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.