『Abstract
The Vani manganese deposit is located in the rugged NW sector
of Milos Island. 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 chamber, which resulted
in huge pyroclastic episode and the deposition of a thick layer
of pyroclastic material within a shallow submarine basin. This
pyroclastic material subsequently compacted to form the volcaniclastic
sandstone, which became the host for the manganese ore beds which
were about 4 m thick in the two sections studied. Hydrothermal
fluids penetrated these sandstone horizons via fractures and fissures
to produce the manganese deposit. The permeable nature of the
sandstone facilitated the retention of the hydrothermal fluids
within these layers. This permitted the fluids to cool slowly
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 resulted in rapid uplift of the area which elevated
the deposit above sea level.
Two generations of manganese oxides have been identified within
this deposit; the first generation consists of pyrolusite and
ramsdellite; the second generation of oxides of the isostructural
series cryptomelane-hollandite-coronadite 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 for the unique characteristics of this deposit.
Although formed in a submarine setting, the deposit shows marked
differences in mineralogy and composition from known submarine
hydrothermal manganese deposits and is most analogous to the epithermal
vein deposits of the southwestern United States.
Keywords: Cape Vani; Milos; Manganese deposit; Hydrothermal fluids;
Volcaniclastic sandstone』
1. Introduction
2. The Vani manganese deposit
2.1. Geological setting
2.2. Mineralogy
2.2.1. First generation of manganese oxides
2.2.2. Second generation of manganese oxides
2.3. The replacement of the centre of feldspars by manganese oxides
2.4. Compositions of the Mn oxides
2.4.1. Mn/Fe ratio
2.4.2. Co-Ni-Cu
2.4.3. Second generation of manganese oxides
2.5. Accessory minerals
2.5.1. Barite
2.5.2. Arseniosiderite
2.6. Vein mineralization in the Mn oxides at Kondaros
2.7. Hydrothermal alteration associated with the deposition of
the Mn oxides
2.7.1. Zones of hydrothermal alteration
3. Mineralization at Triades - Galana
4. Hydrothermal origin of the Vani Mn deposit
5. Discussion
6. Conclusions
Acknowledgements
References