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Sequential opening and filling of cavities formingvesicles, amygdales and giant amethyst geodes in lavasfrom the southern Paraná volcanic province, Brazil andUruguayLéo Afraneo Hartmann a , Lauren da Cunha Duarte b , Hans-Joachim Massonne c , CassianaMichelin a , Leonardo Manara Rosenstengel a , Magda Bergmann d , Thomas Theye c ,Juliana Pertille a , Karine Rosa Arena a , Sandro Kucera Duarte a , Viter Magalhães Pinto a e ,Eduardo Guimarães Barboza a , Maria Luiza C.C. Rosa a & Wilson Wildner da Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Avenida BentoGonçalves, Porto Alegre, Rio Grande do Sul, Brazilb Departamento de Materiais, Escola de Engenharia, Universidade Federal do Rio Grande doSul, Porto Alegre, Rio Grande do Sul, Brazilc Institut für Kristallchemie und Mineralogie, Universität Stuttgart, Stuttgart, Germanyd Geological Survey of Brazil (CPRM), Rua Banco da Província, Porto Alegre, Rio Grande doSul, Brazile Departamento de Geologia, Universidade Federal da Roraima, Rio Branco, Roraima, BrazilVersion of record first published: 06 Aug 2010.

To cite this article: Léo Afraneo Hartmann , Lauren da Cunha Duarte , Hans-Joachim Massonne , Cassiana Michelin , LeonardoManara Rosenstengel , Magda Bergmann , Thomas Theye , Juliana Pertille , Karine Rosa Arena , Sandro Kucera Duarte , ViterMagalhães Pinto , Eduardo Guimarães Barboza , Maria Luiza C.C. Rosa & Wilson Wildner (2012): Sequential opening and fillingof cavities forming vesicles, amygdales and giant amethyst geodes in lavas from the southern Paraná volcanic province, Braziland Uruguay, International Geology Review, 54:1, 1-14

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International Geology ReviewVol. 54, No. 1, January 2012, 1–14

Sequential opening and filling of cavities forming vesicles, amygdales and giant amethyst geodesin lavas from the southern Paraná volcanic province, Brazil and Uruguay

Léo Afraneo Hartmanna*, Lauren da Cunha Duarteb , Hans-Joachim Massonnec , Cassiana Michelina , Leonardo ManaraRosenstengela , Magda Bergmannd, Thomas Theyec , Juliana Pertillea , Karine Rosa Arenaa , Sandro Kucera Duartea ,

Viter Magalhães Pintoa,e , Eduardo Guimarães Barbozaa , Maria Luiza C.C. Rosaa and Wilson Wildnerd

aInstituto de Geociências, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, Porto Alegre, Rio Grande do Sul,Brazil; bDepartamento de Materiais, Escola de Engenharia, Universidade Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do

Sul, Brazil; cInstitut für Kristallchemie und Mineralogie, Universität Stuttgart, Stuttgart, Germany; dGeological Survey of Brazil(CPRM), Rua Banco da Província, Porto Alegre, Rio Grande do Sul, Brazil; eDepartamento de Geologia, Universidade Federal da

Roraima, Rio Branco, Roraima, Brazil

(Accepted 17 May 2010)

The opening and filling of cavities in rocks are the major processes related to the generation and sealing of porosity in oredeposits. This study documents three stages of opening and filling of vesicles and geodes in the basalts and rhyodacites of thesouthern Paraná volcanic province. Each step detailed here is actually part of a sequence of minor hydrothermal events. First,lava degassing at high temperature (1150◦C) formed small (<4 cm) vesicles in the crusts of flow units. In sequence, thesevesicles were partly to fully filled at low temperature (30–150◦C) by hydrothermal minerals, particularly clays and zeolites;this process also sealed the porosity of the lava. Second, the injection of fluidized sand generated new cavities, which werepartly filled with sand; the newly formed porosity was sealed by the low-temperature fluid. Third, intense alteration ofthe basalt or rhyodacite core into a claystone favoured the opening of small to giant protogeodes (0.1 mm to 4 m) bydissolution; cooling of the fluid led to the precipitation of hydrothermal minerals, particularly the spectacular amethyst,calcite, and gypsum-bearing geodes.

Keywords: vesicles; amygdales; giant geodes; amethyst-quartz; Serra Geral Group

Introduction

The opening and filling of cavities in rocks are the majorprocesses related to the generation and sealing of poros-ity in ore deposits. The Paraná volcanic province of SouthAmerica (Figure 1) constitutes one of the largest conti-nental flood basalt provinces in the world and has largedeposits of amethyst and agate (calcite, gypsum) geodes.Specimen-quality minerals, including several zeolites, arealso abundant in the amygdales of the volcanic rocks. Theinvestigation of these different cavities is therefore a majortopic in the geology of volcanic rocks.

Basalts and rhyodacites (including some andesites)from the Cretaceous (133 Ma) Paraná volcanic provinceare included in the Serra Geral Group. These units cover1,300,000 km2 of South America, not only in Brazilbut also in Uruguay, Argentina, and Paraguay (Bellieniet al. 1984). Monthly production of geodes from under-ground mines is nearly 400 tons, mostly from Ametista doSul (Brazil) and Los Catalanes (Uruguay), with commerceconcentrated in Soledade, Brazil.

∗Corresponding author. Email: leo.hartmann@ufrgs.br

The filling of vesicles with zeolites and geodes withquartz and amethyst is recognized as a low-temperatureprocess (50◦C) (see, for example, Juchem (1999), Gilget al. (2003), Duarte et al. (2009), Morteani et al. (2010),and Commin-Fischer et al. (2010)), and generally it isconsidered to represent a single hydrothermal event. Forinstance, Commin-Fischer et al. (2010) observed that thesmall cavities (amygdales) are rarely filled with SiO2

phases, and attributed this feature to restricted water cir-culation. The opening of vesicles in the upper and lowercrust of lavas has long been regarded as the result of lavadegassing at high temperature (1150◦C) (see, for exam-ple, Aubele et al. (1988) and Sahagian et al. (1989)). Theeconomically significant large cavities (geodes) present inthe massive core of the lavas have also been attributedto lava degassing at 1150◦C (Proust and Fontaine 2007a).However, Duarte et al. (2009) showed that these cavitieswere formed at low temperature (150◦C) and thereforehave an epigenetic origin. The hydrothermal processeswere also evaluated by Hartmann (2008).

ISSN 0020-6814 print/ISSN 1938-2839 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/00206814.2010.496253http://www.tandfonline.com

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Figure 1. Geological map of the Serra Geral Group volcanics in South America, including the distribution of sedimentary cover rocks.Palaeozoic–Mesozoic sedimentary rocks (not shown) of the Paraná basin underlie the volcanic group. Precambrian basement is alsoincluded in the white area. Modified after Peate et al. (1992).

The volcanic province is at the top of the sedimen-tary Paraná Basin, which contains the very large Guaraniaquifer directly below the lavas (Araújo et al. 1999). Thisaquifer mostly consists of sandstones of the BotucatuFormation (Scherer 2000). Heating by volcanism provideda widespread, nearly infinite volume of hot water andvapour for basalt alteration and related processes of cavityformation and filling. This aqueous fluid was essentiallyinfinite because only a minor volume of the aquifer wasrequired for the mineralization process, and because theaquifer was continually replenished from the borders ofthe Paraná Basin where the Botucatu Formation is wellexposed (Araújo et al. 1999).

Two main questions remain unanswered (Commin-Fischer et al. 2010) concerning the processes of cavityfilling in mineralized Paraná basaltic rocks: (1) the pres-ence of silica minerals is mostly restricted to geodes(not in amygdales); and (2) mineral zoning (chalcedonyto amethyst) only occurs at the scale of the geode andnot along the lava flows. The epigenetic, low-temperature(∼150◦C) geological environment of basalt alteration andcavity opening reported by Duarte et al. (2009) is consid-ered by Commin-Fischer et al. (2010) to be an adequatedescription of the mineralization process. The state-of-the-science calls, therefore, for an evaluation of the sequenceof the formation of two different types of cavities present

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in the volcanic rocks, the high-temperature vesicles and thelow-temperature protogeodes. We also evaluate the fillingof cavities with silicates, carbonates, sulphates, and nativecopper. We mainly use field observations over a large area(500 × 500 km) of the basaltic-dacitic province in south-ern Brazil and Uruguay. These are supported by scanningelectron microscopy, X-ray diffractometry, and electronmicroprobe data.

This investigation is most significant for the under-standing of the origin of the world-class amethyst geodedeposits of southern Brazil and Uruguay, and of the inter-action of the huge Guarani aquifer with one of the largestvolcanic provinces in the world.

Methods of investigation

Previous investigations of the minerals present in the amyg-dales and geodes were initially evaluated, and these includeseveral unpublished doctoral and master’s theses (Borget1980; Gomes 1996; Scopel 1997; Juchem 1999; Augustin2007; Michelin 2007; Frank 2008), a few articles (Murataet al. 1987; Scopel et al. 1990), and many abstracts pub-lished in regional symposia (e.g. Bossi and Caggiano1974). Them main observations regarding the sequenceof filling of cavities in basalts are well summarized inGustavson (2006), and significant observations are alsomade by Flóvenz and Saemundsson (1993) and Neuhoffet al. (2006). This is the first description of the sequence offilling of cavities in the Paraná basalts.

Field and laboratory works were undertaken over a6-year period (2004–2009). Efforts were concentrated bothin amethyst-mining districts, for example, Ametista do Sulin Brazil and Los Catalanes in Uruguay, and in manyunmineralized road cuts and quarries. Much field work wasdone jointly with the Geological Survey of Brazil (CPRM),for example, Magda Bergmann (2009, unpublished data).Scanning electron microscopy, X-ray diffractometry, andelectron microprobe analyses of hydrothermal mineralswere carried out mostly at the Institut für Mineralogie undKristallchemie, Universität Stuttgart, Germany, whereasthose related to copper mineralization were made at theUniversity of Western Australia in Perth.

Field description

Every basalt lava or rhyodacite unit (including basalticandesite, andesite, and rhyolite) in the Paraná vol-canic province has cavities (amygdales and geodes)partly or completely filled with hydrothermal minerals(Figures 2–11). The internal structure of the lavas (Figures2–4) is typical of inflated pahoehoe lobes in Ametista doSul, but is more complex in Quaraí (Hartmann et al. 2010),because some of the lavas (particularly lava 2, coladaCatalán) have the structure of aa lavas with blocky crustsand massive core. In many outcrops, the upper crust of

the lavas is a stockwork breccia of sand and amygdaloidalbasalt. There is a large number (possibly 120) of lava unitsin the Serra Geral Group, whose total thickness is near1800 m in the depocentre (300 km N of Ametista doSul), and each lava unit has a thickness between 5 and70 m. In a carefully measured sequence of 16 basalt lavaflows along the Serra do Rio do Rastro cuesta on the east-ern border of the volcanic group (Vargas 2006), the totallava thickness is 700 m, which leads to an estimate of120 lava flows in the depocentre farther northwest. Rocktypes are bimodal basic-acid and include basalt, basalticandesite, andesite, dacite, trachyte, rhyodacite, and rhyo-lite, here simply referred to as basalt and rhyodacite, thetwo most abundant rock types in the volcanic province.

In the Ametista do Sul mining district, there are 13basalt lavas (Pinto et al. 2010) exposed in the hills between250 and 600 m elevation above sea level (4 lavas imme-diately below Ametista do Sul town), but an additionalsequence of 800 m of basalt and intercalated rhyodacitewas recognized below the surface during drilling for under-ground water by the Geological Survey of Brazil. Aeoliansandstones of the Botucatu Formation were reached at thebottom of the drill core. On the other hand, in the LosCatalanes gemological district, only 1–4 lava flows arepresent above the aeolian sandstones, and these are onebasal basalt, one andesite, and two basaltic andesites (e.g.Duarte et al. 2009; Hartmann et al. 2010).

Rhyodacite units (including rhyolite and trachyte)occur in the middle portion of the Serra Geral Group andhave the structure of rheoignimbrites (Nardy et al. 2008).Each unit has a thickness of nearly 50 m and attains greatlengths, for example, 30 km. These silicic units have atotal thickness of approximately 250 m. An upper vesicu-lar crust is common in the upper part of the unit, whereasamethyst and agate geodes, when present, always occurbelow the upper amygdaloidal crusts in the upper portionof the volumetrically dominant (70%) massive core.

In the Los Catalanes gemological district, an are-ally restricted basalt (colada Mata Olho) and an andesite(57.5 wt.% SiO2), colada Catalán of Duarte et al. (2009),erupted in the Lower Cretaceous directly over an activesand desert (erg). Colada Catalán has the structure of an aalava. This unit is most important because it hosts the giantamethyst geodes and smaller agate geodes that constitute aworld-class deposit.

The great majority, some 94%, of the lavas are basaltsand basaltic andesites in the Paraná volcanic province withthe classical structure of pahoehoe lavas and lobes withupper and lower amygdaloidal crusts and massive core.Thicker lavas (30–70 m) display vertical joints in the core,whereas thinner lavas do not have cooling joints (Gomes1996). These internal structures of the basalts are similar tothe Columbia River basalts (Long and Wood 1986). In con-trast, in the Los Catalanes gemological district, some of themore siliceous lavas (andesite and basaltic andesite) have

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Figure 2. Model sections of lava units in the two main mining districts in the Serra Geral Group, showing the local stratigraphy andinternal structure of lavas; every lava unit displays upper and lower amygdaloidal crusts (not shown), and the sequence of hydrothermalevents H1, H2, and H3. (A) Los Catalanes gemological district (Artigas) and Quaraí region; lava 1 is a basalt present 30–50 km N ofQuaraí; lava 2 is an andesite, has the structure of aa lava and covers aeolian sandstone in Quaraí; sand is present in dikes, sills, and flows,and also in intertrap dunes; amethyst geodes occur in massive portions of basalt and andesite lavas; lavas 3 and 4 are basaltic andesites.(B) Four basalt lava units under Ametista do Sul town; lava 3 (derrame Veia Alta) is the most intensely mined for amethyst geodes, mostmines at elevation 430 m; an additional sequence of 800 m of basalt and dacite lavas underlies the four locally exposed lavas.

Figure 3. Model section of colada Catalán, Los Catalanes gemological district, and Quaraí region. Silicified sandstone layers andstockwork breccias (amygdaloidal andesite blocks) detailed as H2 events, amethyst geode as H3 event. Core type I (no columnar jointing,mineralized) occurs in thinner flows (<30 m), core type II (columnar jointing present, unmineralized) in thicker flows (>30 m).

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Figure 4. Model cross-section of colada Cordillera, Los Catalanes gemological district, and Quaraí region. Silicified sandstone layersand breccias occur at the top of the flow and at the base of core type II. Amethyst geode ore restricted to core type I. More intenselysilicified core type I constitutes erosional remnants. Box indicates the opening of vesicles during magmatic event M, followed aftercooling by hydrothermal event H1 which filled the amygdales with zeolites. Later, event H3 opened and filled cavities with amethyst.

Figure 5. Musem mine, Ametista do Sul. Classical profile of mine entrance in massive type I core basalt, capped by igneous brecciaand amygdaloidal basalt, covered by intrusive, silicified sandstone breccia and extrusive, silicified sandstone layer. Unmineralized upperbasalt flow with massive, type II core with vertical joints.

the structure of aa lavas. A novel type of structure is thepresence of two types of massive core in the same lava flow.Core type I does not have vertical cooling joints and maybe mineralized with amethyst geodes, whereas core type IIshows vertical cooling joints and does not contain ore.

All rocks in the Serra Geral Group display subver-tical fractures and faults spaced a few millimetres to a fewmetres, formed after cessation of hydrothermal activity.These fractures do not contain low-temperature minerals.

Sequence of events

The initial geological event M (Table 1) involved theflow of lavas over the erg surface; for example, the LosCatalanes gemological district and Quaraí mining district

where the contact of the lavas with sand is exposed overa large distance of more than 100 km. In the depocentreof the Paraná basin, the lava sequence is up to 1800 mthick; for example, 1100 m in Ametista do Sul miningdistrict. The initial cooling, from magmatic temperaturesof 1150◦C, caused degassing and the formation of vesi-cles at magmatic temperature in both the upper and lowercrusts. Aa lavas have a more complex crust and a vesic-ular portion is also present. The core of the pahoehoe oraa lava is commonly more than 70% of the total thicknessand contains a massive core in thinner lavas. In the thinnerlavas, the core does not have a columnar jointing, whichis well developed in thicker lavas. All units are presumedto be highly porous (30%) after cooling to ambient tem-perature, analogous to fresh basaltic lava found in Iceland

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Figure 6. (A) Miolo agate mine, São Martinho da Serra, RS; H2 event, silicified sandstone dike with opening of H2 cavity and its fillingwith quartz. (B) La Bolsa mine, Los Catalanes gemological district, showing brecciated andesite which either opened into a geode (right)or remained closed (left); (C) and (D) Musem mine, Ametista do Sul, RS; amethyst geodes, positioned above a subhorizontal flat-lyingfracture, filled with quartz and amethyst geode above the fracture in (C); amethyst geode along and above flat-lying fracture in (D).

(Flóvenz and Saemundsson 1993), because of the presenceof micro-fractures and vesicles.

In the Paraná volcanic province, no high-temperatureminerals are present in the vesicles or in the altered basalt,andesite, or dacite. Clinoptilolite is fairly common inamygdales and in the matrix of altered rocks (Duarte et al.2009), whereas apophyllite is rare and only present in afew amygdales (Frank 2008). According to Frank (2008),it was considered that the formation of higher tempera-ture minerals was due to the thermal effect of the upperflow. Therefore, the highest temperature of the hydrother-mal event seems to be lower than 150◦C. The vesiclesare therefore presumed to have remained unfilled downto surface temperature after solidification of the lavas.Such unfilled vesicles are dominant in basalt lavas fromHawaii and from the Moon. Stable isotopes indicate thatall hydrothermal minerals in fractures, filled vesicles (nowamygdales), and geodes precipitated from meteoric fluid(nearly pure water) (see, for example, Matsui et al. (1974),Juchem (1999), Gilg et al. (2003), Morteani et al. (2010),and Commin-Fischer et al. (2010)). There is no indica-tion that magmatic water remained in the rock during thetime required to cool the lava from 1150◦C to 150◦C,only to deposit minerals in the pores at 100–130◦C. Anew event of hot water percolation was required to fill

the fractures and cavities with minerals. According to theobservation of Commin-Fischer et al. (2010), Morteaniet al. (2010), and this work, the geodes exhibit in all casesthe same sequence of silica minerals from the wall to thecentre (Figure 10); a thin (0.1–20.0 mm) celadonite rimis succeeded by chalcedony (agate), followed by colourlessquartz and amethyst, and in a few mines in Ametista doSul (3 out of 300) by an overgrowth of gypsite. In the LosCatalanes geodes, sulphates such as anhydrite, gypsum,and barite were neither found nor described in the literature(Morteani et al. 2010). Rare fluorite occurs in both miningdistricts. None of these minerals has been described insideamygdales.

The following hydrothermal events, H, occurred duringa complex succession (Figure 11) of porosity sealing andopening (fault-valve action). The water-rich fluid was prob-ably overpressured during the entire hydrothermal event,but a sequence of structural and mineralogical overprintscan be recognized as H1, H2, and H3 events (Figures 3–10). Two main sealing events occurred after the completionof H1 and H2 and led to the overpressuring of water vapour.The source of water for the hydrothermal events was theGuarani aquifer that underlies the volcanic sequence, asindicated by stable isotopes (Gilg et al. 2003; Morteaniet al. 2010). It is the largest freshwater aquifer in the world

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Figure 7. Highway exposures, Três Passos, RS; structures related to H1, H2, and H3; (A) H2 sand dike (indicated by arrow) intrudedinto M; H1 amygdaloidal basalt and extrudite of sand over the lava, possibly sand volcano above dike; (B) H2, silicified sandstone sillsintruded into the lava, which is covered by sand extrudite; (C) H2 stockwork of sand, injected into M, H1 amygdaloidal basalt; massivebasalt above the sand is a new lava; (D) H2 stockwork of silicified sandstone, containing blocks of amygdaloidal basalt; (E) H2 sandsill (indicated by arrow) emanating from sand dike; (F) H3 quartz-filled, vertical fracture (indicated by arrow), feeding a small H3 quartzgeode; (G) three H3 quartz-filled, vertical fractures, feeding three small H3 geodes (indicated by arrows); (H) H1, H2, and H3 stockworks;H1 is an interconnected structure of amygdales and micro-fractures (visible in the field or under the microscope); H2 is a sand brecciacontaining M, H1 amygdaloidal basalt fragments; H3 consists of interconnected quartz veins; this figure represents the entire sequence ofopening and filling of cavities in the Serra Geral Group.

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Figure 8. Sequence of formation and filling of cavities in Quaraí mining district. (A) Sanga do Tigre, 15 km east of Quaraí town, 500 msouth of dirt road; M vesicles (now amygdales) filled with H1 zeolites inside the circle and filled with H3 quartz outside the circle; H2sand dike indicated by arrows; presence of large H3 geode (possibly 1 m) under the surface, marked by the circle, indicated by georadarscanning of the surface; host rock is an andesite (lava 2, colada Catalán). (B) Same outcrop, showing small amygdales and larger H3geode, both filled with quartz; H1 zeolites dissolved from the amygdales before deposition of H3 silica minerals. (C) Garimpo do Vivi, atthe margin of Rio Quaraí; H3 amethyst filling geode. (D) Garimpo do Vivi, with numerous H3 quartz and amethyst geodes contained inaltered andesite (lava 2, colada Catalán).

Figure 9. Sequence of formation and filling of cavities in São Martinho da Serra; Miolo agate geode deposit. (A) H2 sand dike changesinto H3 agate-filled fracture; (B) H1 zeolite (clinoptilolite, mordenite)-filled amygdales, H2 sand dike and H1 amygdales replaced by H3agate; H3 quartz-filled geode; (C) H2 sand dike cut by a vein of H3 agate; (D) Quartz grains from a sand dike silicified by agate cement.

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Figure 10. Drawing of an H3 geode on a mine wall, hosted insmectite basalt. Only silica minerals present, except for some latecalcite and gypsite. Feeding channel displayed. Model based onoccurrences in several mines, Ametista do Sul, and Cordilleramines, Los Catalanes gemological district (Artigas).

and comprises the aeolian sandstones of the BotucatuFormation and underlying Triassic–Jurassic, sedimentaryformations of the Paraná basin (Zalán et al. 1990; Araújoet al. 1999). Warming of this unique aquifer either bythe Tristan plume or by the concentration of heat beneathGondwana supercontinent (Coltice et al. 2007) provided

the mechanism responsible for the huge circulation cell ofhot water in the basalts and dacites. Field evidence indi-cates that each pulse of lava generation in the mantle and itsascent and extrusion at the surface resulted in a sequenceof three main hydrothermal events, possibly a continuumof fluid overpressure under each basalt flow. These pro-cesses may be similar to those described by Flóvenz andSaemundsson (1993) for Iceland.

Event H1 was the first major hydrothermal processto seal the high initial porosity of the volcanic rock.Micro-fractures and vesicles were sealed by voluminousdeposition of clay minerals (smectites), zeolites, includingheulandite, clinoptilolite, and mordenite indicative oftemperature near 150◦C. Similar to the description ofGustavson (2006), small vesicles were entirely filled withthe first mineral phase to deposit, leaving no room for thenext mineral to precipitate. Larger vesicles have a rim ofthe first clay minerals (smectites), zeolite, succeeded by alayer of the next mineral to precipitate. The largest vesiclesdisplay the entire sequence of deposition of hydrothermalminerals in H1. The core of vesicles from a few flowsis filled with native copper. The thickness of the minerallayer is the same in both small and large vesicles. It isimportant to notice that silica minerals are nearly absent invesicles filled during H1. Although the same minerals weredeposited in the vertical cooling joints, these wide open(1–2 mm) structures remained open after H1, so the corestype II of thicker basalt lavas were not sealed in this event.Successive fluid pulses did not become overpressured inthicker lavas and ascended freely along the fractures to thesurface.

In a similar way as described by Gustavson (2006), thefilled vesicles are interconnected by thin (<0.1 mm) frac-tures that were also filled with clay minerals and zeolitesduring H1. The fractures may be visible in the outcrop andare readily apparent under the microscope.

Figure 11. Evolutionary curves of temperature and water vapour pressure (A) during hydrothermal alteration and mineralization ofbasalt (e.g. Ametista do Sul), basaltic andesite and andesite (e.g. Los Catalanes), and rhyodacite (e.g. Caxias do Sul); (B) inset displayingtemperature evolutionary curve from magmatism to the studied hydrothermal temperatures.

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Table 1. One magmatic (M) and three hydrothermal (H) events related to cavity opening and filling in Serra Geral Group basalts anddacites, southern Brazil, and northwestern Uruguay. Position of sealing events 1 and 2 is indicated in the sequence of hydrothermalprocesses.

Event Process StructuresPorosity (P) –permeability (p) Cavity formed

M Magmatism, 1150◦C Lava flows, dikes, sills High VesicleH Hydrothermalism,

30–150◦CPolygonal faults –

hexagonal, irregularFault seal action, high

P-p evolving to lowP-p

Geode

H1 Basalt alteration, zeolites,and native copperformation, <100–30◦C

Filling of vesicles to formamygdales

Sealing of P-p None

Sealing of lava, event 1H2 Fracturing of basalt,

injection of stockwork offluidized sand, andsilicification, ∼100◦C

Dikes, sills, flows of sand;filling of geodes withsand

Initially high,evolving to low P-p

Geode

Sealing of lava, event 2H3 Alteration of basalt and

silicified sandstone,opening of cavitiesfollowing explosivelifting of upper part oflava, 100–30◦C,dissolution of zeolites

Horizontal fractures,feeding channels toprotogeodes, filling ofgeodes with quartz andamethyst

Very low P-p Giant geode

F Subvertical, transcurrentfaulting

Fault zones, nohydrothermal minerals

High P-p None

G Weathering, soil, gossan,supergene alteration ofamethyst deposits andnative copperoccurrences

Polygonal blocks atsurface, tainted eitherbrown or light green

– –

Event H2 is notable, because it consists of the open-ing of a few cavities and injection of fluidized sand intothe basalt lavas or dacite units. The forceful injectionformed dikes (0.1–1.0 m wide) and sills that evolvedinto stockwork breccias of sand and amygdaloidal basaltin the upper crusts of the lavas. Some dikes and stock-works reached the surface of the sand volcanoes and spreadthin sand sheets over the surface. The lava units have inplaces a thin (0.5–2.0 m) layer of silicified sandstone ontop, observed in several positions in the stratigraphy; thatis, in the 1700 m of volcanic rocks. Overpressured fluidopened some cavities and partly filled them with sand. Thecontinued fluid flow in these high-permeability channels(sand dikes and sills) turned loose, injected sand into verycohesive sandstone, a rock once used by native Indians tomake arrowheads. Thus, at the end of event H2, the lavawas sealed again. A large volume of silica thus becameavailable for dissolution and reprecipitation during the laterhydrothermal events, although some of the silica may beoriginated from the alteration of the basalts as envisagedby Proust and Fontaine (2007b).

Pore fluid again became overpressured under thevolcanic unit and generated the hydrothermal event H3,

whose intense alteration of the lava turned it into aclaystone with more than 60 vol.% of clay (celadonite,smectite). When the water vapour ascended to a depth of10–20 m under the surface, the top of the flow was liftedand the vapour exploded laterally generating a horizontalfracture. In some well-exposed mine walls, this fractureis associated with another fracture that runs up into therock at a 40◦ inclination. Geodes in basalt and andesiticbasalt are at the tips of these inclined fractures, which arefeeding channels of fluid into the cavity. The fracturesremained open to be filled with the same hydrothermalminerals present in the geodes. In aa lavas, on the contrary,horizontal fractures and inclined feeding channels arealso present but are more irregular and are filled with athin veneer of hydrothermal minerals. The fractures andgeodes are filled mostly with silica minerals, includingchalcedony along rim, followed by fine quartz, coarsequartz, and then amethyst. Calcite also occurs in somegeodes. Gypsite may occur with calcite although it fills thegeodes entirely in some of the mines in Ametista do Sul. Itis very significant that zeolites and native copper are veryrare in the giant amethyst geodes of both the Ametista doSul mining district and Los Catalanes gemological district;

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they only occur in amygdales filled during H1. A fewgeodes are present in the silicified sandstones.

During H3, the fluid dissolved the minerals (particu-larly zeolites) in many amygdales previously filled duringH1 and then filled them with silica minerals. This is theonly case where magmatic vesicles were filled with silicaminerals. This was observed in many places as a hetero-geneous process, because some amygdales remain filledwith zeolites whereas others now exhibit silica minerals.The key to the understanding of the outcrop distributionof remnant H1 or newly filled H3 amygdales (both cavitiesformed by degassing during M) is the access of H3 to theH1-filled amygdales.

In an unfractured rock, the H1-filled amygdales remainunmodified. This relationship is also well seen at Ribeirãodo Tigre (Figure 8A and 8B), located 300 m south of theroad, 15 km E of Quaraí, including the formation of largequartz-amethyst geodes during H3. In this outcrop, lava 2(colada Catalán) has H1 amygdales filled with zeolites thatwere injected by H2 sand dikes, followed by H3 dissolu-tion of zeolites and refilling of the vesicles with quartz andamethyst; new cavities (geodes) were also filled with H3quartz at this time.

Colada Catalán is mineralized in the massive core(Figure 8C and 8D) with H3 amethyst geodes in theVivi quarry, 30 km south of Quaraí, on the borderwith Uruguay. The andesite is intensely altered to low-temperature minerals.

The sequence of cavity opening and filling in the south-ern Paraná volcanic province can be modelled (Figure 11)as a continuous process of rock alteration, sealing, cavityopening and filling. The main process occurred shortly(possibly within 1 million years) after cooling of the lava,followed by a main peak at 60 Ma (75 million years afterlava extrusion) related to reheating of the lava pile dur-ing alkaline magma intrusion. Minor filling of the cavitiescontinued to this day.

After cessation of the hydrothermal activity, large sub-vertical, transcurrent faults of event F cut the Serra GeralGroup as observed in satellite images. In the field, the faultsare brittle and the fault zone is commonly 20 m wide ormore, made up of thin (1–50 cm) blocks of volcanic rock.No hydrothermal minerals are present in the major faultplanes, so the faulting must have occurred after cessationof major hydrothermal activity. There is a significant lackof relationships between the faults and the conduits for thefilling of the geodes.

Intense weathering formed thick (1–30 m) soils in theregion during event G, which also formed gossans overthe amethyst deposits and native copper occurrences. Thegossans are observed both in the satellite images and inthe field as brown polygonal structures surrounded by lightgreen low-lying (1–5 m) regions. The brown portions arethe oxidation zone (Fe3+ minerals) and the green portionsare the cementation zone (Fe2+ minerals).

Interpretation

No evidence has been described to support a magmaticorigin for the water responsible for the deposition of thehydrothermal minerals in the province. All evidence (e.g.stable isotopes) indicates meteoric water for the process,which probably originated in the aeolian sandstones ofthe Botucatu Formation (Guarani aquifer). Only mineralsformed below 150◦C are described in the province; forexample, no epidote or chlorite.

The main point of this investigation is that the sequenceof formation and filling of cavities is the same over a largeportion (500 × 500 km) of the Serra Geral Group ofthe volcanic rocks. Vesicles formed at magmatic temper-atures (1150◦C) by lava degassing were later filled duringthe hydrothermal event H1 mostly with zeolites and clays(some native copper). Zeolites are nearly absent from thegiant and small geodes just like silica minerals are nearlyabsent from the filled vesicles (amygdales). We argue thatzeolites should have also formed in the rims of the giantgeodes if these cavities were present in the rock during H1.

Giant geodes mostly lack injected sand, which ispresent in some smaller geodes; only in a few occurrencesis sand present in larger (20 cm) geodes, for example, Saltodo Jacuí, an indication that H2 also opened some cavities.Hence, the giant protogeodes were not present in the rockduring sand injection. The sand was injected during H2 inprocesses similar to those described by Hurst et al. (2003),Fernandes et al. (2007), Shoulders and Cartwright (2007),and Cartwright (2010), and made available to the basaltsa very large volume of silica for dissolution and repre-cipitation in the giant geodes. Some silica may, however,have come from the altered host rock. Event H3 was alsoresponsible for the opening of the giant protogeodes andtheir filling with the valuable silica minerals, calcite, andgypsite.

Geological evidence indicates that the hydrothermalprocesses occurred shortly after lava extrusion, becauseaeolian sand (now the Botucatu Formation) can only befluidized by water as loose grains. Although geologicallypoorly defined, other sediments intercalated in the SerraGeral Group lavas may have contributed to the injectedsand budget. After diagenesis and lithification, this nowlithified sand remained in situ and was not injected upwardsinto the lavas. Hot water probably continues to percolatein the basalts on a smaller scale to this day; for example,there are approximately 100 hot springs within the volcanicregion. Dating of celadonite from basalts (Vasconcellos1998) indicates the beginning of hydrothermal processesclose to the age of volcanism (135 Ma; Innocent et al.1997; Wildner et al. 2006) and protracted alteration witha peak at 60 Ma.

The systematic presence of a similar sequence of open-ing and filling of cavities over a large area (500 × 500 km)indicates that all three steps were needed to form the

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amethyst-filled cavities – filling of the vesicles (H1), injec-tion of sand (H2), and complete alteration of the basalt byhot water. First, anomalous heating of the mantle under thelithosphere caused the generation of the basaltic rocks thatare suitable hosts for the geodes and caused heating of thehuge aquifer and its ascent as hot water and its hot vapour.This hydrothermal process caused the intense alterationof the lavas and the sealing of their initial high poros-ity by the deposition of zeolites in vesicles and fractures.After this point in time, there was an explosive intrusionof fluidized sand into the lavas. After the sealing of thisnewly formed porosity, this hot fluid caused the formationof the giant protogeodes and their filling with the spectacu-lar crystals of quartz, amethyst, calcite, and gypsite. As sooften in ore deposits, only the completion of all the phasesof hydrothermal interaction of channelized fluid with thehost rock leads to an ore deposit.

We have thus solved the two main questions posedby Commin-Fischer et al. (2010): (1) Silica minerals didnot deposit in the vesicles (now amygdales) because thesewere already filled by zeolites during H1 and no spaceremained for additional deposition of minerals; also, per-meability was reduced so the amygdales were not reachedby the silica-oversaturated fluid; silica was only availableto form silicates during H1, not silica minerals; (2) Silicamineral zoning is only observed at the geode scale becausefluid ascended nearly vertically; no zoning in silica miner-als occurred along the mineralized basalt flow because thehydrothermal fluid only percolated horizontally at the minescale but not along the flow for long distances.

Later events were unrelated to the formation of cav-ities and their filling, because the large faults of the Fevent do not contain hydrothermal minerals. Weatheringformed both soil over the entire volcanic sequence andparticularly gossan over amethyst and agate deposits andcopper occurrences, a classical prospecting guide for newdeposits.

Conclusions

Based on extensive field and some key laboratory inves-tigations, we elucidated the structural and mineralogicalevolution of cavities in the Serra Geral Group of the Paranávolcanic province of southern Brazil and northwesternUruguay. This evolution resulted in the formation of world-class deposits of amethyst geodes in Brazil and in Uruguay,which are hosted in lavas from one of the largest volcanicprovinces in the world.

The opening and filling of the giant cavities (geodes)with silica minerals was the third hydrothermal event(H3) that altered the basaltic rocks of the Paraná volcanicprovince in the Cretaceous. A sequence of three hydrother-mal events was required to form and fill the large cavitieswith amethyst. The first event (H1) filled the late-magmatic

(event M) vesicles present in the upper and lower crustsof the lavas mostly with zeolites, which are absent fromthe geodes, and sealed existing fractures. The second event(H2) caused the injection and effusion of fluidized sand.Two sealing events occurred intercalated in the hydrother-mal events after H1 and H2, respectively. The sequence ofhydrothermal events affected most of the Paraná volcanicprovince because of heating and ascent of hot water andvapour from the underlying Guarani aquifer.

AcknowledgementsConselho Nacional do Desenvolvimento Científico e Tecnológico(Brazilian Government) offered systematic support for the inves-tigations (Edital Universal), including field and laboratory workand scholarships to several authors. Fundação de Amparo àPesquisa do Estado do Rio Grande do Sul and CNPq grantedsupport through a project of excellence on: ‘Strategic miner-als from southern Brazil’, coordinated by LAH. The Institüt fürMineralogie und Kristallchemie (Universität Stuttgart, Germany)and the University of Western Australia offered scientific exper-tise and significant support in their laboratories. Paul E. Potter andMatthew Brown are thanked for the English revision.

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