Page 137 - Hydrogeology Principles and Practice

HYDC03 12/5/05 5:37 PM Page 120

120 Chapter Three

BO X
Continued
3.10

Fig. 2 Conceptual cross-section through the Carnmenellis Granite showing the location of tin mines and saline groundwaters.
Isotherms in the granite and its aureole show convective distortion due to groundwater circulation. The suggested circulated route of
groundwater flow is shown as follows: (1) Rapid percolation of recent meteoric water with vertical drainage enhanced by mining. Local
distortion of isotherms by groundwater flow; (2) Ancient meteoric water with high salinity derived from granite–water reactions and
stored in the fracture system; (3) Circulation of mixed groundwater which discharges as springs in the mines. Local enhancement of
thermal gradient by upward groundwater flow. After Edmunds et al. (1985).

In their hydrogeochemical interpretation, Edmunds et al. A second important silicate weathering process affecting the
(1985) concluded that hydrolysis of biotite (K (Mg,Fe) (Fe,Al,Li) 2 hydrogeochemistry is the acid hydrolysis of plagioclase feldspars
4
2
−
+
[Si Al O ](OH) (F,Cl) ) could account for the increase in Cl , Li , K + (eqs 3.45 and 3.46) that contributes the principal sources of Na +
2 20
2
2
6
−
+
and other species in the groundwaters. The Li and Cl are consid- and Ca 2+ to the groundwaters. Both biotite and plagioclase
ered as conservative products of biotite alteration, unlikely to be alteration by weathering produces significant amounts of silica
assimilated by reaction products. The fact that the Li/Cl ratio is only as well as kaolinite and other clay minerals; in some areas, kaolin-
1 : 100 compared to 1 : 2 for unaltered granite suggests that Cl − ization along fractures reduces the available secondary porosity.
release is a relatively easy and rapid process that accounts for the Concentrations of SiO (Table 1) in these groundwaters rep-
2
+
2+
observed salinities over very long timescales, while Li and Mg and resent saturation or supersaturation with respect to chalcedony,
other ions are only released upon a breakdown of the trioctahedral and this tendency for silica deposition is argued to rule out fluid
2+
biotite structure. The depletion of Mg in most of the saline ground- inclusions acting as a potential source of salinity (Edmunds et al.
waters is consistent with the observed biotite alteration to chlorite. 1985).