Contribution to the Mineralogy of the Iron-Rich Mud Sediments of Santorini, Greece

The matrix of the mud sediments consists predominantly of the debris of diatoms and of particles of the volcanic rocks in minor part. The content of iron minerals can reach more than half of the composition of the sediments.

Iron (III)-hydroxide is found in the brown coloured top layers of the sediments. It forms agglomerations of cells and is amorphous with regard to electron diffraction. Goethite is derived directly from it by crystallization. Siderite mineralization follows up to a depth of 40 or 80 em. In lower layers iron sulphides in form of pyrite are stable only. However, cores were taken in the bays of Nea Kameni, showing the predomination of sulphides over the whole length.

Siderites and pyrites as microcrystallites in sizes of 0,1 μm up to 1 μm, mostly arranged in the form of globular aggregations, and reaching diameters of 30 μm in the maximum. The siderite forms incrustations around the debris of diatoms too. The morphology of the framboidal pyrite was especially studied.

The iron mineralization of the sediments is a product of postvolcanic and, last but not least, microbiological processes. It is an example of present-day iron mineralization of volcano-sedimentary type. The investigation of such a progenitor of an iron ore deposit is an important contribution to the knowledge of how iron mineralizations have been formed in the past.

INTRODUCTION

In the Aegean Sea within the caldera of Santorini, especially in the bays of both the volcanic islands of Nea Kameni and of Palea Kameni, the recent formation of iron sediments was first reported by Behrend (1936), Lippert (1953) and Harder (1960, 1969). Mineralogical and chemical investigations have been carried out by Butuzova (1966, 1969) and Bonatti et al. (1971). Detailed studies were undertaker by Puchelt (1972) and Puchelt et al. (1973) in the bay of Palea Kameni, located on the north of the small island. These studies have been continued by Puchelt and his co-workers, including samples from localities on the south coast of Nea Kameni. This research work includes the application of methods of sedimentology, of mineralogy (x-ray diffraction, IR-spectrometry, thermoanalysis, electron microscopy), of geochemistry (chemical composition and trace elements of sediments and pore waters, isotopic composition of sulphur, carbon and hydrogen analysis of gases), of microbiology and bacteriology.

This paper presents the results of the mineralogical investigations obtained by electron optical methods, showing the morphology of an iron-rich mud which represents the progenitor of iron ore mineralization.

The electron optical observation of water-containing colloidal structures was made, using a special modified freeze-etching technique (Geymayer 1974). The mud samples were dried and broken. The broken plain surfaces were the object of the investigations with the aid of scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDAX).

1. IRON SEDIMENTS

Iron-rich sediments which are known to be of a thickness of approximately three meters were sampled by pressing plexiglass tubes into the stratum. The soft and water-rich (60 - 80 % H₂O) sediments consist predominantly of a gel material interbedded by sandy layers of rock particles. Generally the cores show a brown coloured zone of up to 20 cm on the top. The middle zone which is 40 cm to 60 cm thick, mostly shows varigated layers coloured white, grey, yellow, greenish, dark green, brown, reddish, rust red to black. The lower part of the cores is more or less monochrome grey. At Nea Kameni cores with a black top layer are found too. In detail the stratification of the cores shows differences from sampling point to sampling point and between the localities.

The chemical composition of the more carbonatic iron sediments of Palea Kameni varies about 35 % to 45 % Fe₂O₃ and 15 % to 30 % SiO₂. The more sulphidic iron muds of Nea Kameni contain about 20 % to 30 % Fe₂O₃ and 40% to 50% SiO₂.

The pH-Eh-environment is weakly acid (pH 5 - 7) and varies between reducing and oxidizing conditions (-200 / + 300 mV). The salinity of pore solution increases with depth.

The results of scanning electron microscopy show that the matrix of the mud sediments consists of the debris of diatoms (Figs. 1 and 2). It is a source of numerous species of these microorganisms and the object of special microbiological investigations.

2. MINERALOGY OF THE IRON SEDIMENTS

Using instrumental methods of mineral phase analysis the following minerals are identified:

Silicate matrix : Opal and chalcedony, which the diatom debris consists of. Chalcedony is the diagenetic alteration product of opal present in lower layers of the mud sediment. Quartz and plagioclase in traces are components of rock particles.

Iron minerals: Iron (III)-hydroxide (amorphous), goethite; siderite (calciosiderite); pyrite; iron allophan (hisingerite), nontronite, probably vivianite too.

Using electron optical methods and qualitative micro probe analysis the occurrence of iron (III)-hydroxide, siderite, pyrite and nontronite could be detected.

2a. IRON (III)-HYDROXIDE

Fig. 3 shows the electron optical picture of amorphous iron (III)-hydroxide taken by the freeze-etching technique at high magnification. The iron (III) hydroxide forms a three-dimensional structure of cells showing diameters of 0,1 μm or less. Chukrov (1971, 1973) described the amorphous phases of protoferrite and of ferrihydrite (2,5 Fe₂O₃. 4,5 H₂O) as instable colloid minerals also from the locality of Santorini. This observation is partly confirmed. Synthetically precipitated iron (III)-hydroxide shows the same morphology as observed in nature. The transition to the ordered state, such as the formation of goethite or hematite, happens by aging (Geymayer/Schroll (unpublished), see also Schroll 1976). The cell-structure collapses and crystallization starts.

2b. SIDERITE

The siderite forms globular aggregates and shows a tendency for incrustations too. The tabular microcrystallites have grown orientated towards the table face. The contours of the crystal are hexagonal. The sizes of the siderite balls are in the range of 1 μm to 30 μm (Figs. 4 and 5). However, the balls often show smeary surfaces, so that the habit of the microcrystallites could not be recognized so easily and it is quite possible to mistake them for not well developed balls of pyrites. The qualitative micro probe analysis enables a discrimination in such cases. Besides the deficiency of sulphur the siderites are characterized in each case by a content of calcium.

The chemical composition of the siderite can be estimated from the analysis of the total sediment samples containing 50 % siderite. If the concentration of magnesium, calcium and manganese should be related to the mineral siderite the following composition should be given:

The calcium content appears to be remarkably high. It could not be decided exactly, whether Ca⁺² substitutes Fe⁺² totally or whether ankerite is present as a second carbonate phase. The x-ray diffraction diagram of siderites shows the peak 1014 at 2,817 ± 0,001 Å (2,79 Å for siderite according to ASTM 8 - 133).

In comparison the manganosiderite of the ore muds of the Red Sea are characterized by well formed rhombohedrical crystals of an average size of 2 μm to 5 μm. They are precipitations of hydrothermal submarine brines on the bottom of the Red Sea trench taking metals from basaltic rocks.

2c. PYRITE

Above all pyrite occurs in forms of framboids ⁽¹⁾. Scattered crystals are a little larger (perhaps twice) than the grains of the framboids. In one case a pyritoder (120) of 8 μm was found (Fig. 6).

The microcrystals of pyrite are reaching a size of 0,1 μm to 1 μm (Figs. 7 and 8). Clusters of framboids can be observed too.

The framboids are not always closely packed. Some of them have been found showing an empty core or defect and irregular packing. The ordering often seems to be limited to the outer surface.

The habit of the microcrystallites shows two forms:

a)    pyritohedron (120), also in combination with the octahedron (111) or the cube (100)

b)    the octahedron (111), also in combination with the pyritohedron or the cube (see also Love et al. 1971).

The habits seem to be dependent on the locality. The pyritohedron is found primarily in the cores of the sediments of Nea Kameni rich in sulphides. The morphology of the framboids and the habits of the microcrystallites are not well discernible, if the surface is attacked. In the middle zone of the mud sediments the physicochemical conditions are instable. Resolution and recrystallization can be observed. Pyrite framboids and siderite mineralization are coexistent preferably in layers of the middle zone of the mud sediments.

2c 1. FRAMBOID

Framboids are widely distributed in sediments of every geological age, from the early Precambrium up to Recent. They are in favour with the indication of low temperature origin and microbiological processes.

The forming of framboids is not settled in all details. Framboids represent a geometrically organized packing of microcrystallites produced by a kind of pseudomorphis of pre-existent spherical bodies in a reducing environment rich in organic substances, such as organic coazervates or vacuoles of gas (Rickard 1970; Kalliokoski 1974).

The iron-rich muds of Santorini provide a valuable contribution to the explanation of this phenomenon.

3. THE IRON-RICH MUD AS PROGENITOR OF VOLCANO-SEDIMENTARY IRON DEPOSITS

The forming of iron-rich sediments in the caldera of Santorini raises the question of the relation to genetical problems of sedimentary iron deposits, such as the Precambrian iron formations or the Paleozoic iron stone deposits of the type Lahn - Dill. Especially the origin of the Lahn - Dill type has been seen in this connection (Harder 1960, 1964).

In the caldera of Santorini the iron is mobilized by postvolcanic activities producing heat, gases (mainly CO₂) and reactions between sea water and the lava rocks.

However, the assistance of microorganisms is indispensable. The growth of diatoms is advanced by the warmed sea water and by the increased supply of silicon. The debris of the diatoms represents an ideal matrix with large internal surface. The separating of iron (III)-hydroxid is caused by oxydizing of iron bacteria (gallonella ferruginea) living optimally in an environment characterized by low oxygen content, pH range of 6 - 7,5 and relatively high hydrogen carbonate concentrations (Hanert 1972). Bacteria produce carbon dioxide attacking organic matter, so that it is difficult to determine the influence of volcanogenic gases.

The forming of the iron sulphides by reduction of sea water sulphate is a bacterial process too. Only hot gases of volcanic emanations contain H₂S.

The iron mineralization of the mud is low in the concentration of minor elements and trace elements except arsenic. Also the iron stone ores of the Lahn - Dill type show a deficiency of accompanying minor and trace elements. Of course, in the Lahn - Dill ores the concentrations of Ni, Co and Cr are slightly higher because the iron-spending volcanic rocks have been more basic.

It is easy to imagine that the iron-rich mud in the bays of the volcanic islands of the caldera of Santorini may be found as stratiform iron deposits a million years in the future. After the stages of late diagenesis and weak metamorphism the iron mineralizations should be composed of pyrite, siderite, ankerite, hematite, magnetite, chamosite and quartz as gangue mineral.

What we learn, however, is the fact that the part played by the activity of life must not be underestimated in the formation of low temperature sedimentary deposits, perhaps even in the early Precambrium.

- (1). The globular aggregates of pyrite were first interpreted erronously as "siderite" for lack of chemical data (Puchelt et al. 1973). Figures 5, 6, 8 and 9 in this publication should be corrected to represent pyrite.

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