DEPTH CONTRAINTS ON SILICIFIED NORMAL FAULTS AND QUARTZ VEINS NEAR GEBEL ZEIT, GULF OF SUEZ, EGYPT

Basement rocks of the Miocene Gulf of Suez rift were first deformed during the Precambrian African Orogeny producing fabrics that include foliation, fractures, and dikes. ⁴⁰Ar/³⁹Ar age dates of fracture-filling material (dikes) yield a range from 600 to 460 Ma. The fabrics cluster around N-S, N60°W, and N60°E orientations and were reactivated as previously described by Younes and Altamura (1998). Miocene rift faults trend N30°W, perpendicular to the Miocene extension direction and approximately 30° clockwise to the Precambrian NW fabric.

Mesoscale quartz veins and silicified faults from Gebel Zeit were sampled for fluid inclusion analysis. Dominant quartz veins cluster around N60°W followed by N60°E, N30°E, and N-S. Fluid inclusions in quartz from the N60°W set vary in both the number of phases and composition. Two-phase aqueous, 3-phase CO₂-H₂O, and 2-phase hydrocarbon (liquid) -natural gas (vapor) inclusions were found. The variation in composition and type of inclusions suggests that the N60°W fractures were reactivated over a time interval during which migrating fluids evolved or changed. For example, two of the veins are characterized by only 2-phase aqueous inclusions. While primary fluid inclusions in quartz from one vien yielded a mean minimum homogenization temperature (Th) of 412°C, a mean Th of 192°C was obtained for the second. Another N60°W quartz vein contains 2-phase aqueous as well as 3-phase CO₂-H₂O and oil-natural gas inclusions. Therefore, quartz emplacement occurred at various depths and from evolving fluids associated with multiple reactivation of the N60°W-trending fabrics.

Hydrothermal solutions that deposited the quartz vein containing the hydrocarbon inclusions must have time migrated through the fault, mixing with oil/gas phases in any rift or pre-rift section above the basement. While early quartz veins were emplaced at temperatures greater than the oil-maturation window (OMW, ~150°C), fluid temperature of the latter vein didn’t exceed OMW for any length of time. A minimum emplacement depth of 3.4 km is estimated using a geothermal gradient of 44°C/km. The depth to the top of the rift section is 2-3 km whereas that of the pre-rift section is 3-4 km. Although the solutions must not have departed from P & T conditions that characterize OMW, the blue fluorescence of these inclusions suggests a relatively high degree of oil maturity.