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The El Sid Au mineralisation in the Fawakhir area, Eastern Desert, Egypt, is comprised of hydrothermal quartz veins cutting a Neoproterozoic granitoid pluton. The mineralisation is divided into Au-bearing, transitional and late carbonate vug stages. Pyrite-arsenopyrite and streaky pyrite-sphalerite-galena assemblages characterise the early and late episodes of the Au-bearing stage, respectively. These sulphide minerals sometimes contain Au as inclusions.

Fluid inclusions from the Au-bearing stage are H2O-CO2-rich fluids (ca 29-62 mole% CO2 and density, ca 0.51-0.66 g cm(-3)) with moderate to high salinities (ca 12-19 wt% NaCl equiv.). The trapping temperature of the ore occurred between 280 and 350 degrees C, at pressures between 120-170 MPa (similar to 800-1800 m depth). However, fluid inclusions from the pyrite-arsenopyrite assemblage reflect lower homogenisation temperature measurements (T-h) (265-295 degrees C) than those in the streaky pyrite-sphalerite-galena assemblage (330-365 degrees C). The minerals of the transitional and late carbonate vug stages (quartz and calcite) were formed between 180 and 265 degrees C.

Oxygen and H isotope data of fluid inclusions hosted in the quartz and calcite from the Au-bearing stage indicate a rather wide range of calculated delta(18)O(H2O) and delta D-H2O (i.e. +3.2 to +7.8 parts per thousand and -75 to -32 parts per thousand, respectively). The data for the streaky pyrite-sphalerite-galena assemblage exhibit higher delta(18)O(H2O) values (+5.3 to +6.8 parts per thousand) and lower delta D-H2O (-75 to -48 parts per thousand) relative to that of the pyrite-arsenopyrite assemblage. Calculated isotopic temperatures of quartz-calcite fractionation range from 282 to 353 degrees C and are consistent with the trapping temperatures (280-350 degrees C).

The fluid inclusion and stable isotope data imply that Au at the El Sid Gold Mine has been transported as a bisulphide complex. The high salinity and inferred occurrence of CH, in some fluid inclusions collected from the graphite-rich zone along a serpentinite-granitoid contact suggest that the mineralising solutions gained their metal contents through circulation in the fractured zones and incorporation with a geothermal convective system by wall rock interaction. Deposition of Au sulphides has taken place at shallow crustal levels (similar to 800-1800 m) as a result of meteoric/magmatic-metamorphic water exchange with wall rocks through H2O-CO2 immiscibility during fluid pressure drop and decreasing ligand activity. (C) 2000 Elsevier Science Limited. All rights reserved.