The San Leone magnetite skarn deposit consists of lens-shaped bodies interlayered in a Silurian quartzo-pelitic formation metamorphosed by Hercynian granodiorite. Iron oxide deposits of this type can result from iron being introduced into a limestone by metasomatism in the vicinity of an intrusive. On the other hand, iron-rich sedimentary carbonates can be transformed into oxides in the vicinity of an intrusive. San Leone appears to be a deposit where both phenomena have operated. A two-stage model is proposed to explain the evolution of the deposit.1. The lens-shaped deposits consisted initially of iron-rich carbonate sediments with quartz-pelitic and/or pyroclastic intercalations. Diffusion across layering, taking place at the time of metamorphism, at about 550 degrees C and low pressure led to the formation of monomineralic (mainly hedenbergite) or oligomineralic layers. This required local mobility of silicon and iron. The state of the iron oxidation appears to have remained unchanged during this metamorphic stage. Resulting minerals include hedenbergite, andradite, grandite, epidote, vesuvianite, calcite, and magnetite.2. The hydrothermal stage, taking place at about 450 degrees C and also at low pressure, consisted of two kinds of phenomena: alteration of hedenbergite and other metamorphic silicates into quartz, magnetite, ferrotremolite, ilvaite, babingtonite, fluorite, calcite, and scheelite and replacement of the remaining calcite by magnetite. Iron was introduced into the lenses by infiltration, presumably along bedding planes. The silicates show an unusually high iron content, especially the amphibole which appears to represent the purest ferrotremolite yet reported from nature.