2018 | 71/1 | 19–28 | 8 Figs. | 3 Tabs. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society 1. INTRODUCTION Gypsum is by far the most abundant secondary mineral in sedi- mentary settings, sometimes forming the calcium sulfate mineral anhydrite in arid and hot supratidal environments (sabkhas) in the presence of concentrated brines, but rarely at the surface (KINS MAN, 1966; BUTLER, 1969; SHEARMAN, 1985; STROH MENGER et al., 2008; ZAKI et al., 2011; ABRANTES et al., 2016). Gypsum is readily prone to dehydration, producing water and anhydrite, when heated to a temperature that is high enough as a function of the salinity of coexisting fluids (HARDIE, 1967; AZIMI et al., 2011; MA et al., 2015). Thus, the most common process that causes the dehydration of gypsum, and thus the for- mation of anhydrite, is burial to depths below the reaction iso- therm, in settings where fluids can still escape the system. If an- hydrite is uplifted back above the reaction isotherm, it can return to gypsum, provided sufficient water is available. Ancient evap- orites are economically important sources of salts (e.g., gypsum) and metals (e.g., Cu, Zn, and Au), as well as being key indicators for the existence of structural oil and gas traps (WARREN, 2006). Upper Permian evaporite deposits occur in the Dinarides in central Dalmatia (TIŠLJAR, 1992) and can be observed at a num- ber of sites. Three known sites in this region that contain evapo- rite deposits were formerly active quarries for the extraction of gypsum and are thus the focus of this study: the Mali Kukor A mineralogical and petrographic study of evaporites from the Mali Kukor, Vranjkovići, and Slane Stine deposits (Upper Permian, Dalmatia, Croatia) Željko Dedić*, Nikolina Ilijanić and Slobodan Miko Croatian Geological Survey, Sachsova 2, P.O. Box 268, HR-10000 Zagreb, Croatia (*corresponding author: zdedic@hgi-cgs.hr) doi: 10.4154/gc.2018.02 Abstract The evaporite deposits examined in this study are located in the central part of middle Dalmatia, Croatia. In this region, Upper Permian evaporite sediments were deposited under favourable conditions onto the Variscan basement around the northern margins of Gondwana. These se- diments can be subdivided into three members, a lower evaporite unit (an anhydrite member), a middle evaporite unit (a gypsum member), and an upper unit (a clastic member), and are main- ly comprised of secondary gypsum that formed via the hydration of precursor anhydrite rocks. The middle evaporite unit comprises beds of gypsum as well as early diagenetic dolomites that contain gypsum sequences, extending up to 60 m maximum thickness, and overlying clastic se- quences that themselves are up to 20 m thick. These Upper Permian evaporite sediments con- tain horizontal, irregular, gypsum lithofacies that exhibit pronounced enterolithic and boudinage structures. The characteristics of these sediments are indicative of deposition in supratidal and sabkha settings (i.e., early diagenetic dolomites and evaporites) within a shallow epicontinental marine environment with highly varied coastlines, bays, and lagoons. The secondary gypsum seen within this Upper Permian middle evaporite unit displays alabastrine and porphyroblastic secondary textures and includes corroded anhydrite relics; associated minerals include musco- vite, chlorite, potassium (K)-feldspar, quartz, and amphibole. The Upper Permian evaporite se- diments discussed in this study are composed of irregular, locally brecciated secondary gypsum that probably formed as a result of multiple synsedimentary collapse of pre-existing soluble mi- nerals and/or synsedimentary and post-sedimentary tectonics. quarry in Kosovo polje, Vranjkovići quarry near Vrlika, and the Slane Stine quarry near Sinj. The aim of this study is to highlight the depositional environments and phases of diagenetic evolution that led to the formation of these Upper Permian evaporites based on the results of petrographic and sedimentological investigations of beds within the sequences at these sites. The Upper Permian evaporites and adjacent sediments known from middle Dalmatia have been the subject of extensive geological investigations in the past (TIŠLJAR, 1992, ŠUŠNJARA et al., 1992, GABRIĆ et al., 2002). Previous studies have ad- dressed the age of these sequences, their superposition and tec- tonic relationships with surrounding rocks, their mineralogy, pe- trology, and chemical composition, and have determined the conditions and environments of their deposition. The most comprehensive resource available regarding the geology of this area is the basic geological maps of the Republic of Croatia at a 1:100,000 scale, specifically the Knin, Drniš, and Sinj sheets (GRIMANI et al., 1962–1966; PAPEŠ et al., 1962– 1966; GRIMANI et al., 1975; IVANOVIĆ et al., 1977; IVANOVIĆ et al., 1978; RAIĆ et al., 1984). These deposits have also been extensively studied in order to calculate the extent of gypsum re- serves as a mineral commodity; this research has generated de- tailed site data based on exploration drilling and core analyses (LUKŠIĆ et al., 2005). The Upper Permian evaporites and adjacent sediments known from middle Dalmatia were deposited above a Variscan basement in hypersaline marine and/or marine-marginal deposi- Article history: Manuscript received August 09, 2017 Revised manuscript accepted December 20, 2017 Available online February 28, 2018 Keywords: Upper Permian, Evaporite sediments, Gypsum, Anhydrite, Dalmatia G eo lo gi a C ro at ic a Geologia Croatica 71/120 tional settings around the margins of northern Gondwana (TIŠLJAR, 1992). The upper part of the Middle Permian as well as the Upper Permian in this region is characterized by the for- mation of carbonates, notably dolostones and some sabkha evapo- rites (VLAHOVIĆ et al., 2005). Evaporitic rocks within deposits at the Kosovo polje were previously studied by KULUŠIĆ & BOROJEVIĆ ŠOŠTARIĆ (2014) and considered to be part of the Dinaric evaporite mélange. The aim of this study is to highlight the origins of Upper Permian evaporite depositional environments and to discuss their diagenetic evolution based on petrographic and sedimentological investigations of carbonates and evaporites at a series of study sites. 2. GEOLOGICAL SETTING The study area and deposits discussed in this paper are located in middle Dalmatia, Croatia (Fig. 1), within the central part of the outer Dinarides. The Dinarides generally encompass a strati- graphic range between the Middle Permian (or even the Upper Carboniferous) and the Eocene (TIŠLJAR et al., 1992; VELIĆ et al., 2002a; VLAHOVIĆ et al., 2005). It is thought that deposits ranging from the top of the Lower Jurassic (Toarcian) to the top of the Cretaceous can be attributed to the Adriatic Carbonate Platform (AdCP), one of the largest Mesozoic structures of this type within the peri-Mediterranean region (DERCOURT et al., 1993, 2000; VLAHOVIĆ et al., 2005). Figure 1. Map to show the locations of the study sites in middle Dalmatia, Croatia. The reference source for Upper Permian sediments used in this paper is the 1:300.000 Geological Map of the Republic of Croatia (CGS, 2009). G eologia C roatica Dedić et al.: A mineralogical and petrographic study of evaporites from the Mali Kukor, Vranjkovići, and Slane Stine deposits ... 21 The basement of the AdCP can be subdivided into several major sequences, the oldest of which encompass Upper Carbon- iferous to Middle Permian rocks and comprise part of the Va- riscan basement. Siliciclastic deposition prevailed during the Car- boniferous period followed by mixed clastic-carbonate deposits in the Lower Permian and a transition within the Lower/Middle Permian into continental clastic deposition. Similarly, the Middle Permian to Middle Triassic sequence in this region is characterized by carbonate and mixed siliciclas- tic-carbonate deposits that formed an epeiric platform at this time along the northern margin of Gondwana, while the upper part of the Middle Permian and Upper Permian is characterized by car- bonates and sabkha evaporites (VLAHOVIĆ et al., 2005). Ancient epeiric evaporitic platforms (SHAW, 1964; IRWIN, 1965; WETZEL et al., 2013; ORTI et al., 2017) are huge areas of restricted stagnant circulation on the continental margins of sea- ways where extensive evaporite beds periodically accumulated (WARREN, 2006). The Upper Permian sequences of middle Dalmatia comprise three main facies types, carbonates (mainly limestones), evapo- rites (gypsum and anhydrite), and early diagenetic dolomites and clastics including siltstones, sandstones, and rare conglomerates. Exceptional facies types in this region include porous carbonate cavity breccias, so-called “rauchwackes” (ŠUŠNJARA et al., 1992; TIŠLJAR, 1992). 3. METHODS A series of detailed field investigations of Upper Permian evapo- rite units were carried out at the Mali Kukor, Vranjkovići, and Slane Stine sites (Fig. 1). A total of 45 representative samples were selected from processed profiles and subjected to further laboratory analysis; thin sections for mineralogical-petrographic analysis were prepared from all samples, and 24 bulk evaporite samples were selected for X-ray diffraction (XRD) analysis. Preparation of evaporite thin sections required the use of a special method. Samples were first crushed and hand ground with a pestle and mortar before a few grams of each were sieved through a 125 mesh section. Cutting, polishing, and thinning were then carried out using Norland Optical Adhesive 61, glue that hardens under ultraviolet light. All of the thin sections were then examined using a polarizing microscope. Bulk sample mineral compositions were determined using a PANalytical X’Pert Powder X-ray diffractometer equipped with Ni-filter Cu Kα radiation, a vertical goniometer with θ/θ geo- metry, and a PIXcel detector. Scans were performed at 45 kV and 40 mA using both a ¼ divergence slit and anti-scatter slits and with a step size of 0.002° 2θ every four seconds across a range between 4° 2θ and 66° 2θ. Individual minerals were identified on the basis of their reflections following the method outlined by MOORE & REYNOLDS (1997); semi-quantitative mineral anal- ysis was performed using the software RockJock (EBERL, 2003), and the weight percent (wt %) of minerals in each sample was calculated using integrated X-ray intensities. 4. RESULTS AND DISCUSSION 4.1. The lithofacies at Mali Kukor, Vranjkovići, and Slane Stine Detailed field observations enabled the identification of three dis- tinct lithofacies within the quarries, evaporites, carbonates and clastics i.e., siltstones, sandstones and carbonate cavity breccias. The dominant lithofacies in the Slane Stine quarry comprise anhydrite in the lower part of the profile with clasts that range in size between 1 – 5 cm diameter. This basal sequence is around 25 m thick and occurs beneath an alternating sequence of anhy- drite, dolomite, and massive gypsum that marks the middle of the plateau. This overall sequence is then capped by an approxi- mately 15 m thick tectonic zone that comprises green and reddish clastic sediments. The lithofacies within the Slane Stine quarry also includes fragments of carbonate rocks (dolomites) within evaporative (gypsum) sediments that are characterized by entero- lithic folding (Fig. 2). The lithofacies in the Vranjkovići quarry include poorly de- veloped horizontal laminations of secondary gypsum with dolo- mite intercalations that contain organic matter in their lower por- tions. These can be up to 15 m thick and underlie an alternat ing sequence of massive gypsum units including thinly bedded early diagenetic dolomite (ca. 5 m thick) below palaeokarst forms (Fig. 3). The lithofacies in the Mali Kukor quarry comprise dolomites and alternating thinly bedded white and gray gypsum units that contain portions of dolomitic breccia in their lower parts. These units are up to 20 m thick, occurring below a zone of alternating Figure 2. (A) Fragments of carbonate rocks (dolomites) within evaporitic (gypsum) sediments in the Slane Stine near Sinj. (B) An example of enterolithic folding within the Slane Stine deposit. A B G eo lo gi a C ro at ic a Geologia Croatica 71/122 secondary gypsum and anhydrite, which terminates in green and reddish clastic deposits (Fig. 4). In summary, the following basic lithofacies are seen at all three sites: • A laminated evaporite-carbonate facies composed of mil- limetre-centimetre-sized gypsum and anhydrite laminae intercalated with dolomite and dolomicrite and organic matter patches. This facies type is also characterized by horizontal, wavy, and irregular laminations as well as en- terolithic, massive, and boudinage-style lithologies; • An evaporite-carbonate breccia facies that is composed of millimet-metre-sized fragments of dolomite and anhydrite relics. The laminated evaporite-carbonate facies also presents a se- ries of very thin evaporite beds and laminae interbedded, or in- terlaminated, with thinner, dark, lime, muddy structures that con- tain organic material (possibly algal mats) and thinly bedded dolomites. These evaporite laminae range in thickness from 0.1 mm upwards and often disintegrate into fine-grained dolomic- rite and dolomite, organic matter, or anhydrite. Evaporite laminae within the sections studied in this paper also display enterolithic and boudinage features forms of dolomite or organic matter patches intercalated with evaporite laminae. Evaporite-carbonate breccia facies containing fragments of dolomite or anhydrite relics are seen at all three sites; this litho- facies type consists of angular or irregular fragments of various sizes set within a sulfate groundmass. 4.2. A mineralogical and petrographic evaluation of the evaporites A microscopic mineralogical-petrographic study was carried out on the laminated evaporite-carbonate facies described above that comprises millimetre-to-centimetre scale gypsum and anhydrite laminae intercalated with dolomites, dolomicrites, and organic matter. The results of this investigation show that the Upper Permian evaporite units from all three sites discussed in this pa- per are primarily composed of microcrystalline secondary gyp- sum. Samples from all three deposits nevertheless display a va- riety of textures ranging from xenotopic to idiotopic and encompass four types of secondary gypsum: Figure 3. (A) Images from the top of the geological profile at Vranjkovići showing the alternation between massive gypsum and thinly bedded early diagenetic dolomite that terminates in palaeokarst structures. (B) The lower part of the geological profile at Vranjkovići showing poorly developed horizontal laminations of secondary gypsum and dolomite intercalations. Figure 4. The lower zone of the geological profile at Mali Kukor showing alternating thinly bedded white and gray gypsum and organic patches that possibly rep- resent algal mats (A). The upper zone of the geological profile at Mali Kukor showing organic patches and anhydrite embedded within a gypsum mass (B). A B A B G eologia C roatica Dedić et al.: A mineralogical and petrographic study of evaporites from the Mali Kukor, Vranjkovići, and Slane Stine deposits ... 23 Figure 5. Thin-section images of sample Mk-12, an alabastrine secondary gypsum that contains euhedral-to-subeuhedral features around 200 μm in size. Most of the oriented crystals within this sample are relics of fine-grained crystallized secondary dolomite (Dol) as well as coarse-grained crystallized secondary dolomite (Dol). The image on the left (A) is under plane-polarized light while the one on the right (B) is under cross-polarized light). (1) Alabastrine; (2) Porphyroblastic; (3) Fibrous satin spar, and (4) Granoblastic. Within these varieties, fibrous satin spar gypsum is the only type formed from sulfate-rich solutions that develop due to an- hydrite hydration; all the other varieties form via the rehydration of an anhydrite precursor. The mineral abbreviations used in the subsequent text and figures are as outlined by KRETZ (1983). Observations show that secondary gypsum is the most wide- spread form seen in Upper Permian evaporite units at the three sites. This kind of gypsum can be classified into several forms which can be loosely grouped into three stages that comprise a continuous recrystallization sequence. Thus, in stage 1, alabas- trine secondary gypsum directly forms from anhydrite, contains abundant corroded inclusions, and comprises a mosaic of com- plex, interlocking anhedral grains that about one another to such an extent that their grain boundaries are almost impossible to trace. Viewed under crossed poplars, each grain appears fibrous and ill-defined and exhibits an irregular undulating extinction. The bulk of this type of gypsum appears to have been formed at, or very near, the surface as it develops via the weathering of an- hydrite outcrops. When no anhydrite relics are present in stage 1 gypsum, re- crystallization is initiated to form stage 2 or stage 3 granoblastic gypsum. This form is not seen adjacent to anhydrite as it does not form from this starting point. Stage 2 gypsum comprises small, usually anhedral, grains that are characterized by non-undulatory extinction, sometimes with curved re-entrant boundaries. The grain size of this form is usually uniform, with diameters around 50 μm, and the presence of larger subhedral and euhedral crystals are indicative of further recrystallization to stage 3. Thus, stage 3 gypsum, which deve- lops from either stage 2 forms or directly from stage 1, is com- posed of subhedral or euhedral grains about 100 μm in diameter. It is noteworthy that the grain size of this form can be variable, reaching up to 0.5 mm in diameter; many larger grains can be blasts that contain no anhydrite relics, and have well-developed crystal faces and textures that formed subsequent to the finer gypsum matrix (HOLLIDAY, 1970). Figure 6. Thin-section images of sample Mk-16, a porphyroblastic secondary gypsum that contains inclusions of carbonates, dolomites, and calcite (Cal), as well as corroded anhydrite relics (Anh). The image on the left (A) is under plane-polarized light while the one on the right (B) is under cross-polarized light. A B A B G eo lo gi a C ro at ic a Geologia Croatica 71/124 Microscopic thin sections reveal that the textures of alabas- trine gypsum can exhibit erratic and migrating extinction shad- ows when samples are rotated under cross-polarized light (Fig. 5). Examples of this gypsum with euhedral-to-subeuhedral features comprises the most frequent crystal orientations within the samples examined in this study alongside relics of fine-grained crystallized secondary dolomite (Fig. 5). Indeed, as these secon- dary gypsum crystals were most probably formed from anhydrite they can vary from place to place. In early work, OGNIBEN (1957) discussed super-individual examples of alabastrine se- condary gypsum that comprise sub-crystals with shadowy grain boundaries as well as the extinction polarization of aggregates of microcrystalline gypsum less than 70 μm in diameter. The crys- tal size of samples from the Mali Kukor site (sample Mk-12) is around 200 μm, and alabastrine gypsum can include a wide range of related textures which all form a natural grouping, in contrast to the porphyroblastic variety. The crystals of alabastrine gyp- sum are therefore commonly circular, irregular, or form elon- gated patches adjacent to, or within, the porphyroblastic form. The boundaries between the finer-grained alabastrine form and its coarser counterpart also tend to exhibit gradational interpen- etrating contacts indicative of the replacement of larger gypsum crystals by smaller ones. This process of degradation has been described in Miocene evaporites from the Red Sea (AREF, 2003), from Messinian evaporites in Italy (TESTA & LUGLI, 2000), and in pedogenic gypsum crusts (WATSON, 1988; AREF, 2003). In contrast, porphyroblastic gypsum tends to forms both large and small crystals that have generally interlocking margins. In some examples the individual crystals are clearly defined, while in other cases samples can be very coarsely crystalline (Fig. 6). Porphyroblastic gypsum therefore comprises large, an- hedral porphyroblasts up to 1 cm in length that tend to occur as single crystals, although aggregates can also be locally abundant. This form also contains numerous inclusions of corroded anhydrite relics or other associated minerals, and porphyroblasts are sur- rounded by later alabastrine secondary gypsum, often the stage 3 granoblastic form. Other associated minerals can include an- hydrite, dolomite, muscovite, chlorite, amphibole, potassium (K)-feldspar, apatite, rutile, and quartz, while pyrite and organic material are also common in some samples. Hydration veins are also common features of these secondary lithofacies and are often filled with satin spar gypsum (Fig. 7). Observations show that gypsum porphyroblasts do not form to any great extent in pre- sent-day anhydrite settings where the alabastrine type is domi- nant, indicating that these structures are components of an earlier phase of gypsification. This conclusion is corroborated by the fact that similar porphyroblasts have been recorded as comprising the earlier form of secondary gypsum in many areas (FORBES, 1958; HAM, 1962; WEST, 1964). This kind of gypsum is com- posed of large euhedral-to-subhedral crystals that exhibit sharp extinction and occur as individual elements or in groups of two or three crystals. HOLLIDAY (1970) and TAJ (2012) noted that the origin of porphyroblastic gypsum via anhydrite rehydration was evidenced by corroded anhydrite relics in samples from the three sites they investigated (Fig. 6). The fibrous (satin spar) gypsum variety is the only type that forms from sulfate-rich solutions, developed due to the rehydra- tion of anhydrite; all other known types of secondary gypsum form via the rehydration of precursor anhydrite (MAKLOUF et al., 2006). The formation of satin spar gypsum is therefore thought to be the result of the increase in volume that character- izes the transformation of anhydrite to gypsum (SHEARMAN et al., 1972; YEŞİLOVA & HELVACI, 2013). Thus, prismatic an- hydrite laths are often observed as pseudomorphs within host sediment; for example, fibrous (satin spar) gypsum occurs as veins interbedded within evaporites and within the carbonates of the Abu Ruweis Formation (MAKLOUF et al., 2006). Similarly, veins from sample Mk-5c (Fig. 7) are composed of elongate crys- tals arranged perpendicular to walls; in this case, veins of fibrous (satin spar) gypsum are interbedded within a dolomite intraclast and comprise an evaporite facies mineral assemblage in thin sec- tion. Fibrous (satin spar) gypsum is composed of crystals greater than 200 μm in length, while relic anhydrite components greater than 10 μm in size also float within subhedral gypsum crystals that can also sometimes be arranged randomly. It is therefore thought that such randomly oriented crystal veins were originally formed from anhydrite at depth and at high temperatures and pressures, before subsequently being rehydrated to gypsum. A number of previous authors have considered satin spar gypsum to therefore be a by-product of anhydrite rehydration (SHEAR- MAN et al., 1972; TESTA & LUGLI, 2000). Granoblastic gypsum has a limited distribution within the Upper Permian evaporite units discussed in this study, recorded in only the Mali Kukor and Vranjkovići deposits. This form ex- Figure 7. Thin-section images of sample Mk-5c, gypsum veins within a dolomite intraclast (Dol) and a mineral assemblage from the evaporite facies. These images show an anhydrite (Anh) relic floating within subhedral crystals of gypsum (Gp). The image on the left (A) is under plane-polarized light while the one on the right (B) is under cross-polarized light. A B G eologia C roatica Dedić et al.: A mineralogical and petrographic study of evaporites from the Mali Kukor, Vranjkovići, and Slane Stine deposits ... 25 hibits well-defined euhedral-to-subhedral crystals that have sharp and homogenous extinction patterns. Such crystals are almost equigranular in size, range between 10 μm and 50 μm, interlock with one another, and form mosaic textures. These granular crys- tals also exhibit dominantly euhedral faces toward porphyroblas- tic gypsum, indicative of a replacement origin, and this second- ary type is also characterized by the absence of anhydrite relics. It is therefore thought that granoblastic gypsum represents an advanced stage of anhydrite rehydration (WEST, 1964, 1965; HOLLIDAY, 1970; WARREN, 1999, MAKLOUF & EL- HADDAD, 2006, TAJ, 2012). On the basis of the petrographic descriptions presented in this study, it is clear that the Upper Permian gypsum evaporites from selected sites across middle Dalmatia are mainly composed of secondary gypsum that most probably originated from either the hydration (by groundwater) of and/or the surface weathering of precursor anhydrite rocks. It is noteworthy in this context that the term “secondary gypsum” was first used by MURRAY (1964) and then later by HOLLIDAY (1970) and SHEARMAN (1972) to refer to gypsum that formed via hydration of anhydrite. 4.3. The bulk mineralogy of evaporite samples The results of bulk XRD analysis of 24 samples (Tabs. 1–3) shows that the dominant mineral components are gypsum, dolomite, and anhydrite, while calcite, chlorite, K-feldspar, muscovite, quartz, amphibole, and apatite also occur in smaller proportions. Celestine and rutile were detected in just one sample from the Mali Kukor site (sample MK-8). Mineral compositions at the Slane Stine site are dominated by gypsum, anhydrite, and muscovite, with the exception of sam- ples SS-12A and SS-PIR which do not contain anhydrite (Tab. 1). Dolomite is also present in most cases in minor concentrations, but is the dominant phase (alongside gypsum) in sample SS-PIR, while apatite, rutile, and quartz also occur in minor amounts. Gypsum is the dominant mineral phase in all samples from the Mali Kukor site (Tab. 2; Fig. 8), while anhydrite was seen in just one sample (MK-ANH) from this site, and calcite is present in samples MK-3, MK-8, and MK-14. Mineral phases that appear in minor concentrations in the samples from this site include am- phibole, quartz, and muscovite (in samples MK-14 and MK-16), while dolomite is only seen in sample MK-16, celestine is only observed in sample MK-8, and hydroxylapatite is only seen in samples MK-3 and MK-5B. All the samples from the Vranjkovići site contain both gyp- sum and dolomite as their main mineral phases (Tab. 3), while minor amounts of amphibole are also present in nearly all sam- ples (with the exception of VR-6), alongside muscovite in samples VR-11, VR-12, VR-13, and VR-14, K-feldspar in samples VR-5-2, VR-11, VR-12, VR-13, and VR-14, apatite in samples VR-5-1 and VR-5B, and anhydrite in sample VR-11. The microscopic and XRD investigations of Upper Permian evaporite samples discussed here reveal the presence of carbonates Table 1. Bulk semi-quantitative mineralogy of evaporite samples from the Slane Stine site. Sample Gypsum Anhydrite Dolomite Muscovite Chlorite Amphibole K-feldspar Apatite Rutile Quartz SS-2 83 7 1 8 – 1 – SS-3A 86 4 2 8 – – – – – – SS-6A 76 5 3 12 3 – – – 1 – SS-12A 63 – 3 14 7 1 9 – 3 SS-PIR 30 – 60 7 – 3 – – – – Table 2. Bulk semi-quantitative mineralogy of evaporite samples from the Mali Kukor site. Sample Gypsum Calcite Dolomite Amphibole Quartz Muscovite Celestine Apatite Anhydrite MK-3 15 83 – – 1 – – 1 – MK-5B 97 – – 2 – – – 1 – MK-8 80 12 – 2 3 – 3 – – MK-12 97 – – 2 1 – – – – MK-14 88 5 – 2 1 4 – – – MK-16 88 – 3 2 – 7 – – – MK-ANH 67 – – 2 – – – – 31 Table 3. Bulk semi-quantitative mineralogy of evaporite samples from the Vranjkovići site. Sample Gypsum Dolomite Amphibole Apatite K-feldspar Muscovite Anhydrite VR-5-1 87 11 1 1 – – – VR-5-2 68 25 1 – 6 – – VR-5B 79 19 1 1 – – – VR-6 97 3 – – – – – VR-6B 97 1 2 – – – – VR-9A 99 – 1 – – – – VR-11 90 – 1 – 3 5 1 VR-12 79 6 1 – 7 7 – VR-13 69 22 2 – 4 3 – VR-14 66 23 2 – 4 5 – VR-15 94 4 2 – – – – VR-18 50 50 – – – – - G eo lo gi a C ro at ic a Geologia Croatica 71/126 in addition to gypsum. The data show that dolomite is dominant at the Vranjkovići and Slane Stine sites, while calcite is most abundant at the Mali Kukor site. Calcite occurs as regular crystals at the Mali Kukor site (Fig. 4; e.g., sample Mk-16) alongside a mass of secondary gypsum that generally exhibits both alabastrine and porphyroblastic tex- tures associated with corroded anhydrite relics. It is thought that this kind of gypsum forms via the replacement of carbonate min- erals (i.e., dolomite and calcite) with sulfate-rich solutions. The textural characteristics of this gypsum indicate that carbonate minerals (i.e., calcite or dolomite) were originally replaced by an- hydrite, which was in turn replaced by secondary gypsum. In- deed, as this is likely to have been the case, the replacement of carbonate minerals took place at depth under both high tempera- ture and pressure, and favored the formation of anhydrite rather than gypsum (MURRAY, 1964; HOLLIDAY, 1970; KENDALL, 1989; TESTA & LUGLI, 2000, AMADI et al., 2010). The fact that gypsum occurs locally as lenticular crystals scattered amongst dolomite (Fig. 5) probably indicates that it di- rectly replaced carbonate minerals under near-surface conditions (WARREN, 1999). The alternation of gypsum and dolomite as couplets contin- ues upwards through the sections studied here and suggests a “dolomite-gypsum rhythmical pattern” at different levels within outcrop successions. Observations show that each couplet com- prises a dark-gray, thin-to-thickly bedded dolomite overlain by thickly bedded white gypsum; rhythmic couplets are distinct, but irregularly spaced, and are rich in organic matter when they oc- cur, transitioning upwards into nodular gypsum (Fig. 2). 4.4. Interpretation of different gypsum types Experimental studies and modern sedimentary analogues indi- cate that primary gypsum is more commonly precipitated under normal surface conditions (HARDIE, 1967; SHEARMAN, 1985; TESTA & LUGLI, 2000, SCHREIBER & HELMAN, 2005). This means that the anhydrite precursor to this process can be interpreted as a diagenetic product derived from the dehydration of primary gypsum (MURRAY, 1964; HOLLIDAY, 1970, KASPRZYK, 2003); thus, primary gypsum is transformed dia- genetically into anhydrite either during burial (MURRAY, 1964; HOLLIDAY, 1970; TESTA & LUGLI, 2000; KIRKLAND, 2003), or via solar heating as was the case for Miocene evaporites in the northwestern Red Sea (AREF et al., 2003). TIŠLJAR (1992) concluded, for example, that evaporitic conditions around the edges of epeiric marine basins tend to exist when general regres- sive conditions are in place, associated with ongoing coastal sea- ward progradation. The microscopic investigations reported here indicate that most of the observed types of secondary gypsum associated with the evaporite units studied resulted from the hydration of precur- sor anhydrite. The data suggest that all of the types of gypsum seen in evaporite units within the study area (e.g., alabastrine secondary gypsum, porphyroblastic gypsum, fibrous satin spar, and granoblastic gypsum) are of secondary origin, and this ob- servation is confirmed by the widespread presence of satin spar gypsum veins within Upper Permian units. The veins have been considered by many authors to be the result of anhydrite hydra- tion (HOLLIDAY, 1970; KENDALL, 1989; TESTA & LUGLI, 2000). It is therefore thought to be the case that these Upper Perm- Figure 8. Upper Permian evaporite sample XRD patterns from the Mali Kukor site (i.e., samples MK-3, MK-8, and MK-ANH). Abbreviations: Gp, Gypsum; Cal, Calcite; Anh, Anhydrite. G eologia C roatica Dedić et al.: A mineralogical and petrographic study of evaporites from the Mali Kukor, Vranjkovići, and Slane Stine deposits ... 27 ian evaporite units were originally deposited as gypsum anhy- drite? (see below) under restricted shallow marine saline condi- tions, although this may have varied over time. Additional evidence for this conclusion comes from their regular bedding and the fact that they are interbedded with marine carbonates. During uplift, the exposed parts of these Upper Permian anhy- drites were modified to gypsum, although remaining similar to other subsurface examples (ANDREWS, 1992; DALQAMUNI, 1995). 5. CONCLUSION The mineralogical and petrographic evaluations of evaporitic sediment thin sections presented in this study reveals the charac- teristics of evaporitic rocks, indicating their primary diagenetic products, and enables a determination of the changes seen throughout the geological evolution of such sediments. The evaporitic sediments investigated in this study are mainly secondary gypsums that also contain dolomite, anhydrite, muscovite, calcite, quartz, amphibole, and clay minerals. Secon- dary gypsum can occur in various structural types that range from idiotopic to xenotopic, of which alabastrine, porphyroblastic, and granoblastic types are recognized here alongside a fourth fibrous satin spar structure seen in one sample. Alabastrine and porphy- roblastic types most probably correspond to the upper phreatic groundwater zones, while the appearance of gypsum on, or near to, the surface, as well as the presence of anhydrite in deeper parts, reflects the fact that secondary gypsum minerals are the result of anhydrite hydration. This hydration process is initiated at the edges of anhydrite crystal cleavage planes in the form of fibrous satin spar gypsum, especially in tectonically disturbed and cracked zones. The microscopic and XRD investigations reported in this study reveal that the Upper Permian evaporites at all three sites are primarily composed of microcrystalline secondary gypsum. Indeed, the evaporites at the Vranjkovići and Mali Kukor sites predominantly comprise gypsum with very little anhydrite, while gypsum and anhydrite are present in equal quantities at Slane Stine. Microscopic investigations indicate that most of the gyp- sum microfacies within these Upper Permian evaporites are the result of parent anhydrite hydration. The analyses presented in this study also confirm the pre- sence of dolomite and calcite as carbonate minerals. The results of XRD analyses show that dolomite is also present, alongside gypsum and/or anhydrite, at both the Vranjkovići and Slane Stine sites, while calcite dominates at Mali Kukor. The main conclusion of this study is that the total mineral- ogical composition of the investigated evaporitic sediments is in- dicative of several stages of deposition, and that there are key differences between samples from Slane Stine and Vranjkovići and Mali Kukor. 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