Petrography, Eh-pH calculations and the stable isotope composition of oxygen are used to interpret geochemical processes that occurred during iron oxide mineralization and dolomitization along the Menuha Ridge segment of the Paran Fault, southern Israel, adjacent to the Dead Sea Transform (DST). Iron mineralization is strongly localized in the fault zone as ferruginous lenses, whereas Fe dolomitization spreads laterally into the CenomanianTuronian carbonate host rock as stratabound beds. The average oxygen isotope fractionation between syngenetic quartz and iron oxides in the ferruginous lenses gives a temperature of 50 ± 10°C and d 18 O SMOW water ¼ )3.5&; consistent with an origin from metalliferous groundwater flow in the sedimentary basin. Ferroan dolomite initially formed under strongly reducing conditions, but this was followed by oxidation and pseudomorphic replacement of the dolomite by a mesh of fine-grained iron oxides (simple zoned dolomites). This cycle of ferroan dolomite formation and replacement by iron oxides was repeated in complex zoned dolomites. Dolomite oxygen isotope compositions fall into two groups: a high d 18 O group corresponding to the simple zoned dolomites and non-ferroan dolomites and a low d 18 O group corresponding to the complex zoned dolomites. Waterrock calculations suggest that the epignetic dolomites formed under fluid-buffered conditions: the high d 18 O group are indicated to have formed at temperatures of ca. 25°C for waters with d 18 O ¼ )4 to 0&; the low d 18 O complex zoned dolomites at 50-75°C for waters with the same isotopic composition. A kinetic calculation for a complex zoned dolomite-bearing bed indicates that dolomitization must have occurred at high values of the dolomite saturation index. This requirement for high Mg supersaturation and the indication that epigenetic dolomitization is more protracted in stratigraphically deeper formations located closer to the DST is consistent with models proposing that Mg-rich solutions originated in the Dead Sea Rift.

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