A POSSIBLE EXPLANATION FOR THE CORRELATION OF GOLD AND ARSENIC WITHIN PYRITE, ARSENIAN PYRITE, AND ARSENOPYRITE

Invisible gold is a type of gold ore that consists of micrometer-sized blebs of native gold and structurally bound gold in iron sulfides, most commonly pyrite, arsenian pyrite and arsenopyrite, in hydrothermal-type gold deposits. Increasing arsenic concentrations often correlate with increasing gold concentrations in natural samples. This relationship was evaluated by conducting experiments at 200 °C, 300 °C, 400 °C, 500 °C and 600 °C in arsenic-free and arsenic-bearing systems. Pyrite or arsenopyrite seed crystals were surrounded by finely powdered FeS₂ or FeAsS and placed into quartz or silica-based glass vessels with a synthetic hydrothermal fluid and metallic gold for a minimum and maximum of 60 and 370 days, respectively. Gold ions from the hydrothermal fluid were able to diffuse into the seed crystal and become incorporated into the new growth around the seed crystal. Gold concentrations, as determined by using laser ablation inductively coupled plasma mass spectrometry, were found to be higher in the rims of crystals than the cores. Gold concentrations in arsenopyrite and pyrite rims ranged from 0.058 to 0.685 and 0.011 to 0.073 µg/g at 200 °C, respectively. At 600 °C, gold concentrations in arsenopyrite and pyrite rims ranged 0.194 to 2.641 and 0.076 to 0.324 µg/g. The presence of arsenic enhanced the concentration of gold in pyrite and arsenopyrite in the experiments, but the concentrations observed are much lower than those observed in natural samples, which can reach up to 10,000 µg/g. We hypothesize that disequilibrium and a non-stoichiometric induced surficial charge can allow for the addition of gold through chemisorption and/or electrosorption during times of fast crystal growth. These processes will enhance gold concentrations in pyrite and arsenopyrite in zones of high fluid flow and where gold species are being destabilized, often by wall rock alteration.