On the occurrence and wider implications of anomalously low δD fluids in quartz veins, South Cornwall, England

A chemical and hydrogen isotopic study has been carried out on quartz-hosted fluid inclusions from three different quartz±carbonate±sulphide±sulphate veins from southwest England. Two fluid types have been identified in these veins by fluid inclusion microthermometry: (1) a low-temperature (80–150°C) saline (20–27 wt.% NaCl+CaCl2 equiv.) NaCl+CaCl2 brine; and (2) a moderate-temperature (200°C), low salinity (0–5 wt.% NaCl equiv.) fluid. δD values for the brines range from −39‰ to −103‰ and values for the dilute fluids are in the range −46‰ to −79‰. Both fluid types have at least some values of δD which are defined as anomalous, i.e., cannot be directly related to either a low δD source fluid or to modification of a source fluid by common physicochemical processes such as evaporation or boiling. Examination of these data in the given geological context suggest the anomalously low values are unlikely to be attributable to analytical artifacts or post entrapment changes of the fluid inclusions. Similarly, fluid interaction with a suitable mineral or organic phase cannot adequately explain the data. We conclude that an, as yet, unidentified process is responsible for producing low δD fluids in southwest England and that this may be a common phenomenon in low-temperature basinal environments.