Iron mineralization and dolomitization in the Paran Fault zone, Israel: implications for low-temperature basinal fluid processes near the Dead Sea Transform

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 Cenomanian-Turonian 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 degrees C and delta O-18 SMOW water = -3.5 parts per thousand; 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 delta O-18 group corresponding to the simple zoned dolomites and non-ferroan dolomites and a low delta O-18 group corresponding to the complex zoned dolomites. Water-rock calculations suggest that the epignetic dolomites formed under fluid-buffered conditions: the high delta O-18 group are indicated to have formed at temperatures of ca. 25 degrees C for waters with delta O-18 = -4 to 0 parts per thousand; the low delta O-18 complex zoned dolomites at 50-75 degrees 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.