APPLYING LASER SPECTROSCOPY TO MEASURE D/H IN PHYLLOSILICATES: A NEW TOOL FOR EXPLORING HYDROGEN ISOTOPE VARIATION IN GEOLOGIC SYSTEMS

Hydrogen stable isotope ratios (D/H) in minerals provide valuable information on paleofluid history in both surface and subsurface environments. Historically, these measurements have been made using mass spectrometric techniques. Here, we develop and test new methodology for quantification of D/H signatures in phyllosilicates using an OA-ICOS near infrared laser analyzer, operated in a continuous flow configuration. Minerals were dehydrated using a simple quartz tube furnace, and advanced to the analyzer in a dry air carrier gas stream. We analyzed hydrogen isotope ratios of 12 IRMS-verified mineral and water standards and 10 unknown mica, kaolinite, serpentine, and gypsum monomineralic samples. We routinely achieved δD precisions of better than 1.5‰, and were able to perform up to 20 analyses in an hour. We also report preliminary δD measurements of clays associated with hydrothermally altered volcanic units at Waihi, New Zealand. Waihi is an epithermal Au-Ag deposit within the Coromandel Volcanic Zone, that has historically yielded over 6 M oz of gold. δD_VSMOW values in hydrothermal clays at Waihi range from -63.4±0.7 to -78.3±0.7‰. Based upon established mineral thermometric relationships for illite clays, and assuming hydrothermal fluids were of meteoric origin, these results indicate that mineralization at Waihi occurred between temperatures of 218 and 269°C. These values are consistent with prior fluid inclusion temperature estimates at Waihi. Additional measurements over a wider spatial extent will facilitate evaluation of δD signals in the context of fluid circulation, and empacement of Au-Ag ore.