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

Published online by Cambridge University Press:  09 July 2018

G. Leone ,
E. Reyes ,
G. Cortecci ,
A. Pochini  and
J. Linares
G. Leone
Affiliation:
Dipartimento di Scienze della Terra of the University, Via S. Maria 22, 56100 Pisa, Italy
E. Reyes
Affiliation:
Estacion Experimental del Zaidin, CSIC, Prof. Albareda 1, Granada, Spain
G. Cortecci
Affiliation:
Dipartimento di Scienze della Terra of the University, Via S. Maria 22, 56100 Pisa, Italy
A. Pochini
Affiliation:
Dipartimento di Scienze della Terra of the University, Via S. Maria 22, 56100 Pisa, Italy
J. Linares
Affiliation:
Estacion Experimental del Zaidin, CSIC, Prof. Albareda 1, Granada, Spain
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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.

Resume

Resume

Les rapports isotopiques de l'oxygène (18O/16O) et de l'hydrogène (2H/1H) ont été mesurés sur des smectites isolées de bentonite provenant de la Sierra de Gata et de la Serrata de Nijar, Almeria, Espagne. Ces mêmes rapports isotopiques furent également determines pour des échantillons d'eau de source et de puits. Les rapports 18O/16O furent également mesurés sur du quartz, du plagioclase et de la biotite provenant des mêmes bentonites. Les smectites présentent deux groupes distincts de composition isotopique, ce qui exclut leur origine par altération des roches volcaniques aussi bien dans des conditions sousmarines que dans les conditions de surface. On peut expliquer le rapport isotopique par un processus de bentonisation dû à des fluides de basse température d'origine météorique. Les températures de formation sont estimées à 40 et 70°C respectivement pour les gisement de la Serrata de Nijar et la Sierra Gata.

Kurzreferat

Kurzreferat

In 22 abgetrennten Smectit-Proben aus Bentonit-Lagerstätten der Sierra de Gata und der Serrata de Nijar, Almeria, Spanien, wurden die Isotopenverhältnisse 18O/16O und 2H/1H bestimmt, ebenso an einigen Quellwässern. Weiterhin wurden an den Quarz-, Plagioklas- und Biotit-Fraktionen des Bentonits 18O/16O-Verhältnisse gemessen. Anhand der Isotopenverhältnisse konnten die Smectite in zwei ausgeprägte Gruppen unterteilt werden, für deren Herkunft sowohl submarine Verwitterung als auch Veränderungen vulkanischer Gesteine unter den normalen Bedingungen der Erdoberfläche ausgeschlossen werden. Dagegen könnten die gemessenen Isotopenverhältnisse durch einen Bildungsprozeß erklärt werden, der unter dem Einfluß von Niederschlagswasser bei etwa 40°C (Serrata de Nijar) bzw. ca. 70°C (Sierra de Gata) abläuft.

Resumen

Resumen

Se han determinado las relaciones isotópicas de oxigeno (18O/16O) y de hidrógeno (2H/1H) en veintidós muestras de esmectitas procedentes de los yacimientos de bentonita de Sierra de Gata y Serrata de Nijar, Almeria, España. Se midieron, asimismo estas relaciones isotópicas en algunas aguas procedentes de manantiales y pozos asi como en muestras de cuarzo, plagioclasa y biotita separadas de las bentonitas. Las esmectitas presentan dos composiciones isotópicas diferenciadas que excluyen como origen tanto una alteración submarina como una alteración condiciones normales en la superficie de las rocas volcánicas asociadas. Los contenidos de isótopos estables encontrados pueden ser explicados a través de un proceso de alteración producido por fluídos a baja temperatura de orígen meteórico. Los datos indican que las temperaturas de las que se formaron las arcillas son de ∼40°C y ∼70°C para los depósitos de la Serrata de Nijar y de la Sierra de Gata, respectivamente.

Type
Research Article
Information
Clay Minerals , Volume 18 , Issue 3 , September 1983 , pp. 227 - 238
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1983

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