Evidence supporting micro-galvanic coupling in sulphides leads to gold deposition

Electrical micro-junctions in metal sulfides drive electrochemical reactions with passing gold-bearing fluids, resulting in the deposition of gold, even from under-saturated ore fluids. Understanding the role micro-junctions play in the deposition of gold requires (a) imaging the electric field distribution of a galvanic couple near the surface to qualify the existence of an active micro-geo-battery and (b) correlating it with gold precipitation on the p-type cathode side of the junction by mapping the host at minor and trace levels. Here we report on correlating electron back scattered diffraction (EBSD), particle induced X-ray emission (PIXE) elemental maps including micron-scaled gold hot spots with laser beam induced current (LBIC) photocurrent maps of galvanic coupling in natural arsenian pyrite from the Otago Schist in New Zealand. The results provide convincing evidence that sulphide electrochemical interactions can lead to gold electro-deposition. We finish by discussing a simplistic model of the processes involved in reference to the original model of Moller and Kersten (Miner Deposita 29(5):404–413. 1994), and discuss the effects of  temperature in light of recent-reported evidence of electrochemical gold deposition in the formation of hydrothermal gold deposits.