Genesis of bentonites from Cabo de Gata, Almeria, Spain: a stable isotope study

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Abstract
Oxygen (18O/16O) and hydrogen (2H/1H) isotopic ratios were measured on 22 smectite separates from bentonite deposits of the Sierra de Gata and Serrata de Nijar, Almeria, Spain. In addition, 18O/16O and 2H/1H ratios were determined on some waters from springs and wells, and 18O/16O ratios were measured on quartz, plagioclase and biotite samples separated from the bentonites. The smectites displayed two distinct isotopic compositional groups which excluded an origin either by submarine alteration or weathering under normal surface conditions of the associated volcanic rocks. Bentonization processes caused by low-temperature fluids of meteoric origin could account for the stable isotopic contents. Argillization temperatures are estimated at ∼40°C and ∼70°C for the Serrata de Nijar and the Sierra de Gata deposits, respectively.