Petrological, geochemical and structural evidence of fluid-rock interaction in the Siljan Ring

Sammanfattning: The Siljan Ring in Dalarna, Sweden is the site of the largest meteor impact crater in Europe and has long been a topic of discussion regarding methane production. However, the source of this methane and the timing of production in relation to the impact remain unclear. An outcrop of red Ordovician limestone preserved on the edge of a downfaulted zone encircling the crater’s central plateau is crosscut by fractures surrounded by pale-coloured reduction haloes within which precipitates can be observed. These haloes suggest interaction with a reducing agent mobilised within a fluid flow, of which methane would be a prime candidate. A field study was subsequently undertaken to establish the reaction whereby these haloes were formed, as well as the timing of their formation relative to the Siljan impact based upon petrological, geochemical, and structural data obtained in the field. Results from this study show that a methane-bearing hydrothermal fluid mobilised within the fractures has preserved the original mineralogy of the limestone within the reaction haloes whilst the country rock beyond was being oxidised. Pyrite is shown to be preserved within the pale reaction haloes, whilst its oxidation within the country rock is shown to be the source of the limestone’s distinct red colouring. Fracture and bedding orientation at the study site suggest the hydrothermal event to have been simultaneous with the meteor impact, with the fractures forming part of a wider complex network of impact features. Whilst mobilisation associated with the meteor impact is a likely cause of methane release, the exact source of the methane active at the study site is unclear.