Hydrothermal alteration geochemistry of the Betam gold deposit, south Eastern Desert, Egypt: Mass-volume-mineralogical changes and stable isotope systematics

Although gold production is relatively minor from the Arabian-Nubian shield at present, extensive alluvial and lode fields along the western side of the Red Sea in Upper Egypt and northern Sudan were worked out by the ancient Egyptians for thousands of years. In the Eastern Desert of Egypt, numerous but small gold deposits are generally related to auriferous quartz veins commonly associated with brittle-ductile shear zones, generally cutting through the Neoproterozoic crystalline basement rocks and trending in different directions. Gold mineralization at the Betam mine area, south Eastern Desert, is related to a series of milky quartz veins along a NNW- trending brittle-ductile shear zone cutting through successions of pelitic schists, next to a small granite intrusion. Goldsulfide mineralization (pyrite, arsenopyrite, galena, subordinate chalcopyrite and gold) is closely associated with a conspicuous hydrothermal alteration halo. Textural relationships, including replacement and crosscutting of mineral phases and quartz veins record a post-foliation alteration assemblage of quartz+sericite+chlorite+ calcite±albite±epidote. The hydrothermal alteration halo alongside the auriferous quartz veins comprises three distinct zones, namely: (a) distal chlorite-calcite zone (chlorite+calcite±biotite± pyrite±sericite±epidote), (b) intermediate sericite-chlorite zone (sericite+chlorite+pyrite±biotite), and (c) proximal pyrite-sericite zone (quartz+pyrite+sericite±albite). These zones merge to each other gradually, ending outwards into the unaltered metasediments. The pyrite-sericite zone presents the highest gold grade, especially in zones thickly seamed with sulfide-rich quartz veinlets. Mass balance calculations revealed that the pyrite-sericite zone experienced significant metasomatic changes relative to limited mass and volume changes for the chlorite-calcite zone. The overall picture of chemical gains and losses with increase the intensity of hydrothermal alteration is indicative of: (i) addition of SiO₂, K₂O, Na₂O, S, L.O.I., (ii) removal of MgO, and (iii) relatively inert behavior of Al₂O₃, TiO₂, MnO, Fe₂O₃. CaO is variably mobile, slightly gained in the chlorite-calcite and sericite-chlorite zones but depleted in the pyrite-sericite zone. Concnetrations of the trace elements are variable in the different alteration types, but a notable increase in Au, As, Ba, Sr, Rb, V, and Ni in the intensively altered rocks is verified. Investigation of the REE behavior reveals a little modification of their distribution pattern with hydrothermal alteration. Heavy REE are more or less unchanged, whereas light REE are significantly mobile in all alteration types. New geochemical data provide evidence for progressive silicification, sericitization, and sulfidation as a function of gold mineralization. A proposed model for the hydrothermal alteration system for the Betam deposit includes fluctuation in pH and redox state (f_O2), mainly during the wall-rock sulfidation. This might have destabilized gold complexes and lowered gold solubility in solution, hence contributed at least partly in gold deposition in the study area. A decrease in the whole rock δ¹⁸O values from unaltered country metasediments to the distal, intermediate and proximal alteration zones, respectively, might be attributed to rock interaction with an isotopically lighter fluid. In addition, sulfur isotope data of pyrite-arsenopyrite pairs in the mineralized quartz veins and adjacent wall-rock along with the calculated δ³⁴S_ΣS values for the ore fluids suggest derivation form non-homogenous (mixed magmatic and metamorphic) fluids.