  Spontaneous Potentials associated with volcanic activity are often
interpreted by means of the electrokinetic potential, which is usually positive
in the flow direction (i.e. Zeta potential of the rock is negative). The
water-rock interactions in hydrothermal zones alter the primary minerals
leading to the formation of secondary minerals. This work addresses the study
of calcite precipitation in a sand composed of 98% quartz and 2% calcite using
streaming potential measurements. The precipitation of calcite as a secondary
mineral phase, inferred by high calcite saturation indices and by a fall in
permeability, has a significant effect on the electrokinetic behaviour, leading
to a significant reduction in the Zeta potential (in absolute value) and even a
change in sign. The measured decrease in Zeta potential from -16 mV to
-27&plusmn;4 mV takes place as the pH rises from 4 to 7, while it remains
constant at -25&plusmn;1 mV as the pH increases from 8 to 10.5. For pH higher
than 10.5, calcite precipitates and is expected to coat the quartz surface. The
measured Zeta potential vary from -17 to +8 mV for pH ranging from 10.6 to 11.7
depending on the amount of precipitated calcite indicated by the decrease in
permeability. The observed change in sign of the electrical surface potential
rules out the usual qualitative interpretation of SP anomalies in order to
determine fluid circulations, even at pH lower than 9 if calcite is widely
present as a secondary mineral phase, since the electrical surface potential of
calcite depends also on CO2 partial pressure and [Ca2+]. Therefore SP anomalies
as measured in hydrothermal field, without mineralogical analyses of
hydrothermal deposits, and without geochemical fluid survey, should be
interpreted with caution.
