Strontium and lead isotope studies on the origin of metalliferous-mineralization
fluids along the Paran tectonic line (Menuha Ridge, southern Israel).
The isotopic composition of Pb and Sr have been measured in order to explore
the geochemical affinity of the fluids that caused the epigenetic mineralization in
Cretaceous rocks of the Judea Group along the Paran fault line (Menuha Ridge,
Negev Shear zone, southern Israel). The well-defined mineralization phenomena,
which include the formation of iron oxide-rich veins and lenses and ferroan and nonferroan dolomitization of the adjacent marine carbonate rocks, have been studied
extensively. Previous studies attributed the mineralization to the overpressured Mg
and Fe-rich solutions from the underlying clastic Nubian aquifer. The current study
aims to verify this assertion and to point out additional sources.
Dolomite and Fe-oxide samples were collected from three rock formations in
three sites along the fault. The 87Sr/86Sr ratios in the dolomite and Fe-oxides samples
range from 0.7079 – 0.70852. This range is significantly higher than that observed for
marine carbonate sediments of Cenomanian – Turonian age (0.7074 – 0.7076). The
ratios of the Paran dolomites and Fe-oxides are similar to those found in discordant
epigenetic dolomite bodies (0.7082 to 0.7087) in Cenomanian – Turonian rock
formations along the western cliff of the Dead Sea Rift (DSR). Furthermore, there
appears to be a trend between the Mg-content of the dolomites, their Sr composition,
and the distance from the DSR. Thus it is possible that the Mg-rich brines originated
in the Sedom Lagoon (in the DSR), migrated through the carbonate section along the
DSR to the Paran area and brought about dolomitization and modification of the
87
Sr/86Sr values of the host rocks.
The 207Pb/204Pb and 206Pb/204Pb in the dolomites from all three sites fall on
straight line indicating mixing between two end members. The radiogenic, high Pbcontent, end-member has been found in previous studies in the epigenetic dolomites
in Cenomanian – Turonian rock formations near the DSR, thus suggesting that DSR
brines provide this end-member. The Pb-isotope ratios of the Paran Fe oxides plot in
the range of the non-radiogenic, high Pb-content, end-member of the dolomite mixing
line. In addition, the Pb-isotopic composition of igneous rocks from the region have
similar non-radiogenic values. Thus, leaching metals from igneous bodies provides a
potential source for the Pb and other metals such as Mn, Ni,Zn, and Fe. It is possible
that all these metals were transported to the region with Nubian aquifer-like
groundwaters.
In conclusion, the solutions that caused the mineralization phenomenon
originated from two sources: (1) DSR brines which provide the source of the Mg for
dolomitization and the radiogenic Pb and Sr. (2) Fe-enriched groundwaters that
leached metals from igneous bodies during their sub-surface migration.