The formation of the Unterlaussa karst bauxite (Austria) – A re-evaluation of the established model

Abstract

Karst bauxites are the most important aluminium resources of Europe and have therefore been intensely mined and studied in the past. However, the genesis of many bauxite occurrences remains insufficiently explained and untouched by modern models and techniques. The karst bauxite of the Unterlaussa mining district represents the most significant occurrence in Austria and is a prominent member of the European karst bauxites. Its formation has been traditionally described by a chemogenic model. To evaluate this theory and elucidate the formation processes we analysed the geochemical and mineralogical characteristics of three bauxite profiles in the mining district.Based on the geochemical gradients and mineralogical patterns we conclude that the Unterlaussa karst bauxite is an Upper Cretaceous weathering product and not a chemogenic sediment. In this study its formation is explained by in-situ weathering of a clayey polygenic precursor sediment which was deposited on karstified dolostone. Among contributions to this sediment were the weathering products of (ultra)basic rocks, as chromite and an uncommon Cr-mineralization suggest. The formation processes led to a substantial accumulation of critical elements, partial reduction and depletion of Fe(III) (bleaching), pyritization, and resilification. Another feature of the Unterlaussa karst bauxite is a unique U-mineralization which is bound to reduction spheroids.This study exemplifies the high potential of karst bauxites to advance our understanding of weathering processes, to support palaeo-environmental reconstructions and to be of renewed economic interest.