Cryptic silver mineralization in the magnetite-sulfide ore of Sagmurgruvan, Central Sweden

Abstract

The ores of Sågmurgruvan consist of nearly massive magnetite with locally important concentrations of sulfides: pyrite, pyrrhotite, iron-rich sphalerite, galena, very minor chalcopyrite and accessorial cobaltite, linnaeite and Ag minerals. The Ag minerals are native Ag, acanthite, stephanite and pyrargyrite. However, the majority of the Ag is present as minute, nearly submicroscopic, inclusions of native Ag in porous pyrite. Younger granite, in contact with the massive magnetite-sulfide ores, locally contains small amounts of sulfides (pyrite, galena and nearly iron-free sphalerite) and rather large grains of native Ag. The magnetitesulfide ores are interpreted as the metamorphic equivalent of siliceous exhalative-sedimentary ores. Occasional spherical pyrite grains are thought to form relicts of the primary sedimentary texture. The mineralization in the granite is thought to originate from local selective remobilization of the magnetite-sulfide ore, caused by the hydrothermal activity accompanying the granite intrusion. The deposition of native Ag in the porous pyrite is probably a low temperature event, during retrogressive metamorphism.