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Sukari |
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Egypt |
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Super Porphyry Cu and Au
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The Sukari (or Sukarai) gold mine is located 15 km to the SW of the Red Sea port of Marsa Alam in the Red Sea Hills, part of the southern-central Eastern Desert region of Egypt (#Location: 24° 56' 50"N, 34° 43' 1"E).
Numerous small pits distributed over an ~2 km strike on Sukari Ridge are the artifacts of gold mining in Pharaonic and Roman times. Production during this period is estimated to have totalled ~1 t of gold. Small-scale mining was re-established in 1912 to 1914, whilst more substantial operations were undertaken from 1937 to 1951, with recorded production of 4.768 t of gold from underground workings. An exploration Concession Agreement was granted in 1994 and in November 2000, a feasibility study was completed for mining at the Sukari Gold Project. In 2005 an Exploitation Lease covering an area of 160 km2, containing the proposed Sukari mine site and surrounding prospects, was officially granted.
The Sukari deposit lies within the Eastern Desert Terrane, the northern section of the Arabian Nubian Shield which is predominantly composed of juvenile Neoproterozoic crust of oceanic affinity. For a description of the regional setting of the shield and its geology and distribtion of mineralisation, see the separate Arabian Nubian Shield Overview record.
The Neoproterozoic crust of northeastern Africa was consolidated by convergence and the accretion of intra-oceanic island arcs, continental micro-plates and oceanic plateaus (e.g. Gass 1982; Stern 1994; Kröner et al., 1994; Abdelsalam and Stern 1996). The Arabian Nubian Shield to the west of the Red Sea is composed of a basement orthogneisses, psammitic schists and amphibolite complex that underwent amphibolite facies, polyphase metamorphism (Neumayr et al., 1996, 1998; Loizenbauer et al., 2001), structurally overlain by greenschist facies Pan-African Nappes of ophiolites, a mélange-like assemblage of accretionary-wedge like sedimentary rocks, and arc affinity calc-alkaline volcanic rocks.
Exhumation of the structural basement units resulted from combined sinistral strike-slip faults and steep NW- to SE-dipping, NE-trending normal faults known as the Najd Fault System (Stern 1985) that accommodated bulk NW-SE extension in the Arabian Nubian Shield (Stern et al., 1994; Burke and Sengör 1986; Wallbrecher et al., 1993; Greiling et al., 1994; Fritz et al., 1996). This extension resulted in the formation of metamorphic core complex domes including the Meatiq-Sibai- and Hafafit Domes exposing the structural basement (Sturchio et al., 1983; Fritz et al., 1996; 2002; Blasband et al., 2000). Coeval with exhumation of basement rocks, intramontane molasse basins were filled with sediment eroded from the core complexes and Pan-African Nappes (Grothaus et al., 1979; Fritz and Messner 1999). Various granitoids were also emplaced during late Pan-African extension (Greenberg 1981; Bregar et al., 2002).
The sequence in the Sukari district comprises a NNE striking mélange of predominantly calc-alkaline igneous and metasedimentary rocks interpreted to be an accreted island arc. Several bodies of serpentinite, representing accreted slivers of highly deformed oceanic crustal rocks, occur in the hangingwall of the district-scale (~25 km), NNE striking, ESE verging Sefein-Sukari thrust (Akaad, et al., 1993). This structure passes immediately to the east of Sukari, where it separates rocks of the Um Khariga Metapyroclastics (west of Sukari) from the Sukari Metavolcanics (east of Sukari). These rocks are interpreted to be ~770-660 Ma in age (Vail, 1983), and have undergone regional metamorphism to mid-upper greenschist facies.
The volcanosedimentary sequence at Sukari comprises a succession of andesites, dacites, rhyodacites, tuffs and pyroclastics with a calc-alkaline affinity (Akaad et al., 1995). The dismembered ophiolitic succession is composed of a basal serpentinite, followed upwards by a metagabbro-diorite complex and sheeted dykes. The metagabbro-diorite rocks and serpentinites form lenticular, 1 to 3 km2 bodies, as well as small masses concordantly interleaved with the volcanosedimentary arc assemblage (Harraz 1991). All of these rocks are metamorphosed to lower greenschist metamorphic facies, but are intensely sheared and transformed into various schists along shear zones.
The Sukari vein deposit is hosted exclusively by the Late Neoproterozoic Sukari granite porphyry, rather than in the island-arc and ophiolite rock assemblages that together occur within the Wadi Ghadir mélange. The reason for the host being the porphyry, rather than in the in chemically more reactive serpentinitic wall rocks, is interpreted to be the result of the Sukari Porphyry acting as a rigid competent body surrounded by weaker, more ductile rocks. Footwall and hangingwall rocks have taken up strain by development of strong schistosity, almost certainly accompanied by large decreases in volume. The porphyry has taken up strain by development of predominantly brittle fault structures. Porphyry dykes in the hangingwall of the main porphyry body similarly show gold mineralisation of essentially the same character as that in the main porphyry, whilst wall rocks immediately adjacent to those dykes are barren. These dykes range in thickness from a few centimetres to several metres.
Granitoids of the Egyptian portion of the Arabian-Nubian Shield are described as 'older granites' that are 'syntectonic', related to arc magmatism, and 'younger granites' that post-date arc accretion and have monzonitic, syenitic or potassic compositions (Moghazi et al., 1999). Sharara (1999) ascribes the Sukari Porphyry to the latter, based on an age of 615 to 570 Ma by Stern and Hedge (1985). Ghoneim et al. (1999) ascribe an age of 559 Ma ±6 Ma to the Sukari Porphyry and mineralisation age of 522 Ma ±11 Ma (Rb-Sr and Sm-Nd isotopic dating of albitised granite and separated albite). In contrast, Sharara and Vennemann (1999) state that gold mineralisation at several localities in the Eastern Desert region, including Sukari, pre-dates intrusion of 'younger granites'. Based on the composition of Sukari Porphyry, its deformed nature and its contact relationships with surrounding rocks, it is considered to represent a sub-volcanic, 'syn-tectonic' granitoid, representing the roots of a rhyolitic arc volcano.
The Sukari granite porphyry, intrudes both serpentinites after ophiolites and andesite flows, serpentinites and associated volcanoclastic sediments. It is NNE elongated and bounded to the west and east by two steep shear zones that together form a sinistral pair intersecting to the south where the intrusion tapers. The hanging wall sequence to the east is complexly deformed and comprises a mixture of serpentinite, meta-conglomerate, lesser fine-grained metasediments, minor basalt and porphyry dykes or sills that are assumed to be genetically and temporally related to the main Sukari Porphyry. The footwall sequence is devoid of porphyry dykes which is taken to possibly indicate that the entire sequence is overturned. In the hanging wall, individual stratigraphic units, normally comprising more competent units in the serpentinite, can be traced over tens of metres. Surface mapping is, however, complicated by a lack of persistent stratigraphic markers and the fact that, in many places, schistosity is at a high angle to bedding. The footwall sequence within 50 m of the granite porphyry, comprises fine-grained pelitic metasediments with lesser basalts. Outcrop is poor, with low ridges and mounds formed by outcropping serpentinite which suggests that this lithology dominates the footwall sequence. Similar to hangingwall rocks, footwall metasediments and serpentinites show development of very strong schistosity and a lack of stratigraphic markers, combined with poor outcrop, prevents mapping of a stratigraphic sequence.
Where fresh, the Sukari granite porphyry comprises a coarse-grained leucocratic and pink granitoid porphyry with a heterogeneous mineralogical composition, ranging from monzogranite to granodiorite with dominant quartz, plagioclase and potash feldspar and lesser biotite with a trondhjemitic affinity (Arslan 1989). It has a strike length of ~2300 m, ranges in thickness from 100 to ~600 m (Cavaney 2005) and dips at between 50 and 75°E, although porphyry/wall rock contacts are, in places, vertical or overturned. Mineralisation occurs throughout this mass, but is not continuous, with deposition having been influenced by major long-lived structures, the most important of which are tabular sheets of crackle breccia. Ore is associated with more intense reef/lode zones particularly the Main and Hapi reefs that follow these major structures. The Main Reef is located within the the granitic body, close to the contact with the foliated metavolcanic rocks. It strikes NNE at surface and varies from 0.3 to 3 m true thickness. Locally the Main Reef splits into a net of small quartz veinlets or stockwork where it is very rich in sulphides. Both the Main and Hapi reef dip at 35 to 50°E whereas the overall dip of the porphyry footwall contact is about 65°, and as such, are not parallel to that contact. The nature of the mineralised structures changes when they pass from the porphyry into surrounding metasedimentary and volcanic rocks and serpentinite. Their projected extensions cannot be traced into footwall and hangingwall rocks where they may steepen external to the intrusion. Where they cut the porphyry they appear to assume a flatter orientation and only cause relatively small (metres to tens of metres) offsets in both footwall and hangingwall porphyry contacts. A large volume of mineralisation is also associated with stacked brittle veins which occur in zones proximal to the through-going mineralised shears, but are also found distal to these structures.
Representative drill intersections plotted by Franzmann et al. (2015) through the Main and Hapi reef zones vary from 4 to 24 m (not corrected to true thickness) with grades of from 8.4 to 441 g/t Au, averaging 388 mg/t Au. The deposit is divided into 4 zones from south to north respectively, Amun, Ra, Gazelle and Pharaoh Zones.
The Sukari deposit appears to have been developed under a regime of near-horizontal, east-west directed stress and is a complex, multiphase, vein system. Early vein quartz bands have, in places, been contorted into recumbent folds indicating reverse movement. The vein, and in places the wall rock immediately adjacent to it, shows development of sub-vertical compressional 'S' foliation planes, the orientation of which is also consistent with reverse movement. The orientation of large-scale slickensides in sheared porphyry immediately adjacent to the hanging wall of the Main Reef in the mineralised porphyry is also consistently steep, predominantly reverse movement. A variety of plunges have been observed, indicating components of both dextral and sinistral shear. All, however, indicate a ratio of dip-slip to strike-slip components of 8:1 or greater.
Four types of mineralised quartz veins have been observed at Sukari:
• Contorted and banded veins, which are due to multiple deformation and veining episodes, and are similar in character to veins found in low-sulphidation epithermal Au-Ag deposits. The Sukari Main Reef is this type of vein. They follow through-going, long-lived structures with significant displacement of metres to tens of metres. Arsenopyrite tends to be concentrated in these veins. They host the highest-grade mineralisation at Sukari.
• Brecciated veins are the result of brecciation of vein quartz and contain porphyry rock fragments or porphyry fragments in a matrix of vein quartz ±sulphides ±hematite. They are a variant of the contorted veins, and as such also represent through-going structures. In some exposures, breccia zones can be seen developed at the contacts of contorted veins and host porphyry.
• Shear veins, which appear to be rare. The few observed occurrences are centimetre-scale laminar veins with vague contact-parallel layering possibly the result of aligned inclusion trails. They sometimes have bands of sulphides and or hematite developed at vein-wallrock contacts.
• Spaced extensional veins which are distinguished by their short strike lengths and, in places, by internal fabrics indicating purely extensional opening with no cross vein displacement. These veins normally occur as stacked arrays between thin linking shears. The latter are commonly sericite altered and may contain fine sulphides ±hematite, and are interpreted to have acted as fluid conduits. However, their orientation relative to the stress field meant they were under compression during the deformation that accompanied mineralisation and hence have not been filled with quartz veining.
Within the first two vein types three generations of quartz have been recognised within the same veins (Helmy et al., 2004):
• Q1 - Milky quartz, which is coarse-grained and dominates in the centre of the vein;
• Q2 - fine-grained, mineralized quartz, that occurs close to the vein margins, where the vein is intensely sheared, and cut milky quartz;
• Q3 - grey quartz, calcite and albite..
Gold mineralisation is intimately and dominantly related to sulphides, of which pyrite is the most abundant, followed by arsenopyrite, with high gold grades associated with increased arsenopyrite concentrations. Sulphides occur as fine grained, subhedral disseminations in altered porphyry and as blebby sub- to euhedral crystals and finer disseminations in quartz veins, fractures and breccias. Pyrite is found in all of the mineralised zones, and was deposited continuously throughout the various mineralising stages. Three types of pyrite are recognised (Helmy et al., 2004):
• P1 - large subhedral to anhedral pyrite crystals in the alteration zones surrounding zones of veining and in undeformed Q1 quartz veining. This pyrite may be zoned, with As-poor pyrite rimmed by As-rich arsenian pyrite;
• P2 - large (>5 mm) subhedral pyrite in undeformed Q1 milky quartz, commonly with inclusions of quartz and plagioclase and sometimes euhedral with oscillatory zoning between As-poor and arsenian pyrite;
• P3 - small (<0.1 mm) deformed grains in the sheared Q2 quartz veins.
Arsenopyrite is most abundant in zones of higher-grade gold mineralisation, and consequently is common in the contorted and banded veins, and breccias, but is not abundant in the stacked extensional zones and minor quartz veins. It primarily occurs as fine, sub- to euhedral crystals in fine, anastomosing stylolitic veins, and rimming breccia clasts. Inclusions of pyrite in arsenopyrite and vice versa are common, indicating they belong to the same paragenesis (Khalil, 2006). Both contain numerous inclusions of rutile and wall rock fragments, suggesting they formed as a result of sulphidisation of pre-existing rocks during the hydrothermal stage. Pyrite and arsenopyrite have deformation and brecciation textures, whilst abundant late native gold fills stringers and small 'holes' in these deformed sulphides. Some pyrite crystals contain relict pyrrhotite and chalcopyrite, suggesting pyritisation of pre-existing sulphides.
Other sulphides, including galena, chalcopyrite, sphalerite, pyrrhotite occur within the deposit. Galena, which is rare, occurs as coarse subhedral crystals in milky extensional quartz veins, generally in areas of strong gold mineralisation. Sphalerite is sometimes a significant sulphide mineral, and has abundant, randomly associated exsolved chalcopyrite. This sphalerite-chalcopyrite appears to be filling and replacing the older pre-existing pyrite. Galena in pyrite is apparently coeval with the sphalerite-chalcopyrite association. Silver is only present at very low levels (generally <1 ppm) and where greater, has no correlation with gold.
Gold occurs in three distinct positions within the lode system at Sukari (Helmy et al., 2004):
i). as anhedral grains (Gold I) at the contact between As-rich zones within pyrite-arsenian pyrite;
ii). as randomly distributed anhedral grains (Gold II) and along cracks in arsenian pyrite and arsenopyrite, and
iii). as large gold grains (Gold III) interstitial to fine-grained pyrite and arsenopyrite in deformed and sheared smoky quartz.
Gold I and II occurs as electrum and ranges from 1 to 40 µm in diameter, followed by Gold III which is high purity >900 fineness, depleted in silver (Helmy et al., 2004; Franzmann et al., 2015). Microscopic examination suggests the following mineral paragenesis (Khalil, 2006):
• Magnetite and a titanium-bearing minerals, probably titanomagnetite or ilmenite, were formed during the magmatic stage of the Sukari granite porphyry. Pyrrhotite and chalcopyrite relicts seem to also belong to this magmatic stage;
• Contact metamorphism and high strain near intrusive margins, probably as shear zones, produced platy hematite and rutile;
• Pyrite and arsenopyrite, which are of hydrothermal origin, were formed by sulphidisation of the pre-existing hematite and rutile;
• Sphalerite, chalcopyrite, galena and native gold are fracture fill minerals, and are coeval with or later than the deformed pyrite and arsenopyrite assemblage;
• Goethite and anatase were formed under weathering conditions.
Based on fluid inclusions and stable isotope analyses, Sharara (1999) and Sharara and Vennemann (1999) estimate a mineralisation temperature of 370 to 270°C for Sukari and other gold deposits in the district, with gold deposited from a low-salinity carbonic fluid.
Serpentinites in the footwall and hanging wall sequences have been variably carbonatised and silicified to listwaenite, typically a mixture of ferroan or magnesian carbonate and cryptocrystalline silica. The timing of this alteration is not known (Franzmann et al., 2015). The listwaenite does, locally, contain fine-grained pyrite and quartz veins but no detectable gold.
The recorded resource at Sukari in 2004 totalled 18.8 Mt @ 2.14 g/t Au for 40 tonnes (1.3 Moz) of contained gold (Helmy et al., 2004).
NI 43-101 compliant Ore Reserves and Mineral Resources at 31 June, 2018 (Centamin, 2019) were:
Proved + Probable Ore Reserves open pit - 174.2 Mt @ 1.1 g/t Au for 192 t of gold (0.4 g/t Au cut-off)
Proved + Probable Ore Reserves underground - 4.4 Mt @ 5.6 g/t Au for 24.6 t of gold (3.0 g/t Au cut-off).
Measured + Indicated Mineral Resources - 358 Mt @ 0.96 g/t Au for 344 t of gold (0.3 g/t Au cut-off)
Inferred Mineral Resources - 34 Mt @ 0.8 g/t Au for 27 t of gold (0.3 g/t Au cut-off).
The information in this summary was largely drawn from: Franzmann, D., Smith, P., Johnson, N. and Zammit, M., 2015 - Mineral Resource and Mineral Reserve Estimate for the Sukari Gold Project, Egypt; an NI 43-101 Technical report prepared for Centamin plc, 206p. and other sources listed below.
The most recent source geological information used to prepare this decription was dated: 2004.
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.
Sukari
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Selected References:
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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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Botros, N.S., 2004 - A new classification of the gold deposits of Egypt: in Ore Geology Reviews v.25, pp. 1-37.
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Helmy, H.M., Kaindi, R., Fritz, H. and Loizenbauer, J., 2004 - The Sukari Gold Mine, Eastern Desert - Egypt: structural setting, mineralogy and fluid inclusion study: in Mineralium Deposita v.39, pp. 495-511.
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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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Mohamed, H.A., Ali, S., Sedki, T. and Abdel Khalik, I.I., 2019 - The Sukari Neoproterozoic granitoids, Eastern Desert, Egypt: Petrological and structural implications: in J. of African Earth Sciences v.149, pp. 426-440.
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Zoheir, B., Holzheid, A., Zeh, A., McAleer, R., El-Behairy, M., Schwarz-Schampera, U., Graupner, T., Lentz, D.R. and Xiong, F., 2023 - The Sukari Gold Deposit, Egypt: Geochemical and Geochronological Constraints on the Ore Genesis and Implications for Regional Exploration: in Econ. Geol. v.118, pp. 719-744. doi:https://doi.org/10.5382/econgeo.4990.
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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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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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