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Fluid inclusions in amethyst quartz of different geological environments from Brazil

Published online by Cambridge University Press:  19 April 2021

Coralie Heinis Dias Open the ORCID record for Coralie Heinis Dias [Opens in a new window] ,
Mario Luiz de Sá Carneiro Chaves ,
Rosaline Cristina Figueiredo e Silva  and
Sylvio Dutra Gomes
Coralie Heinis Dias*
Affiliation:
Universidade do Estado de Minas Gerais, Departamento de Geociências, Ciências Humanas e Linguagens, Av. Brasília, 1304, Bairro Baú, João Monlevade, Minas Gerais, 35930-314, Brazil
Mario Luiz de Sá Carneiro Chaves
Affiliation:
Universidade Federal de Minas Gerais, Departamento de Geologia, Av. Antônio Carlos, 6627, Belo Horizonte, Minas Gerais, 31270-901, Brazil
Rosaline Cristina Figueiredo e Silva
Affiliation:
Universidade Federal de Minas Gerais, Departamento de Geologia, Av. Antônio Carlos, 6627, Belo Horizonte, Minas Gerais, 31270-901, Brazil
Sylvio Dutra Gomes
Affiliation:
Universidade Federal da Bahia, Instituto de Geociências, R. Barão de Jeremoabo, s/n, Ondina, Salvador, Bahia, 40170-290, Brazil.
*
*Author for correspondence: Coralie Heinis Dias, Email: coralie.dias@uemg.br
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Abstract

Fluid-inclusion studies were conducted on amethyst quartz from three different geological environments: basalt cavities; hydrothermal veins; and granitic pegmatites of Eastern Brazil, to understand the conditions of amethyst crystallisation in each of these environments. In samples from basalt cavities, fluid inclusions are exclusively one-phase aqueous types suggesting a low-temperature formation environment. Crystals from the two other environments show that fluid inclusions can be either one-phase aqueous, two-phase aqueous, three-phase aqueous carbonic, or three-phase aqueous with the presence of precipitated solid halite. The carbonic composition of the system H2O–CO2–NaCl was confirmed by Raman spectroscopic analysis and suggests a metamorphic or magmatic fluid source. Fluid inclusions trapped in samples from hydrothermal veins reveal at least two different fluid generations based on homogenisation temperatures. The first generation has minimum trapping temperatures between 249°C and 391°C. The second generation of lower temperature fluids has minimum trapping temperatures varying from 82°C to 203°C. Fluid inclusions of this group record eutectic temperatures that indicate the presence of Ca and Fe cations in addition to Na. Fluid inclusions trapped in amethyst from a pegmatite body have moderate salinity values between 15 and 25 eq. wt.% NaCl, thus reflecting the elevated salt content in pegmatite-forming fluids. In this sample, the first fluid generation is represented by aqueous fluid inclusions with minimum trapping temperatures ranging from 268°C to 375°C. The estimated eutectic temperatures, generally below –50°C, indicate the presence of Ca cations in addition to Na. Minimum trapping temperatures correspond to temperatures of late-to-post-pegmatitic hydrothermal stages. The second generation records minimum trapping temperatures between 125°C and 247°C. Amethyst from both hydrothermal veins and pegmatite environments contain solid inclusions of hematite, an indication that the mineralising fluid was Fe rich and thus, possibly magmatic in origin.

Keywords

fluid inclusionsamethystbasalthydrothermal veinpegmatite
Type
Article
Information
Mineralogical Magazine , Volume 85 , Issue 3 , June 2021 , pp. 332 - 347
Copyright
Copyright © The Author(s), 2021. Published by Cambridge University Press on behalf of The Mineralogical Society of Great Britain and Ireland

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Footnotes

Associate Editor: Daniel Atencio

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