Dados da Submissão

Título

SILICON AND OXYGEN ISOTOPES UNRAVEL THE FORMATION OF AMETHYST GEODES IN THE PARANÁ VOLCANIC PROVINCE

Texto do resumo

We evaluate the origin of amethyst and quartz in the world’s largest producer – the southern Paraná Volcanic Province (PVP). This early cretaceous volcanic province is related to the initial, evolving position of the Tristan da Cunha hotspot, erupted in SW Gondwana prior to the opening of the South Atlantic Ocean at ca. 134.5 Ma. Compared to other large igneous provinces, the PVP shows a series of unique features. The outstanding massive occurrence of giant (1-4 m large) amethyst geodes in basalts and rhyodacites is, perhaps, the most striking one. The production of amethyst geodes reaches ca. 400 t/month in the state of Rio Grande do Sul, southern Brazil, and in Uruguay. In addition to amethyst geodes, large gemstone-quality agate, chalcedony, selenite and calcite are also found. Counterintuitively, the quartz geodes are hosted mainly in low-Si tholeiitic basalts. We studied a variety of samples from these basalts, because these are more significant economically than the smaller, also-widespread geodes in rhyodacites. A main hypothesis is that the source of Si for the formation of quartz in these rocks in the PVP was the dissolution of sand grains from the Botucatu paleoerg, which lies stratigraphically below the volcanics. However, until recently, little direct evidence of this process was available. To investigate the factors that controlled the formation of quartz in this unique geological setting, we present new δ30Si (n = 313) and δ18O (n=348) isotope data acquired in situ using a CAMECA IMS 1280–HR ion microprobe on a collection of 19 representative samples that range from highly-silicified to unsilicified sandstones (grains and cement), silicified sand injectites, sinter, quartz, agate, chalcedony and amethyst from several localities of the southernmost edge of the PMP – Ametista do Sul and Fronteira Oeste of Brazil and Artigas in Uruguay. All samples were collected in situ from their position in the host rocks in the mines. Our study covers the silica-mineral spectrum from chalcedony, fine quartz, coarse quartz and amethyst from geodes. Oxygen isotope ratios are reported relative to the Vienna Standard Mean Ocean Water - VSMOW (18O/16O = 0.0020052) and silicon isotope ratios are expressed relative to NBS-28 (30Si/28Si = 0.0341465). Values of δ30Si and δ18O range from -3.7 to +1.8 ‰ and -0.7 to +36.9 ‰, respectively. When coupled to available stable isotope fractionation models, our results show that the mechanisms of dissolution and reprecipitation of quartz were related to an epithermal-hydrothermal system, with a relatively homogeneous source for the fluid. Additionally, data support processes such as fluid-rock interaction, boiling of a hydrothermal fluid and water cooling in the formation of geodes. Results and interpretations are in agreement with hydrothermal quartz from Iceland, and the isotope evidence supports the occurrence of a large hydrothermal field in the southern border of the PVP during the Early Cretaceous.

Palavras Chave

Hydrothermalism; Paraná Volcanic Province; geode; epithermal