Figure 4

Cartoon model illustrating a likely scenario during the formation of the Bastnäs-type REE deposits in Bergslagen. These deposits are interpreted to have formed in a c. 1.9 Ga shallow-marine back-arc, sub-seafloor setting associated with extensive felsic volcanism and plutonism. High-temperature hydrothermal fluids, enriched in silica, iron and REE among other components, exsolved from a sub-volcanic magma and reacted with nearby interlayers of limestone and carbonate-bearing BIF. This led to skarn formation and magnetite-REE-precipitation within the carbonate units, while extensive hydrothermal alteration affected the surrounding volcanic host rocks. Over the life of the hydrothermal system there was progressive involvement of surface waters. See text for detailed explanation.