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Albitization and quartz dissolution in Paleoproterozoic metagranite, central Sweden - Implications for the disposal of spent nuclear fuel in a deep geological repository

Petersson, Jesper; Stephens, Michael B.; Mattsson, Hakan and Möller, Charlotte LU (2012) In Lithos 148. p.10-26
Abstract
Hydrothermal alteration resulting in albitization and quartz dissolution has been identified in Paleoproterozoic metagranites down to - 1000 m elevation at Forsmark, Sweden. The alteration features were discovered during investigations to locate a site for the disposal of spent nuclear fuel in a deep geological repository. In general, albitization occurs extensively, but it is also observed locally adjacent to minor intrusive bodies of amphibolite. The altered rocks show a marked decrease in K-feldspar and an increase in quartz relative to the unaltered equivalents, resulting in an epitonalitic composition. Plagioclase is metamorphic in character and generally richer in albite than in the unaltered rocks. It is inferred that albitization... (More)
Hydrothermal alteration resulting in albitization and quartz dissolution has been identified in Paleoproterozoic metagranites down to - 1000 m elevation at Forsmark, Sweden. The alteration features were discovered during investigations to locate a site for the disposal of spent nuclear fuel in a deep geological repository. In general, albitization occurs extensively, but it is also observed locally adjacent to minor intrusive bodies of amphibolite. The altered rocks show a marked decrease in K-feldspar and an increase in quartz relative to the unaltered equivalents, resulting in an epitonalitic composition. Plagioclase is metamorphic in character and generally richer in albite than in the unaltered rocks. It is inferred that albitization was triggered by the input of basic or intermediate melts into the crust during igneous activity close to the peak of regional metamorphism at 1.87-1.86 Ga. The mineralogy of the epitonalites gives rise to an increased thermal conductivity and, thereby, a positive influence for the design and safety of a deep geological repository for spent nuclear fuel. However, the increased frequency of low conductive amphibolite in the albitized volumes, consistent with the proposed mechanism for alteration, gives a negative influence. In sharp contrast to the albitization, a majority of the occurrences of quartz dissolution, which resulted in the formation of episyenite, are located along fracture zones. Quartz dissolution took place between or after 1.8-1.7 Ga, when the bedrock was able to respond to deformation in a brittle manner. Most of the vugs left after the removal of quartz are, to a variable extent, refilled by hydrothermal assemblages, including quartz, albite, K-feldspar, hematite, chlorite and calcite. The geometry and spatial distribution of episyenite argue against an extreme fluid/rock ratio and it is inferred that the fluids had at least a moderate salinity with a temperature in excess of 300 degrees C. The dissolution process was promoted by the generation of secondary permeability localized in columnar or pipe-like volumes. The close spatial connection to fracture zones provides a basis to avoid bedrock affected by this type of alteration and, thereby, reduce the negative mechanical and hydrogeological aspects for a deep geological repository. (C) 2012 Elsevier B.V. All rights reserved. (Less)
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author
organization
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Contribution to journal
publication status
published
subject
keywords
Albitization, Quartz dissolution, Epitonalite, Episyenite, Granite, Spent nuclear fuel repository
in
Lithos
volume
148
pages
10 - 26
publisher
Elsevier
external identifiers
  • wos:000308055800002
  • scopus:84862990640
ISSN
0024-4937
DOI
10.1016/j.lithos.2012.06.001
language
English
LU publication?
yes
id
04ed0267-de16-4f1d-8048-433dfc3b39a5 (old id 3139294)
date added to LUP
2012-11-26 12:09:28
date last changed
2017-01-01 03:14:26
@article{04ed0267-de16-4f1d-8048-433dfc3b39a5,
  abstract     = {Hydrothermal alteration resulting in albitization and quartz dissolution has been identified in Paleoproterozoic metagranites down to - 1000 m elevation at Forsmark, Sweden. The alteration features were discovered during investigations to locate a site for the disposal of spent nuclear fuel in a deep geological repository. In general, albitization occurs extensively, but it is also observed locally adjacent to minor intrusive bodies of amphibolite. The altered rocks show a marked decrease in K-feldspar and an increase in quartz relative to the unaltered equivalents, resulting in an epitonalitic composition. Plagioclase is metamorphic in character and generally richer in albite than in the unaltered rocks. It is inferred that albitization was triggered by the input of basic or intermediate melts into the crust during igneous activity close to the peak of regional metamorphism at 1.87-1.86 Ga. The mineralogy of the epitonalites gives rise to an increased thermal conductivity and, thereby, a positive influence for the design and safety of a deep geological repository for spent nuclear fuel. However, the increased frequency of low conductive amphibolite in the albitized volumes, consistent with the proposed mechanism for alteration, gives a negative influence. In sharp contrast to the albitization, a majority of the occurrences of quartz dissolution, which resulted in the formation of episyenite, are located along fracture zones. Quartz dissolution took place between or after 1.8-1.7 Ga, when the bedrock was able to respond to deformation in a brittle manner. Most of the vugs left after the removal of quartz are, to a variable extent, refilled by hydrothermal assemblages, including quartz, albite, K-feldspar, hematite, chlorite and calcite. The geometry and spatial distribution of episyenite argue against an extreme fluid/rock ratio and it is inferred that the fluids had at least a moderate salinity with a temperature in excess of 300 degrees C. The dissolution process was promoted by the generation of secondary permeability localized in columnar or pipe-like volumes. The close spatial connection to fracture zones provides a basis to avoid bedrock affected by this type of alteration and, thereby, reduce the negative mechanical and hydrogeological aspects for a deep geological repository. (C) 2012 Elsevier B.V. All rights reserved.},
  author       = {Petersson, Jesper and Stephens, Michael B. and Mattsson, Hakan and Möller, Charlotte},
  issn         = {0024-4937},
  keyword      = {Albitization,Quartz dissolution,Epitonalite,Episyenite,Granite,Spent nuclear fuel repository},
  language     = {eng},
  pages        = {10--26},
  publisher    = {Elsevier},
  series       = {Lithos},
  title        = {Albitization and quartz dissolution in Paleoproterozoic metagranite, central Sweden - Implications for the disposal of spent nuclear fuel in a deep geological repository},
  url          = {http://dx.doi.org/10.1016/j.lithos.2012.06.001},
  volume       = {148},
  year         = {2012},
}