The origin of saline groundwaters in the Carnmenellis Granite, Cornwall (U.K.); further evidence from minor and trace elements

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Authors:Edmunds, W. M.; Kay, R. L. F.; Miles, D. L.; Cook, J. M.
Author Affiliations:Primary:
Br. Geol. Surv., Wallingford, United Kingdom
Volume Title:Saline water and gases in crystalline rocks
Volume Authors:Fritz, P., editor; Frape, S. K.
Source:Saline water and gases in crystalline rocks, edited by P. Fritz and S. K. Frape. Special Paper - Geological Association of Canada, Vol.33, p.127-143. Publisher: Geological Association of Canada, Toronto, ON, Canada. ISSN: 0072-1042
Publication Date:1987
Note:In English with French summary. 45 refs.; illus. incl. 1 anal.
Summary:Thermal groundwaters (= or <55°C), some of them saline (up to 19 300 mg/litre total mineralization), occur as inflows in tin mines in the Hercynian Carnmenellis granite at depths between 200 and 700 m below ground-level. These waters are recognized as meteoric and have circulated to depths of approx 1200 m. The saline waters are mixtures, in varying proportions, of a recent, low salinity, tritium-containing component and an older, saline component, possibly with total mineralization as high as 30 000 mg/litre, which is of the order 104-106 years old. In this paper the chemistry of waters occurring naturally in the granite is compared with that of waters which have circulated to a depth of over 2 km in the UK Hot Dry Rock experiment. Acid hydrolysis of plagioclase and biotite is proposed as the principal origin of the groundwater salinity and the distinctive ionic compositions. Enriched Ca/Na ratios compared with bulk rock compositions are achieved in the groundwaters by selective reaction of the more calcic centres of zoned plagioclase. The groundwaters contain high lithium concentrations (= or <125 mg/litre Li+) which can be related to stoichiometric biotite reaction; chloride, which is enriched by a factor of about 20 over Li+, is considered to be derived from hydroxyl exchange for Cl- in the biotite interlayer, without structural breakdown. This reaction also generates H+ which may further drive the hydrolysis reactions. The dissolved concentration of several minor and trace elements (Li, Sr, Ba, F, B, Br, Rb, Cs) can be related to reaction of aluminosilicate minerals. Other elements, such as Mn, As, Al, Cd, Cu, Co, Be, Ge, Ni and Zn, are detected in acidic and shallow minewaters. These elements may either be derived from oxidation of sulphide minerals or released from oxides, the latter possibly being an important adsorption sink for some of the trace elements originally derived from silicate breakdown reactions. (Authors' abstract) [A.W.H.]
Subjects:Carnmenellis Granite; Geochemistry; Ground water; Minor elements; Salinity; Surveys; Trace elements; Cornwall England; England; Europe; Great Britain; United Kingdom; Western Europe
Abstract Numbers:88M/3828
Record ID:1988013969
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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