Skuppesavon,northern Sweden: A uranium mineralisation associated with alkali metasomatism

Summary Skuppesavon represents a uranium mineralisation emplaced within a sequence of metavolcanics dominated by rocks of rhyolitic to trachytic composition. Uraniferous solutions have penetrated along zones previously opened by Na-Ca-rich mineralising fluids which caused albitisation of potassium feldspar, the partial to complete removal of quartz and the crystallisation of new mineral phases. Uranium, which has been transported mostly as uranyl carbonate complexes, has precipitated as uraninite which occurs as impregnations and only very rarely forms fracture infillings. The introduction of Ca-and Ti-bearing solutions, coeval and subsequent to uraninite precipitation, has resulted in the association of uraninite and sphene. The hydrothermal fluids responsible for the observed mineral parageneses have been activated during metamorphism. During the passage of these fluids through the volcano-sedimentary country rocks, elements such as Ca, Na, U and Ti were mobilised, concentrated and transported. Access through the rocks was facilitated by structural weaknesses and rock permeability variations.