THE ROLE OF ARSENIC DURING GOLD DEPOSITION IN HYDROTHERMAL GOLD DEPOSITS

In sulfidic hydrothermal gold deposits, gold is often found in the rims of pyrite crystals as submicrometer inclusions. This “invisible” gold has a strong spatial correlation with arsenic. Research done on Carlin-type gold deposits suggests that gold and arsenic are at highest concentrations in the rims of pyrite crystals, where arsenic is at elevated concentrations, and are nearly nonexistent in the core of the pyrite crystal. This has led to suggestions that there is a link between arsenic and gold, with arsenic possibly facilitating gold adsorption onto the pyrite and increasing the solubility maximum of gold.

This study experimentally tested the relationship between arsenic and gold. Experiments were conducted in glass and silica tubes at temperatures ranging from 200-600 °C. Capsules had three experimental setups. The first was composed of a pyrite seed crystal, crushed pyrite, and gold that was in physical contact with the pyrite seed crystal. The second and third setup consisted of either an arsenopyrite or pyrite seed crystal, crushed pyrite, a layer of crushed quartz, and gold. The first experiment allowed gold to plate directly onto the pyrite, while in the second and third experiments gold was dissolved by a solution and transported through the quartz layer before deposition. The capsules were loaded with 5 wt.% NaCl-KCl-HCl-H₂O fluid. Experiments were sealed under a low vacuum and placed into furnaces. Experiments were conducted for up to 400 days.

The seed crystal as well as powdered sulfide grains were mounted in epoxy and polished for electron microprobe analyses (JEOL 733 - major elements) and laser-ablation inductively coupled plasma mass spectrometry (Au). Experiments that had a pyrite seed crystal had arsenic values ranging from 0-2 wt.%, with arsenic in the outer rims of the seed crystal. Experiments that had an arsenopyrite seed crystal had arsenic values ranging from 40-52 wt.%, with arsenic values decreasing from rim to core of the crystals.