ORIGIN OF THE BENTONITE DEPOSITS OF EASTERN MILOS, AEGEAN, GREECE - GEOLOGICAL, MINERALOGICAL AND GEOCHEMICAL EVIDENCE

The Lower Pleistocene bentonite deposits of Eastern Milos, Greece have been formed at the expense of volcaniclastic rocks under submarine co nditions. Systematic variation of the major chemical elements reveals that the deposits were formed from different precursors which were eru pted from different volcanic centers belonging to at least two separat e volcanic provinces. The volcanic eruptions were probably subaqueous. The major authigenic phases are smectite, K-feldspar, opal-CT and the zeolites mordenite and clinoptilolite. The deposits have a complex hi story and have been affected by hydrothermal alteration. The geologica l features of bentonites coupled by the presence of abundant authigeni c K-feldspar indicate that alteration of the parent volcanoclastic roc ks took place under low temperatures and is probably not related to hy drothermal alteration, which is a separate event. Hydrothermal alterat ion has modified both the mineralogical characteristics and the proper ties of bentonites. Alteration of the parent racks to bentonites was f avoured by high water:wall rock ratios and fluid flow and is associate d with leaching and subsequent removal of Na, K and Ca. The source of Mg was the parent rocks and only small scale Mg-uptake from the sea wa ter has probably taken place. The formation of authigenic K-feldspar h as probably been favoured by a high K+/H+ activity ratio and high Si a ctivity of the pore fluid. Such conditions might have been favoured by the pH conditions and the cooling history of the parent rocks.