The Genesis of the Amethyst Geodes at Artigas (Uruguay) and The
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Int J Earth Sci (Geol Rundsch) (2010) 99:927–947 DOI 10.1007/s00531-009-0439-z ORIGINAL PAPER The genesis of the amethyst geodes at Artigas (Uruguay) and the paleohydrology of the Guaranı´ aquifer: structural, geochemical, oxygen, carbon, strontium isotope and fluid inclusion study Giulio Morteani Æ Y. Kostitsyn Æ C. Preinfalk Æ H. A. Gilg Received: 25 August 2008 / Accepted: 14 April 2009 / Published online: 6 May 2009 Ó Springer-Verlag 2009 Abstract The amethyst-bearing geodes found in the flood Introduction and problem basalts of the Arapey formation at Artigas (Uruguay) were formed as protogeodes by bubbles of CO2-rich basalt- Amethyst occurs in very different geological environments, derived fluids. The formation of the celadonite rim and the such as hydrothermal veins, alpine clefts, miaroles in granitic lining of the geodes by agate followed by quartz and rocks and geodes in basaltic lavas. Brazil and Uruguay are amethyst were driven by the artesian water of the Guaranı´ the world leaders in the production of amethyst and agate aquifer percolating the basalts from below. The tempera- from geodes that are mined from early Cretaceous basalts of ture of the amethyst formation is estimated from fluid the Parana´ Flood Basalt Province (PFBP). The PFBP covers inclusion data to be between 50° and 120°C. Oxygen stable an area of about 1.2 million km2 (Piccirillo et al. 1988). The isotope data suggest a crystallization temperature of calcite most important amethyst mineralization and amethyst min- of about only 24°C. The actual wellhead temperature of the ing is located in the areas of Ametista do Sul/Iraı´(Rio Grande water produced from the Guaranı´ aquifer in the study area do Sul, Brazil) and Artigas (Uruguay). is around 29°C. This contribution predominantly deals with the amethyst deposits in the area of Artigas and consequently with the Keywords Amethyst Á Uruguay Á Guaranı´ aquifer Á geology of Uruguay, but in this context also the geology of Artigas Á Fluid inclusions Á Stable oxygen Á Carbon the surrounding countries has to be considered. Therefore and strontium isotopes Á Parana´ flood basalt province the different names currently used in Brazil, Uruguay, Paraguay and Argentina for the lava flows of the PFBP and the over- and underlying Permian, Triassic, Jurassic and Cretaceous sedimentary formations are given in Table 1.In & G. Morteani ( ) this paper, the Uruguayan names for the different strati- 84424 Isen, Gmain no. 1, Isen, Germany e-mail: [email protected] graphic units are also used when the geology and hydro- geology outside Uruguay is considered. Y. Kostitsyn For the aquifer that underlies the basalts of the Arapey Russian Academy of Sciences, Vernadsky Institute of formation in Brazil, Paraguay, Uruguay and Argentina and Geochemistry and Analytical Chemistry, Kossygin str., 19, 119991 Moscow, Russia consists predominantly of the Tacuarembo´ and Buena e-mail: [email protected] Vista formations, the name ‘‘Guaranı´ aquifer’’ is well established in the national as well as international literature C. Preinfalk and will be used therefore in this paper instead of the name Institut fu¨r Rechtsmedizin der Universita¨tMu¨nchen, Nussbaumstrasse 26, 80336 Munich, Germany Mercosul aquifer (Arau´jo et al. 1999). e-mail: [email protected] The formation of amethysts in geodes of volcanic rocks is still debated. Landmesser (1984) explains the origin of H. A. Gilg the agate druses in Paleozoic to Mesozoic basaltic rocks, Lehrstuhl fu¨r Ingenieurgeologie, Technische Universita¨t Mu¨nchen, Arcisstrasse 21, 80290 Munich, Germany by a ‘‘SiO2 gel’’ diffusion through the pores of the volcanic e-mail: [email protected] rocks. According to Leinz (1949), Juchem (1999), Juchem 123 928 Int J Earth Sci (Geol Rundsch) (2010) 99:927–947 Table 1 Stratigraphy and nomenclature used in Brazil, Uruguay, Paraguay and Argentina for the early Cretaceous lavas of the PFBP and the over- and underlying Carboniferous, Permian, Triassic and Jurassic sedimentary formations (Arau´jo et al. 1999; Pesce 2002) Age Lithology Hydrostratigraphic Uruguay Argentina Brazil Paraguay unit Cretaceous Sandstones and Aquitard Ascencio Fm. Quebrada Bauru Group Acaray Fm. carbonate rocks Menardes Fm. Early Cretaceous Flood basalt (PFBP) Aquitard/ Arapey Fm. Serra Geral Fm. Serra Geral Fm. Alto Parana aquifer Fm. Solari Fm. Solari Fm. Jurassic Sandstone (Subarkosic Aquifer Tacuarembo Tacuarembo Fm. Botucatu´ Fm. Misiones Fm. to quartzose) Fm. Triassic Sandstone(Agillaceous Aquifer Buena Vista Buena Vista Fm. Pirambo´ia/Rosario Independencia subarkose to arkose) Fm. do Sul Fm. Fm. Permo/Triassic Very fine-grained Aquitard Yaguary Fm. Victorino Rodrigues Rio do Rasto Fm. sandstones, Fm. mudstones, siltstones Permo/ Tres Islas/San Chacabuco/Charata/ Carboniferous Gregorio Sachajoy Fm. Fm. et al. (1994), Meunier et al. (1988), Gomes (1996), Scopel • the source of the elements needed to form the et al. (1998), Gilg et al. (2003) and Proust and Fontaine amethysts, calcite, etc.; (2007a, b), the cavities (protodruses) hosting in the Serra • the source and chemistry of the fluids that transported Geral basalts (Brazil), the agate and amethyst druses, are the elements to the site of amethyst (calcite, etc.) produced by the exsolution of magmatic volatiles from the crystallization; basalts. Correˆa et al. (1994) explained the druse formation • the pressure and temperature at the source of the in the Serra Geral basalts of the Ametista do Sul area amethyst-forming fluids and at the site of mineral (Brazil) by a first phase in which volcanic gas bubbles deposition; and produced protodruses that were filled later on in a second • timing of the element mobilization and deposition. phase by hydrothermal solutions of either late magmatic or In the following structural and geochemical characters, unknown origin. In contrast, Bossi and Caggiano (1974) oxygen, carbon and Sr isotope and fluid inclusion data of suggested dissolution of sandstone xenoliths in the basalts the amethyst geodes and of the hosting basalts of the by magmatic fluids as the mechanism that offers the silica Arapey formation (PFBP) in the area of Artigas (Uruguay) needed for the formation of the amethyst druses. will be discussed. This information will be compared with Oxygen stable isotope data shown by Fallick et al. similar information gathered from the Brazilian deposits at (1985), Juchem (1999) and Gilg et al. (2003) on amethyst Ametista do Sul, RS. A genetic model of the Uruguayan from druses in the basalts of the Serra Geral formation amethyst deposits will then be proposed. (Brazil) and by Harris (1989) on amethyst-bearing druses The actual and the paleohydrogeology of the Guaranı´ from the Karroo volcanics (Namibia) suggest a low tem- aquifer has to be discussed in detail because the fluids perature formation for amethyst (120–50°C) as well as for originating from the Guaranı´ aquifer play a key role in the calcite (30°C). Oxygen and hydrogen isotope data of the in the genesis of the present Uruguayan amethyst deposits, water included in agate druses (enhydrous) indicate a as already evidenced by Gilg et al. (2003) for the amethyst- meteoric origin (Matsui et al. 1974). bearing geodes of the Arapey formation at Ametista do Sul The formation of protodruses in the basalts by fluids of in Brazil. less then 150°C and later infilling by amethyst is suggested by Duarte da Cunha et al. (2005). The time span for the Analytical techniques alteration process is assumed to be approximately 1 Ma. In conclusion, the ongoing debate about the genesis of the The chemical composition of the whole rocks was deter- amethyst-bearing druses focuses on: mined at the Technical University of Mu¨nchen (Germany) • the mechanism that opened and shaped the geodes, on Li tetraborate glass discs by routine wavelength dis- offering in that way the space for the subsequent persive X-ray fluorescence using a Siemens SRS 303 deposition of the geode-filling minerals; equipment. 123 Int J Earth Sci (Geol Rundsch) (2010) 99:927–947 929 Rare Earth Elements (REE) contents were analysed on a According to Bellieni et al. (1986) the lavas of the PFBP commercial basis by X-Ral Co. (Canada) using ICP-MS. show a maximum thickness of about 1.7 km and are mainly Oxygen and carbon isotopes of calcites from the druses of tholeiitic composition (90 vol%), but in the uppermost were determined by Hydroisotop Co. (Schweitenkirchen, part of the sequence intermediate (7 vol%) and acid Germany). The analytical precision is estimated to be 0.1– (3 vol%) volcanic rocks predominate locally (Peate et al. 0.2% for both isotope ratios. The isotope values are 1992). reported as permil deviation relative to VSMOW for oxy- The lavas of the PFBP erupted diachronously. The vol- gen and VPDB for carbon. canic activity was driven by the opening of the Atlantic and The isotopic composition of Sr, as well as Rb and Sr the Tristan de Cunha mantle plume (Renne et al. 1992; contents in basaltic rocks and calcite were determined at Turner et al. 1994; Steward et al. 1996). From radiometric the Laboratory of Geochemistry and Geochronology of the data, the lavas of the PFBP erupted between 127 and Institute of Mineralogy and Geochemistry of Rare Ele- 137 Ma in the Lower Cretaceous time (Amaral et al. 1966; ments (IMGRE) in Moscow. The precision of the Sr iso- Creer et al. 1965; Melfi 1967; Cordani et al. 1980; topic composition is about 0.01%, that of the Rb/Sr ratio is Hawkesworth et al. 1992; Renne et al. 1992; Turner et al. about 1%, the blank level for Rb is 0.2 ng, and that for Sr is 1994). In Argentina and Uruguay, the irregular layer of 0.8 ng. sandstone found between the lava flows is known as the Fluid and solid inclusions were inspected in 0.2–0.5- Solari formation (Pesce 2002).