Introduction

The Guarani Aquifer, located in South Brazil, Uruguay, Paraguay and Argentine, present contents of fluoride higher than the accepted potability limits in many wells. The fluoride ion (F−) is the most common form in which fluoride occurs in groundwater, although it may also be complexed with aluminum (AlF2+, AlF2 +, AlF3, AlF4 −), boron [BF(OH)3 −, BF2(OH)2 −, BF3(OH)−], calcium (CaF+), ferric iron (FeF2+, FeF2 +, FeF3) and magnesium (MgF) as well as organic compounds (Nordstron and Jenny 1977). Aluminum (Al3+) and ferric (Fe3+) ions usually are not complexed with fluoride (because of their low solubility) in groundwater and tend to precipitate as gibbsite [Al(OH)3], goethite [FeO(OH)] and in the presence of complexed iron, ferrihidrite (Fe10O15.9H2O). However, aluminum and ferric ions can play an important role in restricted, strongly acidic (pH < 4) and oxidized (Eh > 0.0 for Al3+; Eh > 0.8 for Fe3+) environments (Brookins 1988). Magnesium (Mg2+) is an important agent for fluoride complexation in groundwater while calcium (Ca2+) is not relevant in this process because it forms mainly carbonates and sulfate complex ions. Calcium can also be combined with fluoride resulting in the calcium fluoride ionic species (CaF+), which is unstable in the groundwater (Gaciri and Davies 1993).

The origin of fluoride in groundwater can be related to both natural and anthropogenic sources. There are various minerals which have fluoride in this composition, but the source of this anion in groundwater is mainly related to the solubilization of fluorite (Stumm and Morgan 1981; Ashley and Burley 1995). The black shales of Ponta Grossa and Irati formations in the Paraná Basin contain microcrystalline fluorite. This black shale can be an important sources of fluoride registered in the groundwater of the Guarani Aquifer System. It is important to mention that there are no published studies about the relationships between the origin of Guarani Aquifer groundwater fluoride and the black shales of Irati and Ponta Grossa formations.

The Guarani Aquifer System is constituted by the Botucatu Formation, which represents the main water-bearing unit; the Pirambóia Formation and Rosario do Sul Group (Araújo et al. 1995). More recent studies include the volcanic rocks of the Serra Geral Formation (Machado 2006). These lithostratigraphic units present a mineral composition incompatible with the high fluoride groundwater contents (up to 14 mg/l).

In acid waters (pH < 4), fluoride occurs together with hydrogen (HF and HF2 −), which volatilizes at normal atmospheric pressure, and with silicon (SiF6 2−), which is stable at atmospheric pressure. The behavior of fluoride in groundwater is strongly dependent on pH and the solubility decrease in pH 6.0 up to pH 6.5. The increasing solubility of fluoride at acid pH values is due to formation of complexes with aluminum (AlF2+, AlF2 +) and silicon (SiF6 2−), while higher pH values inhibit the adsorption of fluoride due to negatively charged clay minerals (Brookins 1988).

Hydrogeochemistry studies were carried out in some regions of the Guarani Aquifer (Fraga 1992; Kimmelmann et al. 1989; Machado 2006; Marimon et al. 2007) showing a great compositional variation in this groundwater. The groundwater salinity and fluoride content are higher in the confined portions of the aquifer next to the central zone. Some hypotheses have been presented for the origin of the fluoride in groundwater of the Guarani Aquifer in the Rio Grande do Sul state. Lobo and Costa (1998) suggest a relationship of groundwater fluoride with a saline domo in the region of the Rio Pardinho Valley, while Silvério da Silva et al. (2002) associate it with weathering of micas present in the crystalline basement and Triassic reptile fossils from Santa Maria Formation.

Hydrogeochemistry studies made by Licht et al. (1997) in the Paraná state correlate the high fluoride content of groundwater from Guarani Aquifer with intrusive subvolcanic alkaline rocks in the Paraná Basin.

The Ponta Grossa and Irati Formations are composed mainly of mud rocks, such as lutites, mudstones, siltstones, limestones and black shales, (ca. 90% of total volume) that outcrop in Brazil extending for about 3,000 km from Aceguá in the southernmost part of Brazil to Jaciara and Cuiabá region in Mato Grosso state, in the West Centre-west region of the country (Figs. 1, 2). Besides the high frequency of black shales in Paraná Basin, the total rock geochemical, petrography and mineral chemistry studies are reduced when compared with others sedimentary rocks from Paraná Basin. Probably, this is the principal reason for no establishments in specific literature of the relationship between fluoride in groundwater and black shales.

Fig. 1
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Overview of the study area in the Paraná Basin of South America and the chronostratigraphic framework of the studied interval, modified from Zalán et al. (1990) and Milani (1998)

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Fig. 2
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Occurrence of the Gondwana I Supersequence containing the black shales of the Irati Formation and Paraná Supersequence containing the black shales of the Ponta Grossa Formation. The map shows core sample locations along the cross section A–A′ and selected analytical results

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The current paper presents petrological and geochemical results on black shales interval characterization in Permian and Devonian strata from the Paraná Basin, Brazil, and their relationships with fluoride of groundwater from Guarani Aquifer. The relationship between groundwater fluoride origin and Irati and Ponta Grossa Formation have never been presented in previous studies.

Geological setting

The Paraná Basin is a vast sedimentary basin situated in central–eastern South America, including parts of Brazil, Argentina, Paraguay and Uruguay, with an area of about 1,500,000 km2 (Fig. 1; Zalán et al. 1990). Eight Brazilian states are partially included in the area of the basin (Fig. 2). In its central portion, this basin holds a stratigraphic record over 7.500 m thickness (Fig. 3; Milani 1998).

Six second order allostratigraphic units, each of them comprising a geological record of some tens of millions years, define the stratigraphic framework of the Paraná Basin. The overall record spans from 450 to 65 Ma, but much of the time is taken up by the hiatus that split apart the various supersequences. The Rio Ivaí (Ordovician–Silurian), Paraná (Devonian), and Gondwana I (Upper Carboniferous–Lower Triassic) sequences document major Paleozoic transgressive–regressive cycles, whereas the Gondwana II, Gondwana III and Bauru sequences comprise Mesozoic continental sedimentary successions and associated igneous rocks. Source rocks of Ponta Grossa Formation occur in the second supersequence Paraná, whereas source rocks of Irati Formation are developed in the third supersequence Gondwana I (Upper C–Lower Tr), based on data from Milani (1998).

Paraná supersequence (Devonian)

Furnas Formation constitutes of a blanket-like package of white, medium to coarse-grained, sandstones and quartzite. Cross bedding of various natures is always present, and basal conglomerates occur occasionally. Trilobite trace fossils (Assine 1996) support the interpretation that the unit was accumulated on a shallow marine platform. On the top of the Furnas Formation occur fine-grained sandstones with hummocky cross bedding interfingering with the basal Ponta Grossa strata. The Ponta Grossa Formation is a dominantly shaly section reaching a thickness of almost 600 m, divided into three portions or members. The middle Ponta Grossa Formation, of Eifelian age, is a regressive section of sandy deltaic complexes that prograde from the northeastern border of the basin. The unit is completed by a second package of shales, of Givetian–Frasnian age beyond palynomorphs that served to establish the regional pattern of distribution of time-lines for the Ponta Grossa Formation (Lange 1967; Daemon et al. 1967); this unit presents a content rich in macrofossils that certify its Devonian age.

Gondwana I supersequence (Late Carboniferous–Early Triassic)

The Gondwana I Supersequence is a thick and complex package of sedimentary rocks starting with glaciogenic beds of either marine or terrestrial origin, making up the Itararé Group. This unit is composed by an association of sandstones, diamictites, conglomerates, and shales that reach 1.400 m of maximum thickness, (Milani and Zalán 1998).

The sedimentation included a large component of gravitational mechanisms, driven by the voluminous influx of clastics from those areas laid open by deglaciation. Rhythmic facies are common, as well as syndepositional deformation of strata due to slumpings. França and Potter (1988), using well log data, recognized three major finning-upward cycles in the glaciogenic package, probably reflecting climatic changes of higher order during the overall glacial time. Their cycles define a pattern of southward onlapping, related to a progressive expansion of the basin.

The Irati formation is a shaly section reaching a thickness of approximately 120 m, divided into three portions or members. The Irati formation represents a mixed dynamic system of siliciclastic and carbonatic sequences. The siliciclastic sequence predominates in the eastern portion of the basin and the carbonatic sequence prevails in the northern portion of the Paraná Basin (Araújo 2001).

Tectonically, the Paraná Basin forms a NE–SW ramp (Fig. 3), with a smooth inclination of about 0.0015° towards the southeast in its southern portion. Under this paleographic configuration, inherited because of the installation of a magmatic arc in the Permian, three different lithofacies associations were formed, which developed under depositional domains related to the carbonatic internal, intermediate and distal slope (Araújo 2001).

Fig. 3
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Geologic cross section showing the sequences and structural framework of the Paraná Basin (Assine 1996)

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In terms of sequence stratigraphic interpretation, the lithofacies associations of the Irati Formation represent retrogradational, progradational and aggradational patterns (Milani 1998), which occur in three different fourth order cycles of relative sea level variations. According to this concept, the three depositional sequences of the Irati Formation were formed in transgressive and high system tracts settings (Araújo 2001).

Methods

The characterization of inorganic components in black shales was made with X-ray fluorescence (EDXRF) to determine the elemental concentration, X-Ray diffraction (XRD) to determine the mineralogical components and scanning electron microscopy (EDX, BSE, SE) to determined the elemental modes of occurrence in the shales. Lixiviation and solubilization tests were also performed to estimate the potential of this shale as a font of fluoride registered in the groundwater of Guarani Aquifer.

Sampling

Sample material was collected from outcrops, surface mines and core from exploration boreholes, provided by the Geological Survey of Brazil (CPRM) and the National Department of Mineral Production (DNPM). The study is based on an extensive sampling representing the whole area of outcrops occurrences of the Irati and Ponta Grossa formations along a SE–NW transect in the Brazilian portion of the Paraná Basin (Fig. 2). The samples were collected as full interval channel samples, including parting. Following sampling the material was homogenized according to ASTM (1991) norm D2797 and split for the various analyses.

Solubilization/lixiviation tests

Solubilization and lixiviation tests were done in five samples (Table 5) of black shales of the Irati and Ponta Grossa Formations that presented fluorite in electronic microscopy analyses. The methodology applied for solubility/lixiviation tests are described below. Ten grams of crushed sample (smaller than 200 mesh) were added to 100 ml of distilled water in a closed PVC container. The mixture was stirred mechanically for 30 days for the solubilization of minerals. After this period, the pH was adjusted to a value next to 9.0. The solution was maintained at rest for 48 h for the decantation of solid particles and lixiviation of fluoride adsorbed in minerals and in organic matter. The solutions were then filtered and analyzed for determination of dissolved fluoride concentration. The dissolved fluoride was analyzed through visible spectrophotometry technique using a HACH DR-2010 machine. The content of fluoride extracted from sediments was, calculated by the equation:

$$ F_{{{\text{mg/kg}}}} = F_{{{\text{mg/l}}}} \times Vs{\text{/}}Ma $$

F mg/kg is the mass (g) of fluoride extracted per kg of sediment sample; F mg/l is the concentration of fluoride of the solution; Va is the volume of the solution (ml) and Ma is the mass of sample (g).

X-ray fluorescence (EDXRF)

The major oxides and trace elements were analyzed in Rigaku (RIX-2000) equipment–X-ray fluorescence spectrometry analyzer. The analysis of major oxides was done in glass bricket using 50 kV and 40 mA for a duration of 40 s and 10 s for pick and background lectures, respectively. The analysis of trace elements was done in pellets using 50 kV and 50 mA for a duration of 60 s and 10 s for pick and background lectures, respectively. For trace elements, corrections were done for matrix effects and pick interference.

X-ray diffraction (XRD)

X-ray powder diffraction patterns were obtained with an X-ray diffractometer (D5000 Siemens-Bruker-AXS, 40 kV, 25 mA, Cu Kα radiation, diffract plus® Siemens-Bruker-AXS software was used for data treatment). The sampling of black shales was carried out with steps of 0.02° and its range was 2–72° in Bragg Brentano geometry.

The total mineralogy was analyzed in 22 samples of shales from Irati and Ponta Grossa formations, of which 6 samples were from Ponta Grossa Formation and 16 samples were from the Irati Formation.

Scanning electron microscopy (SEM)

The distribution of the elements in the black shales of Irati and Ponta Grossa Formations inorganic fractions was determined using scanning electron microscopy (SEM).

For the SEM, the following procedures were executed: The samples were fixed on a stub and carbon and gold coated, using a Sputter Coater (SCD 005/BALTEC) with approximately 20 nm of thickness, in order to eliminate any undesirable charge effects during SEM observations. After these procedures, it was possible to carry out the microscopy analysis. The instrument used in this study was standard Philips XL30 model. The images used in this work were obtained in back-scattered electron (BSE) and secondary electron (SE) mode, with a primary scanning beam of voltage of approximately 20 kV. Inorganic inclusions on the surface of source rocks specimens were analyzed using a SEM equipped with an energy dispersive X-ray (EDX) detector.

Petrography

In addition to chemical analysis, routine four samples of black shales were selected for petrographic analysis, which was carry out using binocular polarized microscope Leica DFC-280 with Sony 3CCD digital camera in thin sections and polished sections to identify the main mineral phases.

Results and discussion

The techniques used in this work were applied to evaluate the nature of mineral matter. This study illustrates that the SEM techniques of back-scattered electron scanning (BSE) and microanalysis represent important tools in the field of organic geochemistry. It was possible by means of SEM to locate sedimentary organic matter in source rock and to directly analyze for elemental content using BSE and EDS.

Mineralogy

Irati and Ponta Grossa black shales are fine-grained clastic sedimentary rocks, with a mean grain size smaller than 65 μm that are composed of organic matter, clay minerals (kaolinite and Illite), and contain subordinate amounts of quartz, plagioclase, carbonates (calcite, dolomite and ankerite), phosphate minerals, sulfides (pyrite) in some cases, barite, fluorite and rutile (Fig. 4). These shales usually show laminated bedding (Fig. 4a–c), sometimes deformed by volcanic clasts, ranging from less than millimeters to 1 cm thickness (Fig. 4d). The diffractograms show general patterns in the main and more predominant mineralogy, seeking to illustrate the results of Irati (Fig. 5a) and Ponta Grossa (Fig. 5b) Formations.

Fig. 4
figure 4

Photomicrography of thin sections showing the fabric, structures, textures and mineralogical composition of samples from Irati and Ponta Grossa Formations. a Plan parallel lamination and optical fissility typical of black shales from Irati Formation. b Laminar structure in samples of Ponta Grossa Formation. c Plan parallel lamination and quartz and pyrite crystals in organic matrix in Irati black shales. d Volcanic clast deforming the laminated structure in Irati black shales. e Thin section of calcilutite intercalated in black shale levels showing crystals of quartz, clay minerals and pyrite and organic matter fragments. f Main mineral phases in Irati black shales and a plagioclase crystal involved in diverse grains of quartz, clay minerals and opaque minerals. g Photomicrography of thin sections showing the fabric, structures, textures and mineralogical composition of black shale samples and diverse grains of quartz involved in a matrix composed of muscovite, lamellar smectite and pyrite. h Main mineral phases in Irati black shales and diverse grains of quartz involved in a matrix composed of muscovite, pyrite and very fine lamellar smectite

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Fig. 5
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Examples of samples of Ponta Grossa with SEM micrographs and X-ray characteristic (EDS), respectively, showing the morphology and identification of mineral content. a K-Feldspar and Calcite in sample 04-056. b Rutile and Pyrite in sample 04-079. c Pyrite in sample 04-077. d Fluorite and Ankerite in sample 04-079. e Quartz, Barite and Pyrite in sample 04-079

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The samples analyzed by SEM indicated that Ponta Grossa Formation present main mineralogy like quartz, illite/muscovite, kaolinite and pyrite.

In some samples, the presence of chlorite, K-feldspar, hematite, smectite and pirofilite was observed (Fig. 6). Irati black shales are, in its great majority, formed essentially for generally fine granulometry and constitute of clay-minerals, polymorphous silica, essentially quartz, K-feldspar, plagioclase, carbonates such as calcite, dolomite and ankerite, oxides such as rutile or anatase, sulfites such as pyrite (amorphous, framboidal and cubic crystalline form), sulfates such as barite, and phosphates such as apatite and fluorite (Fig. 7). These results are shown in Table 1.

Fig. 6
figure 6

Examples of samples of Irati with SEM micrographs and X-ray characteristic (EDS), respectively, showing the morphology and identification of mineral content. a Calcite in sample 04-055. b Calcite and Dolomite in sample 04-049. c Pyrite in sample 04-166. d Barite in sample 04-165. e Barite and Pyrite in sample 04-165

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Fig. 7
figure 7

a Illustrative diffractogram of sample from Ponta Grossa Formation (sample 04-074). The figure shows the main mineralogy of the shale analyzed. Q quartz, Il Illite, Gy gypsum, Sm smectite, Pl plagioclase. b Illustrative diffractogram of sample from Irati Formation (sample 04-118) Q quartz, Il illite, K kaolinite. The figure shows the main mineralogy of the shale analyzed

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Table 1 The mineral phases present in the analyzed shales from Irati and Ponta Grossa formations
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Geochemical: Major oxides of shales

The major element distribution is based on whole black shales characteristics; on the other hand, the proportions of the different elements has been determined with the black shales using X-ray fluorescence (XRF) analysis that shows features of black shales of Irati and Ponta Grossa Formations, as can be observed in Table 2.

Table 2 Major constituents of black shales samples from Irati and Ponta Grossa formations
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The identification of the different oxides by XFR methods shows that in Irati Formation, the black shales are richer in SiO2, Fe2O3, MgO, CaO, P2O5 and SO4 than the Ponta Grossa black shales. On the other hand, Ponta Grossa black shales are richer in Al2O3, TiO2 and K2O.

The range for total sulfur was 2.96–4.68 wt%, with an average of 3.82 wt%. The Na2O percent in the Ponta Grossa black shales is very low, generally less than 0.32 wt% but Irati samples have more than 2.12 wt% Na2O. The Al2O3 content is relatively low in Irati samples, with many samples having less than 13.2 wt%, but in Ponta Grossa samples, the Al2O3 content is larger than 25.2 wt%. These elevated Al2O3 concentrations in Ponta Grossa Formation are the results of great quantity of clay minerals in these rocks.

The CaO contents are very low in the Ponta Grossa samples, generally less than 0.03 and 1.66 wt% in Irati samples. The Fe2O3 contents in Irati Formation present an average value of 4.17 up to 12.90 wt%. In Ponta Grossa Formation, the Fe2O3 contents are highly variable ranging from 5.99 to 13.89 wt%. These elevated Fe2O3 concentrations in Ponta Grossa Formation are the results of great quantity of pyrite in these rocks. The values of K2O are low in the Irati samples, generally smaller than 2.26 wt% in Ponta Grossa Formation. The SiO2 content ranges from 60.17 wt% in Irati samples to 57.3 wt% in Ponta Grossa samples.

The X-Ray fluorescence analysis confirms the high content of SiO2 in the black shales and the relatively low MgO, MnO and TiO2 contents with average values of 1.01, 0.02 and 0.03 wt%, respectively, for Ponta Grossa Formation. In the Irati Formation, these vales are 1.03, 0.03 and 0.14 wt%, respectively.

Geochemical: Trace elements of shales

Irati and Ponta Grossa black shales present chemical compositions characterized by high plumbum (up to 780 ppm) and barium (up to 769 ppm) concentrations. The sample concentrations of major oxides and trace elements are presented in Tables 2 and 3, respectively. A comparison of average concentration of Irati and Ponta Grossa Formation with other black shales from other parts of the world is presented in Table 4. The data presented in this table shows that Irati and Ponta Grossa Formations have higher concentrations of Al2O3, Fe2O3, Ba and Pb than black shales from others sedimentary basins. On the other hand, the average concentrations of Mo, V, Ni, Zn, Y, P and Mn are lower, wile the concentrations of Cr, Cu, Co, Sr, Nb, Rb, Ga, Ag, Cd and As are similar when compared with other black shales (Leventhal 1993. In: Engel and Macko 1993).

Table 3 Traces elements of black shales samples from Irati (I) and Ponta Grossa (PG)
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Table 4 Representative element values for selected shale units from Leventhal (1993) modified and average data obtained from Ponta Grossa and Irati shales
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Geochemical: Fluoride solubilization and lixiviation of shales

Microcrystals of fluorite occur frequently dispersed in the matrix of Irati and Ponta Grossa Formations (Fig. 5d). The solubilization of this fluorite can be the most important source of the fluoride dissolved in groundwater of Guarani Aquifer. Solubilization and lixiviation tests were done in samples from the black shales of the Irati and Ponta Grossa Formations. The resultant solutions of these experiments showed high contents of dissolved fluoride. The results obtained indicate solubilization and lixiviation of 8.6–21.80 mg of fluoride per kg of sediment. The concentration of fluoride in the solutions obtained in the solubilization/lixiviation tests present good positive correlation with the content of organic matter in the black shales (Table 4; Fig. 8). This means that the dissolved fluoride is more enriched in groundwaters that circulate through the black shales with higher content of organic matter (Table 5).

Fig. 8
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Correlation diagram for F versus TOC showing high affinity of the fluoride and organic matter

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Table 5 Geochemical: fluoride solubilization and lixiviation of black shales
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The results demonstrate that the black shales can be the main source of the high fluoride content in the groundwater of Guarani Aquifer System. The incorporation of fluoride into the groundwater of Guarani Aquifer is related to deep circulation mechanisms of the groundwater that ascend from the underlying packages of the Paraná Basin through regional faults and fractures. These regional structures are identified and described by many authors, such as Zalán et al. (1990), Milani (1998) and Machado (2006).

Conclusions

This work suggests that the fluoride present in groundwater of Guarani Aquifer results mainly from water that circulates in deeper seams such as Ponta Grossa and Irati Formations. This promotes the dissolution of fluorite and lixiviation of fluoride resulting in anomalous concentrations of this ion in the groundwater from many locales of the aquifer.

The content of V, Mo, Cd and B in the Ponta Grossa Formation shales are low. Regarding the Irati shales, they can be described as the ones that showed higher contents of Y, Ga, Ni, Co, Cu, Sr, Zn, Nb, Rb, Cr, Ba, As and Zr.

The mineralogy of the black shales from the formations of Irati and Ponta Grossa is constituted essentially of quartz, illite, muscovite, kaolinite and smectite, and great amounts of organic matter. Plagioclase, calcite, dolomite, ankerite, apatite, pyrite, barite, fluorite and rutile are also registered. The fluorite appears frequently as cubic diagenetic microcrystals, identified only in the MEV analyses that normally associate with the organic matter.

The chemical composition of these black shales is characterized by the high contents of Al2O3 and Fe2O3 and low content of CaO, and associated with the organic matter, high contents of Pb, Ni, Co, Cu, Zr, Cr, Ba and V were registered.

The high content of microcrystalline fluorite in black shales allows the solubility and leaching of great amounts of fluoride from them, which are characterized as a potential source of high fluoride concentrations registered in the groundwater of Guarani Aquifer System. The interaction of the groundwater circulating in the deep black shales with groundwater from Guarani Aquifer System occurs through great faults and tectonic lineaments that constitute excellent hydraulic connectors between the two sedimentary packages.