Lithostratigraphy and structural framework of the Ediacaran volcano-plutonic complex of the Imiter Ag–Hg mining district (Jbel Saghro, Anti-Atlas, Morocco)
Résumé
New lithostratigraphical and structural data provide a better assessment of the still controversial metallogenetic context of the world-class Ag–Hg Imiter deposit, located in the north-eastern part of the Saghro massif (Anti-Atlas, Morocco). Three Ediacaran volcano-plutonic cycles separated by two major angular unconformities are identified: (1) early plutonic bodies (Imider and Igoudrane) intruding the Middle Neoproterozoic (NP2) of the Saghro Group and overlain by, (2) the monoclinal Imiter Sequence (lower part of Late Neoproterozoic, NP3) emplaced in basinal conditions and consisting from bottom to top of (i) a lower volcano-sedimentary unit (LVSU) with conglomerates, tufaceous siltstones and sandstones, and interbedded andesites, (ii) the >500 m thick compound Imiter Ignimbrite, (iii) an upper volcano-sedimentary unit (UVSU), and (iv) upper andesites, unconformably overlain by (3) an upper silicic sequence (upper NP3) emplaced in continental conditions and made of rhyolitic lava flows and ignimbrites currently dated at ca. 550 Ma. The structural analysis shows that the Imiter sequence was deposited under a N–S extensional regime, then slightly tilted under a NE-SW extension. At the end of the lower NP3 period and prior to an emersion and erosional time lapse, the Imiter sequence was slightly deformed by a NW-SE transpressional regime under which the Taouzzakt and Bou Teglimt plutons (ca. 570 Ma) were emplaced. The upper NP3 rhyolitic volcanism was early controlled by the opening of two major N–S and NW-SE crustal fault systems and further evolved with the emplacement of N–S dykes and NW-SE extensional normal faulting in response to a NE-SW transtensive regime. Two post-Precambrian kinematic regimes have been identified from faults crosscutting the unconformable contact between the NP3 and the overlying Middle Cambrian formations. The first regime is expressed by ENE to E sinistral normal faults, the second one by an echelon NE dextral strike-slip faults. The so-called Imiter fault is a polyphase E-W trending fault complex that is a first-order control on the Ag ore location. Our data show that the main, vertical motion of this fault that lowers the northern side by 300–400 m offset might have been progressive during deposition of the Imiter Sequence, and is sealed by the erosional surface between the lower NP3 and upper NP3 periods. Our data further assign a lower NP3 age for the transpressional regime, responsible for the mineralized Ag veins according to previous microstructural studies at the mine scale. These lithostratigraphic and structural results are likely to be relevant to the Saghro massif as a whole, and thus may help assessing the metallogenic context of the numerous ore deposits found in this area, and guide further ore explorations.