Nature and origin of the Vani manganese deposit, Milos, Greece: an overview

The Vani manganese deposit is located in the rugged NW sector of Milos Isla nd. It occurs within the Vani volcano-sedimentary basin, which is underlain by dacitic domes and flows of Upper Pliocene age (3.5-2.0 Ma). The end of the emplacement of the dacites was marked by the collapse of the magma cham ber, which resulted in a huge pyroclastic episode and the deposition of a t hick layer of pyroclastic material within a shallow submarine basin. This p yroclastic material subsequently compacted to form the volcaniclastic sands tone, which became the host for the manganese ore beds which were about 4 m thick in the two sections studied. Hydrothermal fluids penetrated these sa ndstone horizons via fractures and fissures to produce the manganese deposi t. The permeable nature of the sandstone facilitated the retention of the h ydrothermal fluids within these layers. This permitted the fluids to cool s lowly and deposit the manganese oxides almost quantitatively. Formation of the hydrothermal manganese deposit took place fairly rapidly over a period of several tens of thousands of years at most. Strong tectonic activity res ulted in rapid uplift of the area which elevated the deposit above sea leve l. Two generations of manganese oxides have been identified within this deposi t; the first generation consists of pyrolusite and ramsdellite; the second generation of oxides of the isostructural series cryptomelane-hollandite-co ronadite plus hydrohetaerolite characterized by high contents of K, Ba, Pb and Zn, respectively. This sequence is the result of a two-stage process of formation of the manganese-oxide minerals in which a second high-salinity hydrothermal fluid enriched in Ba, Pb and Zn as a result of the dissolution of sulphide minerals remineralized the original manganese-oxide assemblage . It is this two-stage process of formation. which was mainly responsible f or the unique characteristics of this deposit. Although formed in a submari ne setting, the deposit shows marked differences in mineralogy and composit ion from known submarine hydrothermal manganese deposits and is most analog ous to the epithermal vein deposits of the southwestern United States. (C) 2001 Elsevier Science B.V. All rights reserved.