Constraints of the regional deformation on the hydrothermal veins in Ousdrat and Aït Ahmane ore deposits (Bou Azzer-El Graara inlier, central Anti-Atlas, Morocco): Implications for mineral exploration

Abstract

The overlap of tectonic and magmatic events in the Bou Azzer-El Graara inlier causes a complexity for understanding the absolute and/or relative chronology between the deformation and the hydrothermal system. In the central northern part of the Bou Azzer-El Graara inlier, syn-orogenic ca. 650 Ma Aït Ahmane and Ousdrat plutons have been deformed under NE-SW shortening and with metamorphic conditions. These plutons are affected by fault systems with four mains directions: N015, N090, N140, and N040. The last two fault systems are the most mineralized and they are weakly constrained in the frame of deformation phases affecting the Bou Azzer inlier, and filling style. In addition, Pan-African, WACadomian or Hercynian origin of NE-SW faults is not yet reached. This contribution aims to clarify these issues based on structural, textural and morphological of these faults and veins. Our analyses highlight that most of these structures were formed through open-space filling and crack-seal-slip mechanisms. N140 contacts faults between serpentinite and quartz diorite experienced multiple reactivations during the WACadomian stages. The oldest activation was in ductile condition and later reactivated in a transpression to transtension regime leading to the development of N040 faults as sinistral antithetic faults due to NNE stress evolution. Field observations suggest that N040 structures affect late Ediacaran Ouarzazate Group, but not the Adoudounian Formation, highlight their WACadomian origine. The N040 structures preserve metric to microscopic cataclastic shear zones. This evolution suggests that the hydrothermal system in the Bou Azzer inlier shifted from ductile-brittle hydrothermal style along the N140 foliated contact to a brittle regime along the N040 sinistral-normal faults. Some intra-vein microtextures suggest that this hydrothermal fluid was emplaced in a predominantly extensional and tectonically active environment, as evidenced by extensional relay, syntaxial and antiaxial textures, and elongated grains. Consequently, a detailed analysis of the morphology, textures, filling types, and fault movements is an essential and important tool for establishing the relative chronology of deformation in front of the N140 and N040 veins.