SOME ADDITIONAL ASPECTS OF THE EVOLUTION OF BRINES IN CRYSTALLINE ROCKS GAINED FROM ISOTOPE GEOCHEMISTRY

Groundwaters (fluids) found in crystalline rocks can have a highly variable composition similar to fluids from sedimentary environments. The chemistry of shallow groundwaters is very dilute composed of a mixture of atmospherically recharged precipitation and gases reacting with the existing minerals to produce dilute chemical dissolved loads. The isotopic signature of shallow groundwaters reflects mixtures of yearly precipitation and therefore attests to a young age and recent meteoric origin. The dissolved load tends to be dominated by calcium and sodium balanced by bicarbonate.

Deeper groundwaters in crystalline rocks tend to be different in both their chemical and isotopic signatures. The chemistry of these fluids can be combinations of the cations of calcium and sodium (occasionally enriched with magnesium) and the anions chloride and sulphate. Similar to sedimentary environments, dissolved loads greater than 100 g l^-1 are common. The stable isotopic signatures (¹⁸O and ²H) of the most concentrated fluids are very different than those found in sedimentary formation fluids and age dating techniques have suggested that deep groundwaters are geologically old. A number of recent studies by the authors and others have used a wide variety of stable isotope techniques to attempt to unravel the evolutionary history of these deep brines. Not surprisingly combinations of chlorine, strontium and oxygen isotopes show components related to allocthonous probably paleo seawater intrusions in some cases, as well as the overprinting of rock/water interaction. In the latter case chlorine isotopes have been very useful in showing mixing of different water masses in the subsurface.