The Carls Au-Cu-Mo skarn mineralizitation (Asturias, NW Spain) is located in the Cantabrian zone of the Hercynian Iberian massif. The Carboniferous aged Carls intrusion consists of monzogranite and is placed in a syncline formed by Devonian carbonate rocks at their contact with Silurian quartzites. The resulting skarn occurs at the contact between the monzogranite and the mostly calcitic and minor dolomitic carbonate rocks as an irregular envelope. Samples from the skarn and monzogranite are studied by using optical microscopy, Raman spectroscopy, Electron-Microprobe methods, microthermometry and computer modelling of fluids. A large number of calc-silicate minerals and in addition, a variety of native elements, oxides, sulfides, and sulfosalts was explored in the skarn. Fluid inclusions in quartz, calcite, garnet and clinopyroxene give information about ore generating processes and conditions. The fluids are highly variable in gas-component composition (mixture of CO2-CH4-N2 and H2S) and salinity (H2O-salt). Gas-H2O-salt inclusions (low salinity) are observed in quartz veins. Low saline aqueous fluids are mainly present in calcite and are also included in quartz veins and garnet. High salinity fluid inclusions are observed in diopside and garnet in the presence of CH4, which represent the fluid conditions of the early skarn formation. In addition diopside contains a variety of entrapped solids, which illustrate the transition of a magmatic to a hydrothermal system.