Orogenic gold formation in an evolving, decompressing hydrothermal system: Genesis of the Samut gold deposit, Eastern Desert, Egypt

Samut is a vein-type gold mineralization in the Eastern Desert of Egypt, which shares many geologic features with other orogenic deposits in this region. The deposit has a long history of mining activity dating back to ancient times, but so far has received little attention from the standpoint of geologic study. Here, we present new field, petrographic, microthermometric and microanalytical data from the Samut gold deposit to evaluate its genesis and fluid evolution. Vein quartz at Samut shows abundant evidence of precipitation coinciding with early phases of ductile deformation that show transitions to increasing brittle fracturing and fragmentation through time. In some samples, brecciation of the vein quartz is intense and shows evidence of repeated episodes of fragmentation. This brittle style of deformation culminates with latest-stage brittle opening and loss of confining pressure, resulting in open-space filling textures. Backscattered electron images and electron microprobe analysis reveal three varieties of pyrite in the mineralized quartz veins (Py I, Py II and Py III). Laser ablation ICPMS analysis of pyrites from mineralized quartz veins reveals that the earlier Py I is low in arsenic and gold, whereas Py II and Py III are enriched in both elements. Anhedral Py II is particularly gold-rich, and gives way to blocky, euhedral Py III that is rich in arsenic but not as rich in gold compared to Py II. Fluid inclusions in the least brittle deformed vein quartz indicate that the veins formed at minimum temperatures of ∼300 °C, early in the hydrothermal system. The calculated fluid pressures at homogenization of the aqueous-carbonic inclusions furnish a wide range throughout the paragenesis, perhaps on the order of ∼1 to 3 kbar. The δ ³⁴ S values of pyrites imply sulfur isotopic composition of the mineralizing fluid on the order of +0.6 to +4.9‰ overlapping with the broad range of δ ³⁴ S values calculated for ore fluids in orogenic gold deposits worldwide. Together, these results support a general model in which gold and sulfide deposition was coinciding with an ongoing syn-deformational vein formation, and tending towards the late stages of vein growth prior to loss of confinement. Free-milling gold appears to be mostly related to the later stages of brittle fracturing and re-sealing of the veins during repeated fluid release, as the background fluid pressure evolved from earlier dominantly lithostatic to later fully hydrostatic conditions.