Fine and super-fine gold in mine waste from a coastal placer in southern New Zealand

Abstract

ABSTRACT Reprocessing of mine wastes is a growing procedure around the world, in order to extract more resources from already-mined rocks. Mineralogical and geometallurgical investigations are useful for planned reprocessing, and in this study at Round Hill placer gold mine we characterise fine (<100 µm) and super-fine (micron-scale) particulate gold that has passed through the mine separation stages without being extracted. The gold was discharged with water and suspended sediments to settling ponds, separate from the more voluminous gravel and sand tailings. Authigenic clay and pyrite are abundant in the placer deposit and the settled suspension, and oxidised pyrite yielded gypsum and ferric oxyhydroxide. These minerals coat surfaces of gold particles and are locally intergrown with micron-scale authigenic gold. Most of the gold has toroidal shapes characteristic of beach derivation, and many of these are hollow or clay-filled. Minor super-fine cinnabar and mercury-bearing gold occur with clay. Annealing of gold has liberated silver, and micron-scale authigenic native silver occurs in the settled sediment. Reconcentration of fine and super-fine gold used abundant clean water and slow reprocessing and may be currently uneconomic at this site. The observations on suspended gold loss are relevant to gold recovery at placer mines around the world.