Genesis of amethyst geodes in basaltic rocks of the Serra Geral Formation (Ametista do Sul, Rio Grande do Sul, Brazil): a fluid inclusion, REE, oxygen, carbon, and Sr isotope study on basalt, quartz, and calcite

Quartz-Amethyst Hosted Hydrocarbon-Bearing Fluid Inclusions from the Green Ridge Breccia in the Snoqualmie Granite, North Cascades, WA, USA

“…The role of trace amounts of iron and radiation in the formation of its colour are generally established [11,12]. It is found in a variety of geological settings, e.g., epithermal veins, miarolitic cavities in granites and geodes in basaltic lavas.…”

“…It is found in a variety of geological settings, e.g., epithermal veins, miarolitic cavities in granites and geodes in basaltic lavas. Gem quality amethyst, e.g., from Brazil and Uruguay come from geodes in basaltic rocks of the Parana Continental Flood Basalt Province [12]. Published crystallisation temperatures of amethyst cover a wide range, e.g., (1) 370 to 420 • C from fluid inclusion studies of Brazilian geodes [13]; (2) <50 • C fluid inclusion data from Brazilian amethyst [14]; (3) stable oxygen isotope data from amethyst and agate hosted in the Prana-Etendka volcanics [14] and in Devonian and Tertiary volcanics of Scotland [15,16] indicate temperatures <150 • C; (4) Granite hosted amethyst veins from Thunder Bay, Canada, are considered to have formed between 40 and 90 • C [17]; (5) amethysts from epithermal polymetallic veins have formation temperatures ranging between 150 and 250 • C based on fluid inclusion studies [18][19][20] and (6) higher fluid-inclusion homogenization temperatures (280-400 • C) were reported from amethyst in granite miaroles of the Eonyang deposit, South Korea [21].…”

The genesis of the amethyst geodes at Artigas (Uruguay) and the paleohydrology of the Guaraní aquifer: structural, geochemical, oxygen, carbon, strontium isotope and fluid inclusion study

“…Oxygen stable isotope data shown by Fallick et al (1985), Juchem (1999) and Gilg et al (2003) on amethyst from druses in the basalts of the Serra Geral formation (Brazil) and by Harris (1989) on amethyst-bearing druses from the Karroo volcanics (Namibia) suggest a low temperature formation for amethyst (120-50°C) as well as for calcite (30°C). Oxygen and hydrogen isotope data of the water included in agate druses (enhydrous) indicate a meteoric origin (Matsui et al 1974).…”

“…Landmesser (1984) explains the origin of the agate druses in Paleozoic to Mesozoic basaltic rocks, by a ''SiO 2 gel'' diffusion through the pores of the volcanic rocks. According to Leinz (1949), Juchem (1999), Juchem et al (1994, Meunier et al (1988), Gomes (1996), Scopel et al (1998), Gilg et al (2003) and Proust and Fontaine (2007a, b), the cavities (protodruses) hosting in the Serra Geral basalts (Brazil), the agate and amethyst druses, are produced by the exsolution of magmatic volatiles from the basalts. Corrêa et al (1994) explained the druse formation in the Serra Geral basalts of the Ametista do Sul area (Brazil) by a first phase in which volcanic gas bubbles produced protodruses that were filled later on in a second phase by hydrothermal solutions of either late magmatic or unknown origin.…”

“…The actual and the paleohydrogeology of the Guaraní aquifer has to be discussed in detail because the fluids originating from the Guaraní aquifer play a key role in the in the genesis of the present Uruguayan amethyst deposits, as already evidenced by Gilg et al (2003) for the amethystbearing geodes of the Arapey formation at Ametista do Sul in Brazil.…”