Search

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. [ABSTRACT FROM AUTHOR]

Gabbro-diorite-tonalite±granodiorite complexes, derived from subduction-related, calc alkaline magmas represent a distinct unit in the Precambrian basement of Egypt. These intrusive complexes are commonly intruded into ophiolitic mélange, schistose or gneissic rock assemblages. Well-constrained geological mapping of the Um Eleiga complex, combined with geochemical studies, should enable a better understanding of its genesis, especially its potential as a host for gold mineralisation. This complex represents a composite pluton intruded into the allochthonous, ophiolitic Abu Dahr massif and its associated undifferentiated metavolcano-sedimentary mélange matrix in the southern Eastern Desert of Egypt. Field and petrographic investigations have revealed a compositional continuum from gabbro to granodiorite through diorite and subordinate tonalite. Petrography and bulk rock geochemical data have been used to determine the magma type and tectonic setting of the Um Eleiga complex, whereas the electron microprobe data were valuable for the purpose of determining the pressure–temperature and oxygen fugacity conditions. The compositional continuum, pattern of zonation, shallow magma origin and marked common negative Nb anomalies suggest that a close petrogenetic relationship between the gabbro, diorite, tonalite and granodiorite units of the Um Eleiga complex. Subducted upper mantle wedge is proposed as being the source region for the gabbroic magma, whereas zoning of the complex is attributed to interplay of fractional crystallisation and assimilation. Pressure estimates based on the Al(iv)-in-hornblende geobarometry indicate that the gabbroic and dioritic rocks crystallised under 4–5 kbar, whereas tonalite and granodiorite were formed at 3–4 kbar. The hornblende-plagioclase thermometer indicates a formation temperature range of about 720 to 880°C for the complex. Varied oxidation states are inferred from the mineral assemblages, including magnetite, titanite and ilmenite and checked by Fe/(Fe+Mg) ratios of biotite in the granitoid rocks. The tectonic setting, mineralogical, geochemical characteristics and varied redox state of the Um Eleiga intrusive complex are typical for arc-related intrusives potential for hosting intrusion-related gold in a collisional environment. [ABSTRACT FROM AUTHOR]

Although there is relatively minor gold production from the Arabian–Nubian shield at present, extensive alluvial and lode fields along the western side of the Red Sea in Upper Egypt and northern Sudan were worked out by the ancient Egyptians for thousands of years. In the Eastern Desert of Egypt, numerous but small gold deposits are generally related to auriferous quartz veins commonly associated with brittle-ductile shear zones, generally cutting through the Neoproterozoic crystalline basement rocks and trending in different directions. Gold mineralisation at the Betam mine area, south Eastern Desert, is related to a series of milky quartz veins along a NNW- trending brittle-ductile shear zone cutting through successions of pelitic schists, next to a small granite intrusion. Gold-sulphide mineralisation (pyrite, arsenopyrite, galena, subordinate chalcopyrite and gold) is closely associated with a conspicuous hydrothermal alteration halo. Textural relationships, including replacement and crosscutting of mineral phases and quartz veins record a post-foliation alteration assemblage of quartz+sericite+chlorite+calcite±albite±epidote. The hydrothermal alteration halo alongside the auriferous quartz veins comprises three distinct zones, namely distal chlorite-calcite zone (chlorite+calcite±biotite±pyrite±sericite±epidote), an intermediate sericite-chlorite zone (sericite+chlorite+pyrite±biotite), and a proximal pyrite-sericite zone (quartz+pyrite+sericite±albite). These zones merge to each other gradually, ending outwards into the unaltered metasediments. The pyrite-sericite zone contains the highest gold grades, especially in zones thickly seamed with sulphide-rich quartz veinlets. Mass balance calculations have revealed that the pyrite-sericite zone experienced significant metasomatic changes relative to limited mass and volume changes for the chlorite-calcite zone. The overall picture of chemical gains and losses with increasing the intensity of hydrothermal alteration is indicative of addition of SiO2, K2O, Na2O, and volatile elements (S, CO2), removal of MgO, and relatively inert behaviour of Al2O3, TiO2, MnO, Fe2O3. CaO is variably mobile; slightly enriched in the chlorite-calcite and sericite-chlorite zones but depleted in the pyrite-sericite zone. Concentrations of the trace elements are variable in the different alteration types, but a notable increase in Au, As, Ba, Sr, Rb, V, and Ni in the intensively altered rocks is apparent. Investigation of the rare earth element (REE) behaviour reveals a little modification; heavy REE are more or less unchanged, whereas light REE are significantly mobile in all alteration types. New geochemical data provide evidence for progressive silicification, sericitisation, and sulphidation as a function of gold mineralisation. A proposed model for the hydrothermal alteration system for the Betam deposit includes fluctuation in pH and redox state (fO2), mainly during the wallrock sulphidation. This might have destabilised gold complexes and lowered gold solubility in the ore solution, and hence contributed at least partly in gold deposition in the study area. A decrease in the whole rock δ18O values from un-altered country metasediments to the distal, intermediate and proximal alteration zones, respectively, might be attributed to rock interaction with an isotopically lighter fluid. In addition, sulphur isotope data of pyrite-arsenopyrite pairs in the mineralised quartz veins and adjacent wallrock along with the calculated δ34S∑s values for the ore fluids suggest derivation from non-homogenous (mixed magmatic and metamorphic) fluids. [ABSTRACT FROM AUTHOR]