3. Overview of the Paraná volcanic province

One of the largest continental flood basalt provinces in the world, the Cretaceous (c. 135 Ma: Renne et al. Reference Renne, Ernesto, Pacca, Coe, Glen, Prévot and Perrin1992; Wildner et al. Reference Wildner, Santos, Hartmann and McNaughton2006) Paraná volcanic province covers 120000 km² (Bellieni et al. Reference Bellieni, Comin-Chiaramonti, Ernesto, Melfi, Pacca and Piccirillo1984) or only 917000 km² (Frank, Gomes & Formoso, Reference Frank, Gomes and Formoso2009) in southern Brazil, Uruguay, Argentina and Paraguay. The total volume of lavas extruded and preserved was estimated by Frank, Gomes & Formoso (Reference Frank, Gomes and Formoso2009) at 450000 km³ with an additional volume of 122000 km³ in sills. The Serra Geral Formation has a maximum preserved thickness of 1700 m at the depocentre of the Paraná basin (Zalán et al. Reference Zalán, Wolff, Conceição, Marques, Astolfi, Vieira, Appi, Zanotto, Raja-Gabaglia and Milani1990), situated 300 km north of Ametista do Sul. Each lava unit has a thickness between 5 and 80 m (average 20 m; Peate et al. Reference Peate, Hawkesworth, Mantovani, Rogers and Turner1999) and the total number of preserved lava flows is close to 120. In a carefully measured sequence of 16 basalt lava flows along the Serra do Rio do Rastro cuesta on the eastern border of the volcanic group in the Atlantic coast (T. C. Vargas, unpub. Doctoral thesis, Univ. Federal Fluminense, Rio de Janeiro, 2006), the total lava thickness is 700 m.

The Serra Geral Formation is at the top of the Palaeozoic to Cretaceous Paraná basin, which has a total thickness of 7500 m. The climate remained arid throughout the volcanic event (Peate, Reference Peate, Mahoney and Coffin1997); desert conditions prevailed during the formation of the Botucatu Formation below the lavas (Scherer, Reference Scherer2000, Reference Scherer2002; Scherer & Lavina, Reference Scherer and Lavina2006; Holz, Soares & Soares, Reference Holz, Soares and Soares2008) and during the deposition of the Bauru Formation above the lavas (Paula-e-Silva, Kiang & Caetano-Chang, Reference Paula-e-Silva, Kiang and Caetano-Chang2009). Evidence of lava imprint on the surface of sand dunes is observed in the Quaraí region, similar to other exposures in the province (Petry et al. Reference Petry, Jerram, Almeida and Zerfass2007; Waichel et al. Reference Waichel, Lima, Lubachesky and Sommer2006; Waichel, Scherer & Frank, Reference Waichel, Scherer and Frank2008).

Rock types in the Serra Geral Formation are tholeiitic, bimodal basic-acid and include basalt, basaltic andesite, andesite, dacite, trachyte, rhyodacite and rhyolite, here referred to simply as basalt and rhyodacite, the two most abundant rock types in the volcanic province (basaltic andesite is also abundant). The acid lavas occur as a 200 m thick sequence exposed near the Atlantic coast and dip to the northwest, so they are present at depth in the depocentre. A small volume of cordierite dacites occurs in Jaguarão, Rio Grande do Sul, approximately 100 km east of Quaraí; their age is between 157±5 Ma and 139.6±7.4 Ma (Comin-Chiaramonti et al. Reference Comin-Chiaramonti, Riccomini, Slejko, De Min, Ruberti and Gomes2010), so they may be coeval with the earliest Serra Geral Formation lavas (138–126 Ma).

Both basic and acid lavas belong to either high-Ti (commonly >2 wt% TiO₂) or low-Ti (commonly <2 wt% TiO₂) groups (Peate, Hawkesworth & Mantovani, Reference Peate, Hawkesworth and Mantovani1992). High-Ti rocks (Pitanga, Urubici, Paranapanema and Ribeira types) are common in the northern portion of the Serra Geral Formation and low-Ti rocks (Gramado, Esmeralda) in the southern portion. Although Gramado and Esmeralda-type lavas are abundant at the base of the volcanic stratigraphy, some occurrences are registered in the northern portion of the group. Gramado-type lavas constitute 5–10 vol.% of the volcanic formation, the third most voluminous chemical type. Overall, low-Ti lavas occur at the base, followed by high-Ti lavas (>3 wt% TiO₂, e.g. Pitanga-type) which are capped by intermediate-Ti lavas (2–3 wt% TiO₂). Low-Ti and high-Ti lavas occur intercalated in a few places, for example, the Serra Geral escarpment along the Atlantic Ocean coast in southern Brazil (Mantovani et al. Reference Mantovani, Marques, Sousa, Civetta, Atalla and Innocenti1985; Peate, Hawkesworth & Mantovani, Reference Peate, Hawkesworth and Mantovani1992; T. C. Vargas, unpub. Doctoral thesis, Univ. Federal Fluminense, Rio de Janeiro, 2006).

NW-oriented, vertical feeder dykes up to several hundred metres thick are abundant in the Ponta Grossa arch and also occur along the margin of the volcanic group in the Paraná basin and in the Precambrian basement (e.g. Rocha, Marques & Figueiredo, Reference Rocha, Marques and Figueiredo2005; Marques et al. Reference Marques, Ernesto, Piccirillo, Bellieni, Figueiredo and Min2005; Nardy et al. Reference Nardy, Squisato, Machado and Oliveira2005; Machado et al. Reference Machado, Nardy, Squisato and Oliveira2005). Both high-Ti and low-Ti dykes are found, commonly equivalent to nearby lavas. Most dykes and sills are basaltic but some are acid.

We focus our interest in this investigation in the basal portion of the Serra Geral Formation, specifically the Alegrete facies, which contains several basalt to andesite lava flows in the southern portion of the Paraná basin (Table 1). All lavas in the region are known to be low-Ti, Gramado-type, although Turner et al. (Reference Turner, Peate, Hawkesworth and Mantovani1999) mentioned the presence of Paranapanema (intermediate-Ti) below Gramado-type lavas in a borehole in the town of Artigas. We describe the Alegrete facies on either side of the international border and question the interpretation of Turner et al. (Reference Turner, Peate, Hawkesworth and Mantovani1999) on the diachronous effusion of different lava types in the region.

Table 1.

Selected geological and geochemical characteristics of Quaraí mining district (Brazil) and Los Catalanes gemological district (Uruguay)

4. Field description

The pampas of the border region of Brazil and Uruguay are grasslands with only limited forests along Rio Quaraí and its tributary creeks. Rivers and creeks flow into Rio Uruguay, which runs from north to south at a distance of 100 km from Quaraí. The low-lying hills have typical elevations of 100–300 m above sea level, but are as low as 50 m along the Rio Uruguay. The climate is sub-tropical temperate, with average temperature between 0 and 10°C in winter and 25–35°C in summer; rainfall is approximately 1300 mm/yr, distributed evenly throughout the year.

Soil cover is 0.5–1.0 m thick and geological units are poorly exposed. Most outcrops are isolated, but continuous rock exposures occur along creeks. Colada Catalán is well exposed in road cuts along the BR-293 highway, where its internal structure can be fully observed.

The Botucatu Formation underlies the volcanic sequence and is made up of eolian sandstones formed in a large erg in the Cretaceous, possibly the largest erg in Earth history (Scherer, Reference Scherer2000). These sandstones and the underlying Guará Formation sandstones constitute the huge Guarani aquifer (Araújo, França & Potter, Reference Araújo, França and Potter1999; Soares, Soares & Holz, Reference Soares, Soares and Holz2008). The water table in the desert was probably near the surface as in the Sahara and Kalahari deserts today, so sealing by a lava flow very likely filled the aquifer in a short time, perhaps thousands of years. Filling of the erg's topographic basin (widespread ergs occur in topographic lows) with lavas sealed the aquifer and residual volcanic heat caused the intrusion of dykes and sills of liquefied sand with the formation of sand volcanoes and flows on top of the lavas. Clastic sand dykes and sills are common in the volcanic sequence (e.g. Perinoto et al. Reference Perinoto, Etchebehere, Simões and Zanardo2008), presumably as a result of fluidization processes similar to those proposed by Hurst, Cartwright & Duranti (Reference Hurst, Cartwright, Duranti, Rensbergen, Hills and Maltman2003) and Cartwright (Reference Cartwright2010).

The total thickness of the six lavas reaches 100–150 m in most of the region (Fig. 6), although Turner et al. (Reference Turner, Peate, Hawkesworth and Mantovani1999) directly observed lavas 400 m thick, particularly in borehole DDH210 outside the mining district. Total lava thickness attains almost 300 m in one borehole on the Brazilian side of the border (Fig. 7), although it is less than 100 m in most of the region (Fig. 6). A model geological section displays the variable thickness of the lavas (Fig. 8).

Figure 6.

Lithological and scintillometric logs of boreholes that reached down into the Guarani aquifer; data from the Geological Survey of Brazil (CPRM), year 1986. Borehole numbers shown (e.g. IT-5).

Figure 7.

(a) Lithological and scintillometric log of borehole UR-13; data from the Geological Survey of Brazil (CPRM), year 1986. (b) K₂O v. Th+U of studied lava flows; clear correspondence with scintillometric measurements.

Figure 8.

Model geological section of six lava flows from the studied region.

Five lavas (coladas Mata Olho, Catalán, Cordillera, Muralha and Coxilha) were recognized in the field and positioned in correct stratigraphic succession. Colada UR-13 was positioned between coladas Muralha and Coxilha based on scintillometric results from borehole UR-13 (Fig. 7); it was not observed in the field in this study, although outcrop samples were analysed by Nakamura et al. (Reference Nakamura, Wildner, Shibuya, Masuta, Murakami and Romanini2003).

The intermediate-Ti (~2.5 wt% SiO₂) Paranapanema rocks identified by Turner et al. (Reference Turner, Peate, Hawkesworth and Mantovani1999) as flows below the low-Ti lavas are actually intrusive sills in the sedimentary rocks underlying the Serra Geral Formation volcanics (description by DINAMIGE, unpub.). The stratigraphy of borehole 410 is as follows: 0–36 m = basalt flow; 36–321 m = Botucatu Formation and underlying sandstones; 321–400 m = basaltic sill; 412–652 m = Permian sandstones; 652–783 m = basaltic sill; 783–1687 m = Permo-Carboniferous sandstones. This is consistent with surface geology, because the Botucatu Formation sandstones are exposed in Artigas town, close to the location of the borehole.

This is an important point because the main issue of Turner et al. (Reference Turner, Peate, Hawkesworth and Mantovani1999) was that near Quaraí the apparent stratigraphic relationship between the Paranapanema and Gramado types is reversed compared to elsewhere in the Paraná volcanic province. Because the Paranapanema rocks are now correctly identified as sills, no diachronous geochemical stratigraphy is registered in the region.

Colada Catalán directly overlies the sandstones in the basaltic cuesta situated in the northeastern Los Catalanes gemological district, but regionally it is the second lava (Figs 6, 7). In the Quaraí mining district, colada Mata Olho is observed to underlie colada Catalán in several boreholes (Fig. 6). Along the BR-293 highway, coladas Mata Olho, Catalán and Cordillera overstepped the sand dunes in palaeohighs as shown by the evidence of the direct contact of each lava with the Botucatu Formation sandstone. The added thickness of the three lavas indicates an estimated 100 m elevation for the Cretaceous dune (Fig. 8). This is comparable to the height of some of the modern dunes in the Kalahari desert.

The lava flows are not horizontal in the Los Catalanes gemological district because they display a basinal dip (~3°) to the north, but they are nearly horizontal in the Quaraí mining district. The lava sequence was presumably upwarped along the Rio Grande arch, whose NW-directed axis runs through Alegrete (e.g. CPRM, 2009). Many strike-slip faults cut the basaltic association in both mining districts and cause horizontal displacements of undetermined extent. The presence of a 300 m thick lava sequence in borehole UR-13 (Fig. 7), located close to thinner (<100 m) lavas requires the occurrence of nearly 200 m vertical displacements. Similar strike-slip faults and comparable vertical displacements were measured in the Ametista do Sul region, situated 300 km to the north of Quaraí, with the use of geochemical correlation of lava flows (Pinto, Hartmann & Wildner, Reference Pinto, Hartmann and Wildner2010). Nevertheless, lateral continuity of individual lavas can be observed for tens of kilometres in the Quaraí–Los Catalanes districts.

The most siliceous units (≥56 wt% SiO₂, colada Catalán and colada Muralha) have the structure of aa lavas (Fig. 4), particularly the voluminous and extensive, contorted igneous layering and autobreccias. This structure is particularly well-displayed in colada Catalán; colada Muralha also shows structures of aa lava but this colada covers a smaller portion of the studied region. Similar structures were described in aa lavas by Peterson & Tilling (Reference Peterson and Tilling1980), Kilbum (Reference Kilbum1981, Reference Kilbum and Fink1990), Rowland & Walker (Reference Rowland and Walker1990), Hon, Gansecki & Kauahikaua (Reference Hon, Gansecki and Kauahikaua2003) and Bondre & Hart (Reference Bondre and Hart2008). The presence of well-exposed aa lavas in the Paraná volcanic province is here registered for the first time. The least siliceous lavas such as colada Mata Olho and colada UR-13 have the structure of pahoehoe lavas, including the absence of brecciated crusts, but poor exposure prevents more detailed description.

Colada Mata Olho was observed in some drill holes as the basal unit in direct contact with the Botucatu Formation sandstones in the Itaqui and Alegrete regions (Figs 6, 7). Integrated field work with scintillometric measurements led to the identification of colada Mata Olho in a few windows in regions covered by coladas Catalán and Cordillera. Colada Mata Olho is overlain by colada Cordillera in the Mata Olho Creek near the BR-293 highway where colada Catalán is missing. In other places, colada Catalán and colada Cordillera are in direct contact with the eolian sandstones. The thickness of colada Mata Olho varies between 10 and 15 m, as measured in four boreholes (Figs 6, 7).

Colada Catalán was given special attention because it has the structure of aa lava (Fig. 4); it has locally a basal crust and a massive core, and everywhere an upper crust and typical contorted flow banding over large distances. The crusts are made up of breccias of vesiculated lava, whereas the core has elongated amygdales. The core is thick and contains the amethyst geode deposits of colada Catalán in the Los Catalanes gemological district and in Garimpo do Vivi (Fig. 2), Quaraí mining district. Colada Catalán and colada Cordillera are present in all nine boreholes except IT-08 and IT-05 (Fig. 6), where they were probably eroded or not deposited.

An impressive 1–3 m thick autobreccia, formed during lava emplacement, marks colada Catalán in its upper crust and along most of its 50 × 50 km observed extent (possibly 200 km), with angular blocks and variable cement, as first described in Garimpo do Vivi by Augustin (Reference Augustin2007). The breccia blocks are locally immersed in silicified sandstone stockwork, which is invariably in contact with a 0.5–2.0 m thick, silicified sandstone at the top. Autobreccia lacking sandstone is more abundant and has no sandstone layer on top; the upper contact where observed is with colada Cordillera. Colada Catalán has contorted, igneous flow banding in the upper (below the breccia zone) and lower crusts. Breccia portions are observed in lateral contact with massive lava in vertical sections along the BR-293 highway as part of the same flow. The flow is approximately 30–70 m thick (Figs 6, 7) and its upper breccia portion has a distinctive appearance both in satellite images and in the field, because it consists of 100–300 m wide, 1000–2000 m long, ribbons of light grey (in satellite images) rocks alternating with green (in satellite images) grassland portions. This oustanding feature extends for more than 50 × 50 km into Brazil and possibly over an area of 200 × 100 km. The exposed rock is either a breccia or massive, flow-banded rock. The breccia is composed of angular blocks (10–50 cm large) and is clast-supported, mostly with no visible cement. On the Brazilian side of the border, a chalcedony or calcite cement is present in some places.

Colada Cordillera (20–60 m thick, Fig. 6) hosts the world-class amethyst geode deposits in the Los Catalanes gemological district, but no mineralization is known on the Brazilian side of the border. The structure of the mineralized flow (Fig. 5) consists of upper (10 m thick) and lower (5 m thick) amygdaloidal crusts with a massive core divided into two portions. The upper part of the core is 20 m thick, displays columnar jointing and is not mineralized, whereas the lower part (10 m thick) is not jointed and contains the large deposits of geodes. This structure was described in the field and confirmed by chemical analyses of the rocks and scintillometric measurements. This is a novel type of lava structure previously unknown in the Paraná volcanic province.

Colada Muralha is approximately 30–60 m thick and is mostly massive with amygdaloidal lower and upper crusts with a massive core (Fig. 6). It has the structure of aa lava, but was less studied than coladas Catalán and Cordillera because it covers a smaller area. Colada Muralha is exposed at the surface in four boreholes (Fig. 6) and was probably eroded in the others.

Colada UR-13 was first identified in borehole UR-13 (Fig. 7) which drilled all six lavas, and was then confirmed by chemical analyses of outcrop samples. Its thickness is near 60 m in the borehole (Fig. 7).

Colada Coxilha was little studied because it only occurs in one hill in the northern part of the region and in borehole UR-13 (Fig. 7); it is approximately 60 m thick and has amygdaloidal lower and upper crusts. Because colada Coxilha caps the hills in the region, the upper amygdaloidal crust has been mostly removed by erosion.

Coladas Catalán and Cordillera were strongly altered in their mineralized portions due to percolation of hydrothermal fluids. These fluids originated in the Guarani aquifer and are nearly pure water (Krüger et al. Reference Krüger, Gilg, Taubald and Frenz2009; Morteani et al. Reference Morteani, Kostitsyn, Preinfalk and Gilg2010). The highest temperature (~130 °C) attained by the fluids is delimited by the abundant presence of clinoptilolite, a zeolite, widespread in Los Catalanes mines and identified by stable isotopes (Morteani et al. Reference Morteani, Kostitsyn, Preinfalk and Gilg2010). In the Quaraí mining district, this alteration was caused by similar fluids, causing the filling of vesicles (now amygdales) with zeolites, although intense replacement of zeolites by silica minerals is also observed.

Hydrothermal alteration of all mineralized flows is concentrated in pigeonite and less so in augite; plagioclase is little altered (Duarte et al. Reference Duarte, Hartmann, Vasconcellos, Medeiros and Theye2009). Hydrothermal minerals include clays (celadonite, smectites) and zeolites, as determined by X-ray diffraction and electron microprobe analyses (Duarte et al. Reference Duarte, Hartmann, Vasconcellos, Medeiros and Theye2009).

5. Geochemical results

Six different lava flows (including two aa lavas) are identified in the study area from field stratigraphy and systematic rock geochemistry (Figs 9, 10). Selected chemical compositions are shown in Tables 2, 3; all 124 analyses are shown in Appendix 1 (see online appendix at http://multimediafluids.uam.elogim.com/journals/geo).

Figure 9.

Geochemical composition of lavas from the Quaraí mining district and Los Catalanes gemological district. (a) TAS classification; inset shows chemical composition of 355 volcanic rocks from the Paraná province (Nakamura et al. Reference Nakamura, Wildner, Shibuya, Masuta, Murakami and Romanini2003). (b) TiO₂ v. P₂O₅ diagram; black ellipse (arrow) indicates composition of Los Catalanes gemological district surface samples from Turner et al. (Reference Turner, Peate, Hawkesworth and Mantovani1999); black filled symbols (Cat) are from Los Catalanes gemological district, unfilled (Qua) are from Quaraí mining district. (c) MgO v. TiO₂ diagram indicating the low-TiO₂ compositional type of the lavas. (d) TiO₂ v. Zr diagram; (e) Ti/Zr v. Ti/Y diagram; three samples (within black circle) are sills from a drill core in the town of Artigas (Turner et al. Reference Turner, Peate, Hawkesworth and Mantovani1999). (f) Ba/Nb diagram; Pi – Pitanga, high-Ti lava; Pa – Paranapanema, high-Ti lava; Pi and Pa compositions from other regions.

Figure 10.

Variation diagrams of several elements v. TiO₂. All analyses plotted, irrespective of LOI content. Cat – samples from the Los Catalanes gemological district; Qua – samples from the Quaraí mining district.

Table 2.

Selected chemical analyses of volcanic rocks from the Quaraí mining district and Los Catalanes gemological district

(< DL) – below detection limit; Qua – Quaraí mining district, LC – Los Catalanes gemological district; oxides in wt%, elements in ppm; (–) – not analysed.

Table 3.

Classification and selected geochemical data of six lava flows from the Alegrete facies, Quaraí mining district and Los Catalanes gemological district

SiO₂ and K₂O shown for minimum and maximum LOI of samples from each lava. Oxides in wt%; (< DL) – below detection limit; Class. – classification.

The two most intensely mineralized lava flows in the Los Catalanes gemological district are colada Catalán and colada Cordillera, although up to four mineralized flows were indicated in other studies (e.g. Morteani et al. Reference Morteani, Kostitsyn, Preinfalk and Gilg2010). The chemical analyses of the four mineralized flows published by Morteani et al. (Reference Morteani, Kostitsyn, Preinfalk and Gilg2010) correspond to colada Catalán and colada Cordillera (inset in Fig. 9d), so fewer mineralized flows are known than previously considered.

All six studied lavas are of the low-Ti, Gramado-type (Fig. 9c, e), based on Peate, Hawkesworth & Mantovani (Reference Peate, Hawkesworth and Mantovani1992); three Paranapanema samples (Fig. 9e) from Turner et al. (Reference Turner, Peate, Hawkesworth and Mantovani1999) were actually collected from sills in a borehole in the town of Artigas. Most lavas in the Serra Geral Formation (Bellieni et al. Reference Bellieni, Comin-Chiaramonti, Ernesto, Melfi, Pacca and Piccirillo1984) are basalts and basaltic andesites (>90 vol.%) and rhyodacites; the inset of Figure 9a shows the classification of 355 volcanic rocks from the Serra Geral Formation collected by CPRM (Nakamura et al. Reference Nakamura, Wildner, Shibuya, Masuta, Murakami and Romanini2003) and analysed at ACME (Canada). In some regions, such as the Ametista do Sul region (Gilg et al. Reference Gilg, Morteani, Kostitsyn, Preinfalk, Gatter and Strieder2003; Pinto, Hartmann & Wildner, Reference Pinto, Hartmann and Wildner2010), only basalts occur.

The first lava of the Alegrete facies is a basalt; the other five lavas have the compositions of andesite and basaltic andesite (Tables 1, 2, 3). This classification is based on the SiO₂ content of the rocks (Fig. 9a) that have <2 wt% LOI (loss on ignition); strong SiO₂ and K₂O depletion occurs in the rocks at >2% LOI so they are not used for classification. The inverse correlation of SiO₂ and K₂O with LOI was caused by hydrothermal alteration (smectite, zeolites), so we selected for classification the lava composition at the lowest LOI content of the several samples from the same lava. Hydrothermal alteration is highest in the mineralized rocks. For example, colada Catalán is part of the group of Gramado-type lavas with the highest SiO₂ content and is an andesite, but this can only be recognized in little-altered rocks. These have SiO₂ = 57.5 wt% at LOI = 0.6 wt% but hydrothermal alteration reduced the SiO₂ to 50 wt% with corresponding LOI = 7.6 wt%; from SiO₂ content alone, colada Catalán would be incorrectly classified as a basalt. Normalization of the major analyses to 100% free of volatiles before plotting would artificially increase the contents of the other oxides. The contents of most oxides and trace elements are little affected by alteration (Figs 9, 10). Similar geochemical mobility of SiO₂ and MgO (and Rb) was observed by Wood, Gibson & Thompson (Reference Wood, Gibson and Thompson1976) in Iceland basalts.

All chemical analyses are used in variation diagrams of least mobile elements (Figs 9b, c, 10a–f), in spite of variably low to high LOI content. Some elements, such as Y and Nb, display very small deviations from a linear relationship; Rb and Sr have large deviation from linearity, whereas Th and U have small deviations. This indicates low geochemical mobility of most elements during hydrothermal alteration of the studied basalts.

Titanium and phosphorous are among the least-mobile elements. The TiO₂ v. P₂O₅ plots (Fig. 9) discriminate the six different lavas in the region because each has a characteristic composition. The four lavas from the Los Catalanes gemological district are (in stratigraphic order) colada Mata Olho, colada Catalán, colada Cordillera and colada Muralha, and they correlate with the four lavas in the Quaraí mining district in the same stratigraphic order. As mentioned, coladas UR-13 and Coxilha were only studied to the north of the international border.

Chemical analyses of volcanic rocks from the Los Catalanes gemological district published by Turner et al. (Reference Turner, Peate, Hawkesworth and Mantovani1999) include borehole and outcrop samples. Surface samples collected in the district (U2, U3, U6, U7) are chemically similar (Fig. 9b) to colada Cordillera.

6. Scintillometric results

The scintillometric results provide a fingerprint for the stratigraphic succession of the six lavas and are a direct consequence of the variation in the chemistry of the rocks (Table 4; Figs 6, 7a, b). The siliceous coladas Catalán, Muralha and UR-13 have higher cps, K₂O >1.5 wt% and U + Th >7 ppm. Chemical analyses (n = 3) of Botucatu Formation sandstones by Morteani et al. (Reference Morteani, Kostitsyn, Preinfalk and Gilg2010) yielded average 0.32 wt% K₂O.

Table 4.

Average scintillometric results (cps – counts per second, K + U + Th) from six lavas in boreholes and outcrops, Alegrete facies, Quaraí mining district and Los Catalanes gemological district

(–) – not measured; n – number of measurements; av – average; sd – standard deviation; cps – counts per second. Borehole cps estimated from continuous profiling.

Colada Mata Olho has 49±3.2 cps (counts per second) in outcrops, the lowest among the six lavas, and this corresponds to 51.0 wt% SiO₂. Colada Catalán has 123±10.3 cps and 57.5 wt% SiO₂, whereas Colada Cordillera has 62±4.7 cps and 52.5 wt% SiO₂. Colada Muralha has 94±4.6 cps and 56.0 wt% SiO₂ and colada UR-13 has estimated 45 cps in one borehole and 53.0 wt% SiO₂, whereas colada Coxilha has 85 cps in one borehole and 54.5 wt% SiO₂. Total cps values are mostly a result of K₂O content of the rock (Table 3; Fig. 7b) with minor contribution from Th and U.

The chemical stratigraphy of the six lavas is supported by an equivalent scintillometric stratigraphy (Table 4). Colada Mata Olho directly overlies the eolian sandstones and has the lowest scintillometry (~49 cps) of the six lavas, only higher than the sandstones (~43 cps). Colada Catalán is a stratigraphic guide based on its high value of 123 cps, succeeded by colada Cordillera with low scintillometry (62 cps). Colada Muralha is also a stratigraphic guide, because of its high cps value near 94.