Phosphate deposits of Neogene age in Greece. Mineralogy, geochemistry and genetic implications

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Chemie der Erde 64 (2004) 329–357 www.elsevier.de/chemer

Phosphate deposits of Neogene age in Greece. Mineralogy, geochemistry and genetic implications Michael G. Stamatakis* Department of Geology, Section of Economic Geology & Geochemistry, National & Kapodistrian University of Athens, Panepistimiopolis, Ano Ilissia, 157 84 Athens, Greece Received 26 February 2003; accepted 10 August 2003

Abstract In Greece, several occurrences of phosphates have been located in Mesozoic and Cenozoic sedimentary rocks. The aim of the present paper is to describe the mineralogy, geochemistry and the origin of the phosphates deposited in marine and lacustrine basins of Neogene age in Greece. Phosphates of marine origin formed in an outer shelf-upper slope environment (Palliki Peninsula, Kefalonia Island) as well as in a hemipelagic environment (Heraklion, Crete Island). In both the deposits, the phosphate minerals belong to the apatite group. In Kefalonia Island, phosphatic material accumulations occur in the field as scattered or accumulated vertebrate bones, fish teeth and other biogenic components, hosted in a sandy limestone of Upper Tortonian age. On a microscopic scale, in the groundmass of the limestone, phosphate minerals are present as fillings and secondarily as replacements of foraminiferal and other calcareous microfossil tests. In Crete Island, burrowed cobbles and phosphatic concretions up to 10 cm in diameter have been detected east of Heraklion town. The phosphatic material is hosted in a sandy marlstone that is interbedded with diatomaceous rocks of Middle Pliocene age. Phosphates of lacustrine origin have been formed in the NW–SE-oriented Upper Miocene basins of Florina–Ptolemais and Elassona–Sarantaporo, located, respectively, in western Macedonia and Thessaly, Greece. The phosphate minerals are mainly represented by Ca/Fe phosphates such as anapaite and mitridatite and secondarily by Ca phosphates of the apatite group that are hosted in a thick succession of clayey diatomite rock. They usually form lenticular layers, asymmetric lenses, nodular and botryoidal aggregates, and faecal pellet replacements.

*Fax: +30-210-727-4213. E-mail address: [email protected] (M.G. Stamatakis). 0009-2819/$ - see front matter r 2003 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2003.11.005

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The major and trace element content of a representative set of rocks was studied. Their geochemistry was found to be controlled by the presence of detrital minerals, the host rock mineralogies, and the types of phosphate minerals present. The relatively high amounts of uranium, arsenic and barium detected in some of the phosphate rocks studied are mainly related with organic matter and are comparable to those detected in other phosphate deposits worldwide. In the deposits studied, phosphogenesis was mainly promoted by the decay of organic substances derived from diatoms, fishes and other vertebrates, leaves and trunks, and faecal pellets in a highly reducing environment on or just below the sea or lake bottom, and secondarily by the feeding of the basin with phosphate-bearing nutrients that originated in the land. r 2003 Elsevier GmbH. All rights reserved. Keywords: Phosphates; Apatite; Anapaite; Mitridatite; Neogene; Greece

1. Introduction Cenozoic phosphate rocks of commercial grade have been exploited in several countries located in the Middle East and in North Africa regions. Morocco is the major producer, whereas Jordan, Egypt, Tunisia, Algeria and Israel also play an important role in the world market (Harben and Kuzvart, 1996). Even though most of the deposits worldwide are solely composed of minerals of the apatite group, phosphate is also extracted on a commercial scale as a by-product from phosphorusrich iron ores or magnetite–apatite assemblages (Harben, 1992). Phosphates are one of the few industrial minerals that have no competition from other compounds. In Greece, several occurrences of phosphates have been located in Mesozoic and Cenozoic sedimentary rocks (Skounakis, 1978; Pomoni-Papaioannou, 1994; Pomoni-Papaioannou and Dermitzakis, 1995; Stamatakis and Koukouzas, 2001). The most important of them have been reported from Mesozoic limestones in western Greece. Due to the small reserves of good quality phosphates in Greece, all the Greek companies that produce fertilizers import phosphates mainly from Morocco, Jordan, Togo and the Kola region of Russia. Recently, several phosphate occurrences were reported in lacustrine and marine basins of Neogene age in the Greek mainland and in some islands. In particular, the occurrence of phosphate rocks in the lacustrine environment has been described from Thessaly, Central Greece (site 5, Fig. 1) (Stamatakis and Koukouzas, 2001). However, the presence of the iron-phosphate vivianite was reported as leaves’ replacements in several borehole clayey samples obtained from the exploitation of lignite deposits in Western Macedonia (Kotis et al., 1995, unpublished data). In general, the commercial-grade phosphate deposits contain more than 20% P2O5, composed of Ca-phosphate minerals of the apatite group, such as hydroxylfluorapatite [Ca5 (PO4)3(OH, F)] and/or carbonate fluorapatite (or francolite or amorphous collophane) [Ca5(PO4,CO3,OH)3(F,OH)]. Both minerals occur as nodules or layers hosted in a calcareous or siliceous groundmass, commonly having

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Fig. 1. Location map of the Neogene phosphate occurrences in Greece. Site 1: Palliki Peninsula, Kefalonia Island; site 2: Prassas Hill, Heraklion, Crete Island; site 3: Komnina area and site 4: Vegora lignite quarry, Florina–Ptolemais basin, Western Macedonia; site 5: Giannota area and site 6: Drymos area, Sarantaporo–Elassona basin. For site 5 phosphates and the host rock descriptions, see Stamatakis and Koukouzas (2001).

greenish, brownish, yellowish or whitish colour. Other phosphates containing iron, manganese and or aluminium are more rarely found in sedimentary basins (Nriagu and Dell, 1974; Piper et al., 1995). The most common Fe- and Fe/Ca phosphates of 2+ such occurrences are: vivianite Fe2+ (PO4)2
4H2O 3 (PO4)2
8H2O, anapaite Ca2Fe 3+ and mitridatite Ca2Fe3 (PO4)3O2
3H2O. Phosphogenesis occurs worldwide across a broad range of palaeoenvironments, and some of which were different from modern phosphorite depositional systems (Hiatt and Budd, 2001). Phosphorite ore deposits have mainly a marine origin, even though there are some lacustrine and continental deposits (Harben and Kuzvart,

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1996). Phosphate minerals of the apatite group constitute the majority of the phosphates of commercial grade. In sedimentary basins, they formed: (a) by early diagenetic decomposition of organic matter, occurring in the form of angular to well-rounded nodules and pellets, and also as organic matter replacements sometimes hosted by silica-rich rocks of biogenic origin (diatomite) or their diagenetic products (chert) (Sheldon, 1987; Zelibor et al., 1988; Piper et al., 1995; Deike et al., 1997; Medrano and Piper, 1997; Walker and Owen, 1999), (b) by transfer of phosphates adsorbed onto iron and manganese oxyhydroxides, which are desorbed upon burial and reduction of the iron and manganese compounds (O’Brien et al., 1990), (c) by direct precipitation of inorganic phosphate in the early diagenetic . environment (Follmi, 1996). The aim of this work is to determine the depositional and the diagenetic environment under which the Greek marine and lacustrine phosphates were formed, the mineralogy and chemistry of the phosphates and the host rock, and their possible commercial interest. The trace element content of certain phosphate rocks is critical for their utilization, because the residual phosphogypsum produced during apatite processing may contain elevated amounts of toxic elements, such as arsenic, cadmium, antimony, selenium, thorium and uranium. For this reason, special attention is given to the trace element contents of the phosphate occurrences studied.

2. Geological setting 2.1. Marine basins Phosphate deposits of marine origin occur in Neogene sedimentary rocks on the islands of Kefalonia, Ionian Sea, and Crete, Aegean Sea (Fig. 1). 2.1.1. Palliki Peninsula, W. Kefalonia Island On Kefalonia, a thick-bedded, whitish to yellowish-brown phosphatic limestone outcrops on the Palliki Peninsula in the SW part of the island for more than 20 km2, having a thickness of about 30 m (Fig. 2a). The limestone/phosphate succession is of marine origin, formed in a tropical to subtropical pelagic environment during Upper Tortonian (Georgiadou-Dikeoulia, 1967; Symeonidis and Schultz, 1968). The limestone beds have variable porosity and sometimes host well-preserved single shells of bivalves such as Pecten sp., Flabelipecten sp., Chlamys sp. In contrast to the rarely occurring, large and thick, calcareous, well-preserved shells of bivalves, accumulated ‘‘nests’’ of biogenic components such as white shell fragments of bivalves, greenish phosphatized gastropods, yellowish, greenish and brownish relics of bone fragments and other unidentified biogenic components occur locally (Fig. 2b). In the Skineas–Kontogennada dimension stone quarry of the above area,

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Fig. 2. Field and light microscopy images of the Kefalonia Island phosphates (site 1): (a) Ankerite-rich band in the phosphatic limestone; (b) phosphatic grains hosted in a calcareous groundmass; and (c and d) Foraminiferal tests filled by calcite (bright white) or apatite (dark).

triangle-shaped teeth that belong to sharks such as Carcharodon sp. have been reported (Symeonidis and Schultz, 1968). Brown phosphatized teeth with a spherical shape and a size of less than 0.5 cm in diameter belonging to the vegetarian fish Shellac sp. also occur (Georgiadou-Dikeoulia, personal comm. 2001). In the same quarry, well-preserved fossilized marine vertebrate skeletons and large bone accumulations were recently identified. Even though the limestone contains bivalve shells of shallow marine origin, the predominant fossil species are given by a series of pelagic foraminifers that define a pelagic and rather deep depositional environment (Georgiadou-Dikeoulia, 1967). This mixed fauna most likely has been reported in outer shelf-upper slope marine environments that were favourable for the accumulation of phosphates (Burnett, 1977). 2.1.2. Prassas Hill, Karteros basin, Heraklion, Crete Island The phosphate-bearing beds of Crete occur in the northern part of the Karteros basin, near Heraklion in an area of more than 10 km2 (Fig. 1). The phosphatic material is hosted in yellowish-brown sandstone beds of 2 m thick which alternate with white diatomite beds that have a thickness of 2–7 m and Middle Pliocene age (Georgiadou-Dikeoulia, 1979).

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Fig. 3. Field and light microscopy images of the Heraklion, Crete Island phosphates (site 2). (a) Irregular polished and burrowed cobble of phosphatic materials (apatite) hosted in sandy marlstone. (b) Angular cobbles of burrowed phosphatic material in a channel fill formation in sandy marlstone. Note the burrowed nature of the cobbles in the piece located in the upperright part of the photograph. White shells belonging to Ostrea sp. are also visible at the right of the hammer. (c) Dark phosphatic groundmass and foraminiferal cells fillings interlocked with calcitic groundmass in the phosphatized limestone cobbles. (d) Phosphatized peloids hosted in a calcareous–phosphatic groundmass.

The phosphates occur as (i) small, greyish to dark brown, almost spherical nodules up to 2 cm in diameter, (ii) irregular accumulations of brownish, angular pebbles (cobbles) up to 20 cm in length that form a kind of channel fill within the sandy marlstone groundmass (Figs. 3a and b), (iii) brownish, angular phosphatic sand. The sandstone–diatomite succession of Prassas Hill was most likely deposited in a hemipelagic environment that was characterized by the episodic deposition of reworked detrital material, eventually in gravity-flow deposits having a thickness of about 70 m. The sandy marlstone contains shells of bivalves such as Flabelipecten sp., Ostrea sp., and Pecten sp. and also small fish remains and their scales, and small relics of leaves. The sandstone alternates with diatomaceous rocks that exhibit reworking features such as the presence of abundance of broken diatom frustules and sponge spicules. The phosphatic materials have also undergone reworking and most of them

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are polished and burrowed. Bivalve shells have been reported on the upper surface of large phosphate pebbles, forming colonies. The phosphatic beds were formed in the northern margin of the basin, close to the Mesozoic substrate, and disappear towards the deepest part of the basin to the south. 2.2. Lacustrine basins (Florina–Ptolemais and Sarantaporo–Elassona) Phosphates of lacustrine origin occur on the Greek mainland in the 80 km long system of Upper Miocene lignite basins that extend from NW to SE in Western Macedonia–Thessaly. Phosphates have been reported in Komnina and Vegora, Florina–Ptolemais lignite basin, and in the Drymos and Giannota areas, Sarantaporo–Elassona basin (Fig. 1). In these basins, clayey diatomite rocks more than 150 m thick were developed. They host tabular Ca/Fe-phosphatic masses up to 40 cm thick and lenticular layers, oriented subparallel to the bedding planes (Figs. 4a and b). Nodular to botryoidal phosphate assemblages and randomly scattered spherical nodules up to 2 cm in diameter were also developed (Stamatakis and Koukouzas, 2001; Thewalt et al., 2001). The substrate of the basins was formed by Mesozoic and Palaeozoic gneiss, schist, ultramafic rocks and marble and is

Fig. 4. Field and light microscopy images of the lacustrine phosphates of W. Macedonia and Thessaly. (a) Irregular phosphatic lens composed of dark botryoidal concretions (anapaite/ mitridatite) developed subparallel to the bedding planes in bright-coloured clayey diatomaceous rocks (site 6). (b) Lenticular layers of dark phosphatic material (anapaite) hosted in earthy, weathered clayey diatomaceous rocks (site 4). (c and d) Anapaite radiated fibrous crystal assemblages from sites 5 and 3, respectively.

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unconformably overlain by clayey sediments. The clayey diatomite beds commonly contain numerous plant remains, such as leaves, seeds, branches, and spores partially replaced by iron oxides–hydroxides (Velitzelos and Gregor, 1986, 1990). Borehole data show that at depths of more than 40 m, these biological constituents have been replaced by the Fe-phosphate vivianite (Stamatakis and Koukouzas, 2001). In the lacustrine basins studied, the diatomite rock contains diatom frustules of cylindrical (Melosira sp.) and disk (Cyclotella sp.) shape in sizes varying from 5 to 20 mm. The presence of the diatom species Melosira sp. is characteristic of a deep-water lake environment (Inglethorpe and Morgan, 1992).

3. Rock sampling, sample preparation and analytical methods Twelve bulk rock samples were collected from the phosphatic rocks studied and their host rocks. The phosphate-bearing rocks of Palliki Peninsula, Kefalonia Island, were better exposed in an active quarry that has about 20 m height, located in the Skineas–Kontogennada area. According to colour variations, three distinct bulk limestone rocks were recognized and sampled in the quarry, namely off-white limestone that is 15 m thick (KEF1) and predominates in the stratigraphic column, yellowish limestone of 3 m thick (KEF2) and brownish limestone of 2 m thick (KEF3). The brownish and yellowish lensoid limestone beds are irregularly distributed within the off-white limestone. In the Prassas Hill, Karteros basin, Heraklion, Crete Island a main phosphatic sandy bed of 2 m thickness was recognized, hosted in between diatomite beds. It contained irregularly disseminated phosphatic polished cobbles weighing 50–500 g that were ‘‘bulk’’ sampled to reach a weight of 15 kg (HRA1). The sandy host rock of the phosphate cobbles is represented by the sample HRA2 that also weighs 15 kg. In the lacustrine basins of Vegora and Sarantaporo–Elassona, greenish nodular and botryoidal phosphate assemblages of 1–2 cm in diameter were separated and sampled from their off-white to brown clayey diatomite host rock by hand picking, reaching to a weight of 15 kg. VEG1 and DRY1 to DRY3 are the phosphatic pelletal rock samples extracted from Vegora and Sarantaporo–Elassona diatomaceous beds, respectively. Sample KOM1 and sample VEG2 represent the Komnina and Vegora clayey diatomite beds, respectively, that host o0.5 cm sized phosphate pellets, each sample weighing 15 kg. From each ‘‘bulk’’ rock sample, 2–3 pieces were separated, cut, polished coated by gold to perform scanning electron microscopy and microprobe analysis (SEM-EDS) analysis. The rest of the raw samples were dried, ground and prepared for chemical and mineralogical analysis using the quadrilateral splitting method. The mineralogical composition of the samples (except that of Giannota, see Stamatakis and Koukouzas, 2001) were examined by X-ray diffraction analysis and light microscopy (Geology Department, Athens University, Greece) (Table 1). Major and trace element analyses of the bulk samples (except Giannota) were performed by ICP, XRF, fusion electrochemistry and NAA techniques (ALS Chemex Laboratories, Ontario, Canada) (Table 2a and b). The ultratrace level

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Table 1. XRD mineralogical analysis of the Neogene phosphates and their host rocks in Greece Samples

Minerals Anap Mitrid Ap

Marine HRA1 HRA2 host KEF1 KEF2 KEF3 Lacustrine VEG1 VEG2 ph/host KOM1ph/host DRY1 DRY2 DRY3

MJ TR MD TR MD

MMJ MD MD MD MMJ MMJ MMJ

MJ

CC

Fe-D Ank Qtz

TR MJ MJ MMJ MMJ MMJ

TR TR

Fl

Sm

Verm Ill

MD MD MJ MD

TR MJ MJ MD MD MJ MD MD MD MD MD TR TR TR

Chl

TR TR

TR TR TR TR

Explanatory notes: Anap=anapaite, Mitrid=mitridatite, Ap=apatite group minerals, CC=calcite, FeD=ferroan dolomite, Ank=ankerite, Qtz=quartz, Lf=feldspars, Sm=smectite, Verm=vermiculite, Ill=illite, Chl=chlorite, MMJ=almost exclusive componetcomponent, Mj=major component, MD=medium component, TR=minor to trace component, ph/host=phosphate micro-pellets/clayey diatomite mixture, host=host rock, KEF=Kefalonia samples, HRA=Heraklion samlpes, VEG=Vegora samlpes, KOM=Komnina samlpes, DRY=Drymos samlpes.

method of ICP-MS combined with ICP-AES was used for trace element analysis. Chlorine was measured by NAA and Fluorine by fusion electrochemistry. Major element analysis was performed by XRF. As in the phosphate rocks studied, iron occurs in bivalence and trivalent form, sometimes in the same sample (i.e. iron of anapaite and of mitridatite, respectively); all iron was expressed as Fe2O3t (see Tables 2 and 3). SEM-EDS on polished and thin sections were also performed to identify the texture and mineralogy of the phosphatic rocks (Geology Department, Athens University, Greece) (Table 3 and Figs. 2–5).

4. Mineralogy and texture of the phosphates and the host rock 4.1. Marine basins 4.1.1. Palliki Peninsula, W. Kefalonia Island In Kefalonia Island, irregularly concentrated phosphatized fish teeth, vertebrate bones, shells and other large biogenic constituents were identified by visual

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Table 2. Chemical analyses of Neogene phosphate rocks of Greece (a) Major elements Marine

11.74 1.96 3.60 0.12 1.74 39.72 0.03 0.70 1.15 21.97 1.20 15.10 99.14 0.44 2.35

23.05 4.04 3.92 0.26 4.41 29.07 0.04 0.89 1.31 0.51 0.28 31.20 98.98 1.54 0.11

KEF1

KEF2

KEF3

DRY1

DRY2

DRY3

KOM1

VEG1

VEG2

1.58 0.45 0.51 0.01 0.48 53.65 BDL 0.14 0.27 3.93 0.33 37.47 98.82 BDL 0.46

0.97 0.31 0.24 0.01 0.40 55.55 BDL 0.06 0.19 2.00 0.23 38.41 98.37 BDL 0.26

2.17 0.54 1.57 0.02 0.58 53.07 0.01 0.23 0.28 4.49 0.43 34.89 98.28 BDL 0.49

12.28 3.72 20.24 0.18 0.62 20.69 0.83 0.66 0.36 28.58 0.05 11.52 99.65 BDL 0.60

7.94 2.21 16.98 0.11 0.49 22.94 0.64 0.34 0.21 33.00 0.10 15.91 100.87 BDL 0.03

9.54 2.92 16.90 0.15 0.53 21.68 0.55 0.45 0.23 30.53 0.08 15.77 99.33 BDL 0.03

52.79 11.56 9.25 0.71 5.16 3.20 0.08 1.69 0.72 2.75 0.05 10.20 98.30 0.14 0.05

11.42 3.00 20.49 0.16 0.81 18.57 0.21 0.36 0.36 27.82 1.00 15.17 99.37 BDL 0.06

47.42 12.06 14.72 0.71 2.14 3.91 0.34 1.44 1.45 2.88 0.10 10.88 98.05 BDL 0.06

6.8 783 0.8 22.4 11.2

1.0 344 0.2 11.5 4.2

1.6 557 0.4 17.9 3.5

15.4 1438 0.9 18.9 8.0

10.6 603 0.9 41.1 16.6

(b) Trace elements Marine As Ba Cd Ce Co

35.6 2060 0.4 19.7 11.6

Lacustrine 18.0 110 0.8 18.9 12.9

5.0 10 1.0 12.4 11.7

BDL 7 0.7 6.8 4.40

15.0 10 0.7 16.7 8.1

7.6 365 0.6 52.9 35.0

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SiO2 Al2O3 Fe2O3t TiO2 MgO CaO MnO K2O Na2O P2O5 SO3 LOI Total Cl F

HRA2

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HRA1

Lacustrine

86 2.2 15.6 5.3 0.1 0.6 0.015 12 19.0 13.7 4.3 84 4 32.2 1.3 1 0.2 626 1.2 0.1 3.0 0.4 6.2 72 6.6 11.3 30 21 0.7 10

15 0.5 7.4 1.1 0.1 BDL BDL 19 1.8 0.6 0.5 20 4 4.4 0.3 1 BDL 768 0.3 0.1 1.2 0.1 16.2 19 12.3 52.1 30 1 0.1 10

6 0.2 5.0 0.6 BDL BDL BDL 12 1.4 0.5 0.8 15 2 1.6 0.3 1 BDL 693 0.4 0.1 0.4 BDL 12.1 8 15.1 31.2 12 1 0.1 10

25 0.5 14.8 2.1 0.1 BDL BDL 35 2.6 24.7 0.5 46 3 6.5 2.1 2 BDL 682 0.2 0.1 1.0 0.1 27.4 33 19.2 81.1 72 2 0.1 BDL

17 1.2 22.4 6.4 0.2 0.2 0.010 12 10.4 8.6 1.5 74 5 29.3 0.3 1 0.4 990 0.6 0.2 4.0 0.3 86.2 40 8.2 11.1 76 8 0.4 10

12 0.7 11.2 4.3 0.1 0.1 BDL 6 5.2 0.6 2.5 19 3 15.9 0.1 1 0.2 224 1.0 0.1 2.4 0.1 3.0 18 3.6 5.3 14 3 0.3 10

16 0.9 13.0 5.3 0.2 0.1 0.005 9 6.4 0.6 2.4 25 4 21.1 0.2 1 0.2 269 0.7 0.1 3.0 0.2 8.9 21 2.3 9.5 24 4 0.3 10

593 4.6 57.2 14.0 0.2 0.7 0.045 28 36.4 0.7 3.2 429 12 74.2 0.3 1 0.6 140 0.7 0.2 10.2 0.5 5.5 114 4.8 22.5 98 29 0.3 40

177 0.8 16.8 4.0 0.2 0.1 BDL 11 5.8 0.5 1.0 48 6 14.2 0.8 1 0.2 414 0.4 0.1 2.2 0.2 4.8 31 4.6 13.8 24 6 0.2 40

177 3.6 53.4 15.9 0.2 0.6 0.040 20 29.2 0.6 9.2 157 14 45.9 0.8 1 1.4 229 0.9 0.2 9.2 0.7 4.1 118 4.8 21.3 96 22 0.5 40

All figures are in ppm except Hg (ppb).

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98 1.7 14.4 3.8 0.1 0.1 BDL 18 9.8 12.4 2.4 66 7 23.7 1.3 5 BDL 1540 1.5 0.3 1.6 0.3 139.5 114 13.6 27.1 20 5 0.5 30

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Cr Cs Cu Ga Ge Hf In La Li Mo Nb Ni Pb Rb Sb Se Sn Sr Ta Te Th Tl U V W Y Zn Zr Ag Hg

339

340

(a) Lacustrine VEG1-2

VEG1-3

VEG1-4

VEG1-5 KOM1-1 KOM1-2 DRY1-1

DRY1-2 DRY1-4 DRY2-1 DRY2-2

ND ND ND 1.47 40.10 ND ND 7.49 ND ND ND Ba-MnO

ND ND ND 1.08 53.53 ND ND 3.76 ND ND ND Ba-MnO

31.16 35.53 14.93 2.87 ND ND ND ND ND ND ND MTR

34.88 21.31 29.45 ND 0.32 ND ND ND ND ND ND ANP

38.12 18.00 25.64 0.63 0.24 0.20 ND ND ND ND ND ANP

HRA1-2 29.97 ND 50.40 ND

35.50 20.59 27.70 0.82 0.28 0.22 0.12 ND ND ND ND ANP

38.12 17.64 26.78 0.72 ND ND ND ND ND ND ND ANP

ND 1.40 2.30 1.40 59.70 1.04 ND ND 0.53 0.40 ND TOD

ND ND 3.26 0.81 65.08 0.82 ND ND 0.58 ND ND TOD

34.87 39.22 16.08 1.24 ND ND ND ND ND ND ND MTR

38.32 31.34 18.98 ND ND ND ND ND ND ND ND ANP

39.36 0.92 50.17 ND ND ND ND ND ND ND 5.69 APA

HRA1-3 HRA1-4 KEF1-1

KEF1-2

KEF1-3

KEF1-4

KEF2-1

KEF2-2

KEF3-1

KEF3-2

33.14 ND 53.38 ND

28.05 ND 47.49 ND

28.81 ND 47.31 0.50

29.86 ND 38.45 ND

26.29 0.44 45.73 0.60

27.96 ND 47.86 ND

30.57 ND 49.55 ND

31.24 ND 50.60 ND

(b) Marine HRA1-1 28.69 P2O5 Fe2O3t 0.72 CaO 45.05 MgO 0.91

ND ND ND ND

26.91 1.91 45.09 ND

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P2O5 Fe2O3t CaO MgO MnO K2O Na2O BaO SiO2 TiO2 F

VEG1-1

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Table 3. SEM-EDS microprobe analysis of the lacustrine/marine phosphate rocks of Neogene age in Greece

1.05 1.39 ND ND 2.89 ND 0.19 ND ND ND APA

0.96 1.87 ND ND 6.44 ND 0.44 ND ND ND APA

1.37 1.85 ND ND 2.30 ND 0.22 ND ND ND APA

ND 31.61 67.59 0.67 ND ND ND ND ND ND BAR

1.14 1.88 ND ND 3.61 1.36 ND ND ND ND APA

ND 2.18 ND ND 0.34 0.49 ND ND ND ND APA

ND 1.74 ND ND 0.33 0.29 ND ND ND ND APA

0.89 2.15 ND ND ND ND ND 3.47 6.10 1.86 RE-APA

ND 2.26 ND ND 1.37 1.16 ND ND ND ND APA

1.41 2.56 ND ND ND ND ND ND ND ND APA

1.55 2.18 ND ND 0.30 0.41 ND ND ND ND APA

1.50 2.84 ND ND 0.32 0.37 ND ND ND ND APA

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Explanatory Notes: Ba–MnO=Ba-rich manganese oxides, MTR=mitridatite, ANP=anapaite, TOD=todorokite, APA= apatite group mineral, ND=not detected, Fe2O3t=total iron, RE-APA=Rare earths-rich apatite mineral, BAR=barite. Location of the samples as in Table 1.

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Na2O SO3 BaO SrO SiO2 Al2O3 Cl La2O3 CeO2 Nd2O3

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Fig. 5. SEM images of the phosphate rocks studied. (a) Foraminiferal test filled by apatite (white), sample KEF2 (site 1). (b) Apatite replacing the groundmass of the limestone and filling foraminiferal tests, sample KEF3 (site 1). (c) Apatite has totally replaced the calcitic groundmass of a cobble sample HRA1 (site 2). (d) Foraminiferal tests (left) and voids replaced and filled, respectively, by Sr-bearing authigenic barite sample HRA1 (site 2) (see also Table 3). (e) White todorokite-forming wormy concretions and acicular assemblages in pore fillings, sample DRY1 (site 6). (f) Rhombohedral mitridatite crystals partially rimmed by clayey material, sample DRY1 (site 6).

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inspection. On a microscopic scale, the disseminated limestone phosphatic material was identified using chemical, mineralogical and microprobe analyses. 4.1.1.1. Phosphates. XRD and light microscopy mineralogical analysis of the Palliki Peninsula phosphatic rocks revealed that their main mineralogical constituent is calcite, whereas phosphate minerals are minor to trace components presented by apatite (Table 1). Apatite is mostly developed as replacements of relics of micro- or macrobiogenic material. It has filled tests of foraminifers, and has replaced skeletons of echinoderms and other biological components (Figs. 2c and d). Under the SEM, it was detected that in the phosphate-poorer limestone samples apatite minerals had only filled tests of foraminifers. Secondarily, phosphates were detected as a progressive replacement of calcareous microfossil tests (Fig. 5a). In the richest phosphate samples apatite, besides its development as foraminiferal test fillings, has additionally formed phosphatic microconcretions replacing the calcareous groundmass of the limestone (Fig. 5b). Besides calcite and apatite minerals, trace amounts of quartz, ankerite, siderite and glauconite were also determined by SEM-EDS analysis. 4.1.1.2. Host rock. Borehole analysis and field data show that the phosphatic limestone beds are irregularly distributed in the greyish white sandy to massive limestone succession of the Skineas–Kontogennada areas in Palliki Peninsula (Skounakis, 1978). Light microscopy studies reveal that the limestone is a packstone containing abundant foraminiferal tests, in which most of them retain their minute morphologies. The limestone that hosts the phosphates is thick to non-bedded and sandy to massive in texture. The phosphate-free limestone beds are mainly composed of calcite and minor quartz. 4.1.2. Prassas Hill, Karteros basin, Heraklion, Crete Island 4.1.2.1. Phosphates. As determined by XRD analysis of the phosphatic cobbles and spherical nodules hosted in a sandy matrix in the Karteros basin, apatite group minerals, mainly carbonate-fluor-apatite, is their main constituent. Other minerals identified in minor amounts are quartz, feldspars, ankerite, and ferroan dolomite (Table 1). In general, the small spherical phosphate nodules are poor in clastic materials, composed almost exclusively of apatite minerals and traces of quartz, whereas the irregular phosphate cobbles contain non-assimilated patches of limestone that exhibit a gradual phosphatization from their surface towards the calcitic core of the fragment. In some almost un-phosphatized irregular limestone fragments, thin greenish to black glauconite is developed as a thin coating. Almost all the phosphatic masses show microburrowing textures because of the intense biological activity on the sea-bottom, mainly from bivalves. The examination of thin sections of phosphate rock reveals that apatite minerals have replaced calcareous matrix and/or have filled calcareous microfossils (Fig. 3c). Sometimes phosphatized peloids occur in burrowed phosphate cobbles (Fig. 3d). Apatite has replaced faecal pellets, corals, tests of foraminifers and echinoderms. It has also filled the interior part of the foraminiferal tests. By contrast, the thick calcareous shells of

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gastropods and bivalves, and the siliceous tests of the radiolarians retained their original nature. The examination of polished samples under the SEM reveals the progressive substitution of calcite by apatite (Fig. 5c), as well as the development of Sr-rich authigenic barite that either has replaced tests of foraminifers and other biological constituents, or formed interlocked euhedral tabular microcrystals (HRA 1–4, Table 3 and Fig. 5d). In addition, traces of gypsum and halite were also found (HRA1-1 to HRA1-3, Table 3). The phosphate minerals have almost totally replaced calcitic relics whereas the ankerite-ferroan dolomite groundmass was unaffected. 4.1.2.2. Host rock. The sandy marlstone beds that host the phosphates are mainly composed of quartz, calcite, Ferroan dolomite, ankerite, clay minerals and feldspars (Table 1). Locally, disseminated fine particles of phosphatic sand constitute a minor component of the sandy marlstone. 4.2. Lacustrine basins (Florina–Ptolemais and Sarantaporo–Elassona) 4.2.1. Phosphates In the clayey diatomaceous rocks of these basins, anapaite, mitridatite and minor Ca phosphates of the apatite group were identified (Table 1). In the studied areas, the Ca/Fe-phosphate minerals anapaite and mitridatite occur as small spherical nodules, botryoidal acicular assemblages, tabular masses, lenticular layers, lenses oriented subparallel to the bedding planes and veins crosscutting diatomite beds (Figs. 4a and b). The lacustrine phosphates exhibit the following features. 4.2.1.1. Anapaite. Under the microscope, euhedral neoformed tabular and acicular crystals of anapaite were observed as yellowish-green, massive or radiated aggregates in all types of phosphate rock occurrences (Figs. 4c and d). In thin sections and under the SEM, the anapaite-rich rock is shown to be poor in detrital minerals (Table 3). Isolated quartz, mica, clay and feldspar grains were rarely detected in the anapaite groundmass. In addition, small amounts of todorokite have been detected forming irregular and/or radial fibrous crystal assemblages (Fig. 5e). The microprobe analysis of the anapaite groundmass reveals that its composition is consistent with that of the ideal anapaite (Table 3). 4.2.1.2. Mitridatite. Even though the majority of the Ca–Fe phosphate materials in the lacustrine basins studied are represented by anapaite, XRD analysis reveals that mitridatite is also present, especially in the tabular phosphate masses (Table 1). In polished sections, mitridatite occurs as fine-grained anhedral or rhombohedral crystal assemblages (Fig. 5f). In general, the mitridatite-rich samples are softer than anapaite, and earthy, having a greenish colour. In the mitridatite-rich samples analysed by microprobe, large amounts of very fine-grained detrital minerals are disseminated in the mitridatite groundmass. As a result, the mitridatite groundmass contained less phosphorus than the ideal mitridatite (Table 3).

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4.2.1.3. Ca phosphates. In addition to the Ca–Fe phosphates, minor amounts of apatite group minerals were also found in some samples by using XRD and microprobe analytical techniques (Tables 1 and 3). They mainly formed rims around anapaite nodules or veins and their origin is assumed to be late diagenetic, at the expense of Ca/Fe phosphates. Besides phosphates, brownish-reddish goethite and amorphous Fe-oxide and hydroxide were determined as partial or total alteration products of the iron-rich phosphates and other Fe-rich minerals. 4.2.1.4. Todorokite. Todorokite with the formula (Mn2+,Ca,Mg)Mn4+ 3 O 7
H 2O has been identified by SEM-EDS analysis in the Drymos phosphates forming characteristic kidney-like, wormy, irregular, and/or radial fibrous authigenic crystal assemblages in the anapaite groundmass that forms tabular phosphate masses (Fig. 5e and Table 3a). Todorokite associated with Ca/Fe phosphates has been detected in other basins as well (Medrano and Piper, 1997). 4.2.1.5. Ba-bearing Mn oxides. Manganese oxides rich in Ba, Mg and K were identified in the Vegora phosphates as detrital elongated or rounded grains less than 100 mm in size (Table 3). They probably represent clastic Ba-rich todorokite, psilomelane and/or hollandite that are commonly rich in the elements above, among others (Pirajno and Adamides, 2001). 4.2.2. Host rock In the lacustrine deposits studied, the phosphates are hosted in a homogenous, soft, lightweight brownish grey rock that is characterized as clayey diatomite. Its principal mineral components are opal-A (mainly diatom frustules), vermiculite, feldspar, quartz and clay minerals (Table 1).

5. Geochemistry of the phosphates and the host rock Representative samples of the phosphates and their host rock were chemically analysed to identify their major and trace element composition. The analytical results are shown in Table 2. It is well known that the phosphate deposits exhibit variable enrichments in certain major and trace elements worldwide (Pacey, 1985; Nathan et al., 1997; Plank and Langmuir, 1998). In general, in the samples studied, the highest P2O5 amounts were detected in the phosphates of lacustrine origin, combined with high Fe2O3 amounts, due to the predominance of Ca/Fe phosphates in their bulk mineralogy (Tables 1 and 2). Barium enrichment was observed in Heraklion marine phosphate and in the Vegora lacustrine phosphate. In both samples, relatively high SO3 amounts were detected. SEM analysis has shown that authigenic barite is present in the Heraklion phosphate sample, whereas barium is mainly hosted in Mn oxides in the Vegora phosphate sample (Fig. 5). Besides barium, arsenic and uranium exhibit their highest

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content in the Heraklion phosphate rock, whereas high amounts of nickel, chromium and cobalt detected in the clay-rich phosphatic rock originated from the Komnina lacustrine basin. Rare earths content was low. However, rare earths (Ce, La, Nd)rich apatite was detected by SEM-EDS analysis as bright inclusions o0.5 mm in size within the apatite groundmass of Kefalonia rocks (KEF1-4, Table 3b).

6. Discussion—source and mobility of phosphorus 6.1. General Despite the predominant theory that phosphorus was supplied onto slope and shelf environments by upwelling currents (Kasakov, 1937; Baturin, 1982), there exist several examples worldwide showing that this hypothesis is not applicable for some marine phosphate deposits. Bushinskii (1966) suggested that the main source of phosphorus in marine basins was the land, and that rivers fed the basins with phosphorus. This hypothesis is supported by (a) the presence of sandy horizons that host the phosphates, (b) the development of phosphate deposits close to Cratonic masses, and (c) the presence of phosphates in places where up-welling currents are absent. During major transgressions, a wealth of iron, phosphorus, silica and organic matter, of both terrestrial and marine origin, was provided by rapid flooding of coastal plains, erosion of tropical soils developed during previous lowstand stages, and plankton blooms in nutrient-rich waters (Garzanti, 1991). Bromley (1967) hypothesized that marine phosphorites were restricted mainly to the continental shelf sediments in areas of warm water, low rate of detrital sedimentation and slightly reducing conditions. Bentor (1980) suggested that phosphorite formation seems to be connected with oceanic upwelling, but estuarine areas were also possible sites. The formation of concentrated deposits of pelletal phosphate in the chalk of NW Europe is probably related to the development of an upwelling regime over the epeiric Cretaceous Sea. The phosphatized bioclasts are mainly the faecal pellets of detrital feeding organisms such as fishes, worms and crustaceans (Pacey, 1985). Recent measurements indicate that the contribution of upwelling currents to the formation of phosphates disseminated in sedimentary deposits is less than 10%, the remainder being recycled from dead organisms (Cox, 1997). In marine basins where oceanic upwelling and productivity are limited, phosphates may develop outside microbial cells and also within bacterial cellular structures, formed by slow bacterial assimilation of phosphorus from assaying organic matter in areas of restricted sedimentation (O’Brien et al., 1981; Krajewski et al., 1994; Soudry, 2000). Fossil bacteria have been detected in some Japanese and Indian phosphorite deposits and their role in absorbing phosphorus from seawater and precipitation of apatite is important (Rao and Lamboy, 1996; Ogihara, 1999). The high productivity of a lake results in the increased accumulation of organic matter in the lake sediments (Swirydczuk et al., 1981). It is therefore suggested that

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productivity in lakes and the formation and sequestering of autochthonous, lacustrine organic matter may be important factors for phosphogenesis. In addition, lacustrine sedimentary basins that have received large inputs of detrital terrigenous organic matter, and also have low availability of calcium, neutral to slightly acidic pH, and mildly oxidizing to reducing conditions, phosphate deposits may be formed (De las Xeras et al., 1989). Baturin (1982) suggested that phosphorus comes from the decay of phytoplankton such as diatoms. In general, the origin of phosphorus in marine and lacustrine sedimentary basins can be attributed tothe presence of thick layers of sediments of biogenic origin such as diatomite, to relics of other organic matter, such as pieces of woods, leaves, faecal pellets, fish teeth and bones, and also to the presence of bacteria (O’Brien et al., 1981; Porter and Robbins, 1981; Sheldon, 1987; Ogihara, 1999; Stamatakis and Koukouzas, 2001). Recent phosphorite nodules discovered off S. Africa, Peru and Chile were precipitated in organic-rich diatomaceous ooze (Burnett, 1977; Birch, 1979). Generally, in marine and lacustrine diatomites, Ca phosphates are the major phosphate phases, whereas Fe- and Ca/Fe phosphates only rarely occur (Medrano and Piper, 1997; Tiercelin, 1986). Several Ca-, Fe- and/or Ca-Fe-phosphate deposits are related to the presence and diagenesis of diatomaceous rocks of marine or lacustrine origin (Nriagu and Dell, 1974; Sheldon, 1987; Piper et al., 1995; Stamatakis and Koukouzas, 2001). 6.2. The Greek deposits In Greece, all the occurrences of phosphate rocks studied are located close to the basement rocks that are either limestone or metamorphic rocks. No phosphate material has been reported towards the deepest part of the basins, either lacustrine or marine. 6.2.1. The marine basins The marine basins studied differ in their age, the depositional environment, the host rock mineralogy and the texture, and probably the origin of the phosphates present. 6.2.1.1. The Kefalonia deposit. The Kefalonia phosphatic limestone hosts wellpreserved vertebrate skeletons and shark teeth, and irregular accumulations of micro- and rarely macrofragments of biogenic origin. Most of the bioclasts have been phosphatized, whereas the thick shells of bivalves remained calcareous. Even though the frequently occurring long and thick burrows remain calcareous, they are outlined by a thin film composed of ankerite and phosphates. Organisms with opaline skeletons that may supply phosphorus in the basin such as diatoms, radiolarians and silicoflagellates, or their diagenetic derivatives (opal-CT or chalcedony) were not detected in the phosphatic limestone. Phosphatized microbial remains as those described by Rao and Lamboy (1996) were also not detected.

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Besides the phosphatized bones and teeth, phosphogenesis in Kefalonia is represented by a weakly disseminated apatite in a calcitic groundmass. It is therefore suggested that upwelling currents did not play a significant role in phosphogenesis, but did in the recycling of phosphorus from dead organisms such as fishes and other marine vertebrates. During recycling, in an early stage of phosphogenesis, phosphate ions were released from the decay of organisms after their death, and in late stages reprecipitation of phosphate as apatite took place, either as pore fillings, or as replacements of calcareous groundmass and thin shells. The foraminiferal tests were the main centres of phosphogenesis. Inside the tests, there was the appropriate space that resulted from the organic matter decay apatite to be deposited. The presence of broken phosphatized foraminiferal tests can be attributed to their compaction during diagenesis and/or to reworking and transport (Fig. 2c). 6.2.1.2. The Heraklion deposit. In the Heraklion Karteros basin, the successive alternation of diatomite–marlstone beds suggests a turbiditic deposition. The presence of siderite or ferroan dolomite in association with phosphates defines an environment of low S2 content, low redox potential (Eh) and shallow depth of deposition in the fermentation zone (Tucker and Wright, 1990). Both carbonate minerals are common in phosphate nodules (Horton et al., 1980; Greensmith, 1981). The phosphates of the basin occur as asymmetric, polished and burrowed cobbles and as almost spherical concretions. This type of phosphate deposition preserved in the form of polished-burrowed masses can be characterized as reworked phosphates, and is common in phosphate deposits, whereas the nodular type with no internal structure can be characterized as large faecal pellet (coprolites) replacements. The reworked phosphate masses were probably derived from more massive layers of phosphates that were weathered, transported and deposited as gravity-flow deposits. Phosphate layers frequently become reworked on the sea floor because of changes in . current density (Follmi et al., 1991). Most of the larger phosphate polished concretions are burrowed and contain remains of bones, teeth and other biogenic elements. On their upper surface, colonies of bivalves such as Ostrea sp. grew. As many bivalves prefer to make their colonies on the hard rock and not in the muddy sea bottom, this relationship suggests that the phosphate concretions were reworked and they were exposed on the sea bottom after their compaction. The presence of very small angular grains of phosphate in the sandy marlstone of less than 0.1 mm in diameter suggests that they had undergone reworking and transport from their original site of formation. The spatial relationship of the biogenic silica (diatomite rock) and the phosphatic masses is most likely genetic (Chenney et al., 1979; Sheldon, 1987). It is therefore suggested that the main source of phosphates in Heraklion basin was most likely the decayed organic matter of the diatoms. 6.2.2. The lacustrine basins The clayey diatomite beds and the siltstone in the lacustrine basins of Thessaly and W. Macedonia host biological debris at several levels of the Upper Miocene

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stratigraphic column. Most of the debris has been transformed to Fe, Ca/Fe and Ca phosphates. It is therefore plausible to assume enrichment to phosphorus due to a higher biologic productivity at certain periods of the siliceous sedimentation in the basin. The phosphate occurrences studied were always associated with clayey diatomite layers that contain abundant tree leaves, suggesting that an additional possible source of phosphorus originated from their decay. Iron is usually present in ferric form in lacustrine sediments. When the lake stratifies and the hypolimnion becomes anaerobic (the redox potential falls), the iron is reduced to the ferrous form. The reducing agent is most likely organic matter or iron-reducing bacteria (Brown et al., 1999; Fredrickson et al., 1998; Nriagu, 1972). The PO34 was fixed by Fe2+ which had been derived by the reduction of primary Fe3+. Vivianite is a primary mineral precipitated from the pore fluids in lake sediments (Nriagu and Dell, 1974). Thermodynamic calculations indicate that vivianite precipitation is favourable in deeper anoxic sediment layers (Hupfer et al., 1998). It is usually formed in a strongly reducing environment below the mud–water interface where appropriate Eh values exist (Nriagu and Dell, 1974). The ideal pH values for the precipitation of vivianite from calcium- and iron-saturated pore fluids is 6.4–7.8, whereas anapaite is formed in a similar Eh regime but at more acidic pH values than vivianite (Nriagu and Dell, 1974). However, the stability fields of vivianite, mitridatite and anapaite show that the formation of Fe- or Fe/Ca phosphates depends on the iron and calcium content of the pore fluids. It is therefore assumed that a change in the chemistry of the pore fluids causes the precipitation of anapaite and/or mitridatite, followed by minor apatite at shallow stratigraphic levels. This change was probably enhanced by the action of Ca2+- and HCO3 -rich meteoric waters and/or groundwater of acidic character during uplift and groundwater penetration (Stamatakis and Koukouzas, 2001). In general, mitridatite is formed at the expense of anapaite and vivianite under oxidizing conditions (Nriagu and Dell, 1974). It is also known that anapaite and vivianite turn to iron oxide/hydroxides in surface conditions (Altschuler, 1973). It is therefore suggested that an initial conversion of the Fe2+ phosphate to Ca/Fe2+ phosphate and subsequently to the Fe3+/Ca phosphate and/or to FeOOH at the surface took place under strongly oxidizing conditions. The diverse porosity of original Fe phosphates due to different detrital mineral content probably influenced the development of either anapaite- or mitridatite-rich assemblages. Hence, it is suggested that in layers with low porosity (i.e. clay-rich diatomaceous layers) and reducing environment, the growth of anapaite [iron as Fe2+] was favoured, whereas in layers with higher porosity (i.e. clay-poor diatomaceous layers) and an anoxic environment, mitridatite [iron as Fe3+] was grown, due to the oxidation of the Fe2+ by circulating water in the microenvironment. In the studied areas, vivianite was probably derived from the decay of the diatom cells, faecal pellets and leaves and trunks. When reducing conditions existed immediately below the lake bottom, vivianite may have replaced the biologic matter. The presence of only vivianite in the borehole samples and the low availability of Ca2+ in the lake waters, as is shown by the absence of CaCO3 for most of the

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diatomite rocks studied lead us to assume that the formation of anapaite and/or mitridatite was diagenetic, at the expense of vivianite. Despite the age of the diatomite rocks that is Upper Miocene, biogenic silica diagenesis of the diatomaceous rock has not been important, as shown by the good preservation of the delicate frustules of the diatoms. The poor silica diagenesis suggests that (a) the diatomaceous succession was never deeply buried and (b) they were never exposed to saline depositional and post-depositional conditions. As climate indirectly affects the nature of the lake sediments in several ways, it is believed that a subtropical climate dominated during the deposition of diatomite and phosphates in the region, producing a warm–humid lacustrine environment (Velitzelos and Gregor, 1986, 1990). In conclusion, the phosphate minerals of the Upper Miocene lacustrine basins of Thessaly and western Macedonia may be characterized as diagenetically derived, formed by the decay of organic matter, at the lake bottom. All the marine and lacustrine phosphate occurrences studied are located at marginal sites of the basins. It is therefore plausible to assume a contribution of landderived phosphatic materials to the accumulation of phosphorus in the bottom of the basins. However, the main source of phosphorus was most likely the decayed organisms that lived within the basins, such as decomposed, fish bodies, teeth and skeletons, coprolites, leaves and trunks, and the decay of the organic matter of diatoms and foraminifers.

7. Geochemical analyses and their implications Evaluating the mineralogical and the trace and major element analyses of the bulk samples (phosphates and the host rocks), the following implications were obtained. 7.1. Host rocks The host rock of lacustrine phosphorites is clayey diatomite in all deposits studied. This peculiar rock type that is free of carbonate minerals contained almost equal amounts of minerals of biogenic origin such as diatoms (opal-A), and detrital minerals such as feldspars, quartz, vermiculite and clays (Table 1). As a result of this mineralogy, the host rock has high concentrations of SiO2, Al2O3, Fe2O2, TiO2, K2O and Na2O, and low amounts of CaO (Table 2, see also Stamatakis and Koukouzas, 2001). The higher amounts of K2O and Na2O are attributed to the presence of feldspars, whereas the higher amounts of SiO2 are due to the presence of opal-A, and secondarily to quartz and aluminosilicate minerals. In addition, the trace elements Ga, Hf, Cs, Cu, Li, V, Ni, Nb, Pb, Sn, Tl, Cr, Rb, Th, Zn, Zr and Hg show their highest concentration in these clayey diatomite host rocks (Table 2). It is known that Ga has chemical affinity with Al (Cox, 1997), so its enrichment is attributed to the presence of the aluminosilicate minerals. Li concentration could be attributed to the presence of mica minerals, whereas Ni, Co, Cr, V enrichments can be attributed

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to the presence of minerals related to mafic–ultramafic rocks, i.e. vermiculite and chlorite. High amounts of Zr are related to Hf and are in general fixed in silicate minerals (Cox, 1997). Thorium shows its higher amounts in the diatomite host rock and it is probably related to the presence of traces of heavy minerals of detrital origin. Rubidium is probably related to the potassium feldspars contained in the clayey diatomite. Caesium that sometimes accompanies phosphate minerals is low in the phosphates studied and its highest amounts were related to the host rocks. The sandy marlstone host rock of Heraklion, as well as the phosphate nodules, contains relatively large amounts of sodium and chlorine, due to the presence of fine-grained halite crystals in the rocks. The presence of brackish groundwater and the vicinity of the outcrops studied to the north seashore are the reasons for the development of the disseminated halite crystals. 7.2. Phosphates The P2O5 content of the samples analysed correspond to a range of rock qualities from almost pure phosphates to almost barren host rock (Tables 1 and 2). In general, the rocks poorest in phosphate minerals content (apatite) are those of Palliki, Kefalonia Island (Table 2). Based on the apatite theoretical formula, it was calculated that the sample richest in P2O5 content (sample KEF3) contained less than 20% apatite (Table 2). LOI values are mainly attributed to the presence of hydrated minerals such as mitridatite, anapaite and todorokite, apatite minerals, carbonates such as calcite, ferroan dolomite and ankerite, clay minerals and the presence of halogens such as F and Cl in some phosphate samples. The phosphate rocks studied, especially those from Heraklion basin, contain relatively high amounts of certain trace elements such as uranium, arsenic and barium. This could be attributed to several factors that have diverse effects on particular deposits. Enrichment in elements such as Ba, V, Pb, Co, Y was reported in a protectedisolated supratidal lagoon type of environment for the precipitation and diagenetic alteration of the Cumbrum phosphorite (Banerjee and Saigal, 1988). Huba et al. (1983) suggested that phosphorus and Fe hydroxides seem to be the main sinks for Pb, Cd, and Zn, whereas the organic matter plays a less important role. In the phosphatic rocks studied, the range of the Sr content is normal for the phosphatic groundmass. Barium and sulphur show high values in the phosphates of Heraklion and Vegora, suggesting the presence of barium-rich mineral inclusions in the phosphatic groundmass. SEM analysis on polished samples from Heraklion revealed the presence of authigenic Sr-bearing BaSO4 (Table 3b). As barite is related to biogenic components, it is suggested that barium in the form of barite and/or witherite was absorbed by planktonic organisms from seawater and sunk down on sea bottoms. Ba follows a dissolution–migration–redeposition path, forming a biochemical sedimentary deposit along with phosphates (Gao-Huaizhong, 1998). In the Vegora phosphates, barium is not found in the form of BaSO4, but it is fixed in the lattice of Ba-rich Mn oxides such as psilomelane, todorokite or hollandite that

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are found in the form of very fine-grained particles scattered in the anapaite groundmass, along with disseminated grains of chromite, zircon, pyrite, feldspar and an unidentified Fe, Si, phosphate mineral (Table 3a). As revealed by SEM microanalysis, the increased SO3 and Na2O values measured in the apatite in Kefalonia and partially in Crete are indicative of a partial substitution of PO4 by SO3 and of CaO by Na2O in the apatite lattice (Table 3b). Similar substitutions have been commonly reported in unaltered Tertiary phosphorites of apatite composition (McArthur, 1985). The differences in PO4 substitution by SO3 ions could be attributed to the diversity of the depositional and diagenetic environments. The Heraklion phosphate has the highest uranium content of all samples studied although all were U enriched. The lacustrine phosphates of Drymos–Elassona also have a high uranium content, suggesting that if U is present in solutions it can precipitate in reducing environments in both marine and lacustrine phosphate rocks. It is known that hexavalent uranium forms complexes with ions such as HPO24 , H2PO4 , CO23 and F that enhance its solubility and transport (Kimberley, 1978; Maynard, 1983). Even though uranium precipitates in a reducing environment, when vanadium is present it forms complexes that precipitate under oxidizing conditions (Maynard, 1983). Arsenic is enriched in both phosphates and host rock of Heraklion, following the uranium enrichments. VO34 and AsO34 substitute PO34 ions, whereas uranium and strontium substitute for calcium in phosphates (Cathcart and Gulbrandsen, 1973). The high U contents of the samples studied are not followed by high Th contents, the latter being enriched in the phosphate host rock samples of lacustrine origin. The relatively high amounts of Mo and W measured in the marine phosphate samples studied would have biogenic origin, as both the elements are related to the presence of anaerobic bacteria and enzymes and play a catalytic role in reduction– oxidation reactions (Cox, 1997). MgO is low in almost all phosphate samples. CaO is high in all phosphates and phosphate-bearing limestone samples. The highest CaO content was measured in the apatite-rich samples of marine origin (Tables 1 and 2). The iron content is higher in the Fe–Ca phosphates of lacustrine origin, where it occurs in the Fe2+ (anapaite) or the Fe3+ (mitridatite) form. However, the marine phosphates of Crete contain increased amounts of iron, due to the presence of ferroan dolomite and ankerite. The Mn content is low, especially in the phosphates of marine origin. Despite the absence of Fe/Mn-rich phosphate minerals that usually accompany Fe and Ca/Fe phosphates in lacustrine deposits (Medrano and Piper, 1997), high Mn values were measured in the Drymos–Elassona samples due to the presence of todorokite. The origin of todorokite was secondary as shown by its development on broken surfaces and vugs of phosphate masses, and it most likely formed during microbial reactions by manganese-oxidizing microorganisms (Spilde et al., 2001). Barium-rich Mn oxides were also reported in the Vegora and Komnina phosphates (Table 3b). Selenium is low in all samples studied and only the phosphates from Heraklion show higher content that is probably related to its higher SO3 content (Table 2).

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Cadmium has been reported in high amounts in some phosphate deposits. It is the most enriched trace element in the Negev phosphorites (Israel), even though it was not correlated with P2O5 or has a slightly negative correlation (Nathan et al., 1997). Thinly laminated, non-disturbed pelletal phosphorites have much higher Cd concentrations than the condensed stratigraphic sections that consist of a few amalgamated beds of reworked phosphorites (Nathan et al., 1997). Independent of the depositional environment, cadmium contents are very low in all Greek phosphate samples studied, compared to that of Israel and also the commercial phosphates that have considerably higher concentrations (Nathan et al., 1996, 1997) (Table 2). Cadmium is strongly correlated with Zn in precipitated sphalerite from upwelled seawater in laminated microbially generated phosphorites, whereas in the reworked type of phosphate deposits this correlation is absent (Nathan et al., 1997). As shown in Table 2, Zn has higher values in the lacustrine phosphates than in the marine ones. Cerium is an important palaeoceanic redox indicator, as the insoluble Ce4+ phase is precipitated by oxidation of the Ce3+ in shallow waters of the oxic, highly productive zones (Mazumdar et al., 1999). Even though Ce–Nd–La-rich apatite microinclusions were detected in the Kefalonia phosphatic groundmass, the total Ce content of the Kefalonia samples is low compared with the samples of lacustrine origin. The highest Ce values were measured in the clayey diatomite samples of lacustrine origin (Table 2). On the other hand, the presence of trace amounts of ankerite and siderite in the Kefalonia samples is indicative of an anoxic environment that is not favourable for high Ce concentrations (Mazumdar et al., 1999). The outer shelf-upper slope deposit of Kefalonia contains galena grains and also REE-rich apatite inclusions of less than 1 mm in size in a normal apatite groundmass, whereas barite was not detected. Fluorine is high in the phosphate samples that contain carbonatefluorapatite. Differences in the fluorine enrichment of some samples can be attributed to differences in fluorine content of the original organic matter, and also to the dissolution reprecipitation process, fostered by microbial infestation (Soudry and Nathan, 2000).

8. Commercial potential As indicated by geological, mineralogical and geochemical data, phosphogenesis was enhanced in several stages that included primary deposition, remobilization of phosphate ions, redeposition of secondary phosphates, and also reworking of phosphatic masses that produced polished-burrowed clastic phosphate deposits. In the lacustrine basins studied, the occurrences of phosphates are scarce and volumetrically small in size. This fact, along with the data available from borehole measurements of the Institute of Geology and Mining of Greece and the study of the lignite quarries in the broad Ptolemais–Florina lignite fields, led to the conclusion that the existence of a phosphate deposit of commercial grade is limited. An additional negative factor for the evaluation of the Greek phosphate deposits studied

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is their high iron content due to either Fe/Ca phosphates or to ankerite/ferroan dolomite. It is known that the chemical companies that produce fertilizers utilize phosphate raw material with an exceptionally low iron content. Phosphogenesis in the marine basins of Kefalonia and Crete islands occurred in sedimentary rocks developed over several square kilometres. The phosphate occurrences of Crete Island were only recently discovered near Heraklion town. For the evaluation of these phosphates, a research project supplemented by borehole measurements in the broad Karteros Upper Neogene basin and the neighbouring basins has to be implemented. The Kefalonia Island phosphates studied are more extensively developed in the field. The present measurements represent a small portion of the entire stratigraphic column that has been reported as hosting phosphates. A small research project performed in the past near the area where the present samples were collected revealed that there are only a few phosphate-rich horizons in a phosphate-bearing limestone succession (Skounakis, 1978). However, as commercial sedimentary phosphate deposits worldwide have been located in phosphate-bearing limestone as phosphate layers of few metres thickness (Abed and Fakhouri, 1996), large-scale geological fieldwork supported by borehole measurements in the entire Tortonian sedimentary succession of the Palliki Peninsula is suggested.

Acknowledgements This paper presents the results of a research project on the utilization of the Neogene phosphate deposits in Greece, funded by the Empeirikeio Foundation and the National and Kapodistrian University of Athens (contract 70/4/4505) in the period 2001–2002. The author expresses his thanks to Prof. James Saunders (Auburn University, Alabama, USA), Dr. Anthony Hall (Royal Holloway, University of London, UK) for their comments and revision of an early version of the manuscript . above. Thanks are also due to Prof. Karl Follmi, University of Neuchatel Swiss for his substantial suggestions and thorough revision of the manuscript above. The author also wishes to thank Prof. E. Georgiadou-Dikeoulia for her assistance in determining the fauna of the Kefalonia phosphate rocks, Mr. E. Michailidis, technician of the National University of Athens for his assistance in extracting SEM images and microprobe analysis and Dr. John Mitsis for his help in XRD mineralogical analysis.

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