TY - THES

T1 - The karst bauxite of the Unterlaussa mining area (Upper Austria)

AU - Hampl, Ferdinand

N1 - no embargo

PY - 2018

Y1 - 2018

N2 - Four of the six once existing mining fields close to the abandoned miners´ village Weißwasser northeast of Unterlaussa in Upper Austria are still accessible by underground openings. They operated on boehmitic karst bauxite lenses at the base of the Gosau succession of the basin “Weyerer Bögen”. Twenty-three samples of bauxites and bauxite-related sediments were analysed by means of ICP-MS and XRF. The geochemical samples also include those of four bauxite profiles. In addition SEM, electron microprobe and XRD were used to define the mineralogy and to map element distributions. The XRD analyses reveal that the bauxite-related sediments are not repositioned bauxites but rather immature (not bauxitisized) sediments of which some are probably mineralogically similar to the parent material of the karst bauxite. XRD analyses are also a useful tool to realize a kinship among the different sediments and to show mineralogical differences within a bauxite profile. High values of lithium reaching 1370 ppm positively correlate with K2O in most of the samples indicating that phyllosilicates might be the carrier minerals. In the longest and best-preserved bauxite profile (Almstollen) the REE (except for Sc and Ce) distinctively accumulated in the lowermost two meters of this six-meter-long profile. Rare earth elements are generally highly enriched compared to average crustal values. Chondrite-normalized REE patterns of the Almstollen profile indicate reducing conditions in the lowermost two meters of the bauxite body due to negative Ce anomalies. The upward following samples show clear positive anomalies pointing to oxidising conditions. The karst bauxite of the Unterlaussa mining area can be regarded as an Upper Cretaceous (Turonian) paleosol which was formed in-situ by tropical to subtropical weathering of a precursor sediment on karstified dolostone. This precursor sediment was presumably illite-rich and likely already contained kaolinite. Lateritic material as well as volcanogenic sediments were likely contributors to the parent sediment. Additionally ultrabasic rocks must be assumed in the catchment area of this sediment as chromite and chromium accumulations prove. Conclusively the precursor material can be described as a mixed, fine-grained, polygenetic sediment. Changing conditions of water saturation are responsible for the layered in-situ concretions (spheroids) which give the bauxite its characteristic pisolitic texture. The karst bauxite of Unterlaussa was partly re-silificated, deferrificated (bleached) and pyritized. Both of the latter processes are attributed to microbial activity. Moreover a presumably microbially mediated uranium mineralisation and a massive aluminium-hydroxide bound chromium mineralisation have been identified.

AB - Four of the six once existing mining fields close to the abandoned miners´ village Weißwasser northeast of Unterlaussa in Upper Austria are still accessible by underground openings. They operated on boehmitic karst bauxite lenses at the base of the Gosau succession of the basin “Weyerer Bögen”. Twenty-three samples of bauxites and bauxite-related sediments were analysed by means of ICP-MS and XRF. The geochemical samples also include those of four bauxite profiles. In addition SEM, electron microprobe and XRD were used to define the mineralogy and to map element distributions. The XRD analyses reveal that the bauxite-related sediments are not repositioned bauxites but rather immature (not bauxitisized) sediments of which some are probably mineralogically similar to the parent material of the karst bauxite. XRD analyses are also a useful tool to realize a kinship among the different sediments and to show mineralogical differences within a bauxite profile. High values of lithium reaching 1370 ppm positively correlate with K2O in most of the samples indicating that phyllosilicates might be the carrier minerals. In the longest and best-preserved bauxite profile (Almstollen) the REE (except for Sc and Ce) distinctively accumulated in the lowermost two meters of this six-meter-long profile. Rare earth elements are generally highly enriched compared to average crustal values. Chondrite-normalized REE patterns of the Almstollen profile indicate reducing conditions in the lowermost two meters of the bauxite body due to negative Ce anomalies. The upward following samples show clear positive anomalies pointing to oxidising conditions. The karst bauxite of the Unterlaussa mining area can be regarded as an Upper Cretaceous (Turonian) paleosol which was formed in-situ by tropical to subtropical weathering of a precursor sediment on karstified dolostone. This precursor sediment was presumably illite-rich and likely already contained kaolinite. Lateritic material as well as volcanogenic sediments were likely contributors to the parent sediment. Additionally ultrabasic rocks must be assumed in the catchment area of this sediment as chromite and chromium accumulations prove. Conclusively the precursor material can be described as a mixed, fine-grained, polygenetic sediment. Changing conditions of water saturation are responsible for the layered in-situ concretions (spheroids) which give the bauxite its characteristic pisolitic texture. The karst bauxite of Unterlaussa was partly re-silificated, deferrificated (bleached) and pyritized. Both of the latter processes are attributed to microbial activity. Moreover a presumably microbially mediated uranium mineralisation and a massive aluminium-hydroxide bound chromium mineralisation have been identified.

KW - Unterlaussa

KW - karst bauxite

KW - geochemistry

KW - uranium mineralisation

KW - chromium mineralisation

KW - Unterlaussa

KW - Karstbauxit

KW - Geochemie

KW - Uranmineralisation

KW - Chrommineralisation

M3 - Master's Thesis

ER -