Acid Rock Drainage Caused by Hydronium-Bearing and Fe-Deficient Jarosite: An Example from Lillesand, Southern Norway

Abstract

Acid rock drainage (ARD) represents a serious environmental concern due to potentially extreme lowering of pH and mobilisation of toxic metals. The present paper focusses on S-bearing gneisses in the Lillesand district in Norway in light of 20 years of experience. These Precambrian gneisses were affected by later extensive alterations involving hydrous sulfates. Our objectives were to investigate a) the role of iron sulfide oxidation versus secondary alterations; b) the geological origin of minerals responsible for ARD, and c) discuss consequences for classification of ARD-susceptible rocks. We used a multiproxy approach, involving field work, abrasion pH, water chemistry, X-ray diffraction, petrographic examination, scanning electron microscopy, electron probe micro analysis and S and Pb stable isotopes for targeting the ARD contributors. The main acidifier was a non-stoichiometric Fe-deficient and hydronium-bearing jarosite residing in weak alteration crusts and microveins within gneiss, whereas scarce pyrrhotite and pyrite had little or no effect. Resistance to fragmentation was generally lowest in jarosite-rich material, hence rising the ARD-potential. The alterations were caused both by hydrothermal fluids derived from the nearby Permo-Carboniferous Skagerrak Rift and subsequent Mesozoic deep weathering. Preliminary thermometry suggests that the hydrothermal activity took place at temperatures of 80-95 °C, whilst the weathering took place in a tropical climate. Acid rock drainage susceptibility hinges on mineralogy, field relations, and blast-induced fragmentation, all requiring thorough local rock mass characterization.

Keywords: Acid rock drainage, deep weathering, hydrothermal alteration, jarosite, pyrite, pyrrhotite