Source of Fluids in the Auriferous El Cabaco Mineralized Zone, Southern Salamanca

This paper assesses chemical-mineralogical changes resulting from hydrothermal alteration associated with granite-hosted gold mineralization in southern Salamanca province, Spain. Within the mineralized veins, along planes of quartz growth, two types of fluid inclusions were observed. One type is rich in CH4 with minor CO2, the other is rich in H2O with CO2 (± CH4). These are interpreted as reflecting the immiscibility of an initial fluid rich in H2O-CH4 and some CO2. Inclusions with similar composition are seen at the silicification formed at the granite contact with host rocks. However, differences in P-T conditions and immiscibility of fluids are indicated by the microthermometric study and relation of the inclusions. These are consistent with the temperatures calculated from the arsenopyrite-pyrite geothermometer. Formation temperatures of 445 ± 15° C were deduced for the mineralization at the granite contact and temperatures not exceeding 386° C for the vein mineralization. Metasomatically altered granites are depleted in Na and K in comparison to fresh granites. A gain in CO2 has been measured in the altered rocks. No correlation was found between gold contents and any of the major or trace elements analyzed. δ34S arsenopyrite values suggest a variable source of sulfur. Calculated δDfluid values show significant variability (−66 to −37%0 SMOW), whereas δ18O fluid values show small variation (from +7.6 to +8.4%o SMOW). These values for the fluids are consistent with interaction between magmatic fluids and metamorphic rocks. Gold deposition in quartz veins could be explained by the loss of H2S during fluid immiscibility.