Isotopic determinations have been made on minerals from mineralized veins (including inclusion fluids), altered wall-rocks and host shales from the Clogau-St Davids gold mine and smaller mines in the Dolgellau Gold Belt. The Clogau Formation shales hosting the mineralization contain calcite (mean δ13C −11.5‰PDB) and graphite (mean δ13C −22.2‰PDB) yielding an equilibrium isotopic exchange temperature of 300-315 ±20 °C, a likely metamorphic temperature. Mineralized vein calcites are isotopically lighter than calcite in the shale with mean δ13C of -12.9‰PDB. Mineralized vein quartz oxygen isotopic compositions (9.6-11.0‰ SMOW) are similar to the host shale quartz (9.4-13.6‰ SMOW). Calcites are always too enriched in 18O to be in equilibrium with quartz in the same sample. It is not possible to distinguish whether this is a primary feature or whether the calcite compositions have been reset subsequent to formation. Chalcopyrite and pyrrhotite in one sample from the Clogau-St Davids mine give an equilibrium formation temperature of 300 ± 50 °C. A relationship exists between the composition of mineralized vein sulphur and that in the host shales, though the ore sulphur is always depeleted in 34S relative to the shale host. This is most probably due to a mixing of shale sulphur with a lighter, introduced component during ore formation. Deuterium/hydrogen ratios in fluid inclusion water may also indicate the introduction of a fluid into the shales during mineralization.
The stable isotope data show that mineralization in the Dolgellau gold belt probably involved the introduction of an externally derived fluid into the shales of the Mawddach Group and that it interacted to a significant extent with the shales. This interaction makes it difficult to be certain of the original isotopic 'signature' of the ore-forming fluid, but is was probably derived from an igneous source, either as a primary magmatic fluid or as the result of fluid interacting with igneous material during metamorphism.