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The Origin of Saline Groundwaters in Granitic Rocks: Evidence From Hydrothermal Experiments

Published online by Cambridge University Press:  28 February 2011

David Savage ,
Mark R. Cave  and
Antoni E Milodowski
David Savage
Affiliation:
Fluid Processes Research Group, British Geological Survey, Keyworth Nottinghamshire, NG12 5GG, United Kingdom.
Mark R. Cave
Affiliation:
Fluid Processes Research Group, British Geological Survey, Keyworth Nottinghamshire, NG12 5GG, United Kingdom.
Antoni E Milodowski
Affiliation:
Fluid Processes Research Group, British Geological Survey, Keyworth Nottinghamshire, NG12 5GG, United Kingdom.
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Abstract

Hydrothermal rock-water experiments at 80°, 150° and 250° C have been carried out to investigate the origin of saline groundwaters in the Carnmenellis Granite, Cornwall, UK which have previously been attributed to be derived via dissolution of biotite and plagioclase feldspar by dilute meteoric groundwaters. The nature of product fluid and solid phases in the experiments was dominated by the dissolution of plagioclase and potassium feldspars, coupled with the precipitation of laumontite, calcite and smectite. However, the release of chloride from the rock was minimal, suggesting minor dissolution of the major chloride bearing mineral, biotite. The relative inertness and stability of biotite during the experiments was borne out by direct physical observation of reacted biotites and by consideration of the fluid phase composition in terms of thermodynamic mineral stability diagrams. It is concluded that the experimental and natural systems are buffered under different chemical conditions, which may be due to acidity generated via oxidation of localised sulphide zones within the granite and absent in the experiments. Dissolution of biotite may be an important mechanism for enhancing the salinity of groundwaters in granitic rocks if groundwaters are buffered to be outside the stability of biotite and convective circulation is significant.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 50: Symposium STOCKHOLM – Scientific Basis for Nuclear Waste Management IX , 1985 , 121
Copyright
Copyright © Materials Research Society 1985

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