The Los Santos skarn shows several chemical and mineralogical features common to many other W-bearing calcic mesozonal reduced skarn deposits. Early exoskarn consists of hedenbergite (Hd82-87), grossular (Grs53- 73) and Mo-rich scheelite. The mineral assemblage which formed during the cooling of the system is very complex and is constituted mainly by subcalcic garnet, other hydrated silicates (amphibole, phlogopite, prehnite, epidote-clinozoisite, chlorite, white mica and zeolite) and metallic mineralization with Mo-poor scheelite. The aplitic granite is replaced by a calcic plagioclase and clinopyroxene endoskarn. Stockscheider, breccias and fractures indicate a forceful emplacement and over pressure associated with volatile exsolution from granite magma. The occurrence of zeolites indicates that the hydrothermal fluid circulation probably communicated with the surface as in present-day geothermal systems