Petrological and geochemical evidence for putative ancient microfossils within the Berlins Porphyry, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Berlins Porphyry, located on the South Island of New Zealand, is the result of granitic intrusions, brecciation and hydrothermal alteration of country rock. Exposure of the Berlins Porphyry provides an opportunity to examine high temperature hydrothermal systems at depth (~2km) and to investigate the potential of biological activity in areas of localized hydrothermal activity. All Berlins Porphyry deposits are mineralogically and geochemically similar; however, localized areas within the porphyry show increased hydrothermal activity. Locations for microbial investigations include outer porphyry alteration zones that experienced high levels of hydrothermal fluid alteration. A recently exposed and highly altered porphyry outcrop on the Denniston Plateau contains iron concretions generated during localized hydrothermal mineralization. Here we show the Fe-rich concretions are the result of biomineralization via crystal morphology observations, carbon chemistry analysis and putative microfossils. High resolution petrographic and cathodoluminescence imaging reveals a variety of ancient fossilized burrows present within the magnetite zone. Transects through the concretions demonstrate that the abundance of carbon increases in the magnetite zone and exhibits δ¹³C‰ values characteristic of organic carbon (averaging -26‰ ± 4‰). Alteration mineralogy including recrystallized quartz, muscovite, adularia and kaolinite are present within the Berlins Porphyry iron concretions and suggest a hydrothermal system with microbial activity operating at temperatures between 200-250°C. Overall, these ancient and unusually persevered microbially-mediated Fe-rich concretions potentially represent a significant temperature increase for the habitability of life.