The occurrence and wider implications of anomalously low delta D fluids inquartz veins, South Cornwall, England

A chemical and hydrogen isotopic study has been carried out on quartz-hoste d fluid inclusions from three different quartz +/- carbonate +/- sulphide /- sulphate veins from southwest England. Two fluid types have been identif ied in these veins by fluid inclusion microthermometry: (1) a low-temperatu re (80-150 degrees C) saline (20-27 wt.% NaCl + CaCl2 equiv.) NaCl + CaCl2 brine; and (2) a moderate-temperature (200 degrees C), low salinity (0-5 wt .% NaCl equiv.) fluid. delta D values for the brines range from -39% to -10 3% and values for the dilute fluids are in the range -46% to -79%. Both flu id types have at least some values of delta D which are defined as anomalou s, i.e., cannot be directly related to either a low delta D 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 geologic al context suggest the anomalously low values are unlikely to be attributab le to analytical artifacts or post entrapment changes of the fluid inclusio ns. Similarly, fluid interaction with a suitable mineral or organic phase c annot adequately explain the data. We conclude that an, as yet, unidentifie d process is responsible for producing low delta D fluids in southwest Engl and and that this may be a common phenomenon in low-temperature basinal env ironments. (C) 1999 Elsevier Science B.V. All rights reserved.