Petrography and chemistry of SiO2 filling phases in the amethyst geodes from the Serra Geral Formation deposit, Rio Grande do Sul, Brazil - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Journal of South American Earth Sciences Année : 2010

Petrography and chemistry of SiO2 filling phases in the amethyst geodes from the Serra Geral Formation deposit, Rio Grande do Sul, Brazil

Résumé

The filling process of amethyst-bearing geodes from Serra Geral Formation basalts, Brazil, is investigated by different methods performed on the SiO2 filling phases. Image analysis of quartz–amethyst deposits suggests a single growing mechanism ruled by geometric selection of randomly oriented crystals. Microthermometry of fluid inclusions reveals formation temperature lower than 100 °C, probably lower than 50 °C, and fluid salinity as high as 3 mass% NaCl eq. Composition in REE and trace-elements measured by ICP-MS on acid-digested or laser-ablated samples indicates a common genesis for amethyst, quartz and chalcedony, as well as the absence of significant variations from one geode to another. 87Sr/86Sr data on chalcedony shows that both the host basalt or the Botucatu sandstone are possible silica sources. These data, combined with thermo-kinetic considerations, permit us to discuss the filling process. We argue in favor of the contribution of a mineralized fluid of hydrothermal origin producing a regional silica source which decreased with time. The observed mineral sequence is related to the depletion of silica in the solution.

Dates et versions

hal-04396234 , version 1 (15-01-2024)

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Adriane Commin-Fischer, Gilles Berger, Mireille Polvé, Michel Dubois, Paul Sardini, et al.. Petrography and chemistry of SiO2 filling phases in the amethyst geodes from the Serra Geral Formation deposit, Rio Grande do Sul, Brazil. Journal of South American Earth Sciences, 2010, 29 (3), pp.751-760. ⟨10.1016/j.jsames.2009.10.002⟩. ⟨hal-04396234⟩
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