Peculiar arsenic, copper, nickel, uranium, and yttrium-rich stone coatings in a high mountain stream in the Austrian Alps

Abstract

Peculiar coatings rich in arsenic, copper, nickel, uranium, and yttrium were found to cover stones in a high mountain stream downstream of the confluence of small acidic tributaries, which emerge at the front of the Krummgampen rock glacier (Ötztal Alps, Tyrol, Austria). The hard mineral coatings are characterized by a white color, a x-ray amorphous structure and showed properties of hydrous aluminium silicates like allophane (Al O (SiO ) x n H O) and/or imogolite (Al SiO (OH) ). Main constituents of 2 3 2 2 2 2 3 4 the coatings are aluminium, silica, and sulfur. Contrary to high concentrations of arsenic, copper, nickel, uranium, and yttrium in the stream stone coatings, corresponding element values are below the limit of detection both in the Krummgampen stream and except for nickel and traces of copper – also in acidic outflows from the rock glacier. Stone coatings are neither present in rock glacier outflows nor in the Krummgampen stream upstream of the rock glacier, nor in other streams in the catchment. The coatings are interpreted as a precipitate on or an accretion to stone surfaces in the Krummgampen stream, which is characterized by moderate solute concentrations and a neutral pH. In acidic outflows of the rock glacier high concentrations of dissolved sulfate, calcium, magnesium, silica, aluminium, manganese, and nickel prevail. Some constituents of stream stone coatings like arsenic, copper, nickel, uranium, and yttrium have also been measured in paragneiss rocks in the catchment of the rock glacier which are characterized by heavily weathered accessory minerals monazite, xenotime, and pyrite. It is therefore very likely that minerals which occur in the bedrock of the catchment represent a major source for elements accumulated in stream stone coatings even though processes like element release from the ice-rock matrix in the rock glacier, transport to and accumulation on Krummgampen stream stone surfaces are not yet completely understood.____________________________________________________