Contrasting geology and mineralogy of evaporative encrustations in salt-tolerant ecosystems, Maniototo basin, Central Otago, New Zealand

ABSTRACT

Evaporative mineral encrustations have formed on bare substrates at three different sites in the Maniototo basin. Two sites, at Patearoa and Belmont have salts derived primarily from marine aerosols. The Patearoa site has developed on clay-rich pans formed on a terrace eroded by the Taieri River into clay-altered schist basement, and dissolution of albite from altered schist has resulted in precipitation of sodium carbonate with halite, at pH∼10. The Belmont site lies on the flat distal part of an active alluvial fan, and halite-dominated salt encrustations develop on thin dry crusts through capillary action from below. At the nearby Hamiltons historic placer gold mine, excavations exposed clay-altered schist basement beneath Eocene auriferous sediments. Highly fractured and hydrothermally altered fault zones dominate basement, with secondary ankeritic carbonate and pyrite. Water-rock interaction of shallow groundwater in the basement leads to enrichment in dissolved Mg and sulphate that overshadows the marine aerosol components, and evaporative encrustations are dominated by Mg-rich minerals including brucite and epsomite. The halite-dominated sites host rare halophyte ecosystems, but progressive colonisation by adventive species will eventually cover the bare saline substrates. Similar colonisation at the Hamiltons site will result in natural rehabilitation of the abandoned mine site.

KEYWORDS:

• Rain shadow
• clay
• halite
• brucite
• evaporation
• halophyte
• gold
• unconformity
• geoecology

Acknowledgements

This research was funded by University of Otago and Otago Regional Council, and we thank Euan Hind of ORC for his help. Luke Easterbrook-Clarke and Stephen Read (OU Geology Dept) assisted with construction of DEM images. Access to the Belmont site was facilitated by George Barnish, and access to the Patearoa site was facilitated by Rob Wardle and Charlie Hore. Discussions with Ben Dickie, Mark Hesson, Doug MacKenzie, Rick Sibson, and Guy Simpson over >30 years have helped with understanding of the Hamiltons site. Kate Wardle, and Clement Lagrue and Sasha Roselli (DOC) helpfully provided context of this study. Scanning electron microscopy was done at the Otago Micro and Nanoscale Imaging unit (OMNI), University of Otago. Manuscript presentation was improved by helpful comments from Travis Horton, an anonymous journal referee and the Associate Editor.

Data availability statement

All data relevant to this study are included within this paper and within cited references.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was funded by University of Otago and Otago Regional Council.