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Atalla |
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Egypt |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Atalla gold mine is located ~ 20 km north of the Fawakhir along the Quft-Quseir road in the Central Eastern Desert of Egypt, between the Nile River and the Red Sea coast (#Location: 26° 9' 15"N, 33° 30' 50"E).
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 Central Eastern Desert contains abundant dismembered ophioltic nappes and mélanges, and high strain gneiss-cored metamorphic complexes that are associated with extensive fault/shear zones. Low-angle NW vergent reverse faults, nappes and shear zones in the area are interpreted to be related to an oblique convergent event at ~ 690 to 610 Ma (Andresen et al., 2009, 2010; Augland et al., 2012; Lundmark et al., 2012). These Pan-African nappes are interpreted to have been developed in a thin-skinned tectonic regime during early westward compression, whilst subsequent extensional rebound tectonics led to the exposure of gneiss cores beneath these nappes (Unzog and Kurz 2000; Fritz et al.,2002) at ~623 to 579 Ma (Ar/Ar ages of hornblende and muscovite in some domes). These core complexes are often truncated by NW–SE sinistral shear zones related to the ~615 to 585 Ma Najd Fault System in the Arabian Shield (Stern 1985; Wallbrecher et al. 1993; Shalaby et al. 2005). The Najd Fault System is a large transcurrent fault complex that extends for >1200 km, with a width of ~300 km (Agar 1987). The moderately-dipping thrusts and shear zones in the region are interpreted to have facilitated the intrusion of calc-alkaline I-type granitic intrusions (Fowler 2001). Andresen et al. (2010) and Fowler and Osman (2013) suggest the granitic intrusions cutting the prominent subhorizontal foliation may indicate substantial lithospheric thinning. Greiling et al. (2014) suggested that the weak crust assisted the initiation or reactivation of Najd-related high angle wrench faults/shear zones in the Central Eastern Desert during subsequent lateral tectonic escape, e.g., the Atalla Shear Zone.
Gold mineralisation at Atalla occurs in milky and smoky quartz veins in NE-trending faults which cut a small monzogranite body, the 615±9 Ma (U-Pb) Atalla intrusion. The deposit area is mainly underlain by a NW-SE elongate belt of ophiolitic rocks that are cut by felsic dyke swarms and the Atalla intrusion. Large exposures to the west and north of the deposit are occupied by rocks composed of clasts of metabasalt, serpentinite and metagabbro set within a matrix of metasiltstone. To the east, mafic and intermediate island arc metavolcanic/volcaniclastic rocks dominate, including foliated metabasalt intercalated with andesite tuffs and breccias. At the extreme western margin of the deposit area, rhyolite and rhyodacitic tuffs and ignimbrites of the 610–580 Ma Dokhan group (Wilde and Youssef 2000) are exposed. The arc metavolcanic rocks are unconformably overlain by a successions of purple metagreywacke, siltstone and conglomerate of the Hammamat Sediments which are generally characterised by NW-SE striking bedding. Younger, 588±12 Ma post-Hammamat felsite porphyries (Rb/Sr isochron; Hassan 1998) occur as a NNW elongate body cutting both the ophiolitic and island arc rocks. In the deposit area, these rocks are fine-grained, intensely jointed, porphyritic rhyodacites. Small intrusions of monzo-syenogranite rocks occur, orientated parallel to the NNW-SSE Atalla shear zone.
The ~0.35 km2 Atalla intrusion cuts ophiolitic serpentinites and the Post-Hammamat felsite intrusion. It is composed of medium- to coarse-grained, pale pink monzogranite with abundant xenoliths of older rocks. In the deposit area, the NNW-SSE structural trend is related to marginal shear zones and steeply dipping mylonitic foliation. Strike-slip faults juxtapose the ophiolitic mélange nappe to the Dokhan Volcanics and Atalla felsites. Mineralised quartz veins are confined to NE-SW brittle shear zones that cut the Atalla intrusion and are characterised by sub-horizontal slickensides. The main lode occurs as a 270 m long and ~ 0.5 m thick quartz vein. These veins are associated with pervasive sericite/white mica-altered, silicified, and ferruginated rocks. White mica associated with the mineralised veins have been dated at 601±5.5 Ma (Ar-Ar). Other smaller and less extensive quartz veins are parallel to subparallel to the main lode. The mineralised shear zones are part of the major Atalla shear zone, which deforms the area and extends for >30 km southward in a NNW-SSE direction. Atalla deposit, the Atalla Shear Zone also hosts the El Sid and Fawakhir gold deposits (Zoheir and Moritz 2014).
Ore minerals include an early pyrite-arsenopyrite (±pyrrhotite) mineralisation, partly replaced by a late pyrite-galena-sphalerite-chalcopyrite (±gold/electrum ±tetrahedrite ±hessite) assemblage. Gold occurs as small inclusions in pyrite and arsenopyrite, or more commonly as intergrowths with galena and sphalerite/tetrahedrite in microfractures.
Based on the vein textures, sulphur isotope values, composition of ore fluids, and conditions of ore formation, Zoheir et al. (2018) suggest that the Atalla monzogranite intrusion acted only as a competent structural host for ore deposition from shear-related, metal-rich fluids which migrated upward from depth. The same authors conclude that gold deposition occurred under generally mesothermal conditions, i.e., 1.3 kbar and ~ 280°C, and continued during system cooling to <200°C and pressure decrease to ~ 0.1 kbar.
Reserve and production figures have not readily available. Atalla was one of the important ancient gold sources in Pharonic and Roman times. It is one of >100 ancient gold mines in the Eastern Desert of Egypt.
El Monsef (2019) states that the average grade ranges from 11 to 29.7 g/t Au and averages ~22.75 g/t Ag. El Bouseily et al. (1986) showed that the Atalla gold mineralization is characterised by high Ag contents of up to 92 ppm, and provided a preliminary estimate of the geological reserves in Atalla as 28 t of Au and 78 t of Ag.
The most recent source geological information used to prepare this decription was dated: 2018.
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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Abd El Monsef, M., 2019 - Ore Controls and Metallogenesis of Au-Ag Deposits at Atalla Mine, Central Eastern Desert of Egypt: in Acta Geologica Sinica, doi.org/10.1111/1755-6724.14326, 26p.
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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., 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., 2004 - A new classification of the gold deposits of Egypt: in Ore Geology Reviews v.25, pp. 1-37.
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Eleraki, M., Ghieth, B., Abd-El Rahman, N. and Zamzam, S., 2017 - Environmental assessment of Atalla gold mine area, Central Eastern Desert, Egypt: in Australian Journal of Basic and Applied Sciences, v.11, pp. 77-83.
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Greiling, R.O., de Wall, H., Sadek, M.F. and Dietl, C., 2014 - Late Pan-African granite emplacement during regional deformation, evidence from magnetic fabric and structural studies in the Hammamat-Atalla area, Central Eastern Desert of Egypt: in J. of African Earth Sciences v.99, pp. 109-121.
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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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Zoheir, B., Deshesh, F., Broman, C., Pitcairn, I., El-Metwally, A. and Mashaal, S., 2018 - Granitoid-associated gold mineralization in Egypt: a case study from the Atalla mine: in Mineralium Deposita v.53, pp. 701-720.
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Zoheir, B., Feigenson, M., Zid, J.-W., Turrin, B., Deshesh, F. and El-Metwally, A., 2018 - Ediacaran (~ 600 Ma) orogenic gold in Egypt: age of the Atalla gold mineralization and its geological significance: in International Geology Review doi.org/10.1080/00206814.2018.1463180, 18p.
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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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