Dolomitization patterns of the Liassic platform of the Tazekka Pb–Zn district, Taza, eastern Morocco: petrographic and geochemical study

Dolomitization is the most significant diagenetic process within the Liassic carbonates of the Tazekka district. It is more extensive in the Middle Atlas Causse than in the Middle Atlas domain. Middle Atlas Causse, the dolomites may be syndepositional, early diagenetic, epigenetic (with a paleogeographical control), or hydrothermal. Three hydrothermal dolomites are associated with the ore deposits. Two early phases, type 1 and 2, are separated in time from a phase of saddle dolomite. Saddle dolomite represents a late stage diagenesis post-dating Pb–Zn mineralization. Early dolomites and saddle dolomite were precipitated from similar fluids during three distinct phases but formed by two mechanisms, by a continuum of replacement during Carixian (early hydrothermal type 1 dolomite) and the Toarcian (early hydrothermal type 2 dolomite) and by cement precipitation (saddle dolomite). Precipitation of saddle dolomite cement followed a phase of fracture generation. Middle Atlas the sequential character of changes in dolomite characteristics in the middle formation of the Lower Lias confirms the synsedimentary nature of early diagenesis and seawater is regarded as the source of the Mg. Euhedral dolomite crystals indicate a minor late diagenesis. 3C values of the dolomites range from −0.83‰ to +2.37‰ PDB. There is a general decrease in δ18O values and a corresponding increase in diagenetic intensity from the laminated dolomite (−2.43‰) to the dolomitized calcarenite at the base of reduced series of the Middle Lias (−9.77‰). is a decrease in both δ18O and δ13C values in the ore deposits passing from sterile to mineralized rocks. Mineralized limestones show δ18O values of around −7.66‰. The dolomites of Bou Khalifa average −7.20‰, those of Aı̈n Hallouf −7.96‰, Asdi Ben Zerhla −7.86‰ and Aı̈n Tarselt −8.21‰ have similar average values, but those of Sidi Abdellah have values of around −9.79‰. δ18O values of the hydrothermal dolomites differentiate two groups: the first contains the stratiform ore deposits and the second the orebody of Sidi Abdellah. Saddle dolomites have an average δ18O values of −9.39‰ PDB. These phases possess differing isotopic signatures and apparently formed at different temperatures. l different waters were involved in the formation of the dolomites of the Tazekka district and that contrasting processes were involved in these episodes of dolomitization.