Check the great paper by Petr Drahota and co-authors about low-temperature formation of As sulphides, which has just been published in GCA! Drahota P., Mikutta C., Falteisek L., Duchoslav V., Klementová M. (2017): Biologically induced formation of realgar deposits in soil. Geochimica et Cosmochimica Acta 218, 237-256. (DOI).