From granitoid-hosted Au deposits to carbonate-hosted Hg deposits in Pedrosa, N Spain.

The relation between granitoid-hosted As-Sb-Au and carbonate-hosted Hg deposits in the foreland thrust and fold belt of the Variscan orogen in N Spain has been studied at Pedrosa, the only locality in which both types of deposits are spatially related. Both mineralizations derived from paleocirculating fluids under nearly hydrostatic conditions. The mean deposition temperature seems to be likely 280 °C for early As ore, about 170-180 °C for the late Sb mineralization, and roughly 130 °C for the late Hg mineralization. The application of arsenopyrite and chlorite geothermometers shows systematic deviations of T_h from associated fluid inclusions in quartz. The fluids were dominantly aqueous of moderate salinity, but a carbonic portion has been also observed, partly derived from thermal evolution of organic matter in synorogenic sediments, as reflected by negative values of δ¹³C_PDB in hydrothermal carbonates. The stable isotopic composition of the fluid is partly buffered by the host-rocks.