Gold and associated minerals in the Waikaia placer gold mine, Northern Southland, New Zealand

ABSTRACT

The Waikaia alluvial gold mine is hosted in a Pleistocene paleochannel that is buried beneath younger sediments beside the Waikaia River. The paleochannel sediments were derived primarily from local basement, with a minor component of recycled Miocene quartz pebble conglomerate. The detrital gold in the paleochannel has a wide range of textures, from angular nugget-like particles, through equant rounded and abraded particles, to highly flattened thin flakes that have commonly been folded. The internal grain structures of these various particles reflect differing degrees of particle deformation during transport, followed by variable amounts of post-depositional recrystallisation and associated leaching of Ag from the gold. The diverse gold textures represent passage through different numbers of sedimentary recycling events and associated deformation since the first particles were eroded from the adjacent Otago Schist source area in the Cretaceous. Minor authigenic gold overgrowths occur on the gold particles, typically with authigenic clay. However, the finest sediment component (<10 µm) of mine-related waters is dominated by quartz and albite rather than clay minerals.

KEYWORDS:

• alluvial
• paleochannel
• gold
• placer
• Pleistocene
• Otago Schist

Acknowledgements

This study was kindly facilitated by Noel Becker of Waikaia Gold Ltd, who provided access to the site and samples of gold. Discussions with Donna Falconer, Gemma Kerr, Dave Prior and John Youngson were helpful in compiling the manuscript. Brent Pooley assisted with sample preparation, and Gemma Kerr assisted with the XRD work and with sampling of mine groundwaters. Helpful suggestions from Rob Chapman and Brent Alloway improved the presentation of the paper.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was funded by NZ Ministry of Business, Innovation and Employment (MBIE) via a research contract to CRL Energy, partly subcontracted to University of Otago. Additional financial support was provided by Royal Society of New Zealand via the Marsden Fund (Contract UOO1313).