Postvolcanic Activity of the Santorini Volcano and its influence on Sediment and Ore Formation

The major exhalation components supplied into the bottom water are Fe, Mn and in some places P, Pb and Zn.

Depending on the composition of ore-forming exhalations on the fumarole fields, various types of sediments can be formed. In case of absence of hydrogen sulphide, rusty brown oxidized ores are generated. In places, where along with carbonated springs, hydrogen sulphide emanates, ore accumulations differ in colour and composition. The upper part of the ore sediment is rusty-brown, composed of amorphous ferric hydroxides; the lower one is black, formed by free ferrous hydroxide. It also contains hydrotroilite, pyrite, sulphate and iron carbonate, vivianite.

The Fe content in the upper layer is 30 - 40 %, that in the lower, up to 28 %. Noteworthy is an almost complete absence of titanium and negligible contents of trace elements. Higher contents of phosphorous (up to 1 %) were recorded in the lower layer, and of plumbum (285.10^-4 %) and zinc (up to 139.10^-4 %) in the upper one.

The thinnest sediments of the basin inside the caldera are appreciably enriched in Fe and Mn, as compared to the sediments outside the caldera. The participation of exhalative components in the sediments was determined by the method of dissolution of Fe and Mn free forms. The exhalative iron inside the caldera makes up 54 - 65 % of the total, manganese, 80 - 87 %.

No exhalative components were recognized outside the caldera. The Santorini volcano caldera is a trap for exhalative material.

Thus, the influence of the post-volcanic gas-hydrothermal activity of the Santorini volcano is confined to the area of the caldera, and is pronounced in the formation of the near-foci iron ore lenses of the complex mineral composition, as well as in the enrichment of the intracalderal sediments in iron and manganese.

The influence of volcanism on sediment and ore formation is one of the cardinal problems of geological science. The present-day study of the processes of volcanic and post volcanic activity contributes considerably to the solution of important questions dealing with the genesis of volcanogenic-sedimentary ores, such as the composition of ore-bearing solutions and the distance of their migration, space localization of the ore processes, and the physico-chemical, geomorphological and hydrodynamic conditions of development of deposits, etc.

A classical example is the Santorini volcano, situated in the southern part of the Aegean Sea among the islands of the Greek Archipelago, at a distance of 120 km to the north of the island of Crete. At present, the Santorini volcano is at the postvolcanic stage of development, when the volcanic products (exhalations and hydrothermae) affect the normal course of the sedimentary process, giving rise to the formation of volcanogenic-sedimentary iron-ore accumulations.

The Santorini caldera is an oval basin of 83 km² with a complicated geomorphological structure of the bottom. Three basins, up to 390 m. deep, are distinguished within the depression, separated by more elevated areas. The caldera is surrounded by islands, and in the west by an underwater barrier that crops out in some places in the form of islands (fig. 1). Inside the caldera there are the youngest volcanic islands, Paleo -and Neokameni, composed of lavas of the andesite-dacitic series. The main exhalations of volcanic gases, the hydrothermal springs, and both the surface and underwater fumarole fields are given in Fig. 2.

Underwater volcanic springs of fumarole fields belong to the low temperature type of carbon dioxide mofettes with exhalations of H₂S varying in time and space. The temperature of the water measured at various times by researchers varies within 30 - 40° C (Georgalas and Liatsikas 1932; Harder 1964).

The chemical analysis of the sea water in the fumarole fields showed that the main components migrating in thermal solutions are Fe (10.5 - 2 mg/l), Mn (1.0 - 3.5 mg/l) and SiO₂ (10 -14 mg/l). Some samples yielded higher amounts of P (up to 0.24 mg/l), Al being practically absent in hydrothermae.

Relatively small concentrations of iron in water near the springs are due to a rapid oxidation of its divalent forms and precipitation of particles of amorphous hydroxides.

The most intense manifestations of gas-hydrothermal activity are now observed in fumarole fields in Bay A (fig. 2). The bottom water is coloured red brown here, and on the bottom there forms a continuous layer of ferruginous mud, the thickness of which in the most quiet places, near the hydrothermal springs, reaches 60 - 70 cm. As the distance from the springs increases, the thickness of the layer decreases, but is not less than 40 cm at places accessible for observation. The area of distribution of ore sediments in Bay A is 2 - 2.5 km².

Iron ore accumulations differ in colour and composition. The upper, greater, part of the ore sediment is presented by rusty-brown and orange-red, very thin mud, its thickness in visible areas being 25 - 40 cm. The redox potential of this layer is +260 - + 270 mv, pH -5.2. It consists of X-ray amorphous ferrihydroxides, an admixture of volcanic glass, rare fragments of andesites and pumica, skeletons of diatoms, sponge spicules and small pellets of opal. The Fe₂O₃ content in the rusty-brown sediment is 44.87 - 53.34 %, FeO - 0.43 - 0.57 %, SiO₂ - 8.77 - 17.48 %, Mn - 0.01 - 0.04 %, P₂O₅ - 0,14 - 0,25 %. A complete absence of titanium and extremely low concentrations of trace elements (V, Cr, Ni, Co, Cu) are characteristic too. Some samples showed higher contents of Pb (285 X 10^-4 %) and Zn (139 X 10^-4 %).

The rusty-brown layer, visibly over 20 cm thick, is underlain by black-greenish mud; its Eh is -40 mv, pH 5.2. This sediment has a more complicated mineral composition: a predominant component is free X-ray amorphous ferrichydroxide; in addition, there are carbonates and ferric sulphates, hydrotrolite, pirite, vivianite, elementary sulphur and an admixture of ashy particles, siliceous organisms and opal pellets. The Fe₂O₃ + FeO content is 28.32 - 37. 21 %, SiO² - 22.8 - 24.3 %, Mn - 0.01-0.06 %, P₂O₅ - 073 - 1.64 %. There is practically no titanium, the amount of trace elements being negligible.

In bays B.C.F. only oxide ores are distributed, the thickness of ore sediments varying from several to 40 cm. According to the datum of E. Bonatti, the iron content in Bay C is 27 %, Mn - 0.2 %, P - 0.8%, those in Bay F being 40 %, 0.07 and 0.3 % respectively; the titanium content is 0.009 - 0.03 % (Bonatti et al., 1973).

The mechanism of formation of Santorini iron-ore sediments in Bay A is as follows: the main ore component (iron) is supplied with low temperature carbon dioxide hydrothermae in the divalent form. When hydrothermae with sea water containing oxygen, ferrous iron becomes oxidised; jelly flakes of amorphous iron hydroxide falling onto the bottom form a layer of rusty brown very thin ferruginous mud. Hydrosulphuric springs supply H₂S; the latter, passing through a ferruginous sediment, interacts with ferric hydroxides. According to experimental studies, if pH is below 4.5 - 4, hydrosulphur transforms the trivalent iron into a divalent form following the equation: 2Fe (OH)₃ + H₂S + 4H ⁺ = 2Fe^{2 +} + S + 6H₂O. The reduction is accompanied by a higher pH of the solution.

When pH reaches 4.5 - 5, a sulphide formation begins. Iron ions reduced by hydrosulphur are partly bound with CO₂, forming small particles of ferruginous carbonate, and partly with SO₃ and P₂O₅, forming sulphates and vivianite.

The principal part of iron in a sediment is present in the form of free ferrous hydroxides.

The ore sediment at the given stage of existence is in a position of unstable equilibrium, being under the control of two differently directed processes - surface oxidation at the expense of sea water oxygen and the process of oxidized form reduction by volcanogenic hydrogen sulphide.

High silica contents in ore sediments are related to its supply together with thermal solutions; sedimentation of SiO₂ proceeds mostly biogenically, this being reflected in the wide distribution of skeletons of various siliceous organisms in an ore.

Low manganese concentrations in near-foci ferruginous sediments are related to its greater, as compared to iron, geochemical mobility. Due to this fact the sedimentation of the major portions of volcanogenic manganese takes place beyond the fumarole fields.

The supply of phosphorus into a basin takes place unevenly, this being pronounced in its different contents in the upper and lower beds of an ore sediment.

Extremely low concentrations of V, Cr, Ni, Co and Cu in ore sediments and their slight enrichment in plumbum and zinc enable us to assume that the first elements in the hydrothermal composition are not flown out, whereas Ph and Zn are partly of a volcanogenic origin.

Before speaking of the influence of the volcanic and post-volcanic activity of Santorini on sedimentation, let us analyze briefly the main types of bottom deposits of the regions studied. The space localization of sediments inside and outside the caldera are presented in fig. 3.

The inshore very shoal parts are characterized by the distribution of volcano-terrigenous and biogenic sands. They surround volcanic islands in a narrow circle (ring). In the places outside the caldera carbonate organogenic sands predominate with a small admixture - of volcano -terrigenous material; inside the caldera the arenaceous deposits consist mostly of fragments of volcanic rocks and pyroclastic material.

Silt is the predominant type of sediment inside the caldera. Fine-silty deposits are developed in depressions (basins), whereas coarse siltstones are recorded in more shoal-water parts. Outside the caldera the silt deposits occupy a large area between inshore coarse-grained deposits and calcereous-argillaceous muds of the Cretan Basin. The main components of silty sediments are fragments of volcanic rocks (pumice, andesite-dacites), some minerals composing them (feldspars, pyroxene, magnetite), volcanic glass and biogenic material.

The quota of the biogenic constituent considerably increases as it becomes more distant from the Santorini volcanic zone, the carbonate content in sediments simultaneously increases, as well. The main reason for the low carbonate contents in intracalderal muds is probably the gas-hydrothermal activity of the volcano that is responsible for the supply of large amounts of CO₂ into the sea water, thus preventing sedimentation of carbonates.

The amount and composition of the pelitic fraction in sediments of various parts of the sea are different. The sediments of the Santorini zone, on the whole, are characterized by a low content of the pelitic fraction. Outside the Caldera its participation in sediments gradually increases, up to 60 % in pelagic muds of the Cretan Basin. The composition of fine fractions of sediments of various sectors of the sea is different too. Inside the caldera the major components of the pelitic fraction are feldspars and volcanic glass, i.e. finely ground pyroclastic material, a small admixture of clay minerals being present too. In muds outside the caldera the fraction <0.001 mm is composed mostly of clay minerals. Dioctahedral hydromica prevails among these minerals, small amounts of chlorite, kaolinite and montmorillonite having been recorded as well.

The distribution of Fe, Mn, Ti, and trace elements (V, Cr, Ni, Co, Cu) in sediments inside and outside the caldera (table 1) was studied in order to elucidate the scope and forms of volcanic activity of the Santorini volcano affecting sedimentation.

A comparison of the Fe and Mn contents in sediments of the same granulometric type inside and outside the caldera shows a significant enrichment of intracalderal deposits in these elements. In this case the maximum Fe and Mn concentrations are confined to the finest, fine-silty muds of the intracaldera basins, particularly to the deepest, northern, one (fig. 4, 5).

For the determination of the amounts of hydrothermal Fe and Mn in sediments the method of the dissolution of their free forms by chlorinated alcohol was used; in so doing the hydrothermal forms were separated from the "terrigenous" included in the lattice of minerals. It turned out that hydrothermal iron in the sediments of the caldera basins amounted to 54 -65 % of the total, and manganese 80 - 87 %. The manganese modulus (Mn/Fe) of muds inside the caldera is 0.06, that outside the caldera 0.03, in ore sediments 0.0003. Small Mn contents in ores and considerable enrichment of fine sediments in Mn are due to its greater geochemical mobility as compared to iron.

Muds inside the caldera are also rich in some trace elements V and Cu; this enrichment appears not to be related directly to hydrothermal activity.

Higher V and Cu contents in the caldera sediments may likely be due to their absorption from sea water by hydrothermal iron hydroxides, localized within the caldera area.

Outside the caldera no Fe and Mn were recognized. Thus, the caldera of the Santorini volcano is a natural trap for elements supplied by ore-bearing solutions, preventing the outflow (evacuation) of these elements into the open sea. In the case of favourable geomorphological conditions (quiet places, deep bays) a part of the ore material subsides and becomes buried directly near the volcanic focus, and forms a lens of high quality ore that is almost devoid of any admixture of pyroclastic material, as it was observed in some bays near the Neo Kameni island. The major part of the iron suspension is dispersed by currents and distributed in intracalderal sediments in accordance with hydrodynamic laws and the morphology of the sea bottom highly saturating (enriching) the thinnest sediments of the basins. The ore accumulation taking place in the Red Sea, where the ores are localized within small basins - traps of hydrothermal material - testifies to the importance of the geomorphological environment and the occurrence of volcanogenic-sedimentary ore deposits.

As a result of studying the character of distribution and behaviour of elements in the Santorini volcanically active zone, we can say that outside the caldera the influence of volcanism manifests itself in the origination of solid volcanic products. The post-volcanic hydrothermal activity does not in practice affect the composition of sediments outside the caldera. Its effect is strictly confined to the area of the caldera, and is emphasized in the accumulation of iron ores within the small near-island bays inside the caldera, as well as in the enrichment of intracalderal sediments in hydrothermal iron and manganese.

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