Dolomites in the Tazekka Pb-Zn district, eastern Morocco:: polyphase origin from hydrothermal fluids

Many different types of water and processes have been proposed for the formation of dolomites. The three phases of hydrothermal dolomites in the Middle Atlas Causse were investigated to elucidate their formation processes. The first two of these are associated with sphalerite and galena in stratiform and open space-filling deposits. These formed early in the history of the deposition of the Pb-Zn mineralization and commonly reveal a paragenetic overlap. A later phase, post-dating Pb-Zn mineralization, is reflected in saddle dolomite. All three phases show a decrease in delta(18)O and delta(13)C values passing from sterile (unmineralized) to mineralized rocks, and isotopic signatures are independent of the carrier facies. However, early-formed dolomites can be separated into two distinct groups on the basis of delta(18)O values. Type 1 dolomites host stratiform ore deposits, whereas type 2 dolomites host an open space-filling ore-body. Later saddle dolomites are more depleted in O-18 than either of these. The early hydrothermal and saddle dolomites precipitated from similar fluids during three distinct events, but formed by two mechanisms: replacement (hydrothermal dolomite) and cement precipitation (saddle dolomite). They show different isotopic signatures and apparently formed at different temperatures. Field data, petrographic and stable isotope results suggest a continuum of replacement, during the Carixian for the early hydrothermal dolomite 1, and during the Toarcian for early hydrothermal dolomite 2, followed by a cement precipitation phase for saddle dolomite.