Anthropogenic Hg‐Au residues from placer mining compared to natural Au‐Hg sources, South Island, New Zealand

ABSTRACT Amalgamation of gold (Au) with mercury (Hg) has been occurring around the world for thousands of years, and global modern uncontrolled use of the process for gold concentration is causing widespread Hg contamination. Despite the environmental significance of HgAu‐amalgamation, there has been little characterisation of solid materials that escape the processes and contribute to the Hg contamination. We describe herein the morphological and compositional characteristics of solids sampled through amalgamation processes obtained from small modern active mines in New Zealand under well‐confined conditions. Liquid Hg droplets resisted amalgamation because of clay coatings on gold, and finely particulate gold (especially <100 µm) was only superficially coated by HgAu‐amalgam with relict liquid Hg droplets. Recovery of purified gold and liquid mercury after amalgamation was done at high temperature (>360°C) by retorting, and at room temperature with acid treatment. Acid treatment yielded variably crystalline bronze‐coloured Au with ∼10% Hg, whereas retorting yielded pale yellow porous amorphous (‘sponge’) Au with <1% Hg. Most anthropogenic products are distinguishable from natural Hg‐bearing materials, but some micron‐scale features of retorted gold are indistinguishable from natural authigenic gold. Materials from all stages of the amalgamation process can be discharged to the environment, contributing to elevated dissolved Hg.