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Origin of the bentonite deposits of eastern Milos, Aegean, Greece: Geological, mineralogical and geochemical evidence

Peter W. Scott, T. Marcopoulos

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Summary

The Lower Pleistocene bentonite deposits of Eastern Milos, Greece have been formed at theexpense of volcaniclastic rocks under submarine conditions. Systematic variation of the major chemicalelements reveals that the deposits were formed from different precursors which were erupted from differentvolcanic centers belonging to at least two separate volcanic provinces. The volcanic eruptions wereprobably subaqueous. The major authigenic phases are smectite, K-feldspar, opal-CT and the zeolitesmordenite and clinoptilolite. The deposits have a complex history and have been affected by hydrothermalalteration.The geological features ofbentonites coupled by the presence of abundant authigenic K-feldspar indicatethat alteration of the parent volcanoclastic rocks took place under low temperatures and is probably notrelated to hydrothermal alteration, which is a separate event. Hydrothermal alteration has modified boththe mineralogical characteristics and the properties of bentonites. Alteration of the parent rocks to bentoniteswas favoured by high water : wall rock ratios and fluid flow and is associated with leaching andsubsequent removal of Na, K and Ca. The source of Mg was the parent rocks and only small scale Mguptakefrom the sea water has probably taken place. The formation of authigenic K-feldspar has probablybeen favoured by a high K§ + activity ratio and high Si activity of the pore fluid. Such conditionsmight have been favoured by the pH conditions and the cooling history of the parent rocks.

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