A fluid inclusion and stable isotope study of the world class imiter silver deposit (Morocco)
Résumé
The Imiter silver mine is a world-class deposit located in the Eastern
part of the Anti-Atlas (Morocco). The deposit is hosted by Cryogenian
black shales and Ediacaran volcanics, and unconformablyoverlain by
Paleozoic sedimentary rocks. The paragenetic sequence for the gangue
mineral is composedby amacroscopically white quartz (Qz1) associated
with the deposition of galena and scarce silver mineralsas matrix
of hydraulic breccias, followed by a grey quartz (Qz2) associated with
the main silver deposition event and a first generation of scarcedolomite
(Dol1). A third episode is characterized by massive pink dolomite precipitation
(Dol2) with minor quartz (Qz3) and base metals. Finally, a
surficial event affects the mineralisation and is responsible for a secondary
ore enrichment.
Microthermometry study coupled with Raman analyses reveals that
fluid inclusions are CaCl2-dominated. The general trend is a decrease
of temperature from Qz1 (250°C) to Dol1 (80°C) then Dol 2 (probably
below 70°C) depositions. Fluid salinity increases from Qz1 (6.3 to 19.0
wt % NaCleq) to Dol1 (27.2 % wt NaCleq), and decreases with the precipitation
of Dol2 associated with quartz 3 (15 %wt NaCleq). The 18O
isotopic composition of fluids in equilibrium with quartz and dolomitesuggestsamagmatic,
metamorphic and/or basinal origin for the fluid at
the origin of Qz1, and that meteoric / surficial basinal brinesmay be at
the origin of Dol2.
The proposed model to explain the ore forming processes in Imiter is a
mixing of Ca-brines, probably providing chloride complexes, with a hot
NaCl fluid of lower salinity, leading to the precipitation of silver.The
results favour the important role of calcic brines, whose origin remains
matter to debate in such a giant depositand for whom age of emplacement
is strongly discussed.