc Terranes were derived from depleted mantle sources, whilst the Eastern Arc Terranes were
derived either from mantle with minor continental contamination, or was derived from mantle with a different composition.
The Eastern and Western Arc Terranes began sinistral approach and transpressive collision at ~680 Ma and were fully amalgamated by ~640 Ma. This collision is represented by the narrow north-south to NNW-SSE trending Nabitah Mobile Belt centred on the Nabitah Fault and its extensions. Deposition continued in post-amalgamation marine and terrestrial basins, mainly within the Eastern and Western Arc terranes respectively, the largest of which was the >600 x 120 km Murdama marine basin. Sedimentation in the Murdama Basin ceased at ~620 Ma when the sequence was metamorphosed to greenschist and lower amphibolite facies (Cox et al., 2011). Volcano-sedimentary deposition continued in the terrestrial bains, mainly over the Western Arc Terrane, until ~550 Ma.
The entire Arabian-Nubian Shield was accreted to the eastern margin of the Saharan Metacraton across the sinistral, transpressional arc-continent Keraf Suture at ~650 to 560 Ma (Abdelsalam et al., 1998; Bailo et al., 2003), one of the principal tectonic elements in the final assembly of Gondwana. This collision is interpreted to be partly coeval to the proposed ~650 to 630 Ma collision of Azania with the East African margin in the southern Mozambique Belt (Collins and Windley, 2002; Collins and Pisarevsky, 2005; Collins, 2006; Collins et al., 2010).
By ~630 Ma, north-trending shear and shortening zones had developed in the southwestern Arabian-Nubian Shield in response to this collision. In contrast, to the NE, in much of the Arabian section of the shield and Eastern Desert Terrane, which were more distal to the zone of collision, NW-SE trending shear zones, the Najd Fault System, dominated. This system of sinistral, strike-slip continental transform faults is up to 400 km wide with an exposed length of 1100 km and inferred buried extent of 2000 km. In the NE, convergence and Najd transpression/transtension led to crustal buckling producing linear structural highs with domes of gneissic infracrust overlain by supracrust composed of Tonian to Mid Cryogenian island-arc ophiolitic and volcano-sedimentary assemblages forming metamorphic core complexes.
Post-collisional tectonic relaxation and rebound is marked by further metamorphic core complexes of Late Cryogenian to Ediacaran gneiss, chiefly orthogneiss, but locally also significant paragneiss. These complexes occur along both NE-SW, NW-SE and north-south trending early to middle Cryogenian structural zones and as belts and domes along or close to NW- to WNW-trending fold structures.
Metamorphism in the Arabian-Nubian Shield, as summarised above, ranges from greenschist facies to the east of the Red Sea, in the core of the Barka-Nakfa terrane and in the Midyan-Eastern Desert composite terrane to the north, predominantly dated at 650 to 605 Ma. Metamorphic grade increases to granulite facies dated between 580 and 540 Ma in much of the Gebeit-Gabgaga and Haya terranes, and persists as amphibolite and granulite facies in the exposures further to the south into the West and South Ethiopian terranes. These southern extensions however, includes blocks of both amphibolite and granulite facies dated in the older 650 to 605 Ma range (Johnson, 2014).
Mineralisation
A range of gold, copper and base metal deposits and occurrences are distributed across the Arabian Nubian Shield, as follows:
Porphyry Cu-Au deposit
The significant Jebel Ohier porphyry Cu-Au deposit has been delineated, within the Gebeit Terrane of the western Arabian-Nubian Shield in Sudan. It is located just to the east of the major NNE-SSW Hamisana Shear Zone that forms the western margin of the terrane, and is associated with a suite of 816 to 812 Ma diorite to granodiorite porphyries that intrude a thick succession of andesitic volcanic and volcaniclastic rocks within an juvenile intra-oceanic arc setting.
Volcanic hosted massive and/or disseminated sulphide deposits
Deposits belonging to this family are widely distributed throughout the shield. At least fifty VHMS deposits and occurrences are known in the Nubian half of the shield, to the west of the Red Sea, many of which are clustered in districts. As many others are found in Arabia, to the east of the Red Sea. These include deposits that are primarily Cu deposits with variable Zn and Au, such as: Jabal Sayid, Al Masane, Al Hajar and Jadmar, the Wadi Bidah Mineral District (Shaab Al Taare, Rabathan, Gehab and Mulhal) and Kutam in Saudi Arabia; Hassai/Ariab in Sudan; Bisha-Hambok cluster (Bisha Main, Bisha Northwest, Hambok, Harena and Ashelli) and the Asmara Cluster (Debarwa, Emba Derho, Adi Nefas and Kodatu) in Eritrea.
Other examples include a cluster in Egypt in the WNW-ESE trending, 712 Ma, Shadli Volcanic Belt, ~90 km south of the Sukari gold deposit (see below), the largest of which is Um Samiuki (0.3 Mt @ 11.5% Zn, 1.15% Cu, 1.1% Pb, 100 g/t Ag with local Au to 0.3 g/t; Botros, 2015; Shalaby et al., 2004; Helmy, 1999; Searle 1975).
Many of these deposits have initially been exploited as 'gold only' mines to the base of oxidation at depths of 80 to 100 m below surface. Some have then been mined for Cu and Zn as well as gold in the hypogene sulphides.
Volcanic hosted epithermal gold and/or polymetallic deposits
The same arcs as host the VHMS deposits also contain generally low sulphidation gold and/or polymetallic deposits, sometimes overprinting earlier VHMS mineralisation. These include: Mahd Ad'Dhahab and Al Amar in Saudi Arabia. Other possible epithermal gold and/or polymetallic deposits include Gupo, Adi Rassi and the vein mineralisation at Kodatu, all in the Asmara District of Eritrea. These Eritrean deposits are generally referred to as orogenic in the literature. Similarly, Surour et al. (2014) interpret Bi'r Tawilah in Saudi Arabia to be epithermal, whilst other interpretations regard it to be orogenic.
Volcanic sequences hosting VHMS and/or epithermal mineralisation range in age (after Volesky et al., 2017) from 855 to 770 Ma (e.g., Bisha, Emba Derho - Asmara District); 855 to 815 Ma (e.g., Wadi Bidah); 825 to 795 (e.g., Al Hajar and Jadmar); 825 to 745 Ma (e.g., Jabal Sayid, Hassaï); 780 to 700 Ma (e.g., smaller VHMS/epithermal deposits in Midyan-Eastern Desert Terrane); 750 to 730 Ma (e.g., Al Masane, Kutam); 690 to 615 Ma (e.g., Al Amar).
Orogenic and intrusion related gold deposits
Hundreds of gold workings (>250) found throughout the Arabian-Nubian Shield provide evidence of a ~5500 year history of gold mining. The bulk of the mineralisation worked was as orogenic gold deposits and occurrences, and alluvial/colluvial gold shed from them. Despite the spread and number of workings, Klemm et al. (2001) estimate that prior to modern mining, only ~18 t of gold was won from these workings, with 7 t during Pharaonic times. The remainder was equally distributed between both Ptolemaic and Arab times, neglecting the very low production rates of Predynastic and Roman-Byzantine times.
There are a suite of orogenic deposits, mainly in Saudia Arabia, where quartz vein and disseminated mineralisation is spatially associated with carbonate altered ophiolitic serpentinites rocks (listvenites) formed along suture zones during terrane amalgamation, e.g., Hamdah, Mansourah, Masarrah, Bi'r Tawilah. The bulk of the deposits and occurrences, however, 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 VHMS mineralisation. Key examples include: Al Sukhaybarat, Bulghah, As Suq, Ad Duwayhi, Ar Rjum Goldfield and Jabal Ghadarah (gold bearing quartz veining in listvenite altered ultramafic rocks; >0.33 Mt @ 3.2 g/t Au; Harbi et al., 2006) in Saudi Arabia; Sukari, Hamash and the small historic Atalla and Dungash deposits in Egypt; Galat Safur and Gabgaba/Qbgbih in Sudan; and Koka in Eritrea.
Orogenic, structurally controlled gold deposits developed within Neoproterozoic 'greenstones' are found in western Ethiopia, including Tulu Kapi, Kurmuk, Lega Dembi and Sakaro in Ethiopia.
Pegmatite hosted Lithium and Tantalum
The principal example of which is Kenticha in southern Ethiopia.
Sediment-hosted copper deposits
Historically significant copper deposits are hosted by a generally flat-lying to very shallow-dipping passive margin succession deposited over the northern margin of the Arabian-Nubian Shield for much of the period from the Late Neoproterozoic to the present. During that time, this succession has alternated from emergent, to fluviatile coastal plain to lagoonal to brief intervals in shallow marine settings. It has a high proportion of permeable sandy facies with lesser shales and carbonates, and is punctuated by a number of unconformities marking the emergent intervals when erosion has removed large parts of the sequence. The main deposits flank the north-south oriented Dead Sea Rift Valley, which was developed from the Late Cretaceous to the present and marks the boundary between the Arabian and African plates. Copper mineralisation is largely stratabound, but is almost completely composed to supergene copper minerals such as chrysocolla, paratacamite and malachite, dependent upon the host. This mineralisation has been mined since as early as 10 000 BCE at Feinan in northwestern Jordan, but while not having been mined in modern times has a modest remaining resource. Approximately ~100 km to the south, in southern Israel, the Timna Valley is located on the opposite side of the Dead Sea Rift Valley, and was mined from at least the 5th millennium BCE. Modern mining has been undertaken, and a modest resource remains. Around 170 km to the SW, in the Bir Nasib District of southwestern Sinai Peninsula, in Egypt on the edge of the NNW-SSE Suez Rift, the same succession has produced both ornamental turquoise and copper metal since ~3500 BCE. Significant Cu mineralisation within in this sequence is located at a number of stratigraphic levels, ranging from the Lower to Middle Cambrian, Carboniferous to Lower Cretaceous, whilst younger, structurally controlled vein mineralisation cuts Jurassic to Eocene rocks within the Dead Sea Rift Valley. At both the Feinan and Bir Nasib Districts, copper mineralisation overlaps lenses of supergene Mn-Fe mineralisation, which in the latter is commercially exploited.
Description of these deposits are available from links to separate records above, whilst an additional more detailed overview of the Arabian section of the Shield can be studied from the Central Arabian Gold and Base Metal Region record. Most deposits listed are shown on the geological/tectonic plan above.
The most recent source geological information used to prepare this decription was dated: 2019.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Abdelsalam, M.G. and Stern, R.J., 1993 - Tectonic evolution of the Nakasib suture, Red Sea Hills, Sudan: evidence for a late Precambrian Wilson Cycle: in Journal of the Geological Society, London, v.150, pp. 393-404.
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Abdelsalam, M.G., Liegeois, J.-P. and Stern, R.J., 2002 - The Saharan Metacraton: in J. of African Earth Sciences v.34, pp. 119-136.
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Agar, R.A., 1992 - The tectono-metallogenic evolution of the Arabian Shield: in Precambrian Research v.58, pp. 169-194.
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Barrie, C.T., Abu Fatima, M. and Hamer, R.D., 2016 - Volcanogenic Massive Sulphide-Oxide Gold Deposits of the Nubian Shield in Northeast Africa: in Bouabdellah, M. and Slack, J.F. (eds.), 2016 Mineral Deposits of North Africa, Mineral Resource Reviews, Springer International Publishing Switzerland DOI 10.1007/978-3-319-31733-5_17, pp. 417-435.
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Botros, N.S., 2015 - The role of the granite emplacement and structural setting on the genesis of gold mineralization in Egypt: in Ore Geology Reviews v.70, pp. 173-187.
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Botros, N.S., 2015 - Gold in Egypt: Does the future get worse or better?: in Ore Geology Reviews v.67, pp. 189-207.
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Bowden, S., Gani, N.D., Alemu,T., O Sullivan, P., Abebe, B. and Tadesse, K., 2020 - Evolution of the Western Ethiopian Shield revealed through U-Pb geochronology, petrogenesis, and geochemistry of syn- and post-tectonic intrusive rocks: in Precambrian Research v.338, 14p. doi.org/10.1016/j.precamres.2019.105588.
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Fritz, H., Abdelsalam, M., Ali, K.A., Bingen, B., Collins, A.S., Fowler, A.R., Ghebreab W., Hauzenberger, C.A., Johnson, P.R., Kusky, T.M., Macey, P., Muhongo, S., Stern, R.J. and Viola, G., 2013 - Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution: in J. of African Earth Sciences v.86, pp. 65-106.
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Greenwood, W.R., Stoeser, D.B., Fleck, R.J. and Stacey, J.S., 1983 - Late Proterozoic island-arc complexes and tectonic belts in the southern part of the Arabian Shield, Kingdom of Saudi Arabia: in Department of the Interior, U.S. Geological Survey, Open File Report 83-296, 47p.
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Harbi, H.A., Eldougdoug, A.A. and Al Jahdli, N.S., 2006 - Geology and Geochemistry of Jabal Ghadarah Ophiolitic Melange, Zalim Quadrangle, Central Saudi Arabia: in Journal of King Abdulaziz University, Earth Sciences, v.17, pp. 117-153.
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Johnson, P.R., 2014 - An Expanding Arabian-Nubian Shield Geochronologic and Isotopic Dataset: Defining Limits and Confirming the Tectonic Setting of a Neoproterozoic Accretionary Orogen: in The Open Geology Journal, v.8, pp. 3-33.
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Johnson, P.R., 2006 - Explanatory notes to the map of Proterozoic Geology of western Saudi Arabia: in Saudi Geological Survey, Technical Report SGS-TR-2006-04 76p.
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Johnson, P.R., Andresen, A., Collins, A.S., Fowler, A.R., Fritz, H., Ghebreab, W., Kusky, T. and Stern, R.J., 2011 - Late Cryogenian-Ediacaran history of the Arabian-Nubian Shield: A review of depositional, plutonic, structural, and tectonic events in the closing stages of the northern East African Orogen: in J. of African Earth Sciences, v.61, pp. 167-232.
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Johnson, P.R., Halverson, G.P., Kusky, T.M., Stern, R.J. and Pease, V., 2013 - Volcanosedimentary Basins in the Arabian-Nubian Shield: Markers of Repeated Exhumation and Denudation in a Neoproterozoic Accretionary Orogen: in Geosciences (MDPI), v.3, pp. 389-445.
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Johnson, P.R., Zoheir, B.A., Ghebreab, W., Stern, R.J., Barrie, C.T. and Hamer, R.D., 2017 - Gold-bearing volcanogenic massive sulfides and orogenic-gold deposits in the Nubian Shield: in S. Afr. J. Geol. v.120, pp. 63-76.
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Klemm, D., Klemm, R. and Murr, A., 2001 - Gold of the Pharaohs - 6000 years of gold mining in Egypt and Nubia: in J. of African Earth Sciences v.33, pp. 643-659.
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Reischmann, T. and Kroner, A., 1994 - Late Proterozoic island arc volcanics from Gebeit, Red Sea Hills, north-east Sudan: in Geologische Rundschau, v.83, pp. 547-563.
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Reischmann, T., Bachtadse, V., Kroner, A. and Layer, P., 1992 - Geochronology and palaeomagnetism of a late Proterozoic island arc terrane from the Red Sea Hills, northeast Sudan: in Earth and Planetary Science Letters, v.114, pp. 1-15.
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Stoeser, D.B., Whitehouse M.J. and Stacey, J.S., 2001 - The Khida Terrane - Geology of Paleoproterozoic Rocks in the Muhayil Area, Eastern Arabian Shield, Saudi Arabia: in Gondwana Research v.4, pp. 192-194.
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Stoeser,D.B. and Frost, C.D., 2006 - Nd, Pb, Sr, and O isotopic characterization of Saudi Arabian Shield terranes: in Chemical Geology, v.226, pp. 163-188.
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Wipfler, E.L., 1996 - Transpressive structures in the Neoproterozoic Ariab-Nakasib Belt, northeast Sudan: evidence for suturing by oblique collision: in J. of African Earth Sciences v.23, pp. 347-362.
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Zoheir, B.A., Johnson, P.R., Goldfarb, R.J. and Klemm, D.D. 2019 - Orogenic gold in the Egyptian Eastern Desert: Widespread gold mineralization in the late stages of Neoproterozoic orogeny: in Gondwana Research v.75, pp. 184-217.
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