Origin of the reduced, amethyst-mineralized lower tier of the Cordillera Flow, southern Paraná Volcanic Province

Abstract

The structure of two superposed layers within a redox-altered basaltic flow is restricted to the Paraná Volcanic Province among large igneous provinces. World-class amethyst-geode and agate-geode deposits occur in reduced Tier 1 of the Cordillera Flow (> 100 km long). Satellite and drone-sourced imagery, several field surveys, and 11 new geochemical analyses were used in this study. The lower Tier 1 (10 m thick) consists of massive rocks without cooling joints, whereas the barren Tier 2 is massive with colonnade. The flow is identified from the stratigraphy and chemical composition (e.g., 1.3 wt.% TiO₂). The presence of poikilitic microphenocrysts of magnetite (mostly plagioclase inclusions) is a distinctive feature. Tier 2 rocks are chemically similar to the original lava; Tier 1 displays a decrease in SiO₂ and K₂O contents and an increase in Fe₂O_3T. Reducing and acidic hot water formed amethyst and agate deposits in Tier 1. Curved shapes (erosional remnants with agate) were formed by silicification of the lower portion of the flow. The interaction of the Guarani Paleoaquifer with the lava flow formed the two layers: the geodes and the curved portions. Other provinces may have similar features where hot aquifer water was available below the basalt.

KEYWORDS:
Reduced Tier 1; Cordillera Flow; Paraná Volcanic Province; Guarani Paleoaquifer; hydrothermalism; amethyst-geode mineralization

1 INTRODUCTION

The structure of two superposed layers within a redox-altered basaltic flow is restricted to the Paraná Volcanic Province (PVP) among intraplate, large igneous provinces (LIPs). The province (Fig. 1) contains large extents of this structure. The PVP is a major LIP and is the world’s largest producer of amethyst and agate geodes (> 400 t/month) (Hartmann, 2014, 2022; Hartmann & Baggio, 2014). The PVP is the sole LIP in the continents hosting world-class amethyst- and agate-geode deposits. The largest producer, with 300 active underground mines, is the Veia Alta Flow (basalt: 51.0 wt.% SiO₂) in Ametista do Sul, Brazil (Gilg et al., 2003; Gilg et al., 2014; Rosenstengel & Hartmann, 2012). Additionally, two flows in Artigas, Uruguay (Duarte et al., 2009, Duarte et al., 2011; Morteani et al., 2010; Techera, 2011; Techera et al., 2007), are also significant producers. The Catalán Flow is the primary host of amethyst and agate geodes, containing the largest (< 7 m) and most valuable amethyst geodes in the province. Smaller (< 2 m) amethyst geodes are produced from the Cordillera Flow. The Catalán Flow is a quartz andesite (57.5 wt.% SiO₂) (Santos & Hartmann, 2021), while the Cordillera Flow is a basaltic andesite (52.5 wt.% SiO₂) (Hartmann et al., 2010). On the Brazilian side of the border, the Cordillera Flow shares similar characteristics with Artigas, but deposits remain concealed in the hills of the pampas (Bergmann et al., 2020; Hartmann et al., 2010). The origin of the protogeodic cavities and their filling has been described (Duarte et al., 2009) as the result of hot (150–50°C) water activity from the Guarani Paleoaquifer (Gilg et al., 2003). This water altered the basalt to smectite-rich rock, and subsequently, amethyst and agate geodes were deposited in the cavities after the temperature lowered to 50°C (Juchem, 1999) or to 50–120°C (Morteani et al., 2010). Teixeira et al. (2018) registered even higher temperatures (230°C) in carbonate veins from Permian sedimentary rocks in the underburden of the NE part of the Serra Geral Group (SGG). Additionally, temperatures exceeding 130°C were determined using C and O isotopes and fluid inclusions in fumaroles from the Permian Teresina Formation in eastern São Paulo State (Varejão et al., 2022).

Figure 1
(A) Geological map of the SGG and the underlying Botucatu Formation. The external limit of the Paraná Basin is indicated on the map, which is sourced from Hartmann and Cerva-Alves (2021). The location of Fig. 1B is also shown. (B) Geological map of the Fronteira Oeste Rift, simplified from the study by Silva et al. (2004).

Basaltic and rhyodacitic rocks from the SGG have been studied for their structures and geochemistry; however, less attention has been given to the hydrothermal alteration related to geode formation and silicification of these rocks. The pampas of southernmost Brazil (BRA) and northern Uruguay (UY) display the flat-lying contact between the underlying Botucatu Paleoerg and the overlying volcanic rocks. The interaction of the aquifer with the volcanic rocks over a large area (> 15,000 km²) may display previously undescribed features of intraplate basalts. Several such features have been described by Hartmann and Cerva-Alves (2021), Hartmann et al. (2021), Hartmann et al. (2022a), Hartmann et al. (2022b), and Hartmann et al. (2022c). Extensive flows in this region exhibit two layers of altered rock, containing geodes in the massive core and curved structures at the base of the flows.

This study focused on the Cordillera Flow along the border of BRA and UY, an extensive (> 100 km) basaltic andesite flow hosting significant amethyst-geode deposits in Artigas, UY, within the lower portion of the massive core (Tier 1). The flow also features curved structures in erosional remnants. The goal was to understand the development of a reduced Tier 1 in the lower portion and an oxidized Tier 2 in the upper portion of the flow. Amethyst and agate geodes occur in Tier 1 and are absent from Tier 2. Methods included satellite imagery, field surveys with RPA flights, petrography, and scanning electron microscopy. New whole-rock chemical analyses (n = 11) were integrated into previously published datasets. The results indicate that Tier 1 was altered, deformed, and mineralized by hot, reducing, and acidic Guarani Paleoaquifer fluids, resulting in chapel-shaped cavities partly filled with amethyst. Agate geodes formed in curved structures at the base of Tier 1.

2 GEOLOGICAL SETTING

The geology of the studied region corresponds to the southern portion of the PVP (Fig. 1), which includes the SGG and related sills and dikes. Covering an area of 1.0 million km², this volcanic group is predominantly located in BRA but extends into UY, Argentina, and Paraguay. In the studied region of BRA, the SGG is represented by the Alegrete Formation (Bergmann et al., 2020; Hartmann & Cerva-Alves, 2021; Hartmann et al., 2010; Wildner et al., 2007), while in UY, it corresponds to the Arapey Formation (Techera, 2011). Underlying the basaltic units (BRA) is the Botucatu Formation (BRA) or Tacuarembó Formation (UY), both consisting of eolian sandstones. The upper units of the Botucatu Formation were deposited during the Early Cretaceous, coheval with the effusion of the earliest lavas. The eastern part of the studied region (6,500 km², BRA) was surveyed by Bergmann et al. (2020) as part of the broader BRA portion of the cuesta (15,000 km²).

The volcanic rocks occur near the top of the Paraná Basin (460–60 Ma, Ordovician to Late Cretaceous) (Scherer, 2000; Scherer et al., 2023). The Bauru Group from the northern part of the basin occurs sparsely in the studied region. The SGG, composed of basaltic (97.5%) and rhyodacitic (2.5%) rocks, reaches a thickness of approximately 1,000 m in large portions of the central depocenter of the Paraná Basin (Frank et al., 2009). The maximum measured thickness observed in a borehole in Cuiabá Paulista (Licht, 2014) in São Paulo state is 1,722.5 m. In the studied region, the volcanic rocks have an added thickness of < 300 m (commonly 100 m), as measured in boreholes by the Geological Survey of Brazil (Hartmann et al., 2010).

Following effusion, the volcanic group and the underlying sedimentary rocks were tilted WNW. Internal faults related to lithosphere rupture led to rifting and the basculation of numerous blocks (Hartmann & Cerva-Alves, 2021), resulting in the formation of the Cuesta de Haedo (Chebataroff, 1951; Lisboa, 1990; Müller Filho, 1970; Suertegaray, 1998; Verdum et al., 2012).

The identification of six flows in the cuesta was based on drill logs from the Geological Survey of Brazil in the 1980s, combined with field surveys, rock geochemistry, and scintillometry (Hartmann et al., 2010; Hartmann et al., 2021). Apatite fission-track studies estimated that approximately 1,000 m of rocks were present in the overburden along the eastern limit of the Fronteira Oeste Rift (Bicca et al., 2020). The same flows were observed on both sides of the international border along a north–south traverse in the eastern half of the Cuesta de Haedo, as confirmed by satellite imagery and field verification in BRA and northernmost UY (Techera, 2011; Techera et al., 2007). In UY, the mineralized portions of these flows occur at different elevations due to normal faulting: the Catalán Flow at 190–200 m and 150–160 m, and the Cordillera Flow at 40–250 m (Techera, 2011).

The first three extensive flows, particularly the first (Catalán Flow) and the second (Cordillera Flow), cover the top of the backslope, with remnants of the two additional superposed flows (Muralha and Coxilha Flows) present locally. The positioning as the first (Catalán) and second (Cordillera) flows is important due to their proximity to the underlying heated aquifer. Productive mines in Artigas were studied in these flows by Duarte et al. (2009) and Hartmann et al. (2010). Comprehensive geological descriptions were provided by Hartmann et al. (2010) for all flows, Hartmann et al. (2024a) for the Catalán Flow, and Hartmann et al. (2024b) for the Muralha Flow.

The Botucatu Formation eolian sandstones are exposed in erosional windows amid lava flows (Santos et al., 2010; Silva et al., 2004). Sand sea structures, such as those described by Amarante et al. (2019), resemble active sand deserts (Al-Masrahy & Mountney, 2013; Cosgrove et al., 2021). The lowest exposed stratigraphic unit in the region is visible in a few erosional windows of the Guará Formation (Angonese et al., 2024). U–Pb dating of the cover SGG places its formation at 134.5 Ma (Gomes & Vasconcelos, 2021; Hartmann et al., 2019; Pinto et al., 2011). Similar geological relationships between eolian units and lava flows have been described by Nogueira et al. (2021) in the Parnaíba Basin. Bertolini et al. (2020) provided a detailed description of the paleoerg, while Cosgrove et al. (2021) examined eolian deposits more broadly. The Guarani Aquifer consists of eolian and intercalated fluvial sedimentary rocks (Hirata & Foster, 2020).

The Cordillera Flow was the second pahoehoe lava covering extensive areas of the Botucatu erg during the Early Cretaceous, occurring in the southern part of the province along the Cuesta de Haedo (Figs. 2 and 3). Preceding the Cordillera Flow, the initial Catalán Flow (described by Hartmann et al., 2024a) had already covered most of the erg. These two are the main flows present at the surface of the Cuesta de Haedo (Fig. 4). The Cordillera basalt consists of a thick (20 m) Tier 2 in the upper part of the core with columnar-jointed massive rock (characterized by curved fractures). This core is locally overlain by a 10 m-thick amygdaloidal crust, which has been eroded at hill tops. Gomes (1996) classified this colonnade in Ametista do Sul as a type-II core. Significantly, the vertical fractures within the core acted as conduits for the rapid movement of hot water from the aquifer to the surface. Despite these fractures, the core remained largely unaltered, showing minimal reaction between the hot water and the massive rock in the center of the columns (Duarte et al., 2009). The number of flows is variable in the cuesta because the UR13 Flow does not occur in some parts of the region.

Figure 2
Satellite image of Cuesta de Haedo in the border swath of Brazil and Uruguay. Reference locality shown – Montes de Haedo (UY).

Figure 3
Drill core log (UR13, Brazilian Geological Survey), displaying six flows (Serra Geral Group) in the Fronteira Oeste Rift and the underlying eolianite (Botucatu Formation-turned Guarani Paleoaquifer). Scintillometric counts per second (cps) down the drill hole are characteristic of each geological unit. Sand injectites, an extrudite, and a mineralized layer are shown.

Figure 4
(A) The Cordillera (Co) Flow covering the Catalán (Ca) Flow. The Co plateau is encircled by an apron of reduced Tier 1 (greenish), followed internally by a rim of yellowish-gray columnar-jointed, oxidized lava (Tier 2). The surface of the plateau (greenish in the image) is underlain by the top of the columnar-jointed massive core. Red-fractured Tier 2 (core) of Ca is below Co. Light gray (in the image) Tier 1 of Ca (Ca1) occurs along creeks. Springs (dark, nearly black in the image) mark the position of the unconfined Cati Aquifer, which is flatlying at the contact of Co with Ca. Co is little fractured. Center of the image near 30°32’22.99”S, 56°07’25.76”W. (B) Sequence of three lava flows––Catalán Flow (Ca), Cordillera Flow (Co), and Muralha Flow (Mu). Springs indicating the position of a flat-lying aquifer at the contact of Ca and Co. White (in the image and gray in the field) rims of Co and Mu are distinctive. Center of the image at 30°14’54.89”S, 55°54’45.80”W.

The mineralized lower core is exposed in mines located in the Cordillera region of UY. The barren upper core is also exposed in these mines, where Hartmann et al. (2010) integrated field geology, scintillometry, and whole-rock geochemistry to describe both high-temperature structures of the core as parts of a single lava flow. This identification contrasts with descriptions by Bergmann et al. (2020), Techera (2011), Techera et al. (2007), who assigned these structures to two superposed lava flows. Additionally, Coladas 4 and 5 of Techera (2011) correspond to the Cordillera Flow of this work, while Colada 3 is the Catalán Flow and Colada 6 is the Muralha Flow in our interpretation.

Additional insights into rift processes in the region are discussed in the works by Hartmann and Cerva-Alves (2021), Hartmann et al. (2021), Hartmann et al. (2022a), Hartmann et al. (2022b), Hartmann et al. (2022c), Hartmann et al. (2023), Hartmann et al. (2024a), and Hartmann et al. (2024b), which also examine the formation of numerous hot springs at the surface of the paleodunes. Hot water conduits were also observed at the top of the Cordillera Flow. Both the Cordillera Flow and, locally, the Muralha Flow (both classified as basaltic andesites) submerged the highest sand mounds (draas) (Bergmann et al., 2020; Hartmann et al., 2010).

The nomenclature of pahoehoe flows was applied to the PVP by Waichel et al. (2006). Reidel et al. (2013) named flow units as follows. A lava “lobe” is the smallest coherent package of lava, lava “flow” is a single outpouring of lava, and a lava “flow field” is the composite unit formed from multiple separate outpourings. In the studied region, all identified units are considered flows, as no small lobes or flow fields were observed.

Rifting occurred in the region 12–15 Ma after the effusion of the SGG lavas (Stica et al., 2014). The three basal flows that covered the erg exhibit layer-cake geometry and can be identified through satellite images and field observations (Figs. 4–9).

Figure 5
Geological relationships (GoogleEarth images) between the Cordillera and Catalán Flows. Coordinates of the center of images: 30°25’49.24”S, 55°57’56.39”W. (A) Distinct light green hill tops (Tier 1 transitional to Tier 2) with narrow white rims (Tier 1) of the Cordillera Flow. Red to brown colors for Tier 2 of the Catalán Flow; white margins of creeks for Tier 1. (A) Visualization of the two flows. (B) Geological map displaying the distribution of the Cordillera and Catalán Flows in the same image as (A). (C) GoogleEarth’s detailed image of one region shown in (a). (D) GoogleEarth image showing the detailed relationships of the ellipses and arcs of the basal portions (Tier 1) of the Cordillera Flow, located above Tier 2 of the Catalán Flow. Agate geodes occur in the eluvium of curvilinear Tier 1. (E) Image of the same area as (A); image from CubeSat (March 12, 2024), composition bands 6, 3, and 1. (F) The same area as (E) showing the presence of the Muralha Flow and its distinction (intense green color and a wide white rim) from the Cordillera Flow.

Figure 6
(A) Field photograph of the erosional remnant arc of the basal portion of the Cordillera Flow (Co1)––grass-covered ground, sided by bushy Catalán Flow (Ca2). Photograph in the area shown in Fig. 5. Exposed rocks are basaltic andesite from the Cordillera Flow and some agate. No rock or agate is exposed in Ca2. (B) Massive smectite+chabazite+kaolinite exposure associated with Co1. The white arrow at a distance indicates the position of (A). (C) Mining front in the Cordillera Flow, Los Catalanes Gemological District (UY), displaying the two structures of the core. The graphite-colored, massive, lower Tier 1 is mineralized in amethyst geodes. The brownish gray, massive, upper Tier 2, with vertical colonnade cooling joints, is barren in geodes. The upper crust was eroded, and the lower crust is below the ground. A 1-m thick hydrothermal breccia makes up the upper portion of Tier 1. Photograph: CPRM.

Figure 7
Geological relationships between the Cordillera Flow and the underlying Catalán Flow. (A) GoogleEarth image (based on Landsat8); (B) Image displays the distinct green color of the Cordillera Flow; image from CubeSat (March 12, 2024), composition bands 6, 3, and 1.

Figure 8
Two aerial images obtained by a remotely piloted aircraft, displaying with high precision (5 cm) the distribution of the Cordillera Flow and its relationship with the lower Catalán Flow and the overlying Muralha Flow. (A) Digital elevation model of the area; (B) RBG color image; geographic coordinates near agate-geode occurrence (red dot): 30°19’40.48” S, 56°05’57.77” W.

Figure 9
(A) Field photograph of common geological relationships at the contact of the Cordillera Flow (Tier 1) with the underlying Catalán Flow and the lowermost Botucatu Formation. (B) Photomicrograph (crossed nicols) of a microporphyritic sample from Tier 2 of the Cordillera Flow. Blocky augite (0.3 mm) and laths of plagioclase (< 0.5 mm) set in a holocrystalline matrix of fine-grained (0.1 mm) plagioclase, augite, and altered magnetite. A flow-identifying porphyritic microphenocryst of magnetite in the lower left corner displaying plagioclase inclusions.

Thin (1–3 m), silicified erosional remnants of Tier 1 (Cordillera Flow) exhibit curvilinear patterns, as seen in Planview (Fig. 5). Tier 1 also features rock outcrops and blocks containing numerous agate geodes (Fig. 6A). The white coloration of Tier 1, evident in both satellite images and field observations, is attributed to the abundance of smectite, chabazite, and some kaolinite (Fig. 6B). Silicified basal portions of the Cordillera Flow host agate geodes. The presence of amethyst geodes above the agate geodes within the flow has not been ascertained, as no mining operations occur on the Brazilian side of the border. Currently, only the amethyst-bearing layer is mined in the Cordillera Flow in UY.

3 METHODOLOGY

Previous investigations in the region (Bergmann et al., 2020; Duarte et al., 2009; Duarte, 2011; Hartmann & Cerva-Alves, 2021; Hartmann et al., 2010; Hartmann et al., 2021; Hartmann et al., 2022a; Hartmann et al., 2022b; Hartmann et al., 2022c; Hartmann et al., 2023; Leitzke et al., 2023; Morteani et al., 2010; Pertille et al., 2013; Techera et al., 2007; Techera, 2011) were reviewed, supplemented by observations from several field surveys and additional laboratory data. Satellite imagery from Google Earth, based partly on Landsat 8, was particularly relevant for geological surveying of the flat-lying pampas.

Evidence from studies by Duarte et al. (2009), Hartmann et al. (2010), Hartmann et al. (2021), and Santos et al. (2010) was integrated with interpretations of the internal structure of the flow. Whole-rock geochemical analyses of 11 samples (location in Table 1) were conducted at ActLabs-Canada. The methods used were FUS-ICP, gravimetry for loss on ignition (LOI), and FUS-MS for trace elements. These analyses were combined with data from Hartmann et al. (2010) (ACME Laboratories Canada) and Bergmann et al. (2020) (SGS Geosol Laboratories, Brazil). Major elements were analyzed using X-ray fluorescence and inductively coupled plasma (ICP) and trace elements by mass spectrometry (MS).

Table 1.
Geographic coordinates of analyzed samples from this study.

Sample	S coordinate	W coordinate
L8	30°28′51.8″	56º00′06.7″
L16	30°29′15.8″	56º07′10.6″
B1.1	30°32′27.6″	56°07′57.7″
C1.1	30°14′49.3″	55°55′14.9″
C2.2	30°14′49.3″	55°55′05.8″
C8.2	30°25′52.1″	55°58′07.6″
C11.2	30°25′52.1″	55°57′59.6″
L11	30°28′02.5″	55°57′24.5″
L12	30°28′06.5″	55°57′30.4″
A7.2	30°40′40.1″	55°51′54.2″
A7.3	30°40′40.1″	55°51′54.2″

The field identification of flows utilized a portable gamma spectrometer, the Super Spec RS 125 from Exploration Radiation Detection Systems, as developed by Hartmann et al. (2010). Total scintillometric measurements of K%, eU, and eTh (and potentially other elements) were recorded as counts per second (cps). The values for the main units were as follows: Catalán Flow (120 cps), Cordillera Flow (80 cps), Muralha Flow (90 cps), and Botucatu Formation sandstones (40 cps).

Aerial surveys covered areas up to 500 m × 500 m using a RPA model Fimi X8. Flights were conducted at an altitude of 120 m with 80% image superposition. Image processing was performed using Metashape software.

4 RESULTS

4.1 Field relations

The Catalán and Cordillera Flows cover the largest part of the exposed surface of the cuesta in both BRA and UY. They exhibit a flat-lying, tabular geometry, dipping approximately 10° to the WNW. The volcanic stratigraphy of the cuesta resembles a layer cake, beginning with the effusion of the Mata Olho Flow in interdunes. Remnants of the Muralha Flow are found near hilltops, locally covered by the Coxilha Flow. The Cordillera Flow (Fig. 4) sustains the relief in most hills (Coxilhas and Cerros) of the Fronteira Oeste Rift due to extensive erosion of the upper flows in many areas.

The longest stretches of unpaved roads in the region are located at the erosional top of the studied flow. Rocky exposures occur in hilltops. The contact between the two tiers of the studied flow is wavy and foggy, as seen in satellite images. Field observations, although limited by poor rock exposures, have documented the interfingering of the two tiers in some outcrops.

The studied flow occurs along the eastern portion of the backslope of Cuesta de Haedo, as observed in satellite imagery and verified in field surveys in BRA and in the northern part of the volcanic province in UY. The top of the vertically jointed core of the Cordillera Flow outcrops at hilltops due to erosion of the overlying 10-m thick amygdaloidal crust. In satellite images, the flow appears grass green with white rims. Vertical cooling joints are well-exposed in quarries and road cuts along highways. Weathered rock with cooling joints appears yellowish-brown, as first pointed out by Morteani et al. (2010). The lower portion of the core is massive and lacks cooling joints, forming a reduced whitish apron approximately 50 m wide and 10 m thick, which appears in satellite images in contact with the red upper Tier 2 of the Catalán Flow. This contact surface can be marked in the field. The studied flow has a thickness of 40–50 m, corresponding to an upper (mostly eroded) 10-m thick amygdaloidal crust, 20–30 m of vertically jointed core at the top (Tier 2), 10 m of the underlying massive core (Tier 1), and a thin (1 m) lower amygdaloidal crust. Amethyst and agate geodes are limited to Tier 1, as observed in the Cordillera mine and other mines within the Los Catalanes Gemological District (Fig. 6A).

The lower Tier 1 rocks of the Cordillera Flow exhibit a microcrystalline texture with graphite-gray color, while weathered crusts appear light gray. Tier 2 consists of fine-grained aphanitic rocks that range from light-to-medium gray with blue tones. Weathering imparts a yellowish-brown hue to the rock along cooling joints, occasionally transitioning to orange or rose tints. The cores of cooling prisms exhibit a little-altered, fine-grained crystalline texture.

A quaternary rocky horizon occurs in many places in the Cordillera Flow as a stratigraphic marker. These surfaces are exposed as flat-lying rocky surfaces devoid of soil and vegetation. This horizon has a regional distribution and is displayed in small areas of variable sizes (e.g., 100 m × 100 m). The geological importance of the horizon requires additional studies regarding evolutionary processes.

The internal structure of the flow exhibits magmatic undulations exposed in the Artigas mines, reminiscent of domes and basins (Bergmann et al., 2020; Hartmann et al., 2010). These structures are here referred to as plateaus (flat-lying tops) and pits. The exposed surface of the studied flow at hilltops appears patchy in satellite images and is also intersected by linear structures corresponding to fault traces. Normal faults with 2–4 m downthrows have been documented within Tier 1 at the mineralized level in the Uruguayan mines.

Extensive erosion of the flow has left remnant patches (1–2 m thick) of the silicified basal crust and lowermost core with varied patterns in Planview, including circles, ellipses, arcs, bands, and irregular layers (Figs. 5 and 6A). Two interflow structures—the top portion of the Catalán Flow and the basal portion of the Cordillera Flow—were intensely silicified. The curvilinear patterns in these structures resemble those described for the silicified upper amygdaloidal crust of the Catalán Flow by Hartmann et al. (2021). Their differentiation requires familiarity with satellite image features and field validation (e.g., scintillometry – 120 cps for the Catalán Flow and 80 cps for the Cordillera Flow). Hydrothermal alteration of the residual patches increased the gamma-ray emission count; for instance, the altered amygdaloidal portions may reach 200 cps. Nevertheless, identification of the flow can be made from measurements in the massive core.

An aquifer occurs either at or below the contact of the Catalán and Cordillera Flows. This is the Cati Aquifer, which is flat-lying and is named after the headwaters of Arroio Cati in the Estância do Cati (a ranch) region. One geographic reference point is 30°23’52”S, 56°01’31”W, where numerous springs are observed at the flat-lying intersection of the aquifer with the hill surface. This Cati region also hosts many agate garimpos (artisanal mines). In hilly areas, these springs are positioned approximately 10 m below the lower contact with the underlying flow (see Figs. 4C–4E). Due to limited rock exposures, precise spring placement within the flow structures remains challenging.

4.2 Whole-rock geochemistry

All SGG basalts in the rift are tholeiitic. The geological relations of samples analyzed chemically by Bergmann et al. (2020) were observed in satellite images and in several outcrops in the field. Diagrams of TiO₂ vs. P₂O₅—oxides with stable contents during hydrothermal alteration (Fig. 10)—led to the insertion of the samples into the classification of flows within the proposal of Hartmann et al. (2010). The Catalán Flow was described by Hartmann et al. (2024a), while the Muralha Flow, which overlies the Cordillera Flow in certain locations, was detailed by Hartmann et al. (2024b).

Figure 10
(A) Lava flow classification based on the TiO₂×P₂O₅ contents of whole-rock chemical analyses from this work. Compositional ellipses and stratigraphic flow numbers are from Hartmann et al. (2010). All samples are from the Fronteira Oeste Rift. Analyses from Bergmann et al.’s (2020) (not shown) plot mostly within the compositional fields of the Catalán, Cordillera, and Muralha Flows. (B) A large decrease in the SiO₂ content of rock samples with an increase in LOI. Red Tier 2 rocks have SiO₂ content close to the original lava, whereas medium gray Layer 1 rocks have much lower SiO₂ content. Tier 2 is oxidized, has vertical colonnade cooling joints, and is barren in amethyst or agate geodes. Tier 1 is reduced, massive, without vertical cooling joints, and contains all amethyst and agate geodes known from the Cordillera Flow. Tier 1 rocks with amethyst geodes in the mines have a high LOI and a low SiO₂ content. Chemical analyses from this work have LOI ~1.5 wt.% with plot dispersed in the diagram.

Tier 1 and Tier 2 of the studied flow exhibit contrasting compositions in the La Bolsa mine, Cordillera region (UY), as well as in many other exposures (Fig. 10). The specific collection and analysis of samples from the well-exposed rock section of the mine (Fig. 11) additionally revealed an increase in Fe₂O₃T content during alteration, from 11.5 to 11.9 wt.% in reduced Tier 1 compared to oxidized Tier 2. Overall, this flow contains less iron than the Catalán Flow (Fig. 11).

Figure 11.
(A) Whole-rock chemical analyses from this work (Cordillera Flow) with plot dispersed in the Fe₂O_3T diagram because the LOI is ~1.5 wt.%. An increase in the Fe₂O₃T content of rocks from the Cordillera Flow (Co), comparing data from Tier 1 and Tier 2 of La Bolsa mine (UY) (from Duarte, 2011; Hartmann et al., 2010). (B) Data for the higher-Fe₂O_3T Catalán Flow (Ca) displayed for comparison; two chemical analyses from this work are also displayed.

The Cordillera Flow is a basaltic andesite (52.5 wt.% SiO₂), classified from low-LOI samples (Hartmann et al., 2010). Analyses from this study yielded similar results (Table 1). The identification of the three main flows in the region was based on field relations and flow chemistry. The Catalán Flow has the highest TiO₂ content (1.7 wt.%) in the cuesta. The Cordillera Flow ranges from 1.1% to 1.2 wt.%, and the Muralha Flow has a content of 1.5 wt.%. Several other elements can be used to support the classification of these flows (Table 2).

Table 2.
Chemical composition of whole-rock samples from the Serra Geral Group in the studied Fronteira Oeste Rift (this work). Oxides in wt.%, elements in ppm. Samples A7.2 and A7.3 are from the same outcrop.

Sample Flow	L8 Ca	L16 Ca	B1.1 Co	C1.1 Co	C2.2 Co	C8.2 Co	C11.2 Co	L11 Co	L12 Co	A7.2 Mu	A7.3 Mu
SiO2	58.35	57.78	53.89	51.46	51.46	53.89	53.46	52.68	52.17	55.69	55.13
TiO2	1.78	1.75	1.22	1.10	1.08	1.16	1.16	1.17	1.17	1.53	1.46
Al2O3	12.56	12.42	14.05	14.15	14.14	13.71	13.72	13.81	13.75	13.07	12.74
Fe2O3T	13.89	13.75	11.32	11.17	11.47	11.76	11.81	11.92	11.50	12.55	12.47
MnO	0.18	0.19	0.19	0.18	0.18	0.17	0.16	0.18	0.15	0.17	0.17
MgO	2.15	2.11	5.34	6.27	6.14	4.99	5.23	5.58	5.34	4.07	4.00
CaO	5.52	5.59	8.91	9.87	9.96	8.66	8.56	9.17	9.22	7.51	7.70
Na2O	2.69	2.71	2.52	2.29	2.29	2.62	2.63	2.17	2.43	2.59	2.45
K2O	2.56	2.50	1.33	1.03	1.03	1.29	1.30	0.67	1.49	1.82	1.17
P2O5	0.22	0.23	0.23	0.13	0.14	0.14	0.12	0.14	0.15	0.22	0.22
LOI	0.95	0.73	1.44	1.30	1.33	1.27	1.51	2.32	1.95	1.12	0.93
Total	100.80	99.74	100.50	98.95	99.22	99.64	99.66	99.8	98.9	100.3	98.97
Sc	34	33	36	37	37	37	37	37	36	35	34
Be	3	3	1	1	1	1	1	1	1	2	2
V	433	428	348	307	308	333	336	340	319	370	365
Ba	502	851	382	246	246	303	299	229	285	364	344
Sr	195	206	212	203	203	204	200	222	202	218	208
Y	37	39	25	24	24	25	25	27	25	29	29
Zr	214	205	118	103	104	115	115	116	114	151	145
Cr	-	-	100	120	130	40	40	40	110	-	-
Co	39	41	43	43	42	44	43	44	45	41	41
Ni	-	-	60	60	60	40	40	40	50	30	30
Cu	100	90	140	120	110	110	130	120	190	140	150
Zn	130	150	110	90	90	100	100	100	100	120	110
Ga	21	21	19	18	18	18	19	19	19	20	21
Ge	2	2	2	2	2	2	2	2	2	2	2
Rb	96	98	50	36	38	46	46	32	52	67	68
Nb	14	13	8	6	7	7	7	7	7	11	11
Sn	3	3	2	1	1	1	1	2	1	2	2
Cs	3.5	2.5	1.8	1.3	1.3	1.6	1.2	1.8	2.0	2.2	2.2
La	33.5	38.4	18.4	15.0	14.7	16.4	16.4	18.2	17.0	23.5	23.7
Ce	68.4	78.6	38.2	30.6	30.4	33.6	34.3	37.0	33.5	49.0	49.1
Pr	8.73	9.57	4.92	3.9	4.0	4.19	4.42	4.76	4.56	6.17	5.99
Nd	34.4	37.4	20.0	16.3	16.7	17.7	17.6	19.1	18.2	24.4	24.8
Sm	7.3	8.5	4.7	3.9	4.1	4.2	4.2	4.7	4.4	5.7	5.6
Eu	1.84	1.87	1.34	1.18	1.12	1.15	1.22	1.27	1.28	1.48	1.49
Gd	6.8	7.7	4.4	4.1	4.1	4.2	4.2	4.7	4.5	5.4	5.3
Tb	1.1	1.2	0.8	0.7	0.7	0.7	0.7	0.8	0.8	0.9	0.9
Dy	6.6	7.5	4.8	4.3	4.4	4.3	4.3	4.7	4.5	5.6	5.5
Ho	1.3	1.4	1	0.8	0.9	0.8	0.8	0.9	0.9	1	1
Er	3.5	3.9	2.8	2.5	2.4	2.4	2.4	2.7	2.5	3.1	3.1
Tm	0.51	0.55	0.39	0.35	0.34	0.34	0.34	0.37	0.35	0.44	0.44
Yb	3.3	3.7	2.5	2.2	2.1	2.2	2.2	2.4	2.3	2.8	2.8
Lu	0.50	0.57	0.37	0.36	0.36	0.32	0.34	0.35	0.35	0.43	0.43
Hf	5.4	5.5	3.2	2.7	2.7	2.9	3.0	3.0	2.9	4.0	3.9
Ta	1.0	1.1	0.6	0.4	0.4	0.5	0.5	0.5	0.5	0.9	0.9
W	-	-	-	-	2	-	2	-	15	-	2
Tl	0.5	0.5	0.2	0.2	0.2	0.3	0.2	0.5	0.3	0.3	0.3
Pb	13	14	7	-	5	6	7	7	6	9	10
Th	8.8	9.0	4.5	3.2	3.3	4.2	4.3	4.3	3.9	6.1	6.0
U	1.9	2.4	1.1	0.7	0.7	1.0	1.0	0.9	0.9	1.7	1.6

• Ca: Catalán Flow; Co: Cordillera Flow; Mu: Muralha Flow; (-): below detection limit (DL); DL of elements: Cr < 20 ppm; As and Pb < 5 ppm; Mo < 2 ppm; Ag and Sb < 0.5 ppm; In < 0.2 ppm; Bi < 0.4 ppm; W < 1 ppm.

5 DISCUSSION

The study focused on understanding the origin of the two tiers: the amethyst-geode mineralization and the curved structures in the lower Tier 1 of the Cordillera Flow. Rock alteration occurred due to the intense percolation of hot water from the Guarani Paleoaquifer into and over the volcanic rocks, as interpreted in previous studies (Hartmann et al., 2022c; Hartmann et al., 2024a; 2024b). Dune collapse and an overall reduction in sandstone porosity led to the forced upward movement of water, similar to descriptions of stratigraphically equivalent formations (predating Gondwana rupturing) in South West Africa (Salomon et al., 2016). The initial porosity of basalt flows is known to reach 30 vol.% (Flóvenz & Saemundsson, 1993). The first hydrothermal event (H1) sealed the porosity, resulting in the forceful injection of fluidized sand during H2 (Duarte et al., 2020), followed by the slow percolation of hot water that altered the rocks and formed amethyst and agate geodes (H3). Hot water was oxidizing and slightly acidic during H1, becoming reducing and less acidic during H2 (sand injection), and finally, reducing and acidic during H3. These processes have been described by Duarte et al. (2009), Hartmann et al. (2012a), Hartmann et al. (2012b), Hartmann et al. (2012c), and Hartmann et al. (2023).

Field relationships, combined with chemical analyses of rocks conducted for this study, enabled the identification of the lava flows, with a particular focus on the Cordillera Flow. The Catalán Flow, the first to cover the active dunes, belongs to the low-TiO₂ (Gramado) chemical type of the SGG (Hartmann et al., 2010). The Cordillera Flow was the second to cover portions of the dunes and most of the first flow (1.2 wt.% TiO₂). The third flow (Muralha Flow) has 1.5 wt.% TiO₂. This study confirms the consistency of a layer-cake stratigraphy for the first lava flows in the Cuesta de Haedo. Each flow is distinguishable by its stratigraphy, internal structure, and chemical properties.

Stratigraphic positioning of the flows resolves eventual ambiguities in classification. Each sequence of three flows in the SGG, including the Fronteira Oeste Rift, is unique in terms of chemical composition and scintillometric properties. Minor and trace elements such as TiO₂, P₂O₅, Zr, Y, Nb, and Th are particularly useful for identification. For rocks with less than 1.5 wt.% LOI, the contents of several major elements such as SiO₂, Fe₂O_3T, MgO, CaO, and K₂O are also distinctive. At higher LOI, the contents of these oxides are significantly altered. For comparison, different flows in the Columbia River Province are distinguished by their contents of TiO₂, P₂O₅, Cr, MgO, CaO, Zr, and Ba (Reidel et al., 2013).

High-LOI samples (Fig. 10) collected from the walls of amethyst-geode mines show a significant decrease in SiO₂ content, reaching 51 wt.% or even 50 wt.% with an increasing LOI. Fe₂O_3T shows a poor correlation with LOI due to minimal alteration in most samples. Correlation is positive between LOI and CaO, MgO, and Sr, while negative correlations are observed with K₂O, Cu, and Rb. Elements such as Zr, Y, Nb, Th, and U show little or no correlation with LOI (Hartmann et al., 2010; their Figs. 9 and 10, not shown here) and remain stable during alteration. The compositions of the four analyzed flows align with the findings of Hartmann et al. (2010; their Fig. 9B) based on a larger database (TiO₂ vs. P₂O₅).

Reddening of the Cordillera Flow along fractures (Tier 2) occurred during the first hydrothermal event, followed by whitening (Tier 2) during the second and third events. An explanation is required for the division of the flow into Tier 2—upper colonnade-structured core—and Tier 1—mostly lower massive core. An evolved understanding of this process is presented here, based on Hartmann et al. (2024a, 2024b), who described the Catalán and Muralha Flows. Reddened rock from the Cordillera Flow is the barren oxidized Tier 2, while the reduced, whitened (by lichen-covered rocks in images and also by clay minerals and zeolites in altered rocks; Fig. 6B) Tier 1 contains world-class mineralization of amethyst geodes. The division into two tiers in the Los Catalanes Gemological District is comparable to that in the less-exploited Fronteira Oeste of Brazil in Rio Grande do Sul.

Both the extensive red and white alteration tiers of the lava flow, the world-class amethyst-geode mineralization, and the curved structures are unique to the PVP among LIPs in the continents. These singular features in LIPs are attributed to the large volume of water available below the volcanic rock, which was essential for the alteration processes. This large water availability is not found in the underburden of other intraplate volcanic provinces. Additionally, a substantial volume of water is required for both the alteration processes and the injection and eruption of hydrothermal fluids laden with fine sand (Montanaro et al., 2022).

Silica minerals were deposited after the alteration of the oxidizing (hematite-reddened rock) to a reducing (little pyrite, abundant clay minerals; light gray to graphite-colored rock) condition of the aquifer. For instance, calcite crystals are occasionally embedded within the celadonite rim of the geode (Morteani et al., 2010), indicating reducing conditions of the ion-transporting fluid (King, 1998). Calcite continued crystallizing throughout the reducing conditions, along with some pyrite. Under these conditions, the geodes were partly filled sequentially with:

• chalcedony;

• fine quartz;

• coarse quartz;

• amethyst.

A direct observation (Fig. 6C) in the La Bolsa mine in the Cordillera Flow leads to a similar interpretation for the geodes from the Catalán Flow.

A large, heated aquifer contained in eolian sandstones interacted with the first three lava flows of the SGG, a major LIP, yielding several results (Fig. 12). There was no significant mechanical interaction between the liquid magma and the silicified sand underneath. Similar observations have been documented throughout the stratigraphy and areal extent of the volcanic group (Hartmann et al., 2023).

Figure 12.
Stratigraphic sequence of first three SGG lava flows covering the Botucatu Paleoerg composed of the Guará and Botucatu formations. One type of collapsed paleodune with the interdunes is displayed; a more variable design is present in the rift. Hot water flux is indicated in the paleodunes and into the overlying volcanic rocks by blue lines. Amethyst- and agate-geode mineralization in Tier 1 of each lava flow is indicated, mostly above the top of paleodunes. Springs are indicated near the contact between the lava flows. Hot springs at the surface of the lava are indicated; similar features occurred on top of every individual unit (paleodune, lava 1, lava 2, and lava 3). Sand injectites and extrudites are omitted for clarity.

Hydrothermal processes have been identified in the origin of the geodes (Duarte et al., 2009; Duarte et al., 2011), although Bergmann et al. (2020) concluded that diagenetic processes were predominant. Oxidation–reduction of rocks and the formation of geodes were described for the Catalán Flow by Hartmann et al. (2024a) and for the Muralha Flow by Hartmann et al. (2024b). The transition of hot water to reducing conditions involved multiple processes, including the availability of nanocrystals of calcite on eolian sand grains, interaction with organic molecules in the sand, and the dissolution of H2 from basalt during hot water percolation (Hartmann et al., 2023; Stevens & McKinley, 2000).

Water for hydrothermalism originated from below, rather than directly from rain or rivers in the hyperarid Botucatu desert (Amarante et al., 2019). After being covered by the first extensive lava flow (Catalán Flow), the erg was filled with freshwater (Hartmann & Cerva-Alves, 2021). However, an interpretation persists that liquid lava interacted thermally and mechanically with the loose sand (Proust & Fontaine, 2007; Rios et al., 2023; Rodrigues et al., 2023). The percolation of water through the Serra Geral volcanic aquifer likely resembled processes observed in the Columbia River Volcanic Province in the USA (Burns et al., 2014).

An analogous search for the hydrothermal process can begin in the oceanic realm due to the availability of vast volumes of water interacting with volcanic rocks (Hartmann et al., 2024b). In the Juan de Fuca Ridge, tholeiitic basalt underwent alteration at temperatures below 100°C, resulting in sequentially oxidized and reduced rocks (Marescotti et al., 2000). Initially, seawater circulation in fractures caused intense oxidation of pillow basalts. This initial alteration event was triggered by normal, alkaline, and oxidizing seawater. Basalt alteration in this ridge (Marescotti et al., 2000) initially occurred under oxidizing conditions, resulting in the formation of Fe-oxyhydroxides, amorphous Fe-hydroxides, celadonite, and celadonite-bearing mixtures (Mg-rich saponite). Subsequent water diffusion through the rock led to progressive basalt reduction. During this reduction process, Fe-saponite and minor sulfides formed, followed by widespread carbonate crystallization. The rock–fluid system transitioned from “water-dominated” to “rock-dominated.”

The upper layers of the oceanic crust near the ridge were altered to include a top oxidized layer and a lower reduced layer, further evolving into a greenschist facies layer (Alt, 1995). Subsequently, conditions shifted from oxidizing to reducing, leading to the formation of Mg-rich saponite and, eventually, Fe-rich saponite. The reducing alteration process additionally produced carbonate and sulfide minerals.

Similar alterations occurred in continental rocks. In the Thunder Bay area of Canada, an initially oxidizing, slightly acidic fluid altered plutonic rocks along veins, resulting in the formation of hematite and sulfates (McArthur et al., 1993). Later, the fluid became reducing and acidic, leading to argillic alteration and the deposition of amethyst along with minor sulfides.

In the PVP, basaltic andesite alteration led to the formation of amethyst and agate geodes in the Cordillera Flow, as well as in the underlying Catalán Flow. Under oxidizing conditions, hydrothermal fluid precipitated Fe-oxyhydroxides, amorphous Fe-hydroxides, along with celadonite and celadonite-bearing mixtures (Mg-rich saponite). The Eh–pH conditions of the fluid during sand injection and effusion were reducing, as evidenced by the common occurrence of pyrite in the groundmass of hydrothermal breccias. This reducing alteration resulted in the formation of Fe-rich saponite, carbonate (calcite), sulfides (pyrite, some chalcopyrite), quartz, chalcedony, amethyst, agate, and traces of fluorite and graphite. Sulfides are commonly found in the Catalán Flow and less frequent in the Cordillera Flow, as reported by Techera (2011).

Similar mineralogy indicative of reducing conditions is observed in the Tevinskoye agate deposits within volcanic rocks in Kamchatka, Russia. In addition to quartz and other silica minerals, Svetova et al. (2023) described carbonates, zeolites, calcite, clinoptilolite, native copper, covellite, chalcopyrite, and pyrite, which also occur in Tier 1 of the flows in the Uruguayan mines.

Paleodunes in the underburden of basalts in the Etendeka region of Namibia underwent alteration under oxidizing to reducing conditions. In the Cuesta de Haedo, the paleoerg sandstones have not been examined for oxidation–reduction. The sandstones from the Etendeka are stratigraphically equivalent to the Botucatu Paleo erg prior to the rifting and drifting of continents. These paleodunes were altered by hot water-led oxidation–reduction processes (Grove et al., 2017); some dunes were red, and others were white. The red alteration occurred first, followed by the white alteration.

Based on similar descriptions elsewhere, this study discussed the origin of the two tiers in the studied region, using the study by Hartmann et al. (2024a) and Hartmann et al. (2024b) as foundational references. The research draws upon previous research by Grove et al. (2017), Hartmann et al. (2024a) and Hartmann et al. (2024b) to explore the origin of these features.

The processes are interpreted through the lens of integrated oxidation–reduction reactions, involving the availability of free electrons and variation in oxidation number, as well as acid–base reactions, involving the transfer of H⁺ between reactants in the presence of hot water. The alteration environment can be described with the support of Eh–pH diagrams.

In short, a substantial volume of water is necessary to alter basalt and form amethyst geodes. In the southern PVP, water was available in the Guarani Paleoaquifer, which underlies the province (Hartmann et al., 2023). The aquifer is approximately 160 m thick in the region and was continuously replenished with freshwater from scarce rainfall in the hyperdry Botucatu Desert and the subsequent Serra Geral Desert. The aquifer provided an almost infinite volume of water for hydrothermal processes. At the time of alteration and mineralization, a vast hot spring field (> 20,000 km²) was active in the region (Hartmann et al., 2022b; Hartmann et al., 2022c). Thousands of hydrothermal bowls are present at the top of the Botucatu Paleoerg, evidencing the Early Cretaceous’ hot spring activity. The heat required to warm up the aquifer to an estimated 150°C likely originated from the mantle during the generation of basaltic magmas, including heat from cooling sills in the Paraná Basin and the underlying continental crust. This activity led to an increased geothermal gradient within the basin. The Guarani Paleoaquifer possibly reached temperatures of 100–50°C near the surface. Comparably, larger hydrothermal vents originated in the eolian dune during the Miocene volcanism in the Western Desert of Egypt (Mazzini et al., 2019).

The Cati Aquifer plays a crucial role in understanding the origin of the geodes in Tier 1. This aquifer is considered comparable to the Tabatinga Fracture and Aquifer in the Veia Alta Flow of Ametista do Sul. The fractures were the feeding channels for the mineralizing fluids. In Brazil’s largest amethyst-geode district, the aquifer serves as a major guide for the opening of new galleries.

In some locations in the studied area, water surges at the contact between flows. These springs result from rainwater moving into the Cordillera Flow from the top of hills, initially concentrating in the flat-lying fracture aquifer near the base of the Cordillera Flow. From there, it flows downward through faults and fractures, eventually reaching the upper crust of the Catalán Flow of variable permeability. However, the downward movement of underground water is interrupted upon encountering the sealed top layer (Tier 1) of the Catalán Flow. Lateral water flow leads to spring emergence, leaving distinct marks on the soil surface of the Catalán Flow, as visible in satellite images. There is no evidence of a connection between this aquifer and the Guarani Aquifer. Nevertheless, the sustained flow of underground water due to rain persists throughout the year, supported by the subtropical, humid climate of the region.

We have thus described the activity of a major, complex hydrothermal cell in the Fronteira Oeste of Brazil, specifically in Rio Grande do Sul, extending into North West UY) during the Early Cretaceous. This hydrothermal cell resided in the sandstones of the Botucatu Formation and partly in the underlying Guará Formation. The heating of the aquifer led to a three-part (H1, H2, and H3) hydrothermal event following each lava effusion. The intense oxidation of the Cordillera Flow by hot water was succeeded by an economically significant reducing event, which resulted in the formation of a world-class amethyst- and agate-geode province.

We now address the possible endowment of the pahoehoe Cordillera Flow in internal magmatic squeeze-ups that formed during lava effusion. The domes (lava rises or squeeze-ups) and basins (lava-rise pits), described as “domes and basins” by Bergmann et al. (2020), serve as indicators of the presence of amethyst-geode and agate-geode deposits within the core of the examined hill. Miners refer to the dipping surfaces that delimit the structures as levante (rise) and rebaixe (dip). The lava rises are more akin to plateaus than domes (Hartmann et al., 2017; Sheth et al., 2024; Walker, 1991; our observations). The squeeze-ups display larger volumes (and thickness) of basaltic andesite (sealed during H1) compared to intervening basins. Effusion rates (Peters et al., 2024) significantly impact flow inflation through internal squeeze-ups. These larger volumes are necessary for the expansion of the altered basalt to form protogeode cavities, later filled with valuable minerals (Hartmann et al., 2014).

The control of mineralization by rises (levante) and basins (rebaixe) is a magmatic structure resulting from flow inflation. Still poorly characterized in the PVP, including the Cordillera Flow, this structure may have major control on mineralization processes in the region. Additional studies are required to identify and distinguish the structure from the alteration caused by oxidation and reduction.

The analysis of the processes involved in the formation and hydrothermal alteration of the Cordillera Flow was conducted. The movement of lava created a favorable structure within the flow, allowing for the confinement of hot water during reducing alteration. This confinement led to the formation of agate geodes and giant amethyst geodes. The effusing pahoehoe lava separated into lower and upper amygdaloidal layers and an internal massive core. During cooling, the core is further divided into two major structures of the massive basalt: an upper colonnade-structured core and a lower unfractured core. The lower core possessed the necessary structure to act as a seal after hot water percolation, subsequently undergoing alteration to form protogeodes.

The processes described here may have occurred in other intraplate basaltic provinces, provided the presence of a strong aquifer below the volcanic layers. Nevertheless, other provinces may display the results of parts of the processes where less water was available for rock oxidation, sand injection, and reduction.

6 CONCLUSIONS

The lower layer of the extensive Cordillera Flow was formed and mineralized into amethyst geodes and agate geodes through the action of hydrothermal fluids originating from the underlying Guarani Paleoaquifer. This process was driven by heated, reduced water, after initial oxidation of the flow. The origin of the two layers (Tier 1 and Tier 2) observed within the core and the rounded structures at the base of this major lava flow in the PVP have been elucidated. These unique structures among LIPs worldwide were formed after cooling the lava to ambient temperature. The oxidizing hot water from the Guarani Paleoaquifer altered the basaltic rock into Fe + 3 oxyhydroxides but caused little whole-rock chemical modification. Subsequently, water + sand slurry was injected into the altered rock. Focused injection along dune tops and silicification generated the rounded structures at the base of the flow. The chemical alteration was significant in the reduced layer, but the contents of some elements remained constant after alteration, allowing identification of the flow. These structures are restricted to this province due to the presence of a heated, major aquifer underneath the volcanic rocks. Similar features might occur in other basaltic provinces if a strong aquifer exists in their underburden.

ACKNOWLEDGMENTS

Landowners are acknowledged for their hospitality and access to their properties. Eduarda Klabunde and David Shiguekazu Kanazawa participated in the field work. The Geological Survey of Brazil allowed the use of a scintillometer. Conselho Nacional do Desenvolvimento Científico e Tecnológico of Brazil (CNPq) provided a research grant (number 403556/2021-0) and a research scholarship and accompanying grant to L. A. Hartmann. The PDF files of the reports by Techera et al. (2007) and Techera (2011) and by Bergmann et al. (2020; in Portuguese) are available from DINAMIGE, UY (in Spanish), or from the Serviço Geológico do Brasil, BRA (in Portuguese), and from the first author of this article.

ARTICLE INFORMATION

• Manuscript ID: 20250007. Received on: 25 JAN 2024. Approved on: 16 FEB 2025.
  How to cite: Hartmann, L. A. & Renner, L. C. (2025). Origin of the reduced, amethyst-mineralized lower tier of the Cordillera Flow, southern Paraná Volcanic Province. Brazilian Journal of Geology, 55, e20250007. https://doi.org/10.1590/2317-4889202520250007

• Funding
  Conselho Nacional do Desenvolvimento Científico e Tecnológico of Brazil (CNPq) provided a research grant (number 403556/2021-0)

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Edited by

• Scientific Editor:
  Carlos Grohmann https://orcid.org/0000-0001-5073-5572

• Associate Editor:
  Valderez Pinto Ferreira https://orcid.org/0000-0003-3220-7113

Publication Dates

• Publication in this collection
  02 June 2025
• Date of issue
  2025

History

• Received
  25 Jan 2024
• Accepted
  16 Feb 2025