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118 Chapter Three

Freeze, R.A. & Cherry, J.A. (1979) Groundwater. Prentice- Mazor, E. (1997) Chemical and Isotopic Groundwater Hydro-
Hall, Englewood Cliffs, New Jersey. logy: the applied approach, 2nd edn. Marcel Dekker, New
Krauskopf, K.B. & Bird, D.K. (1995) Introduction to Geo- York.
chemistry, 3rd edn. McGraw-Hill, New York. Stumm, W. & Morgan, J.J. (1996) Aquatic Chemistry: chem-
Lloyd, J.W. & Heathcote, J.A. (1985) Natural Inorganic ical equilibria and rates in natural waters, 3rd edn. John
Hydrochemistry in Relation to Groundwater: an introduction. Wiley, New York.
Clarendon Press, Oxford.

Table 3.15 Mean chemical composition of groundwaters from European and African granitic massifs as presented by Tardy (1971).

Location Number of pH HCO − 3 Cl − SO 2− SiO 2 Na + K + Ca 2+ Mg 2+
4
−1
−1
−1
−1
−1
−1
−1
−1
samples (mg L ) (mg L ) (mg L ) (mg L ) (mg L ) (mg L ) (mg L ) (mg L )
Norway 28 5.4 4.9 5.0 4.6 3.0 2.6 0.4 1.7 0.6
Vosges 51 6.1 15.9 3.4 10.9 11.5 3.3 1.2 5.8 2.4
Brittany 7 6.5 13.4 16.2 3.9 15.0 13.3 1.3 4.4 2.6
Central Massif 10 7.7 12.2 2.6 3.7 15.1 4.2 1.2 4.6 1.3
Alrance Spring F 77 5.9 6.9 <3 1.15 5.9 2.3 0.6 1.0 0.4
Alrance Spring A 47 6.0 8.1 <3 1.1 11.5 2.6 0.6 0.7 0.3
Corsica 25 6.7 40.3 22.0 8.6 13.2 16.5 1.4 8.1 4.0
Sahara 8 6.9 30.4 4.0 20 9 30 1.8 40 –
Senegal 7 7.1 43.9 4.2 0.8 46.2 8.4 2.2 8.3 3.7
Chad 2 7.9 54.4 <3 1.4 85 15.7 3.4 8.0 2.5
Ivory Coast (dry season) 54 5.5 6.1 <3 0.4 10.8 0.8 1.0 1.0 0.10
Ivory Coast (wet season) 59 5.5 6.1 <3 0.5 8.0 0.2 0.6 <1 <0.1
Malagasy (High Plateaux) 2 5.7 6.1 1 0.7 10.6 0.95 0.62 0.40 0.12
Hydrogeochemical characteristics of the Carnmenellis Granite, BO X
Cornwall, England 3.10

The Carnmenellis Granite and its aureole in south-west England waters generally issue from cross-courses with discharges between
contain the only recorded thermal groundwaters in British granites 1 and 10 L s −1 at depths between 200 and 700 m below surface.
and occur as springs in tin mines. Most of the groundwaters are The discharge temperatures of up to 52°C are typically in excess of
−1
−1
saline with a maximum mineralization of 19,310 mg L (Edmunds the average regional thermal gradients of 30°C km in the granite
et al. 1984). The Carnmenellis Granite forms a near-circular outcrop and 50°C km −1 in the aureole. As proposed by Edmunds et al.
of the Cornubian batholith (Fig. 1) which was intruded about 290 (1985) and shown schematically in Fig. 2, the temperature anomaly
Ma into Devonian argillaceous sediments. The rock is highly frac- implies that ancient, warmer saline fluids are upwelling by con-
tured. Beneath a weathered zone of variable thickness, the granite vective circulation and mixing with recent, fresh, shallow ground-
is characterized by secondary permeability. Most groundwater flow waters. The driving force for the current circulation system is the
and storage occurs in open horizontal fractures and is shallow in hydraulic sink created by the former mining operations. Also, the
depth (commonly above 50 m) and localized. existence of old, flooded mine workings locally increases the sec-
The granite is composed of coarse- and fine-grained porphyritic ondary porosity of the rocks.
muscovite-biotite granite, the former being more common, and has Chemical analyses of the four mine waters and two shallow ground-
undergone a long history of alteration. The granite is enriched in water samples are given in Table 1. The fresh, shallow groundwater
volatile elements (B, Cl, F, Li) compared with many other granite ter- is generally of good quality, has a low total dissolved solids content
rains. There is extensive hydrothermal mineralization of Variscan and may be acidic (pH < 5.5). The most important features of the
age which has produced economic vein deposits of Sn, Cu, Pb and hydrochemistry of the Carnmenellis Granite, in addition to the high
+
−
−
Zn. The principal mineral lodes occur in a mineralized belt north of Cl concentrations, are the depletion of Na relative to Cl , the
+
the Carnmenellis Granite (Edmunds et al. 1985). enhanced Ca 2+ levels and especially the significantly enriched Li ,
+
−1
Saline groundwaters are encountered in four accessible mines with Li values as high as 125 mg L . The unusual chemistry com-
(Fig. 1 for locations) in the granite or its thermal aureole, as well as bined with stable isotope data demonstrates a meteoric origin for all
in several disused mines, all at the northern margin. The saline the groundwaters that excludes seawater as the source of the salinity.