ABSTRACT

The Middle-Upper Jurassic section in Israel contains scattered areas of dolomitization within predominantly limestone formations. One of these dolomitization sites, in the subsurface in the Ashdod-Gan Yavne area is associated with karst features as well as Mississippi Valley type mineralization (saddle dolomite-fluorite-quartz-bituminous clay-calcite) and hydrocarbons.

Four distinct carbonate phases--limestone, coarsely crystalline brown host rock dolomite, white sparry saddle dolomite, and white vein calcite--were analyzed for stable isotopes of carbon and oxygen. These phases are believed to represent successive stages of diagenesis--epigenesis of the Ashdod-Gan Yavne deposits. Water taken from fluid inclusions from saddle dolomite and calcite "pairs" was analyzed for stable isotopes of hydrogen as well.

In all but a few samples, the carbon isotope ratio +1 ± 1 was identical for all the carbonate phases studied. The oxygen isotope ratio for the limestones in all but a few cases was much heavier (-4.5 to -7.0) than that of the host rock dolomites (-5.7 to - 11.3) and is interpreted as reflecting dolomitization following limestone deposition. The oxygen isotopes in the host rock dolomites show geographic as well as stratigraphic variations. The host rock dolomites in the Zohar Formation in the Ashdod area have characteristically less negative ¹⁸O values (-6.7 to -9.0) than the host rock dolomites in the Shderot Formations in the Ashdod area and the dolomites from both the Zohar and Shderot Formations in the Ashqelon area (¹⁸O < -9.2). The oxygen isotope results can be interpreted as evidence for either two different events of dolomitization, one regional and the other restricted to the Ashdod area, or a single event of dolomitization affecting two separate aquifers which differ in their oxygen isotope composition.

Saddle dolomites, which were earlier thought to be a separate phase of epigenesis representing the earliest stage of mineralization postdating dolomitization, showed isotopic values identical to the host rock dolomites and showed the same formational and geographic variations as the host rock dolomites. Isotopic signatures of the vein calcites--believed to be a much later stage of mineralization showed ¹⁸O values of (-5.6 to -10.8) similar to the trend expected for dolomite-calcite pairs in the Zohar Formation and the opposite trend in the Shderot Formation.

A large range of D values (-38 to -58) was measured from fluid inclusions from saddle dolomites and vein calcites. This range is significantly different from the hydrogen isotope composition presently found in formation waters as well as local meteoric water.