Morphology and structural evolution of fne beach gold in comparison to detrital platinum, southern New Zealand
- Author
- Palmer, M.; Craw, D.
- Year
- 2024
- Journal / Source
- Mineralium Deposita
- Volume
- 59
- Pages
- 69-83
- Summary
- Beach placer gold has been mined around the world historically, but extraction of fine (~ 100 µm) gold particles is notoriously difficult. This study illustrates morphological and mineralogical changes that transform fine gold during aeolian processes on windy beaches and contribute to mine concentration inefficiencies. Sandblasting on exposed beaches in southern New Zealand has caused extreme attenuation of edges of gold fakes that were previously transported in rivers for>200 km. Flakes have been transformed into complex but compact toroids and spheroids with thin (~20 µm) internal and external strands of attenuated gold. Most of the gold within the attenuated strands has recrystallised to fne (micron-scale) undeformed grains with little or no Ag (<1 wt%). Some coarse (>40 µm) gold grains remain from the precursor fuvial particles, and these retain original Ag contents (1–10 wt%). These coarse grains show substantial internal crystallographic deformation and sub-grain formation, although some of these strain efects may have been inherited from fuvial transport. Co-existing detrital platinum minerals are much less malleable than gold during sandblasting and have only minor (10-µm scale) toroidal deformation on edges of fuvial fakes. The complex frameworks of the fne toroidal and spheroidal gold particles can include air, water, and clay, which lowers their average density and so they commonly foat on water and are readily entrained with other heavy minerals. The fne particle size, compact shapes, and clay coatings also resist mercury amalgamation.