H2O – CO2 – CH4- BEARING FLUID INCLUSIONS IN QUARTZ: INSIGHTS INTO THE ORIGIN AND EVOLUTION OF TWO DIFFERENT HYDROTHERMAL AU DEPOSITS FROM THE EGYPTIAN EASTERN DESERT

Shear-related, mesothermal gold deposits of Um Egat and Dungash in the Egyptian Eastern Desert are hosted in altered greenschist facies metavolcanic ± metasedimentary rocks of Pan-African age. Both deposits are similar in alteration style, structural control, and mineralogy. Gold is related to boudinaged quartz veins, where incipient recrystallization is common. Ore mineralogy includes pyrite, arsenopyrite ± pyrrhotite ± chalcopyrite ± galena. Au is disseminated in the alteration haloes, but also occurs in the veins as needles or blebs in fractures in altered arsenopyrite ± pyrite (Um Egat) or pyrrhotite (Dungash), usually next to fragments of altered country rocks.

Fluid inclusions in vein quartz occur in clusters, or along trails. Three types of fluid inclusions were identified based on preliminary microthermometry and laser micro-Raman spectroscopy: (i) three phase aqueous - carbonic (H₂O-CO₂±CH₄), (ii) two-phase carbonic (CO₂±CH₄±N₂), and (iii) two phase, CO₂-bearing, aqueous inclusions. Homogenization temperatures (T_h) for the two-phase carbonic inclusions fall in two distinct groups: > 300°C, and between 120 – 200°C, whereas for the two-phase aqueous inclusions, T_h is 120 to 200°C, and > 250°C. In all inclusions, the aqueous fluid has a low salinity (< 8 weight% NaCl equivalent). Inclusions from the same trail or cluster are often characterized by different degrees of fill or different T_h values.

Field, petrographic, and microthermometric data suggest that low salinity aqueous-carbonic fluids interacted with graphite – bearing metasediments to form CH₄. These reduced fluids leached Au as they circulated through the metavolcanics, carrying it as bisulfide complexes. A drop of pressure during the migration of these fluids to shallower depths led to phase separation. Interaction of these fluids with the country rocks precipitated Au - bearing sulfides. As in Arsenopyrite and Al in chlorite geothermometers constrain the conditions of Au precipitation between 340 and 250°C. Post depositional deformation caused the stretching, leakage, and decrepitation of some of the fluid inclusions increasing their T_h values to > 250°C, and remobilized the Au depositing it as globules of higher fineness in secondary sites.