Field and Laboratory Analysis of the Mercury Content of Gold: Prelude to the Trial of a Premium to Artisanal Miners for Gold Concentrated without Mercury in Madre de Dios, Peru

Using mercury (Hg) is the least expensive way of concentrating gold (Au) produced at the artisanal scale, even though this is already illegal in several countries. The use of Hg results in occupational exposure of miners and probably exposure of the general population to methyl mercury accumulated in fish. Colombia has forbidden the use of Hg since 2018, but this has only led to a flourishing black-market originating in Bolivia. There is little reason to expect a different outcome of the same Hg interdiction slated within a few years in Peru. Our group is planning a randomized-controlled trial to explore the viability of combining field measurements of the Hg-content of Au with blockchain certification and a premium >10% awarded to artisanal miners as an incentive to forgo the use of Hg.

Two conditions must be met for this trial: (1) it must be possible to measure the Hg-content of Au with sufficient sensitivity in the field before awarding a premium, and (2) the Hg-content of Au concentrated gravimetrically using shaking tables instead of amalgamation must be very low. To explore these conditions, we brought a handheld SciAps Z300 laser induced breakdown spectrometer (LIBS) to the Madre de Dios region of Peru. We measured Hg/Au ratios in the raster mode across a single unprocessed Au flake from Madre de Dios and could not detect a Hg/Au peak. The spectrum was comparable to that of a pure Au foil sample in the vicinity of the Hg emission line. We also analyzed a handful of sponge Au/doré samples obtained directly from a mining concession and local gold shops. The doré was produced by Hg amalgamation and showed a distinct Hg/Au peak, but further processing and smelting by the gold shops can largely eliminate this peak. The implication is that doré would have to be tested before processing by a gold shop if LIBS verification is a requirement for a premium to be given to the artisanal miner.

We reanalyzed the doré samples in the laboratory by both X-ray fluorescence (XRF) and inductively-coupled plasma mass spectrometry (ICP-MS) after dissolution in reverse aqua regia. ICPMS data indicate Au content >90% for all doré samples and a Hg content of ~0.02%. XRF analysis confirmed the high Au content of the samples, but the detection limit for Hg content was below 0.1%. Therefore, we conclude that LIBS will be more suitable for the planned field trial than XRF.