Deep Fluids in the Continents: II. Crystalline Rocks

The phrase “crystalline rock” has become a commonly used term to describe igneous and metamorphic rocks. Groundwaters (fluids) found in crystalline rocks can have a highly variable composition similar to fluids from sedimentary environments see (Chapter 5.16). 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 (Garrels, 1967; Paces, 1972; see Chapters 5.14 and 5.04). 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 (Jacks, 1973). 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 chloride and sulfate anions (Jacks, 1978). Similar to sedimentary environments, dissolved loads greater than 100 g L²¹ are common. The stable isotopic signatures (¹⁸O and ²H) of the most concentrated fluids are very different from those found in sedimentary formation fluids, and age-dating techniques have suggested that deep groundwaters are geologically old (Fritz and Frape, 1982). The detailed characteristics of deep groundwaters in crystalline rocks were relatively unknown until the late 1970s, but the mining industry certainly knew of these highly mineralized, gas filled, often explosive fluids. Reports from the late nineteenth and early twentieth century mention very corrosive fluids.