A newly discovered fossil stratabound hydrothermal manganese deposit at Aspro Gialoudi, N.W. Milos, Greece

The white smoker Mn-Ba-Pb deposit at Aspro Gialoudi in NW Milos is shown to be a fossil stratiform exhalative hydrothermal deposit analogous to the Vani manganese deposit which is located about 1.3 km to the NE. Both deposits are located proximal to fault systems. However, the Vani manganese deposit is adjacent to the NW-SE-trending Vromolimni-Kondaros Fault which marks the western margin of the Gulf of Milos and is one of the major faults on Milos, whereas the Aspro Gialoudi deposit is adjacent to the relatively minor dimensions NE-SW fault on the west coast of Milos. Both the Aspro Gialoudi and Vani manganese deposits formed in a similar manner, namely by transport of hydrothermal fluids through the adjacent fault systems into a reservoir of volcanoclastic sandstone to produce a deposit initially consisting of pyrolusite and occasionally ramsdellite, which were later replaced by cryptomelane, hollandite, coronadite and hydrohaeterolite. Because the NE-SW fault on the west coast of Milos is minor compared to the Vromolimni-Kondaros Fault, the Aspro Gialoudi manganese deposit consisted mainly by cryptomelane, hollandite, coronadite and hydrohaeterolite, is very much smaller than the Vani deposit. The Aspro Gialoudi deposit has not been described before because of its remoteness and small size.
   The hydrothermal manganese deposit at Aspro Gialoudi was formed little earlier to roughly contemporaneously with the Vani manganese deposit at about 1.8 Ma. by similar processes and are considered to be integral parts of the same hydrothermal system.
   Movement along the NE-SW fault located on the western margin of Milos triggered the hydrothermal activity which resulted in the formation of this deposit which seems to be controlled by alternating cycles of deposition of sulfides and hydrothermal manganese oxides within the fault which were probably the result of alternating periods of waxing and waning of seismic activity along that fault. The hydrothermal fluids penetrated the volcaniclastic sandstone which hosts the Aspro Gialoudi deposit along fractures and fissures, which led to the formation of this deposit in two stages as at Vani. The intense lowtemperature, hydrothermal activity would have been relatively short lived. Furthermore, based on the REE geochemical data the hydrothermal fluids that formed the Cape Vani Mn oxides and hydroxides formations were mainly seawater. Although the Cape Vani formations were in contact with oxidising seawater for a sufficient period of time, our geochemical data (depletion of Mg, Ca in relation to the continental crust) suggest that there was no significant alteration of these formations by seawater after their deposition.
   In general, the compositional data show that the elements most enriched in the Aspro Gialoudi, Vani and Vani Dome deposits relative to the average continental crust (Wedepohl 1995), lie in the sequence As>Pb>Sb>Cd >Mn>Tl>Mo>Zn>Cu>Ba>Be>Sr>Co>Bi. All the other elements analyzed are depleted in these deposits relative to the continental crust. The chalcophilic elements, the siderophilic elements Mo and Co plus barium and strontium are therefore the most strongly enriched in the hydrothermal Mn deposits relative to the average continental crust.