Abstract
The Holocene Tiwai Spit on Foveaux Strait is a 12 km by 2 km gravel‐dominated strand plain, ∼10 m above present sea level. Gravel clasts are dominated by well‐rounded quartz cobbles, pebbles and granules derived from Otago Schist 200 km to the north in evolving river drainage systems during late Cenozoic. Localised erosion and re‐accretion, driven by longshore drift, storm waves and strong prevailing westerly winds, has created variably truncated packets of subparallel gravel beach ridges that have developed on a decadal time scale. Storm wave erosion has formed heavy mineral concentrates (black sand) on the seaward beach immediately below the eroding portions of the spit, while heavy minerals have been diluted in sediment accretion zones. Black sand concentrates consist principally of rounded and subangular magnetite and ilmenite, with abundant Otago Schist spessartine‐rich and Fiordland almandine‐rich garnets, plus minor chromite. Most beach gold is fine (<200 µm) flakes that are extremely thin (micron scale) as a result of extreme hammering by gravel clasts on the beach that obscured almost all authigenic gold. Rare non‐flaky metallic particles include Hg‐rich and Ag‐rich gold, and platinum alloys. The spit constitutes a low‐grade gold deposit that is distinctly different from beach gold occurrences elsewhere along Foveaux Strait.