GRANITOID-HOSTED LODE GOLD DEPOSITS IN THE CENTRAL EASTERN DESERT OF EGYPT

Auriferous lode deposits in the Arabian-Nubian Shield share many broad characteristics, including common spatial coincidence with granitoid intrusions, metal associations, and local structural control. Gold deposits closely associated with Late Neoproterozoic granitoid stocks are reported at several localities in the central block of the Eastern Desert, Egypt (e.g., Samut, Atud, Um Rus, Fawakhir, and Atalla mines). Emplacement of these stocks into ophiolitic mélange and island arc metavolcanic-plutonic terranes was likely concomitant with the ~596-565 Ma, post-accretionary transpression along major wrench structures, such as the Najd-related shear zones. The granitoids vary from monzodiorite to syenogranite, with calc-alkaline, metaluminous to peraluminous, medium- to high-K geochemistry.

Gold-bearing milky to grayish, massive and laminated quartz±carbonate veins associated with silicification/sericitization alteration occur along the contact zones between the granitoids and mafic/ultramafic country rocks. Structural control is evident from the spatial association of auriferous quartz veins with brittle-ductile shear zones, and the laminated appearance of the high-grade veins. Mineralogy of the veins includes As-bearing pyrite, arsenopyrite, and pyrrhotite, as well as subordinate chalcopyrite, sphalerite, galena, tetrahedrite, and stibnite. Gold and electrum are intergrown with pyrrhotite as inclusions in pyrite and arsenopyrite, but more commonly are associated with galena, tetrahedrite, and aurostibite along fractures.

Fluid inclusions in the mineralized quartz veins demonstrate an evolving low salinity H₂O-CO₂-NaCl±CH₄(±N₂) fluid, which infiltrated into the shear zones and deposited gold through immiscibility and phase separation due to pressure fluctuations, at conditions of ~260-375°C, and ~1-2.5kbar. Wallrock sulfidation played a significant role in the low grade gold in the hydrothermally altered rocks surrounding the main veins. Despite the granitoid hosts, structural control, mineral associations, and alteration assemblages suggest that the studied deposits are best classified as orogenic. Although precise dating of the ores remains lacking, existing evidence suggests gold deposition during latest Proterozoic to earliest Paleozoic deformation.