Publication: Estudio textural e isotópico de los sulfuros diseminados en los sedimentos de la Cuenca de Cameros (La Rioja, España)
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Publication Date
1999
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Sociedad Geológica de España
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Abstract
Los materiales lutíticos de la Cuenca de Cameros presentan en su composición mineralógica muy pequeñas cantidades de sulfuros diseminados (pirita y pirrotita). La pirita sedimentaria aparece fundamentalmente formando framboides, cristales euhedrales y sustituyendo la concha de fósiles. En
ocasiones, se encuentra pirrotita junto a la pirita, producida por un proceso de rotura térmica de la
pirita sedimentaria durante el metamorfismo. El valor de 834S de la pirrotita es siempre mayor que el de la pirita en la misma muestra, con un rango de diferencias que oscila entre 4,5 y 57,7 %0. Las piritas sedimentarias se generaron por reducción bacteriana de sulfatos sedimentarios. Su composición isotópica varía en función del sector de la cuenca considerado. En la zona oriental, las composiciones
isotópicas son positivas y muy variables, con un rango que oscila entre 2,6 y 19,7 %0. En el sector occidental las composiciones isotópicas son siempre negativas y muy constantes (-18 %0). Estas diferencias están relacionadas con dos factores: a) variaciones en la tasa de subsidencia entre ambos sectores y b) transformación de la pirrotita en pirita durante el retrometamorfismo
Pyrite and pyrrhotite have been found in small quantities scattered in the lutitic sediments of the Cameros Basin. The sedimentary pyrite appears as framboids, euhedral crystals and replacing fossil shells. Occasionally, together with the sedimentary pyrite it is possible to find pyrrhotite generated by the thermal breakdown of the sedimentary pyrite during the metamorphism. The sulphur isotopic signature of the pyrrhotite is always higher than the pyrite in the same sample, ranging from 4.5 to 57.7 %0. The sedimentary pyrite was generated by the bacterial reduction of sbdimentary sulphates. The isotopic composition of the pyrites varies depending on the zohe of the basin considered. On one hand, in the east sector, the isotopic compositions are positive and very variable ranging from 2.6 to 19.7 %0. On the other hand, in the west sector the 834S values are always negative and quite constant ("'-18 %0). These differences are related with two factors: a) variations in the subsidence rate between both sectors, and b) transformation reactions of pyrrhotite to pyrite during the retrogressive metamorphism.
Pyrite and pyrrhotite have been found in small quantities scattered in the lutitic sediments of the Cameros Basin. The sedimentary pyrite appears as framboids, euhedral crystals and replacing fossil shells. Occasionally, together with the sedimentary pyrite it is possible to find pyrrhotite generated by the thermal breakdown of the sedimentary pyrite during the metamorphism. The sulphur isotopic signature of the pyrrhotite is always higher than the pyrite in the same sample, ranging from 4.5 to 57.7 %0. The sedimentary pyrite was generated by the bacterial reduction of sbdimentary sulphates. The isotopic composition of the pyrites varies depending on the zohe of the basin considered. On one hand, in the east sector, the isotopic compositions are positive and very variable ranging from 2.6 to 19.7 %0. On the other hand, in the west sector the 834S values are always negative and quite constant ("'-18 %0). These differences are related with two factors: a) variations in the subsidence rate between both sectors, and b) transformation reactions of pyrrhotite to pyrite during the retrogressive metamorphism.