A new source of isotopically heavy sulphur in lode-gold deposits of NW Spain: a batch volatilization process in the conversion of pyrite to pyrrhotite by contact metamorphism

The conversion of pyrite to pyrrhotite is identified in this study as a new source of isotopically heavy sulphur in thermal domes, Contact metamorphis m of a sequence of pyrite-rich shales by the Penedela blind intrusion (NW S pain) causes the conversion of diagenetic pyrite to pyrrhotite at a tempera ture of 150 degrees C. This process occurred in a closed system with total consumption of the diagenetic pyrite and production of H2S-rich fluids. The H2S liberated in this unidirectional reaction shows a relatively heavy del ta(34)S, in agreement with a batch volatilization process. The mixture of t his H2S-rich fluids with derived sulphur from Lower Cambrian carbonates pro duced gold-bearing pyrite at a temperature of 340 degrees C. The contributi on from these two different sulphur sources was 86% and 14%, respectively. The metamorphic fluids migrated through permeable zones such as fractures a nd shear zones, producing a lode-gold deposit.