Basem Zoheir

Fluid evolution in the El-Sid gold deposit, Eastern Desert, Egypt

Abstract Auriferous quartz (±carbonate) veins in the El-Sid mine cut through the western margin of the Fawakhir granitic intrusion and the immediate country ophiolites. Gold mineralization is spatially and temporally associated with ENE–WSW fault/shear zones developed late in the deformational history of the area. Field and microscopic studies suggest two distinct ore stages; namely an early gold-Fe–As-sulphide, and a late gold-base metal mineralization. New microthermometric and Raman data suggest gold deposition as a result of a complex history of fluid immiscibly, dilution of low-salinity aqueous-carbonic fluids in the early mineralization stage, while wall-rock alteration and pressure loss precipitated Au from intermediate-salinity aqueous ore fluids during the late stage. Fluid inclusion isochoric reconstructions, combined with oxygen and sulphur isotope data, indicate conditions of 320±20 °C and 1.3±0.2 kbar for the early gold-Fe–As-sulphide mineralization, and c. 200±15 and 0.6±0.9 kbar for the late gold-Zn–Pb–Cu-sulphide stage. The clockwise evolution path in pressure-temperature space likely documents gold mineralization under post-peak metamorphic conditions. The calculated sulphide δ34SH2S equilibrium values −9.04‰ to –4.75‰, may refer to a variable redox state of sulphur in the ore fluids from the early to late mineralization stages. The stable isotope signature of the vein quartz and calcite suggest mixed magmatic and metamorphic fluid sources (δ18O H2O=+4.9‰ to +7.4‰). Unusually low δ13C values of calcite in the late mineralization (−13.9‰ to −14.7‰) may reflect input of magmatic CO2 and/or oxidized carbonaceous material in the infiltrating fluid.

Re-Os geochronology of gold mineralization in the Fawakhir area, Eastern Desert, Egypt

We report the first Re-Os data on gold-associated arsenopyrite from mesothermal gold-quartz veins in the ancient Egyptian Fawakhir–El Sid gold mining district in the central Eastern Desert. This mining district has an ~5000-year-old history and is displayed in the Turin Papyrus Map (about 1150 BC), which is widely acclaimed as the world’s oldest geographic map, as well as the oldest geologic and mine map. The Fawakhir–El Sid district is part of a regional NNW-trending shear corridor (15 km wide) that hosts several other historic gold mines associated with left-lateral wrench structures and related granite intrusions. Vein-style gold mineralization is hosted within and at the margin of an I-type and magnetite-series monzogranite, the Fawakhir granite intrusion, and a Pan-African (~740 Ma) ophiolite sequence. The ore mineralogy of the mineralized quartz veins includes pyrite-arsenopyrite-pyrrhotite-sphalerite-galena-chalcopyrite-electrum plus a number of tellurides of Ag, Au, and Bi. The 187Re/188Os versus 187Os/188Os regression on 5 points of arsenopyrite gives an age of 601 ± 17 Ma with an initial 187Os/188Os of 0.24 ± 0.07 (2 σ; MSWD = 17). This age coincides within error with the U-Pb age on zircon from the Fawakhir monzogranite (598 ± 3 Ma). The age coincidence and the hydrothermal Te and Bi metal signature suggest a foremost role of granite-related fluids in the quartz-vein system.

Geochemistry and geochronology of the ~620 Ma gold-associated Batouri granitoids, Cameroon

The Batouri gold mining area in southeastern Cameroon is part of the Adamawa–Yadé Domain of the Central African Fold Belt (Pan-African). It is underlain by a variety of granitic rocks, including alkali-feldspar granite, syeno-monzogranite, granodiorite, and tonalite. Geochemical data suggest that these rocks formed by differentiation of I-type tonalitic magma under oxidizing conditions in a continental volcanic arc setting. U–Pb dating of zircons from gold-associated monzogranite-granodiorite at Kambélé gave concordant ages of 619 ± 2 and 624 ± 2 Ma, while Ar–Ar dating of alkali-feldspar granite yielded a non-plateau maximum age of 640–620 Ma. These ages imply that the Batouri granitoids were emplaced during the collision of the West African Craton and the Congo Craton. The geochemical characteristics of the Batouri granitoids as well as their oxidized state (magnetite series) are typical of gold-associated felsic rocks in subduction settings elsewhere. The similarities in age, composition, and geochemical affinities of these granitoids with those reported from other localities in the Adamawa–Yadé Domain reinforce the earlier assumption that the granitic rocks of this domain represent parts of a regional-scale batholith, with commonly small-scale, high-grade auriferous quartz veins in structurally favourable sites. The spatial and temporal association of gold mineralization and the Batouri granitoids may suggest potential for regional-scale, high-tonnage, granite-related gold ore.

ASTER-based mapping of ophiolitic rocks: examples from the Allaqi–Heiani suture, SE Egypt

ABSTRACT The Allaqi–Heiani suture is an ~250 km-long ophiolite-associated fold-and-thrust belt in the South Eastern Desert of Egypt. It comprises imbricate thrust sheets of ophiolites (serpentinite, amphibolite, metagabbro, and metabasalt) and island-arc metavolcanic/metasedimentary rocks. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)-based mafic–ultramafic indices are combined with band ratioing (BR) and relative absorption band-depth techniques to comprehensively map the ophiolitic rocks along the eastern part of the Allaqi–Heiani suture. The applied techniques are found efficient in delineating the ophiolitic rocks in the study area, despite their highly tectonized and variably carbonate-altered character. Identification and mapping of the exposed mafic–ultramafic rocks were possible using shortwave infrared (SWIR) band ratio 4/8 and thermal infrared (TIR) band ratio 12/14. The refined mafic index (MI) provides an efficient discrimination of the ophiolitic mafic rocks, while the mafic-ultramafic indices (M1 and M2) show high sensitivity to ophiolitic ultramafic units. The mafic–ultramafic indices enabled an efficient discrimination of mafic ophiolites from mafic island-arc rocks. Combining mafic indices and SWIR band ratio 4/8 allowed an enhanced delineation of the ophiolitic mafic–ultramafic rocks from other rock units in the area. Detailed mapping of the ophiolitic units in the ophiolitic terranes was obtained using the relative absorption band depth (RBD8) together with the MI and band ratio 4/8. This approach offers a quick method to discriminate among ophiolitic rock units and to map their structural contacts, i.e. in such hitherto poorly studied and hardly accessible areas.

Geochemistry and mineral chemistry of lode gold mineralisation, SE Egypt: implications for ore genesis and exploration

Orogenic, lode gold mineralisation in the South Eastern Desert of Egypt is related to quartz veins spatially and temporally associated with conjugate NW- and NE-trending brittle–ductile shear zones. These structures are assumed to be linked to a regional transpression deformation which occurred late in the tectonic evolution of the area. In the Betam deposit, gold is confined to quartz(±carbonate) veins cutting through tectonised metagabbro and metasedimentary rocks in the vicinity of small granite intrusions. The ore bodies contain ubiquitous pyrite and arsenopyrite, in addition to minor disseminated chalcopyrite, pyrrhotite, galena, tetrahedrite and rare gold/electrum. New ore microscopy and electron microprobe studies indicate that most free-milling Au is intimately associated with the late-paragenetic galena–tetrahedrite–chalcopyrite assemblage. An early Fe–As sulphide assemblage, however, shows minor traces of refractory gold. New mineralogical and geochemical data are used to better constrain on possible element dispersions for exploration uses. This study indicates that parameters that most consistently define primary dispersion of gold in the mine area include pervasive silicification, sericite and carbonate alteration. The trace element data of gold lodes reflect a systematic dispersion of gold and certain base metals. Low-cost, extensive exploration programs may use elevated concentrations of Ag, Sb, Cu and Pb as tracers for Au ore zones in the Betam mine area and surroundings.

Epigenetic BIF-hosted gold lodes at the Abu Marawat area, Eastern Desert, Egypt: integrated mineralogical, structural control and fluid inclusion studies

Abstract Numerous massive and sheared, milky quartz veins cut a sequence of Neoproterozoic island arc metavolcanic/volcaniclastic rocks and related banded iron formation (BIF) at the Abu Marawat area, central Eastern Desert of Egypt. Sulphide-bearing quartz veins and related hydrothermal breccia masses display a range of textures including sheared, boudinaged and recrystallised quartz, quartz with open space filling and microbreccia, implying a complex history of crack-seal processes characterising the relationship between mineral deposition and a major N–S-trending shear zone, during a late brittle–ductile deformation event that affected the area at ∼550 Ma. Gold-base metal mineralisation is associated with brecciation and fracturing of the iron ore bands, close to silicified shears and related quartz veins. The auriferous quartz lodes are characterised by the occurrence of visible pyrite-chalcopyrite±pyrrhotite±sphalerite±galena mineralisation. Gold is refractory in pyrite and chalcopyrite, but rare visible gold/electrum and telluride specks were observed in a few samples. Hydrothermal alteration includes selectively pervasive silicification, pyritisation, sericitisation, carbonatisation that are confined to a delicate set of veins and altered shears, and a more widespread propylitic alteration assemblage (quartz+||chlorite+pyrite+calcite±epidote). The predominance of hydrated silicate phases (i.e. chlorite and sericite) within and adjacent to the shear zones points toward fixation of H2O during the shear zone development, while CO2 was consumed to form carbonate in halos around the chlorite-dominated zones, leading to variations in the H2O/CO2 ratio of the ore fluid with progression of the hydrothermal alteration. Ore textures, including hydrothermal quartz intimately associated with replacement of magnetite by pyrite, suggest the introduction of ore fluids via epigenetic conduits into the iron-rich wallrocks. Fluid inclusion textural and microthermometric studies indicate heterogeneous trapping of a low-salinity (1·4–6·7 wt-% eq. NaCl) aqueous solution that coexisted with a carbonic fluid. Evidence for fluid immiscibility during ore formation includes variable liquid/vapour ratios in inclusions along individual trails and bulk inclusion homogenisation into liquid and occasionally to vapour at comparable temperatures. The trapping conditions of intragranular aqueous-carbonic inclusions approximate 264–378°C at 700–1300 bar. Similar temperature estimates have been obtained from Al-in-chlorite geothermometry of chlorite associated with sulphides in the mineralised quartz veins. Fracturing enhanced fluid circulation through the wallrock and related BIF, allowing reaction of the S-bearing ore fluid with iron oxides. This caused formation of pyrite, a rise in Eh and concomitant Au precipitation, which was enhanced by fluid immiscibility as H2S partitioned preferentially into the carbonic phase. The ore fluids have probably originated from felsite intrusions, whereas Abu Marawat gold deposit and the post-Hammamat felsites is assumed in view of their communal relationship with the N–S fault/shear trend and high geothermal gradient calculated from the fluid inclusion data.