Opal-a Speleothems of Wei-Assipu-Tepui, Roraima Province, Brazil

Bol. Soc. Venezolana Espel. v.39 Caracas dic. 2005

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Opal-a Speleothems of Wei-Assipu-Tepui, Roraima Province, Brazil

Franco Urbani1, Phillipe Compère2 & Luc Willems2

1Universidad Central de Venezuela. Fac. Ingeniería. Escuela de Geología. Lab. de Geología y Geoquímica.

Caracas 1053 y Sociedad Venezolana de Espeleología. Apartado 47028. Caracas 1041A. urbani@cantv.net

2Université de Liège, Laboratoire de Morphologie Ultrastructurale,

Département des sciences et gestion de l'environnement, allée de la Chimie, 3, B-4000 Liège 1,Belgium.

3University of Liège. Laboratoire de Pétrologie Sédimentaire,

Département de Géologie, B20, B-4000 Liege, Belgium.Lucwillems65@versateladsl.be

ABSTRACT

Coralloid-shaped speleothems samples were collected from a small cave in a mountain of the Roraima massif, Brazil. They were mineralogically identified as opal-A and show two concentric growth zones: A massive light colored inner one, and a darker outer zone with a spongy texture. On the surface the presence of some filaments probably of algae suggest biomineralisation. Chemically the outer zone shows impurities with high values of Cl and P. The leaching of bird nest guano may be the source of P. The two zones were probably formed in two different climatic conditions, the outer one in the current wet and vegetated period, and the inner zone during a former drier period.