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GEOREF RECORD

Amethyst geodes in the basaltic flow from Triz Quarry at Ametista do Sul (Rio Grande do Sul, Brazil); magmatic source of silica for the amethyst crystallizations

Dominique Proust and Claude Fontaine
Amethyst geodes in the basaltic flow from Triz Quarry at Ametista do Sul (Rio Grande do Sul, Brazil); magmatic source of silica for the amethyst crystallizations
Geological Magazine (July 2007) 144 (4): 731-739

Abstract

The amethyst geodes observed in the tholeiitic basaltic flow from the Triz quarry at Ametista do Sul (Rio Grande do Sul, Brazil) show particular wall-layering infillings with, from the outside inwards, celadonite, chalcedony, fine-grained quartz and large-sized amethyst crystals. The primary fluid inclusions analysed in the amethyst crystals yield a 152 to 238 degrees C crystallization temperature range. The amethyst geodes are always located in the massive, fracture-free, basaltic part of the lava flows and surrounded by a limited wall-lining alteration halo, the thickness of which depends on the geode radius. The geochemical balances calculated from fresh to altered basalt in the geode environment indicate that the amount of SiO (sub 2) released from the alteration halo is always sufficient to produce the siliceous rims in the geodes without requirement for extraneous silica supply. The results point to a volcanic origin for the amethyst geode infillings through basalt "autoalteration" by authigenic high temperature hydrothermal fluids. These fluids can originate from (1) unmixing of volatile coumpounds from melt through pressure release, (2) cooling to supercritical hydrous fluids with "autoalteration" of the surrounding basalt and (3) migration of the residual hot fluids from the basalt to the geodic cavities through the pressure gradient between the geodic cavity and the rock.


ISSN: 0016-7568
EISSN: 1469-5081
Coden: GEMGA4
Serial Title: Geological Magazine
Serial Volume: 144
Serial Issue: 4
Title: Amethyst geodes in the basaltic flow from Triz Quarry at Ametista do Sul (Rio Grande do Sul, Brazil); magmatic source of silica for the amethyst crystallizations
Affiliation: Universite de Poitiers, Faculte des Sciences, Poitiers, France
Pages: 731-739
Published: 200707
Text Language: English
Publisher: Cambridge University Press, London, United Kingdom
References: 45
Accession Number: 2007-088811
Categories: Economic geology, geology of nonmetal depositsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
S33°49'60" - S27°00'00", W57°40'00" - W49°30'00"
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Cambridge University Press. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200740
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