The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz
veins are characterised by irregular walls with a steeply
plunging ridge-in-groove lineation. Shear geometry of
rootless intra-folial folds and asymmetrical strain shadows
around the quartz lenses suggests that vein emplacement
took place under a brittle–ductile shear regime, clearly postdating the amphibolite-facies regional metamorphism.
Hydrothermal alteration is pervasive in the wallrock
metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wirelike rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trailbound aqueous–carbonic (LCO2+Laq±VCO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (LCO2±VCO2) and aqueous H2O–NaCl
(L+V) inclusions occur along inter-granular and transgranular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain
boundary migration. Homogenisation temperatures of the
trans-granular H2O–NaCl inclusions are much lower
(130–221°C), implying different fluids late in the shear
zone formation. Fluid densities calculated from aqueous–
carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm3, and the resulting isochores suggest trapping
pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–
pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains fS2 to the range of 10−10 to 10−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and fO2 through pressure fluctuation and CO2 effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins. |
