Dinaride evaporite mélange : Diagenesis of the Kosovo polje evaporites

Permian to Triassic age and floored by the various Adriatic carbonate platform units. The tectonic history of the Dinaridic evaporite melange is related to long term burial and Palaeogene to Neogene exhumation processes. Evaporiterocks of the Kosovo polje deposits are associated with carbonates, clastic and minor albitized, subvolcanic, neutralto basic rocks. They are capped by a clayey Quaternary cover of variable thickness, and a several metres thick cavernouscarbonate breccia – rauhwacke. Evaporite rocks show two distinct facies: laminated evaporite-carbonate, composed of evaporite and dolomicrite (± organic matter, pyrite, halite) intercalations, and evaporite-carbonate breccia, composed of fragments of laminated evaporites, carbonate and siltite, cemented by massive gypsum ± sulfur, occurringat the shallower levels and related to the emplacement. The Quaternary cover is composed of 47-88% clayey material, comprising illite, kaolinite and most likely vermiculite, 9-49 % carbonate and 3-4% evaporite minerals. The thickness of the evaporite rehydration zone correlates negatively with the thickness of the overlying Quater nary cover and with the amount of clayey material within it. Rauhwacke are composed of partly dedolomitized dolostone and leachedgypsum fragments, cemented with late stage calcite. Diagenetic processes are related to early diagenesis and the burialphase began with the formation of diagenetic halite from an oversaturated subsurface or surface (lagoon) brine, and was followed by the biochemical reduction of evaporite sulphate to sulfide and formation of pyrite within organic-richcarbonate laminae. These processes triggered early dolomitization (in a shallow burial realm), and the formation of idiomorphic planar-e type of dolomite crystals at temperatures below 50-60°C. Planar dolomite suppressed diagenetichalite. With an increase in burial depth, gypsum dehydrates to anhydrite and when temperatures exceed ~50 °C, precipitation of fine grained non-planar –a type of dolomite began. Halite molds are partly replaced with non-planar-atype of dolomite. During regional uplift and exhumation, anhydrite rehydrated to gypsum under the influence of low-temperature undersaturated fluid (meteoric water), whereas hydrogen-sulfide oxi dized to elementary sulfur, observed as cement in the evaporite-carbonate breccia. Rauhwacke at the uppermost part of the deposit are formed by severe tectonic movements associated with the gypsum-driven dedolomitization process.


INTRODUCTION
During the collisional phase of the orogenic belts, evaporite layers, due to the low shear resistance of the associated minerals, usually control the location of major detachment zones acting as the lubricant for large-scale displacements (e.g.DA-VIS & ENGELDER, 1985).In the area of the Northern Calcareous Alps (SPÖTL & HASENHÜTTL, 1998;SCHROLL & NEUBAUER, 2013), and Dinarides (HERAK, 1973) evap-A Dinaridic evaporite mélange is located in front of the Palaeogene to Neogene large trusts fronts (Fig. 1).This mélange is mainly composed of gypsum and anhydrite associated with various types of sedimentary to magmatic rocks.The age of the evaporites is considered to be Permian to Triassic by most of the authors (HERAK, 1973;ŠUŠNJARA et al., 1992;ŠIFTAR, 1986).The tectonic history of the Dinaridic evaporite mélange is related to long term burial and Palaeogene to Neogene exhumation processes.These processes were followed by various types of diagenetic reactions and alteration, transformations and volume changes (WARREN, 2006 and references therein), including formation of secondary evaporites, dehydratation to rehydratation reactions (gypsum -anhydrite transition), dolomitization to dedolomitization, cementation and precipitation of authigenic minerals such as clays or quartz.All of the listed reactions are highly dependent on temperature, pressure, pore-water geochemistry, water-table level, mineralogical composition and thickness of the overlying deposits as well as the regional uplift rate.
In this paper we present a set of mineralogical, petrographical and chemical data from selected drill-cores of the Kosovo polje gypsum quarry, Knin, Croatia.The purpose of this study was to examine the types and intensities of the diagenetic changes related to the emplacement of the Koso vo polje evaporite mélange.

Dinaride evaporite mélange
The Adriatic Carbonate platform represents the most external sector of the Dinarides, a southwest-vergent Cenozoic orogen to the northeast of the Adriatic Sea (Fig. 1).It is composed of a thick carbonate succession (at some places >8000 m) of Middle Permian to Eocene age (VLAHOVIĆ et al., 2005;PAMIĆ et al., 1998 and references therein).Onset of deposition of siliciclastic, carbonate and evaporite deposits began with an epeiric type platform, detached from the Gondwana supercontinent (i.e. the South Tethyan Megaplatform; sensu VLAHOVIĆ et al., 2005), and was followed by the Late Permian to Early Triassic large-scale tectonic reorganization and intra-plate magmatism (PAMIĆ, 1984).The Uppermost Triassic early to late diagenetic Hauptdolomit or Dolomia Principale represents the latest sequence deposited on the Megaplatform, prior to its Early Jurassic disintegration.Carbonate deposition beginning in the Middle Jurassic, already belongs to the Adriatic carbonate platform sensu stricto.
Therefore, we can summarise the following: a Dinaric evaporite mélange is composed of Permian to Triassic evaporites and associated sedimentary and magmatic blocks (tens of metres to hundreds of metres in size), sometimes embedded within evaporites and thrusted onto the younger Adriatic carbonate platform units.
Most of the Dinaride evaporite mélange outcrops are located within karst poljes and valleys such as: Vrličko, Kninsko, Petrovo and Drniš karst polje within the Lika and Dalmatia areas and neighbouring regions in SW Bosnia and Herzegovina (Fig 1).These outcrops are large, economically significant and under exploitation (> 1 Mt;GABRIĆ et al., 2002).They are composed of gypsum in the upper part and anhydrite in the lower part, and are covered by Rauhwacke and Quaternary sediments (mostly clay) of variable thickness.The amount of anhydrite increases with depth, and generally, within the investigated deposits, gypsum occurs within the uppermost 20 -40 m (GABRIĆ et al., 2002).This is due to partial rehydration of anhydrite to gypsum under the influence of meteoric water related to regional uplift and erosion.

Geology of the Kosovo polje
Kosovo polje is a small size karst field (33 km 2 ) located in the central part of the Adriatic carbonate platform at an elevation of 300 -350 m above sea level.It is surrounded by up to 1000 m-high hills, composed of early Triassic to Late Cretaceous limestone and dolostone, intercalated with clastics at some locations.About 30% of the total Kosovo polje surface is represented by Late Permian to Early Triassic evaporite mélange units (Figs.2a, b).The evaporite mélange is overlain by Quaternary deposits and Rauhwacke of variable thickness.The Kosovo polje is bounded by thrust faults (HERAK, 1973) covered by overstep carbonate-clastic flysch sequences; the Promina deposits (TIŠLJAR, 1992;ŠUŠNJARA et al.1992).During the exploitation of the evaporite (at Mali Kukor, Veliki Kukor, Sijećko, Bulavato open pits) numerous subvertical to vertical faults were discovered (LUKŠIĆ et al., 2005 1 ), illustrating strong tectonic activity related to the emplacement of the evaporite complex (sensu HERAK, 1986).
representative drill cores, over 70 hand specimens were selected for mineralogical, petrographical and XRD analyses.All of the studied cores display the following successions: (i) Quaternary clays, (ii) rauhwacke (not always present) (iii) gypsum zone, (iv) gypsum-anhydrite transition zone and (v) anhydrite zone.Geological logs with sample locations are presented in Fig. 3.
The layey Quaternary cover is composed of millimetre to centimetre sized carbonate and evaporite fragments within a predominantly clayey matrix (Fig. 4a).Cover sediments display variable thickness, from 3.5 to 34 metres, which correlates with the depth of gypsum and the gypsum-anhydrite transition zone.Within drill-cores with a thick cover (20 -30 m), the gypsum zone is only several metres-thick and increases to >50 m with decreasing thickness of the cover deposits.At some locations, the immediate contact with the underlying evaporite is hidden by a <1 -6 m thick layer of cavernous and sometimes clayey breccia, e.g.rauhwacke.The breccias are composed of various-sized fragments of limestone, dolostone (sometimes dedolomitized) and partially leached gypsum, ce-mented with calcite and are occasionally cavernous (Figs.4b,c).Their origin is related to the leaching of the tectonicallydisrupted evaporites and most likely driven by percolating meteoric water (TIŠLJAR, 1992).Evaporites show basically two types of macro-facies: (i) laminated evaporite-carbonate facies, composed of millimetre to centimetre gypsum/anhydrite laminae intercalated with dolomite/dolomicrite ± organic matter ± siltite, the dolostone member is usually several times thinner than the sulfate one (Figs.

MATERIALS AND METHODS
Twelve representative drill were selected (from over 100) based on the following criteria: (i) depth of the gypsum, gypsum-anhydrite and anhydrite zones, (ii) associated rocks and macroscopic fabrics, (iii) composition and depth of the overlying deposits and (iv) geographical position of the drill core within the exploitation field.Thin sections of evaporite samples from the gypsum, gypsum-anhydrite transition and anhydrite zones, as well as from the overlying Rauhwacke deposits of the Kosovo polje quarry (n = number of samples; n = 45; Table 1) were examined under the optical microscope and analyzed by Qwin image analysis software (IM 50 v.1.20).Fluid inclusions were classified according to the criteria summarized by ROEDDER (1984) and SHEPHERD et al. (1985).In addition, carbonate-rich samples were stained according to EVAMY & SHERMAN, 1962 (n = 8; Table 1) to detect various carbonate minerals.Clay mineral analysis of the Quaternary cover was performed at the Croatian Geological Survey, Zagreb (n = 4; Table 1).Drill-cores were selected according the apparent thickness of clayey cover; three drill-cores having similar cover thickness, (10 -16 m being the average cover thickness within the deposit), and a single drill-core with a cover thickness of 34 m.A Philips vertical X-ray goniometer (type X'Pert) was used, equipped with Cu tube and graphite crystal monochromator with the following experimental conditions: 45 kV, 45 mA, primary beam divergence 1/4º, continuous scan (step 0.002º 2Q/s).Obtained spectra were analyzed using X‫׳‬Pert HighScore plus 2.1 PANalytical B.V. software.Selected Quaternary samples were analyzed according to the BOUD-INE & FERNALLD (1973) method.After carbonate and sulphate dissolution, orientated fraction <2mm was analyzed 4 times: dry, swelled with ethylene -glycol for 12 hours; heated for ½ an hour at 400°C and at 550°C.Clay minerals were determined using a clay mineral identification flow chart.Further analyses such as cation-capacity exchange or glycerin swelling is needed for the positive identification of vermiculite.

Chemical analysis
The amount of evaporite within all the investigated samples is uniform and low (3 -4 vol %), however, the amount of carbonate is highly variable, ranging from 9 to 49 vol %.Eventually, the amount of clayey material ranges from 47 to 88 vol % (Table 2).

XRD analysis
Results of the X-ray diffraction are presented at Fig. 6, and indicate the predominance of an illitic component, the presence of kaolinite and probably vermiculite (MOORE & REYNOLDS, 1997).

Rauhwacke
Rauhwacke are cavernous carbonate-gypsum breccia usually yellow-brownish in colour.They are composed of up to   several centimetre-sized carbonate and partially leached gypsum fragments.Vugs, cavities and veinlets are partly cemented by late stage calcite (according to Evamy and Sherman, 1962; Fig. 7).Purple decorated calcite after staining usually occur as anhedral, coarse grained infill (0.45×0.75 mm in size; Figs.7a, b).Dolostone fragments are partly dedolomitized.Dedolomite occurs as very-fine grained anhedral crystals, 0.05×0.2mm in size, usually along previous cracks and veinlets of the fragments (Fig. 7a).Evaporite fragments are completely rehydrated and sporadically cavernous.

Microfabrics
Microfabric descriptions of the evaporite facies are as follows: (i) Laminated evaporite-carbonate facies consist of millimetre -centimetre thick anhydrite to gypsum laminae (up to 80 vol% of total rock) with intercalations of thinly (d<0.2mm-several mm) often disintegrated laminae of fine-grained dolomicrite/ dolomite, organic matter and siltite (up to 20 vol% of total rock; Figs.8a, b).Anhydrite regularly appears as nematoblasic and foliated textures, and is often found suppressed by gypsum along the grain boundaries or vinelets (Fig. 8c).Apart from gypsum and anhydrite, these rocks contain dolomicrite laminae with subordinate amounts of organic matter, pyrite, quartz and rock salt.Rock-salt occurs as fine to medium-grained cement, enveloped by organic matter.Dolomicrite laminae are often found disintegrated and composed of dolomite   addition, carbonate fragments sometimes contain organic matter, clays, (? authigenic) quartz or rock salt textural types as chevron fabrics or individual molds overgrown by the coarse-grained gypsum or a-type to e-type dolomite (Fig. 9c).Fragments of laminated evaporite are carbonate-rich.Classification of dolomite/dolomicrite has been done following the criteria of SILBLEY & GREGG (1987).We have observed three dolomite types (Fig. 10): (i) fine-grained anhedral dolomite or non-planar-a type of dolomite, <5mm (Fig. 10a); (ii) fine-to medium-grained subhedral dolomite or planar-s type of dolomite, 5 -20mm (Fig. 10b); and (iii) medium-to coarse-grained euhedral dolomite or planar-e type of dolomite (Fig. 10c).Non-planar-a type and subhedral-s type of dolomite appears regularly throughout entire evaporite succession, whereas the planar-e type of dolomite prevails at the shallower levels of the evaporite succession, mostly related to the evaporite-carbonate breccia facies.Carbonate staining shows evidence of minor dedolomitization at the uppermost levels of the deposit (EVAMY & SHERMAN, 1962).Dedolomites are usually of radial appearance enveloped by randomly orientated fine-grained gypsum (Fig. 10d).

Fluid inclusion petrography
Coarse-grained granoblastic to porphyroblastic gypsum contains primary and pseudosecondary monophase, liquid fluid inclusions (according to the criteria of ROEDDER, 1980 andSHEPHERD et al., 1985), located within three-dimensional clusters and subparallel trails within a single crystal, respectively (Figs. 11a, b).Anhydrite and dolomite do not host visible fluid inclusions.

DISCUSSION
Figure 12 presents a model for the evolution of the Kosovo polje evaporites, based on the aforementioned data.Laminated facies composed of millimetre to centimetre thick intercalations of evaporite and dolostone (sometimes bearing organic matter, pyrite, quartz, halite) were interpreted by TIŠLJAR (1992) as having originated from Late Permian sabkha-type or lagoon-type of the environment.Due to burial, the thickness of the overlying deposits increased (i.e. increasing lithostatic pressure and temperature), triggering the onset of the following diagenetic processes: (i) precipitation of secondary diagenetic halite from supersaturated solutions within carbonate, organic rich laminas, (ii) dolomitization and (iii) dehydratation of gypsum.The latter two processes led to strong volume decreases of 12% and 39%, respectively, followed by additional micro-cracking and re-  crystallization, especially within the dolomicrite member.
Severe tectonic movements during the exhumation phase led to the formation of the evaporite-carbonate breccia at the upper parts of the evaporite complex.Exhumation processes were followed by widespread rehydratation of gypsum and partial dedolomitization, mainly in the uppermost levels of the deposit.Description and clarification of these diagenetic processes is given below.

Diagenetic halite
During the evaporation of seawater in the restricted pools or sabkhas, a normal succession would precipitate calcium carbonate, gypsum and halite at a brine concentration of about 1.8, 3.8 and 10.6 times greater than seawater, respectively (McCAFFREY et al., 1987).Therefore, primary halite laminae or deposits would have to occur superimposed on the gypsum -anhydrite laminae or layers.However, within the Kosovo polje deposit, halite is associated with organic-rich dolomicrite/dolomite laminae or fragments and is sometimes overgrown by gypsum and a-type to e-type dolomite.Based on their textural characteristics, both types are interpreted as secondary (WARREN, 2006), and originated from supersaturated subsurface or surface (lagoonal) brines that differ in chemistry and salinity from the primary brines that precipitated carbonate laminae.

Dolomitization
Diagenetic processes in the shallow burial realm are controlled strongly by early dolomitization which is either attributed to the seepage-reflux model (e.g.MATTES & CONWAY MORRIS, 1990) or triggered by reducing conditions induced from removal of SO 4 2-by sulphate reducing bacteria (e.g.GINGRAS et al., 2004).According to the seepage-reflux model, dolomitization occures prior to replacement of connate water within permeable carbonates at the lagoon floor with hypersaline, heavy lagoon brine (ADAMS & RHODES, 1960).On the other hand, bacteria induced reducing conditions accompanied by metal enrichment additionally faciliate dolomite precipitation (MIRSAL & ZANKL, 1985).These types of processes likely occured within organic-rich, pyriteassociated carbonate laminae of the Kosovo polje.The existance of idiomorphic planar-e type dolomite crystals indicates precipitation at temperatures below 50 -60°C, which again, points to early formation in a shallow burial realm.
The predominant non-planar-a type of dolomite/dolomicrite are likely to be related to burial dolomitization at temperatures exceeding ~50°C (SIBLEY & GREGG, 1987;WAR-REN, 2000).When evaporite successions reach depths of 2-3 km, temperature increases above ~60°C which enables dolomitization by solutions with lower Mg/Ca ratios.Therefore residual evaporite brines or seawater can become dolomitizing solutions (WARREN, 1999).The amount of non-planar-a type of dolomite generally increases with depth, whereas volumetrically, minor planar-e type of dolomite generally occurs in the uppermost gypsum or gypsum-anhydrite zone.

Gypsum dehydration
Gypsum dehydration begins under similar conditions to dolomitization.When temperatures within the evaporite exceed 60°C (HARDIE, 1967;JOWETT et al., 1993;WAR-REN, 2006) or the thickness of the overlying deposits exceeds 1000 m (SHEARMAN 1995) primary sabhka-type of gypsum will start to dehydrate, leading to the formation of anhydrite.In addition to temperature (and lithostatic pressure), the depth of gypsum to anhydrite transition depends on pore water salinity and pressure and heat flow.Under high salinity pore water this transition is plausible at rather low temperatures (T >40°C; YAMAMOTO & KENNEDY, 1969), whereas overlying conductor rocks imply transition at relatively shallow depths (>400 m; JOWETT et al., 1993).Water liberated via this process is a Ca-saturated and halite under saturated brine, escaping into the subjacent evaporite or clastic sediments.This type of brine is most likely responsible for dissolution of halite and formation of molds within the Kosovo polje carbonates.Dissolution of halite could also occur during regional uplift and flushing of evaporites by meteoric water.

Pyrite and sulfur
Although minor, the occurrence of pyrite within organic rich intercalations of the dolomicrite member and elementary sulfur as cement of the evaporite-carbonate breccia brings us some insight into the geological environment.Within the evaporite deposits, metal sulfides and sulfur are formed as the result of the biochemical or thermochemical reduction of evaporite sulphate to sulfide, followed by oxidation to native sulfur (e.g.MACHEL, 2001;ORTI et al., 2010;ZIE-GENBALG et al., 2010 and references therein).Biochemical reactions include low-temperature (up to 60 -80°C) bacterial reduction of evaporite sulphate to hydrogen sulfide, whereas thermochemical reactions envisage hydrocarbonsdriven thermal reduction of evaporite sulphate to hydrogen sulfide at temperatures of approximately 100 -140°C.In both cases, a part of the liberated hydrogen sulfide will rapidly react with available metals, especially Fe which is commonly associated with burial sedimentary environments.SO 4 2-removal -metal enrichment processes faciliate dolomite precipitation (MIRSAL & ZANKL, 1985).After the system runs out of iron, hydrogen sulfide will oxidize to native sulfur under the influence of meteoric or other oxygenated surface or subsurface waters.Within the Kosovo polje, pyrite is observed exclusively associated with the organic rich intercalation of the dolomicrite member, implying an active role of organic matter within the reduction process.However, elementary sulfur occurs as cement of the evaporite-carbonate breccia (together with prevailing gypsum) and related to tectonic emplacement of the evaporites, thus indicating late stage hydrogen-sulfide oxidation.Maximum burial temperatures of the Kosovo polje evaporites are not clear, however, burial history revealed from deep drill-cores (>4500 m) of the nearby Ravni Kotari antiform imply that the burial temperature of the "alterations of the early-diagenetic dolomite, anhydrite, limestone and argilitic-limestone" did exceed the oil-window (~120°C), at least for the Ravni Kotari structures (sensu GRANDIĆ et al., 1997).The absence of saddle (baroque) dolomite from the Kosovo polje succession indicates that higher burial conditions (exceeding 60 to 150°C) were not achieved; therefore, low temperature biochemical reduction of evaporite sulphate is more probable.

Rehydration
Rehydration of anhydrite to gypsum occurs during exhumation conditions under the influence of meteoric water.This process led to the formation of granoblastic or porphyroblastic gypsum, volume increase and enterolithic folding, visible at both the micro and macro-scales.Rehydrated gypsum encloses relics of anhydrite.The fluid inclusion assemblage, discovered within granoblastic to nematoblastic gypsum of the Kosovo polje, retains halite undersaturated monophase, liquid-only inclusions, most likely of meteoric-water origin.The width of the rehydration zone (gypsum zone and anhydrite to gypsum transition zone), shows a negative correlation with the thickness of the overlying Quaternary cover, as well as with the amount of clayey material within it.Drillcores with thick cover (20 -30m) have a rehydration zone only several metres thick, whereas with decreasing thickness of the cover deposits, the rehydration zone exceeds 50 m in thickness.Quaternary cover is composed predominantly of carbonates and illite-kaolinite clayey material with the addition of swelling clay and evaporites (<4 vol %).The carbonate to clay ratio of the Quaternary cover also varies strongly, and seems to be related to the thickness of the underlying rehydrated evaporite.Within the drill-cores where the cover thickness is similar, clay-rich cover material (> 80 vol % of clays) are underlain by a thin gypsum (<10 m) and anhydrite to gypsum transition zone (<15m), whereas drillcores with a carbonate-rich cover (~50 vol % of clays) display increasing thicknesses of rehydrated evaporates (>30m).
Formation of fibrous or satinspar gypsum within veinlets, cavities or fractures crosscutting the coarse-grained gypsum and anhydrite of the Kosovo polje is most likely to be related to late stage fluid flow.

Karstification and dedolomitization
Formation of rauhwacke subsequent to evaporite rehydration follows when an increased amount of gypsum is leached under the influence of meteoric water from the uppermost evaporite-carbonate breccia.Gypsum dissolution processes control and facilitate dissolution of the associated dolomite, followed by calcite precipitation throughout a process described in the literature as gypsum-driven dedolomitization (BISCHOFF et al., 1994 and references therein).The precipitation of calcite removes dolomite-supply carbonate ion and gypsum-supply calcium ion from the solution, thereby causing further dolomite and gypsum dissolution.Finally, this type of process, together with tectonic movements resulted in the formation of rauhwacke deposits (several metres thick), on top of the evaporite succession of the Kosovo polje.

Correlation with the Burrano formation, Northern Apennines
The Permian to Triassic Dinaric evaporite mélange shows notable similarities with the Upper Triassic Burano Evaporite formation, located in the Northern Apennines (Secchia River Valley; LUIGI, 2001 and references therein) as follows: (i) The Burrano and Dinaric evaporite formations are both strongly tectonized sequences with limited vertical and lateral continuity, composed of intercalations of gypsum±anhydrite and boudinage dolostones with minor halite.Evaporite-carbonate intercalations of the Burano formation are generally thicker (metre to decametre), and interpreted as subaqueous (LUIGI, 2001) whereas within the Dinarides, intercalations appear to be much thinner (millimetre to centimetre), and are considered to be of sabhka origin (TIŠLJAR, 1991).In addition, Dinaridic dolomicrite laminae contain organic matter, pyrite, quartz and rock salt.(ii) Both formations are thrusted onto younger Mesozoic formations and interpreted as a main decollement horizon during the Oligocene-Miocene shortening at the Adriatic-Dinaric domain.(iii) Dinaride evaporites are associated with magmatic and sedimentary rocks, while the Burano evaporites are associated with greenshist types of metasediments related to thrusting, and suffered secondary Mg-metasomatic replacement with temperatures around 300°C.(iv) Due to surface exposure and leaching, both evaporite formations are covered with cavernous dedolomitized breccia-rauhwacke.

CONCLUSION
Evaporite rocks of the Kosovo polje deposits are associated with carbonates, clastic and minor albitized subvolcanic neutral to basic rocks and represent a part of the Dinaric evaporite mélange.They are capped by clayey Quaternary cover and cavernous carbonate breccia -rauhwacke, several metres thick.Evaporite rocks show two distinct facies: laminated evaporite-carbonate facies, composed of evaporite and dolomicrite (± organic matter, pyrite, halite) intercalations, related to primary sabhka deposition, and evaporite-carbonate breccia facies, composed of fragments of laminated evaporites, carbonate and siltite, cemented by massive gypsum ± sulfur, occurring at shallower levels and related to the emplacement.Quaternary cover is composed of 47 -88% clayey material, consisting of illite, kaolinite and most likely vermiculite, 9 -49 % carbonate and 3 -4% evaporite.The width of the evaporite rehydration zone correlates negatively with the thickness of the overlying Quaternary cover and with the amount of clayey material within it.Rauhwacke are composed of partly dedolomitized carbonate and leached gypsum fragments, cemented with late stage calcite.Diagenetic processes postdating primary sabhka deposition of the evaporite formation are related to early diagenesis, burial and the emplacement phase as follows: • formation of diagenetic halite from oversaturated subsurface or surface (lagoon) brine biochemical reduction of evaporite sulphate to sulfide followed by formation of pyrite within organic-rich carbonate laminas.• early dolomitization in a shallow burial realm within organic-rich, pyrite-associated carbonate laminas controlled by bacteria induced reducing conditions and formation of idiomorphic planar-e type of dolomite crystals at temperatures below 50 -60°C dissolution of diagenetic halite and formation of molds with planar-e type of dolomite • onset of gypsum dehydration at temperatures of 40 -60°C • onset of burial dolomitization of carbonate mud at temperature exceeding ~50°C and precipitation of fine grained non-planar-a type of dolomite • replacement of planar-e type of dolomite within molds with a non-planar-a type of dolomite • rehydration of anhydrite to gypsum under the influence of low-temperature undersaturated fluid (meteoric water) • late stage oxidation of hydrogen-sulfide to elementary sulfur, occurring as cement in the evaporite-carbonate breccia • formation of rauhwacke throughout a gypsum-driven dedolomitization process.
5a,b); and (ii) evaporite-carbonate breccia facies, composed of millimetre to decametre-sized fragments of laminated sulfate, carbonate and siltite cemented by massive gypsum and occasionally sulfur, most likely originating during emplacement (Figs.5c,d; already recognized by TIŠLJAR, 1992).An evaporite-carbonate breccia with below 5 vol% of carbonate, prevails in the uppermost levels of the evaporite succession, followed by the laminated facies down dip.

Figure 3 :
Figure 3: Geological columns (drill cores) of the Kosovo polje evaporites with sample locations, displaying regular successions composed of Quaternary clays, rauhwacke, rehydration zone (gypsum zone and gypsumanhydrite transition zone) and anhydrite zone (with permission of the Knauf company).

Figure 7 :
Figure 7: Photomicrographs of the rauhwacke staining showing (a) very-fine grained anhedral dedolomite crystals along previous cracks and veinlets; (b) anhedral, coarse grained infill of the purple decorated calcite.

Figure 8 :
Figure 8: Photomicrographs of the laminated evaporite-carbonate facies consisting of (a, b) millimetres -centimetres thick anhydrite to gypsum laminae with <0.2mm -several mm thick intercalation of fine-grained, disintegrated dolomicrite/dolomite, organic matter and siltite; (c) nematoblasic and foliated anhydrite suppressed by fibrous gypsum along the grain boundaries or veinlets; (d) gypsum laminae composed of granoblastic to porphyroblastic gypsum, while primary laminated structure remain visible.

Figure 9 :
Figure 9: Photomicrographs of the evaporite-carbonate breccia consisting of carbonate or laminated evaporite fragments enclosed within massive gypsum showing: (a, b) mostly disintegrated carbonate fragments composed of micron-sized dolomite/dolomicrite crystals, anhydrite and gypsum crosscut by a network of irregular veinlets filled with fibrous (satin-spar) or granoblastic gypsum to anhydrite; (c) halite mold suppressed by the coarse-grained gypsum and a-type to e-type dolomite; (d) coexistance of granoblastic and porphyroblastic gypsum.

Figure 10 :
Figure 10: Photomicrographs of the dolomite/dolomicrite. Classification follows criteria according to SIBLEY and GREGG (1987): (a) fine-grained anhedral dolomite or non-planar-a type of dolomite; (b) fine-to medium-grained subhedral dolomite or planar-s type of dolomite, and (c) medium-to coarsegrained euhedral dolomite or planar-e type of dolomite and (d) Bunch-like radial dedolomite enveloped by fine-grained gypsum from the uppermost part of the evaporite sequence.

Figure 11 :
Figure 11: Fluid inclusion petrography of the coarse-grained granoblastic to porphyroblastic gypsum showing (a, b) three-dimensional clusters and subparallel trails primary and pseudosecondary monophase, liquid fluid inclusions (according to criteria of ROEDDER, 1980 and SHEPHERD et al., 1985).

Figure 12 :
Figure 12: Model for the evolution of the Kosovo polje evaporites.See text for discussion.

Table 1 :
Petrography of the analyzed samples of the Kosovo carst polje with marked analysis types.

Table 2 :
Chemical analysis of the Quaternary cover samples.