REE-bearing minerals in Drava river sediments , Slovenia , and their potential origin

Monazite and xenotime are major ore minerals for rare earth elements (REE), Y, and actinides, a powerful tool for radiometric dating and indicators of the geological genesis of the parent rock. In this contribution, we present results of microanalysis of monazite and xenotime mineral grains found in heavy mineral sand fraction from Drava river. Investigated grains of monazite(-Ce) were found to be exhibiting varying concentrations of Th, indicating mixed hydrothermal (or greenschist-amphibolite grade) and igneous origin. Monazite was found to be isomorphically replaced by cheralite (CaTh(PO4)2) and subordinately by huttonite (ThSiO4), forming monazitecheralite and monazite-huttonite solid solution. Chemical composition and the degree of isomorphic replacement of monazite grains are indicating their source from rocks of Eclogite belt and Granatspitz massif in Tauern Window, Austria. On the other hand xenotime(-Y) shows consistent concentrations of Y while concentrations of associated heavy rare earth elements (HREE) vary, indicating the origin of mixed affinities.


Introduction
Heavy mineral sand deposits or placers are waterborne deposits of minerals with high specific density, formed as a result of water flow dynamics, either by wave motion, long-term coastal currents or by unidirectional flow associated with rivers.The principal mechanism in the formation of heavy mineral deposits is a natural separation of low and high-density minerals in a manner where low-density minerals are removed by flowing medium thus concentrating high-density minerals (Robb, 2004).Placer deposits are known as a significant source of many industrially important minerals, i.e., diamond, gold, tantalite-columbite, cassiterite, rutile, ilmenite, uraninite, zircon and rare earth element (REE)-bearing minerals (Robb, 2004;Rao & MisRa, 2009).Principal ore minerals mined for REE are monazite, bastnaesite, and xenotime (Voncken, 2016).Monazite is a phosphate mineral of light rare earth elements (LREE) with generalized formula CePO 4 , which besides Ce contains variable proportions of La, Nd, and Pr.Based on predominant LREE incorporated in monazite a suffix is given to denote the most abundant auxiliary element (i.e., monazite(-Ce)) (Voncken, 2016).Monazite occurs as an accessory mineral in peraluminous granites, syenitic and granitic pegmatites, quartz veins, carbonatites, migmatites and paragneisses (FöRsteR, 1998a;bReiteR, 2016).Monazite is considered to be a major host of LREE and smaller fractions of Y, heavy rare earth elements (HREE), and actinides (bea, 1996).Therefore it presents a powerful tool for monazite-based radiometric dating (HaRRison et al., 1995;ZHu & o'nions, 1999;scHandl & GoRton, 2004;GRand'HoMMe et al., 2016).Xenotime, in contrast to monazite, beside Y contains also a significant amount of HREE, among which most commonly occurring are Dy, Yb, Er, and Gd while Tb, Ho, Tm, and Lu are occurring less often (FöRsteR, 1998b;ŠVecoVá et al., 2016;Voncken 2016).Depending on its geological genetic environment, xenotime is considered to be a primary source of HREE and actinides and it is a common accessory mineral in many non-basic igneous rocks, most granitic rocks and granitic pegmatites where it accounts for significant fraction of Y and HREE of bulk rock composition (bea, 1996;FöRsteR, 1998b), while it is also present in migmatites and high-grade metamorphic rocks (FöRsteR, 1998b).Minerals such as monazite and xenotime are considered to be of economic importance especially when enriched in actinides (i.e., U and Th) for their use in nuclear industry, however they may pose an environmental hazard due to increased natural radiation (alaM et al., 1999;Vassas et al., 2006;Rao & MisRa, 2009).Regarding actinides, monazite is known to more often contain Th, while xenotime is more likely to contain U and also smaller amounts of Th (Van eMden et al., 1997;kiM et al., 2009;deeR et al., 2013).This research aims to present the results of microanalysis performed on REE-bearing minerals from heavy mineral fraction of Drava river and to make an attempt to elucidate their potential provenance.

Geographical and geological background
Our case study area is situated near village Zlatoličje in NE Slovenia, along Drava river (Fig. 1a).River Drava has its source in Innichen (San Candido) in the Puster Valley of South Tyrol, Italy, and flows eastwards through East Tyrol and Carinthia regions in Austria into region Styria in Slovenia and further along Croatia-Hungary border, where it joins the Danube near the city of Osijek (tockneR et al., 2009).Along its path, it has a number of tributaries with their sources in Hohe Tauern, i.e., Isel, with its source beneath Grossvenediger (3674 m) joining Drava near Lienz and Möll with its source near Heiligenblutt below Großglockner (3798 m).Knowledge of drainage area and therefrom eroded rocks, of the river and its tributaries combined with in-detail chemical analyses of minerals, both from parent rock and sediments, is crucial for determining the provenance of the grains downstream.The region of Hohe Tauern, in detail discussed by scHiMd et al. (2013) andscHaRF et al. (2013), is considered to be the source of gold found in fluvial sediments of river Drava (beRManec et al., 2014), therefore broader area of Hohe Tauern could also be considered as a source of minerals in heavy mineral sands.
In the area of our case study, fluvial sediments of river Drava are deposited onto Cenozoic basement mainly represented by Upper Miocene (Tortonian to Messinian) marlstones and sandstones paleogeographically a part of Central Paratethys and Pliocene sand, gravel and conglomerate with intermediate coal beds deposited in the Lake Pannon realm (Fig. 1b; Pleničar et al., 2009).The Quaternary period is represented by sediments of Drava reflecting changes in flow dynamics and migrating river channels, thus most common sediments are granulometrically heterogeneous gravel and sand of the river channel, floodplains sediments and organic clay of oxbow lake facies (Fig. 1b; Mioč & Žnidarčič, 1989;Pleničar et al., 2009).

Field sampling
Sampling was conducted in the river bank gravel, in the river channel and in the distal parts of river bars to ensure a high yield of the heavy mineral fraction.Three bulk samples of initially 2 kg each, were washed to remove any plant detritus and clay particles.Bulk samples underwent manual gravitational separation, which was achieved by panning to obtain a relatively pure heavy mineral concentrate.Particles larger than 5 mm were removed manually.Heavy mineral concentrate was further processed by heavy liquid separation in which we used lithium meta-tungstate heavy fluid (LMT) with density [σ = 2.95 g/cm 3 ] to further remove light mineral grains.The remaining mineral grains were, for the needs of optical and electron microscopy, vacuum-cast into two component epoxy resin.Grinding and polishing were performed stepwise using silicon carbide 240 and 1200 grit for grinding and 9 μm, 3 μm and 1 μm diamond paste for polishing to high gloss finish.

X-ray diffraction analysis
Identification and quantification of major minerals forming bulk fluvial sediment and heavy mineral sands were performed using X-ray powder diffraction analysis (XRD, PW 3830/40, PANalytical B.V., Almelo, the Netherlands).Powdered samples were inserted into a holder with divergence slit for incident X-rays at 0.04° and 0.2 mm wide slit for diffraction path.Samples were analyzed in the angular span between 3° -70° 2θ with step 3°2θ /60 s.Source of X-rays was copper anode with the generator set at 30 mA and 40 kV, producing radiation with λ=1.5406Å.Quantification of entities was achieved using Rietveld refinement (goodness of fit: 2.085, R-expected: 13.95, R-profile: 19.97), individual FWHM function and Pseudo Voigt function.Identification of mineral structure was performed using ICSD Database FIZ Karlsruhe (Pan ICSD 2.x).

Optical microscopy and scanning electron microscopy
Identification of minerals, observation of morphology and grain size were performed by Zeiss Imager Z1.m polarizing optical microscope, additionally equipped with in-lens calibrated digital camera for accurate scaling.Scanning electron microscopy (SEM) was performed by using JEOL JSM-5800 with thermionic source under high vacuum (5x10 -6 bar).Preliminary mineral identification was made using backscattered electron imaging (BEI), where relative contrast in the image is directly related to mean atomic number of the investigated sample.Semi-quantitative chemical analysis of the mineral grains was performed using Oxford Instruments energy-dispersive X-ray spectrometer (EDS, mod.6841, Oxford Instruments Ltd.).The operating conditions for EDS analyses were 20 kV, 10 mm working distance and 0° tilt.Beam current was adjusted to yield a dead time of 20-30 %, while the live time was set at 60s.Data reduction was performed using atomic number-absorption-fluorescence matrix correction (ZAF).Prior to SEM observation samples were coated with few nm of amorphous carbon to improve the conductivity of the sample and prevent charging.

Results
Bulk fluvial sediment is uniformly composed of quartz, calcite, dolomite, chlorite, mica, plagioclase and traces of kaolinite and amphibole group minerals (Fig. 2a) as determined by XRD analysis.Bulk sediment samples with an initial mass of 2 kg each, yielded 46.2 -79.7 g of the heavy mineral fraction after final treatment with LMT.Heavy mineral sands are present in the fraction between 0.06 and 0.5 mm corresponding with very fine sand to coarse sand.Prevailing minerals in the heavy mineral fraction are garnets accounting for 92 wt%, followed by hematite (6.7 wt%) and rutile (0.7 wt%) (Fig. 2b).Besides aforementioned minerals also diopside, epidote, ilmenite, hematite, magnetite, gold, pyrite, sphalerite, barite, chalcopyrite, molybdenite, monazite and xenotime were identified using optical or scanning electron microscopy, which overall account for less than 0.1 wt% of heavy mineral fraction, which is below detection limit of XRD analysis (Fig. 2b).

Discussion
Heavy mineral fractions of fluvial sediments from Drava river are predominantly composed of garnets, hematite, rutile and other minerals in traces (e.g., gold, monazite, xenotime, etc.) that account for less than 0.1 wt% of analyzed samples.Heavy mineral fraction is in size order of 0.06-0.5 mm corresponding with very fine to coarse sand and accounts for 2.3-4 wt% of bulk fluvial sediment.
Investigated monazites from Drava river show variable concentrations of REE and Th.Among REE, La, Ce, and Nd were identified in all analyzed samples, and their relationship can be summarized in the following manner based on their abundance in monazite: Ce > La > Nd.REE plotted in the ternary diagram show that majority of the analyzed grains are shifted towards Ce-rich corner thus indicating cerium monazite -monazite(-Ce) variety (Voncken, 2016) (Fig. 5a).
Monazite shows variable content of ThO 2 (0-7.20 wt%) of which highest concentrations were measured in larger isolated monazite grains.Detrital monazite texture, mode of occurrence and its geochemistry have been found to point toward its genetic environment, therefore, enabling separation of igneous monazite from its hydrothermal counterpart (buRnotte et al., 1989;scHandl et al., 1994;WanG et al., 1994). scHandl & GoRton (2004) demonstrated that hydrothermal monazite can be distinguished from igneous monazite by combining petrographic analyses with advanced electron microscopy techniques, performed on monazite mineral grains, and concluded that hydrothermal monazite (or greenschist-amphibolite metamorphic facies monazite) generally contains lower concentrations of ThO 2 (< 1 wt%), compared to igneous monazite, in which concentrations of ThO 2 were found to be significantly higher (3 ->5 wt%).Based on these facts it can be concluded that monazite in heavy mineral fraction from Drava river originates from two or more genetically different sources, hydrothermal (< 1 wt% ThO 2 ) and igneous (3 ->7.20 wt% ThO 2 ) source.Furthermore, monazite grains with highest ThO 2 concentrations are relatively larger and show high mineral liberation factor, indicating longer transport distance than their lower ThO 2 content counterparts.
In order to delineate possible intercrystalline substitution and intercrystalline partitioning behaviour of REE, Y, Th and U in monazite, composition of detrital monazites from Drava river was plotted in the diagram 4×(P+Y+REE) versus 4×(Si+Th+U), proposed by HaRloV et al. (2008), which presents the degree of cheralite (CaTh(PO 4 ) 2 ) or huttonite (ThSiO 4 ) isomorphic substitution in monazite (Fig. 5b).Majority of the investigated detrital monazites from Drava river show slight or significant monazite-cheralite isomorphic substitution; some studied specimens even plotted above monazite-cheralite substitution line indicating the transition of monazite to cheralite.Only a small number of analyzed monazites plot near monazite-huttonite isomorphic substitution line.Observations using BEI image contrast did not show any huttonite or cheralite exsolutions within monazite, thus indicating they rather form solid solutions with monazite.Diagram of LREE versus U+Th+Pb (Fig. 5c) shows one primary field and another rather discrete.Fields are divided from each other by a virtual horizontal line at 0.84 LREE apfu (atoms per formula unit).Monazites in the primary field have a relatively high concentration of REE but somewhat lower concentrations of Th as their counterpart below 0.84 LREE apfu.
Xenotime grains show relatively consistent Y and variable HREE concentrations and lack presence of any actinides (i.e., U and Th).Among   6b).The diagram shows similarity in monazite-cheralite isomorphic replacements in studied Drava river monazites and monazites from Eclogite belt, while the degree of replacement in monazites from Granatspitz massif and Pohorje vary extensively, with only some points exhibiting a nearly similar degree of isomorphic substitution.Based on observed similarity in chemical composition and degree of monazite-cheralite isomorphic replacement, it can be concluded that majority of monazites, found in Drava fluvial sediments, are derived from localities in Tauern Window, among which rocks of Eclogite belt seem to be their most likely source.

Conclusions
1) Heavy minerals account for 2.3-4 wt% of bulk fluvial sediment of Drava river.Heavy minerals are present in a grain size fraction between 0.04 and 0.5 mm and predominantly consist of garnet group minerals, hematite, rutile and other minerals that account for less than 0.1 wt%.
2) Monazite is classified as monazite(-Ce) based on prevalent content of Ce compared to La and Nd.The variable content of Th in monazite suggests it originates from at least two different parent rocks or geological settings.The first group is igneous monazite where relative concentrations of ThO 2 are 3 ->7.20 wt% and second, hydrothermal or greenschist-amphibolite metamorphic grade monazite, with concentrations of ThO 2 < 1 wt%.Monazite was found to be forming monazite-cheralite and subordinate monazite-huttonite isomorphic replacements.No phase separation was observed using BEI, indicating solid solution relationship rather than mineral exsolution.3) Based on relatively high abundance of Y compared to ∑HREE, xenotime grains from heavy mineral fraction from Drava river can be classified as xenotime(-Y), which most probably originates from various sources or geological backgrounds.
4) Chemical composition and degree of isomorphic replacement in Drava river monazite correspond significantly with monazites from Tauern Window, indicating the possible origin of monazite from Eclogite belt and subordinately from Granatspitz massif, while Pohorje Mts. can be disregarded as a potential source of monazite.

Fig. 2 .
Fig. 2. Column diagram of the mineral composition of (a) bulk fluvial sediment and (b) heavy mineral fraction.

Fig. 3 .
Fig. 3. SEM (BEI) images of monazite mineral grains (Mnz) in polished section of the heavy mineral fraction from Drava river.Monazite is occurring as individual liberated grains (a-c) or as inclusions in other minerals, i.e., quartz and garnet (d-f).

Fig. 6 .
Fig. 6.Comparison of chemical composition (a) and the degree of isomorphic replacement in monazite (b) between studied Drava river monazites and monazites from Tauern Window and Pohorje Mts.
metabasites that were metamorphosed during Alpine plate collision.Monazites, presented by uHeR et al. (2014), were acquired from granitic pegmatites emplaced in UHP rocks in Pohorje Mts, north-eastern Slovenia.Comparison of chemical composition of monazites from our case study with that of monazites from localities mentioned above are presented in the diagram Pb+Th+U versus LREE (Fig. 6a).The diagram shows three affinity fields marked in Roman numerals (I, II, III).The chemical composition of Drava monazites significantly corresponds with monazites originating from Eclogite belt and Granatspitz massif in Tauern Window.Affinity field (i), containing the majority of Drava monazites, corresponds greatly with monazites from Eclogite belt studied by HoscHek (2016), while monazites in affinity field (ii) correspond significantly with monazites from Granatspizt massif (FinGeR et al., 1998).Affinity fields (i) and (ii) are divided by 0.84 LREE apfu virtual horizontal line as established above based solely on Drava monazites.Monazites from Pohorje Mts.(uHeR et al., 2014) on the other hand form a separate group (iii) below 0.70 LREE apfu which does not contain any of the investigated Drava river monazites.The degree of monazite-cheralitehuttonite isomorphic replacement is shown in 4×(Si+Th+U) versus 4×(P+Y+REE) (Fig.

Fig. 6 .
Fig. 6.Comparison of chemical composition (a) and the degree of isomorphic replacement in monazite (b) between studied Drava river monazites and monazites from Tauern Window and Pohorje Mts.

Table 1 .
Relative concentrations of major oxides in monazite acquired by semi-quantitative EDS analysis given in wt%.Values marked by »*« represent formulae for 4 oxygen per formula unit (apfu).

Table 2 .
Relative concentrations of major oxides in xenotime acquired by semi-quantitative EDS analysis given in wt%.Values marked by »*« represent formulae for 4 oxygen per formula unit (apfu).