Abstract : Various types of rocks from the so-called Canaveilles Cambrian sequence are transformed into skarns near the Costabonne Hercynian granite (Eastern Pyrenees, France). This work focuses more specifically on skarns formed after calcitic marbles and massive dolostones (first part). In the latter case, the more frequently encountered zoning shows the succession: dolostone / calcite + forsterite / diopside / salitic clinopyroxene / grossularitic garnet. The various minerals encountered are analyzed by electron microprobe. Andraditic garnet may have developped earlier, before grossularitic garnet, and metasomatic zoning may have changed with time. Alterations of primary minerals produce amphiboles, quartz, sulphides and a tungsten mineral: scheelite. In a second part, a theoretical reflection is proposed on the formation of skarns (discussion of Korzhinskii's model) and a generalized phase rule for a system of metasomatic zones is presented. The third part is devoted to analyzing the conditions of pressure, temperature etc. that prevailed during the skarn formation, and the discussion of isotopic composition (S, C, O) of skarns and enclosing rocks. The isotopic composition highlights an external source, both for carbon, sulfur and oxygen ("deep" origin) for the fluid that has percolated. Measurements of the isotopic compositions of other skarns of the Pyrenees, with and without scheelite (Salau, Roc Jalère, Lacourt), indicate that the condition for tungsten mineralization is the presence of a fluid having such "deep" isotopic signature. The comparison between the skarns from Salau and Costabonne highlights the reducing role of carbon marbles from the country rock on the nature of the mineral associations of the Salau skarns.