Structural controls on gold deposits in the Egyptian Nubian Shield: insights from the Atalla-Sid Shear Belt, Central...

Within the Egyptian Nubian Shield (ENS), the majority of gold deposits are orogenic, occur in a wide range of tectonic settings and commonly exhibit distinct spatial and temporal distributions. Gold mineralization is predominantly hosted within quartz and/or carbonate veins and veinlets emplaced along late Neoproterozoic structural features, including megashears, fractures, stockwork systems, brecciated and mylonitized zones, hinges of thrust- and shear-related folds, and tectonic contacts. The Atalla-Sid Shear Belt (ASSB) is one of the eye-catching gold-bearing megashears in the ENS encompassing two historic gold mines (Atalla and Sid). In this work, detailed field/structural investigation was carried out and integrated with paleostress and fracture analysis to establish a tectonic model and to provide a template for the ENS analogous terranes. Four paleostress tensor stages are indicated in this zone. Stage 1 involves transpressional regime which with a prolate shape of the stress ellipsoid (R'= 1.5) and NNW-SSE SH_max (SH_max = N153°E) coinciding with the Najd Shear orogeny. Stage 2 marks a switch to transtensional regime with a oblate shape of the stress ellipsoid (R'= 1.5) and ENE-WSW SH_max (SH_max = N074E). Stage 2 played a significant role in controlling gold distribution. The transition between both stages is likely triggered by crustal relaxation and a change in the far-field stress orientation. Stage 3 represents extensional regime (R'= 0.5) and ENE-WSW Sh_min (Sh_min = N064°E); and Stage 4 indicated a compression regime (R'= 2.5) and NE-WSW SH_max (SH_max = N039°E). Stages 3 and 4 represent the final phases of the Neoproterozoic Orogeny that played a role in the final structural geometry of the belt without a noticeable role in gold mineralization. Results obtained from the analysis of fault pattern data and processing of photographic fracture networks demonstrate three-abundance orientations (NW, NE and E) of fracture population in Atalla area, and two orientations (NW and NE) in Sid area. It is recommended to consider stage 2-related shearing in further exploration programs in the ENS.