Greenstone-hosted lode-gold mineralization at Dungash mine, Eastern Desert, Egypt

摘要

The auriferous quartz ± carbonate veins at Dungash mine, central Eastern Desert of Egypt, are confined to ～E-trending dilation zones within variably foliated/sheared metavolcanic/volcaniclastic rocks. The vein morphology and internal structures demonstrate formation concurrent with a dextral shear system. The latter is attributed to flexural displacement of folded, heterogeneous rock blocks through transpres-sion increment, late in the Neoproterozoic deformation history of the area. Geochemistry of the host metavolcanic/metavolcaniclastic rocks from the mine area suggests derivation from a low-K, calc-alka-line magma in a subduction-related, volcanic arc setting. In addition, chemistry of disseminated Cr-spi-nels further constrain on the back-arc basin setting and low-grade metamorphism, typical of gold-hosting greenstone belts elsewhere. Mineralogy of the mineralized veins includes an early assemblage of arsenopyrite-As-pyrite-gersdorf-fite ± pyrrhotite, a transitional pyrite-Sb-arsenopyrite ± gersdorffite assemblage, and a late tetrahedrite-chalcopyrite-sphalerite-galena-gold assemblage. Based on arsenopyrite and chlorite geothermometers, formation of gold-sulfide mineralization occurred between ～365 and 280 ℃. LA-ICP-MS measurements indicate the presence of refractory Au in arsenian pyrite (up to 53 ppm) and Sb-bearing arsenopyrite (up to 974 ppm). Abundant free-milling gold associated with the late sulfide assemblage may have been mobilized and re-distributed by circulating, lower temperature ore fluids in the waning stages of the hydrothermal system. Based on the isotopic values of vein quartz and carbonate, the calculated average δ~(18)O_(H2O) values of the ore fluids are 5.0 ± 1.4‰ SMOW for quartz, and 3.3 ± 1.4‰ for vein carbonate. The measured carbonate δ~(13)C values correspond to ore fluids with δ~(13)C_(CO2) = -6.7 ± 0.7‰ PDB. These results suggest a mainly meta-morphic source for ore fluids, in good agreement with the vein morphology, textures and hydrothermal alteration. The calculated δ~(34)S_(H2S) values for early, transitional, and late sulfide assemblages define three distinct ranges (～1.5-3.6‰), (～0.4-1.0‰), and (-3.7‰ to -1.9‰), respectively. The systematic evolution towards lighter δ~(34)S values may be attributed to recrystallization, or to ore fluid buffering under variable physicochemical conditions. The shear zone-related setting, mineralogy and isotopic characteristics of gold mineralization in Dungash mine are comparable with other orogenic gold deposits in the region (e.g., Barramiya deposit), which may suggest a regional setting controlling gold metallogeny of the region. This setting should guide future exploration programs in the central Eastern Desert province.