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

摘要

Although there is relatively minor gold production 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 mineralisation 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. Gold-sulphide mineralisation (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 distal chlorite-calcite zone (chlorite + calcite + - biotite + - pyrite + - sericite + - epidote), an intermediate sericite-chlorite zone (sericite+chlorite + pyrite + - biotite), and a 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 contains the highest gold grades, especially in zones thickly seamed with sulphide-rich quartz veinlets. Mass balance calculations have 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 increasing the intensity of hydrothermal alteration is indicative of addition of SiO_2, K_2O, Na_2O, and volatile elements (S, CO_2), removal of MgO, and relatively inert behaviour of Al_2O_3, TiO2, MnO, Fe_2O_3. CaO is variably mobile; slightly enriched in the chlorite-calcite and sericite-chlorite zones but depleted in the pyrite-sericite zone. Concentrations 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 apparent. Investigation of the rare earth element (REE) behaviour reveals a little modification; 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, sericitisation, and sulphidation as a function of gold mineralisation. A proposed model for the hydrothermal alteration system for the Betam deposit includes fluctuation in pH and redox state (fO_2), mainly during the wallrock sulphidation. This might have destabilised gold complexes and lowered gold solubility in the ore solution, and hence contributed at least partly in gold deposition in the study area. A decrease in the whole rock delta~(18)O values from un-altered country metasediments to the distal, intermediate and proximal alteration zones, respectively, might be attributed to rock interaction with an isotopically lighter fluid. In addition, sulphur isotope data of pyrite-arsenopyrite pairs in the mineralised quartz veins and adjacent wallrock along with the calculated delta~(34)S_(sum s) values for the ore fluids suggest derivation from non-homogenous (mixed magmatic and metamorphic) fluids.