Mineralogical and geochemical characteristics of mudstones in the Jersovec chert deposit Mineraloške in geokemične značilnosti muljevcev iz nahajališča roženca Jersovec

Jersovec chert deposit is a part of the transitional zone between the internal and the external Dinarides. Tethyan cherts are often interlaid with fine-grained material, as is also the case in Jersovec, where chert beds interchange with several thin, up to 10 cm thick layers of fine-grained sedimentary rocks. The source of this material is often questionable and interpreted to be terrigenous, volcanogenic or even combination of both. In order to determine origin and depositional environment of fine-grained material, detailed mineralogical and geochemical analyses were performed. Fine-grained sedimentary rocks were characterised by X-ray diffraction (XRD), scanning electron microscopy/ energy dispersive X-ray spectroscopy (SEM/EDS) and inductively coupled plasma emission spectroscopy (ICPES) and mass spectroscopy (ICP-MS). All samples are mineralogically very similar but differ in mineral ratios. They contain quartz and clay minerals, predominantly illite/muscovite and chlorite group minerals. SEM/EDS analysis additionally revealed zircon, monazite, Ti-oxide (probably rutile) and iron oxides/hydroxides in all samples, whereas chromite, ilmenite, xenotime, apatite and baryte were found only in some of them. The average sizes of accessory minerals range from 3–20 μm. Two samples differ from the others by their brown colour and higher Fe2O3 and lower SiO2 contents. All other samples are green. Chemical analysis showed that they consist mostly of SiO2 (46.3–69.3 %), Al2O3 (15–24.1 %) and minor contents of K2O (2.8–3.4 %), Fe2O3 (2.1–8.4 %), MgO (1.6–2.3 %), TiO2 (0.5–0.7 %), CaO (0.3–1.4 %), Na2O (0.1 %) and P2O5 (up to 0.5 %). Position of all samples in the Zr/TiO2 vs. (V+Ni+Cr)/Al2O3 diagram points to terrigenous origin and sedimentation on the continental margin. According to the chemical weathering indices (CIA and CIW) material has been subjected to the intense and long weathering, supported also by the index of compositional variability (ICV), which corresponds to the compositionally mature material, and by rounded zircon and ilmenite grains.


Introduction
Sedimentary rocks with cherts are very common Mesozoic pelagic sediments of the entire Tethys (Baumgartner, 2013). In Slovenia, they are found in the transitional zone between the internal and the external Dinarides. The stratigraphy of several Tethyan chert outcrops has been extensively studied, whereas the studies on the finegrained clastics, interbedded with cherts, are not very common (Barrett, 1981;Baltuck, 1982Baltuck, , amodeo, 1999di leo et al.,2002a, b;Halamić et al., 2005;Peh & Halamić, 2010, SkoBe et al., 2013. The origin of the fine-grained material could be either terrigenous and/or volcanogenic; it reflects the depositional environment, ranging from a shelf to a mid-ocean ridge or continental margin (BoStröm, 1973;murray, 1994;girty et al., 1996;andreozzi et al., 1997;di leo et al., 2002a). Therefore, it helps to interpret the palaeoenvironmental and palaeogeographic position of the area in Tethys realm.
In Jersovec deposit in Dolenjska region, chert is still being quarried. Here, chert beds interchange with thin, up to 10 cm thick layers of fine-grained sediments. The origin of this fine-grained material has not been determined yet. In the vicinity, very similar clastic material of Jurassic age has been found and studied in the abandoned quarry near Izvir village in Krško depression (SkoBe et al., 2013), in Italy, where the interbedded material is also of Jurassic age (Barrett, 1981), and in Croatia, where the clastics are of Triassic and Jurassic age (Peh & Halamić, 2010).
The aim of the paper is to characterise the fine-grained material from Jersovec deposit, to establish its provenance and depositional environment, and to compare it with similar rocks from neighbouring deposit near Izvir in Krško depression.

Geological setting
The investigated area is located in the south-eastern Slovenia, in the vicinity of Mirna and Trebnje, Dolenjska region. The area is a part of the transitional zone between the internal and the external Dinarides (Fig. 1). In the area several outcrops of cherts are reported, the closest one to Jersovec is Izvir (Fig 1).
The rocks from Jersovec deposit are of upper Triassic age, the so called »transitional« beds (the expression is used for rock of indefinable age, lay-ing between upper Triassic and lower Cretaceous beds), lower Cretaceous and Quaternary age (Šolar, 1991). Upper Triassic beds are characterised by light grey »Bača dolomite« with chert nodules and beds, and crisscrossed with calcite veins. Upper parts of the dolomite are strongly weathered (Šolar, 1991). The transitional beds are represented by dolomite-chert breccia, bedded and laminated cherts and beds of muddy sandy to sandy muddy chert gravel. Cretaceous beds consist of quartz sandstones, shales and micritic limestones transiting to calcarenites and containing chert nodules and beds. The youngest is Quaternary alluvium (Šolar, 1991).
Jersovec quarry is intersected by the NW-SE (Dinaric) fault and therefore the rocks have been tectonically deformed. Also, the older faults with W-E direction had caused some minor deformations (Šolar, 1991).
Between the chert beds there are very thin, at most up to 10 cm thick layers and lenses of finegrained green to brown sediments. The colour is interpreted as a result of different degree of oxidation of some iron mineral(s) (Šolar, 1991). No mineralogical and geochemical studies of these fine-grained materials have been performed yet.

Materials and methods
Altogether, 7 samples were taken at two sites, located about 50 m from each other (Figs. 2 and 3). All samples were dried at 50 °C for two weeks and afterwards pulverised by hand in agate mortar to a grain size of <0.063 mm for geochemical and XRD analysis. Fragments of original rock samples were also prepared for SEM/EDS analysis. Heavy mineral fraction was prepared from one sample (Je-01) using bromoform (density 2.89 g/cm 3 ) and then inspected by SEM/EDS.
Mineral composition of all samples was determined by powder X-ray diffraction (XRD) at the Faculty of Natural Sciences and Engineering, Department of Geology, University of Ljubljana. XRD measurements were conducted using a Philips PW3710 diffractometer (PANalytical B.V.) equipped with CuKα radiation and a graphite monochromator. The X-Ray radiation was generated at a voltage of 10 kV and a current of 10 mA. Data were recorded in the range 2º ≤ 2Θ ≤ 70º. Mineral compositions of the samples were determined by X'pert Highscore Plus database.  A whole rock geochemical analysis of major oxides and several trace elements, including rare earths was carried out by inductively coupled plasma emission spectroscopy (ICP-ES) and mass spectroscopy (ICP-MS) at Bureau Veritas Commodities Canada. The analytical quality based on international standards and one replicate sample (Je-06-1) is satisfactory, with the precision and accuracy error below 10 %. Precision error >10 % was established for CaO, Cu, Hg, Pb, Sn, Ta, U, W and Zn, due to their low content. These elements, and additionally Sb and total sulphur (TOT/S), whose contents were below detection limit (0.1 and 0.02 %), were omitted from further study. Also contents of MnO were below detection limit in some samples. As MnO is necessary for one of the used discriminant diagrams, half of the detection limit value was used for the calculation.
Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) of original rock fragments and one sample of heavy mineral fraction was carried out at Geological Survey of Slovenia. The samples were coated with carbon and then observed in high vacuum, in backscattered electron (BSE) mode by JEOL JSM 6490LV SEM, coupled with an Oxford INCA EDS at an accelerating voltage of 20 kV, working distance 10 mm and acquisition time of 60 s. Minerals were determined using known mineral databases (anthony et al., 2009; Barthelmy, 2010).

Results and discussion
Macroscopically, the colour of the samples varies from green and greenish grey (5GY 7/2, 5GY8/1) to brown (5YR 5/6, 5YR 3/4). The mineral compositions of all samples are very similar ( Fig.  4) with prevailing quartz and illite/muscovite (illite and muscovite cannot be distinguished), and possible minor presence of chlorite group minerals. Samples differ only in mineral ratios. The sample Je-05 contains less quartz than others and also seems to be of lower crystallinity, whereas samples Je-02 and Je-06 contain the maximum content of quartz.
SEM/EDS analysis supports the results of XRD analysis and additionally reveals the presence of some accessory minerals (Table 1). Quartz grains are the most abundant in all samples, followed by clay minerals. The average grain size of quartz is approximately 15 μm. Also, clay minerals are abundant in all samples. Micas, most probably muscovite, and minerals of chlorite group, are around 10 μm in size with frequently pseudohexagonal shapes. In all samples, zircon, iron oxides/hydroxides, monazite and Ti-oxide mineral, most probably rutile, have been detected. Rutile grains are euhedral, sometimes elongated, prismatic and around 7 μm in size (Fig.  5a). They are more abundant in samples Je-01, Je-02, Je-03 and Je-07 (Table 1). Iron oxides/ hydroxides are around 5 μm in size and more abundant in samples Je-03 and Je-05. The majority of zircon grains are around 20 μm in size, but sometimes larger grains are also present (>150 μm) -some rounded grains point to long transportation (Fig. 5b). Zircon grains are more abundant in samples Je-04, Je-05 and Je-07 (Table 1). Monazite is frequently found in clusters of 5 μm (Fig. 5c). Chromite is detected in some samples (Je-01, Je-04, Je-06 and Je-07) -it is angular and about 5 μm in size (Fig. 5d). Ilmenite was detected only in samples Je-01 and Je-04. It has an average grain size of 10 μm, with the excep-tion of one rounded grain (sample Je-01) larger than 150 μm, which could indicate a long transportation (Fig. 6a). About 3 μm large xenotime grains were found in samples Je-06 and Je-07 (Fig. 6b). Apatite, most probably fluorapatite of 70 μm in size, has been found only in one sample (Je-01; (Fig. 6c), and one very small baryte grain (~ 2 μm) in sample Je-04. The heavy minerals in sample Je-01 represent approximately 0.95 % of the total sample weight. According to mineral compositions and grain sizes all investigated samples are mudstones.   The layers and lenses of fine-grained sedimentary rocks from Jersovec are macroscopically quite similar to up to 2 cm thick shale intercalations from the closest chert outcrop at Izvir. However those from Jersovec do not have fissile structure. Mineralogically, fine-grained materials from both Jersovec and Izvir consist of quartz, clay minerals (illite and minor chlorite); however K-feldspar was found additionally in some samples from Izvir (SkoBe et al., 2013).
The geochemical composition of all the samples is generally similar (Table 2). In all samples SiO 2 prevails, with the contents ranging from 46.3 % (Je-05) to 69.3 % (Je-06), followed by Al 2 O 3 (15 % in Je-06-1 to 24.1 % in Je-05) and K 2 O (2.8 % in Je-02 to 3.4 % in Je-05). The content of Fe 2 O 3 is very variable, from 2.1 % in Je-06-1 to 8.4 % in Je-05. The content of MgO is from 1.6 % (Je-02) to 2.3 % (Je-05). The content of CaO is <1 %, except in sample Je-01 (1.4 %). The values of Na 2 O, TiO 2 , P 2 O 5 and MnO are all <1%. With regard to SiO 2 and Al 2 O 3 , samples Je-01, Je-03 and Je-05 differ from others, as they are poorer in SiO 2 and enriched in Al 2 O 3 . However, with respect to Fe 2 O 3 content and content of majority of other elements, sample Je-01 differs from Je-03 and Je-05 and is more similar to the other samples. On the contrary, Je-03 and Je-05 have more Fe 2 O 3 and also elevated contents of some trace elements, i.e. As, Ba, Ni, Pb, Sr, U, V and REE, and higher values of loss on ignition (LOI). Both samples macroscopically differ from others by their brown colour. Colour and geochemical characteristics could indicate more intense weathering. Further, the results of SEM/ EDS analysis have shown the highest abundance of iron oxides/hydroxides in mentioned samples.
In a discrimination diagram of andreozzi et al. (1997) and di leo et al. (2002a, b), Jersovec samples plot in the field of terrigenous sediments (Fig. 7).  According to 100×Fe 2 O 3 /SiO 2 -100×Al 2 O 3 /SiO 2 discriminant diagram (murray, 1994), depositional environment of the material is continental margin (Fig. 8). Exception is sample Je-05, which is placed outside the diagram due to its high Fe 2 O 3 and low SiO 2 content.
The Jurassic bedded cherts from Izvir are the closest to the investigated area. Comparison of Jersovec mudstones with the Izvir shales shows similar geochemical characteristics. The differences are in higher content of K 2 O in Izvir shales reflecting the presence of K-feldspar. In Izvir area, the mixing of marine and meteoric waters during late diagenesis caused alteration of K-minerals, which were already present in sediments, into K-feldspars (SkoBe et al., 2013). This process has not been detected in Jersovec. According to geochemical analysis, the material from Izvir is of terrigenous and not volcanic origin. Sediments in Izvir area, which is also part of the transitional zone that formed by rifting of thinned (micro)continental margin (RižnaR, 2005), were deposited on a Tethyan passive margin due to rapid subsidence, originally as silica-rich carbonate beds intercalated with mud (SkoBe et al., 2013). As is in the case of Jersovec mudstones, the hydrothermal origin is also excluded for Izvir shales (SkoBe et al., 2013). The average chemical index of weathering (CIW) is 95.7 and is similar to Jersovec samples. It indicates an intense and prolonged source of weathering. Also, index of compositional variability (ICV) points to compositionally mature material (SkoBe et al., 2013).

Conclusions
The fine-grained material from Jersovec chert deposit is macroscopically very similar, but different in colour, ranging from prevailing green (5GY 7/2, 5GY 8/1) to sometimes brown (5YR 5/6, 5YR 3/4). Nevertheless, mineral composition of all the samples is very similar. Quartz and clay minerals, that is illite/muscovite and minerals of chlorite group, dominate, but mineral ratios vary. Further, SEM/EDS analysis showed the presence of Ti-oxide (probably rutile), zircon and monazite in all investigated samples and chromite, ilmenite, xenotime, apatite and baryte in some of them. The average sizes of accessory minerals range from 3 to 20 μm. Larger grains of some minerals (e.g. ilmenite, zircon) are rounded due to transportation. The results of SEM/EDS analysis are strongly supported by geochemical results; the contents of SiO 2 and Al 2 O 3 are the highest, followed by K 2 O, MgO, TiO 2 , CaO, Na 2 O, P 2 O 3 , whereas Fe 2 O 3 is variable. Two samples, Je-03 and Je-05, differ from others, as they are poorer in SiO 2 and enriched in Fe 2 O 3 and some trace elements, i.e. As, Ba, Ni, Pb, Sr, U, V, REE, and Al 2 O 3 and loss on ignition. Also, both samples differ from others macroscopically, as they are brown in colour. The colour and geochemical characteristics could indicate more intense weathering of the two samples as it is also proven by weathering indices. Based on the mineral compositions and grain sizes, all samples are mudstones.
According to the results of the geochemical analysis, discriminant diagrams show terrigenous origin of the mudstones from Jersovec. The material was deposited on the continental margin. The average indices of chemical alteration (CIA) and weathering (CIW) are 79.4 and 93.3, respectively. Both indicate an intense and long weathering. Weathering processes have been also confirmed by well-rounded minerals documented by SEM/EDS. The index of compositional variability (ICV; average value is 0.54) demonstrates the compositional maturity of the material. The similar results have been obtained in the clastics from the vicinity, near Izvir in Krško depression. Fine-grained material from both locations has comparable mineral and geochemical attributes, which point to terrigenous origin and the sedimentation on the continental margin. The difference is the presence of authigenic K-feldspar in Izvir samples as a result of later diagenetic processes. K-feldspars have not been detected in Jersovec samples.