Cryptic Superfine Placer Gold Populations, Round Hill Mine, Southern New Zealand

Superfine (50–<1 µm) detrital gold and platinum group mineral (PGM) particles at the Round Hill mine near Foveaux Strait were locally derived (<20 km) from as‐yet undefined basement sources. This superfine component contrasts with the dominant coarser (typically >100 µm) gold and PGM particles at the mine that have been transported for >100 km in regional drainage systems. The small (superfine) particles have travelled shorter distances than the larger particles, and this superfine component has particle morphologies that are absent from the coarser fraction. Also, superfine euhedral PGM particles are rare in the coarser fractions. The cryptic (hitherto unrecognised) superfine gold populations include abundant euhedral, angular or irregular electrum, palladium‐bearing gold and fragments of more distantly derived toroids. Many electrum particles (up to 50 wt% silver) have partial or complete rims of low‐silver gold that were emplaced prior to their local sedimentary transport. Superfine gold and PGM particles have been saved at the mine despite their strong propensity for floating in or on water in the processing plant. Floating superfine gold may be more common in natural rivers than has previously been considered, as saving and observing such superfine gold during prospecting and mining is extremely difficult.