Abundant veins filled by calcite, celestite and pyrite were found in the
core of a 719 m deep borehole drilled in Oftringen near Olten, located
in the north-western Molasse basin, close to the thrust of the Folded
Jura. Host rocks are calcareous marl, argillaceous limestone and
limestone of the Dogger and Malm. The delta O-18 values of vein calcite
are lower than in host rock carbonate and, together with
microthermometric data from fluid inclusions in vein calcite, indicate
precipitation from a seawater-dominated fluid at average temperatures of
56-68A degrees C. Such temperatures were reached at the time of maximum
burial of the sedimentary pile in the late Miocene. The depth profile of
delta C-13 and Sr-87/Sr-86 values and Sr content of both whole-rock
carbonate and vein calcite show marked trends towards negative delta
C-13, high Sr-87/Sr-86, and low Sr content in the uppermost 50-150 m of
the Jurassic profile (upper Oxfordian). The Sr-87/Sr-86 of vein minerals
is generally higher than that of host rock carbonate, up to very high
values corresponding to Burdigalian seawater (Upper Marine Molasse,
Miocene), which represents the last marine incursion in the region. No
evidence for internally derived radiogenic Sr (clay minerals) has been
found and so an external source is required. S and O isotope composition
of vein celestite and pyrite can be explained by bacterial reduction of
Miocene seawater sulphate. The available data set suggests the vein
mineralization precipitated from descending Burdigalian seawater and not
from a fluid originating in the underlying Triassic evaporites.