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摘要
Gold-bearing sulfide-quartz veins cutting mainly through the Atalla monzogranite intrusion in the Eastern Desert of Egypt are controlled by subparallel NE-trending brittle shear zones. These veins are associated with pervasive sericite-altered, silicified, and ferruginated rocks. The hosting shear zones are presumed as high-order structures of the Najd-style faults in the Central Eastern Desert (similar to 615-585 Ma). Ore minerals include an early pyrite-arsenopyrite (+/- pyrrhotite) mineralization, partly replaced by a late pyrite-galena-sphalerite-chalcopyrite (+/- gold/electrum +/- tetrahedrite +/- hessite) assemblage. Gold occurs as small inclusions in pyrite and arsenopyrite, or more commonly as intergrowths with galena and sphalerite/tetrahedrite in microfractures. Arsenopyrite geothermometry suggests formation of the early Fe-As-sulfide mineralization at 380-340 A degrees C, while conditions of deposition of the late base metal-gold assemblage are assumed to be below 300 A degrees C. Rare hessite, electrum, and Bi-galena are associated with sphalerite and gold in the late assemblage. The early and late sulfide minerals show consistently a narrow range of delta S-34 aEuro degrees (3.4-6.5) that overlaps with sulfur isotopic values in ophiolitic rocks.
The Au-quartz veins are characterized by abundant CO2 and H2O +/- CO2 +/- NaCl inclusions, where three-dimensional clusters of inclusions show variable aqueous/carbonic proportions and broad range of total (bimodal) homogenization temperatures. Heterogeneous entrapment of immiscible fluids is interpreted to be caused by unmixing of an originally homogenous, low salinity (similar to 2 eq. mass % NaCl) aqueous-carbonic fluid, during transition from lithostatic to hydrostatic conditions. Gold deposition occurred generally under mesothermal conditions, i.e., 1.3 kbar and similar to 280 A degrees C, and continued during system cooling to < 200 A degrees C and pressure decrease to similar to 0.1 kbar.
Based on the vein textures, sulfur isotope values, composition of ore fluids, and conditions of ore formation, we suggest that the Atalla monzogranite intrusion acted only as a competent structural host for ore deposition from shear-related, metal-rich fluids migrated up from depth. This model is also presumed for most granitoid-associated Au deposits in the region, considering the similarity in their structural control, alteration pattern and mineralogy, and chemistry of the ore fluids.
- 出版日期2018-6
