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A km-scale illite alteration zone in sedimentary wall rocks adjacent to a hydrothermal fluorite vein deposit

Published online by Cambridge University Press:  09 July 2018

O. Brockamp  and
N. Clauer
O. Brockamp*
Affiliation:
Fachbereich Geowissenschaften, Universität Bremen, Postfach 330 440, D-28334 Bremen, Germany
N. Clauer
Affiliation:
Centre de Géochimie de la Surface (CNRS-ULP), 1, rue Blessig, F-67084 Strasbourg, France
*
*E-mail: [email protected]
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Abstract

In a study of wall-rock alteration in the 1.4 km long Würmtal adit at the Käfersteige hydrothermal vein deposit (northern Black Forest, Germany) illite was found to be the only clay mineral within the Bunter sandstone and intercalated claystone. Illite occurs mainly as a detrital mineral in the claystone, whereas it is hydrothermally neoformed in the sandstone, either in the pores or as an alteration product of K-feldspar. The extensive occurrence of authigenic illite along the entire 1.4 km long profile confirms that the fluids migrated far into the sandstone.

The authigenic illite formed during a first pulse of high-temperature fluids (Th of ~220°C) with a low salinity (~1 wt.% NaCleq). These fluids also dissolved Sr and Rb from detrital illite of the claystones at the vein. A later hydrothermal pulse with a lower temperature (~70–150°C) and higher salinity (21–28 wt.% NaCleq) silicified the sandstone adjacent to the vein and caused partial substitution of OH- by F- in the structure of the detrital and neoformed illite along the profile.

Within analytical error, the K-Ar dates for the neoformed illite of the <2 µm fraction are the same along the profile (~150 Ma). During this hydrothermal process, the age of the detrital illite within the claystone was reset from 310 to 190 Ma. The illite-rich <0.2 µm fractions yield ages of ~142 Ma (sandstone) indicating a Jurassic origin. The uniform age data for illite in the sandstone and in the claystone are probably due to extensive migration of hot fluids through the wall rocks.

The hydrothermal fluids are attributed to recycled meteoric water and brines that ascended from the basement into the cover rocks during the opening of the North Atlantic and/or the nearby Tethys area.

Keywords

alterationK-Ar datingillitefluid inclusionsBlack ForestGermany
Type
Research Article
Information
Clay Minerals , Volume 40 , Issue 2 , June 2005 , pp. 245 - 260
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2005

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