ANCIENT HIGH TEMPERATURE MICROBIAL ACTIVITY IN THE BERLINS PORPHYRY, NEW ZEALAND

The Berlins Porphyry located in New Zealand is the result of granitic intrusions, brecciation and hydrothermally altered country rock that occurred during the Cretaceous at ~2km depth. The recent exposure of an outer alteration zone containing magnetite concretions on the Denniston Plateau was investigated to assess its formation. Based on a wide variety of imaging, the magnetite-rich concretions appear to be microbially-mediated as supported by the presence of trace fossils and magnetite morphologies typical of biomineralization. Transects through the concretions demonstrate that the abundance of carbon increases in the magnetite zone and yields δ¹³C‰ values (averaging -26‰ ± 4‰) characteristic of organic carbon. No geochemical and/or mineralogical evidence suggests that low temperature alteration occurred concurrently with magnetite formation. Based on a coupled petrologic approach, the most conservative (i.e., lowest) temperature range experienced in this system is between 200 and 250^oC. Overall, these unusually preserved magnetite concretions are best explained as a microbially-mediated deposit operating at temperatures exceeding the current habitability limit for life.