Distribution of Scheelite in Magnesian Skarns at Traversella (Piemontese Alps, Italy) and Costabonne (Eastern Pyrenees, France): Nature of the Associated Magmatism and Influence of Fluid Composition

Abstract

The Costabonne and Traversella scheelite-bearing skarns are compared on the basis of associated igneous rock composition, early metasomatic columns, parageneses of the scheelite and differences in the fluid composition. Both series of intrusive rocks are of calc-alkaline affinity and are the result of a process of fractional crystallization. These rocks are of granitic composition at Costabonne and of dioritic composition at Traversella. The early metasomatic columns are similar: dolomite/forsterite + calcite/pyroxene/garnet, but the silicates are richer in Fe and Mn at Costabonne. At Traversella, Fe is introduced during a later hydrothermal stage (magnetite). Two generations of scheelite are found at Costabonne, one associated with the garnet and a second in association with amphibole, quartz and calcite as an alteration product of pyroxenes. At Traversella, scheelite is introduced during the hydrothermal stage by replacement of calcite in the outer zones. These differences in scheelite distribution may be explained by differences in fluid compositions under similar pressure and temperature conditions. At Costabonne, activities of Ca and Si were higher and activities of Mg and W were lower than at Traversella.