Shear zone-related, mesothermal gold deposits of Um Egat and Dungash in the Egyptian Eastern Desert are hosted by greenschist facies metavolcanic and/or metasedimentary rocks of Pan-African age. Both deposits comprise boudinaged quartz veins that show evidence of incipient recrystallization, and are similar in alteration style, structural control, and mineralogy. The ore mineralogy includes pyrite, arsenopyrite ± pyrrhotite ± chalcopyrite ± galena; gold occurs both in the veins, usually included in arsenopyrite, pyrite, or pyrrhotite next to fragments of altered country rocks, or disseminated in the alteration haloes. Arsenic in arsenopyrite and Aliv in chlorite geothermometers indicate that wall-rock alteration and ore mineral precipitation occurred at temperatures between 400° and 250°C.
Fluid inclusions in vein quartz occur in clusters, or along trails. Three types of fluid inclusions were identified
based on petrography and laser micro-Raman spectroscopy: (1) two-phase carbonic inclusions with CO2 + CH4 ± N2 ± H2O, (2) two-phase aqueous inclusions, and (3) three-phase aqueous-carbonic inclusions with CH4. Final melting of ice (Tmf) for most inclusions occurs at temperatures between –4° and 0°C, indicating a low salinity (<6.5 wt % NaCl equiv) for the aqueous fluid. Homogenization temperatures (Th) for the twophase aqueous and aqueous-carbonic inclusions range from 120° to >300°C, but cluster into two distinct groups for each type of inclusion. Inclusions from the same trail or cluster are commonly characterized by different degrees of fill or different Th values.
Field, petrographic, and microthermometric data suggest that low-salinity aqueous-carbonic fluids interacted with graphite-bearing metasedimentary rocks to form CH4 at T >400°C and P >3 kbars. These reduced fluids leached gold as they circulated through the metavolcanic rocks, carrying it in the form of bisulfide complexes. Interaction of these aqueous-carbonic fluids with the country rocks caused hydrothermal alteration and precipitated gold-bearing sulfides in the alteration zones. A drop of pressure during the migration of these fluids to shallower depths along the shear zones led to phase separation at T !300°C and P !2.3 kbars. Quartz crystallizing over a range of lower temperatures and pressures trapped carbonic and aqueous fluids as separate inclusions in clusters along pseudosecondary and secondary trails. Postdepositional deformation caused decrepitation of some inclusions, and the stretching and leakage of others, increasing Th to >250°C. Deformation also remobilized the gold, depositing it as globules of higher fineness in secondary sites. |
