Frank, Mark R.
M.S. (Master of Science)
Department of Geology and Environmental Geosciences
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. Invisible gold is found in hydrothermal-type gold deposits, can make up a large percentage of the gold found in that deposit, and has been noted to be related directly to the arsenic concentration of the crystal, with increasing arsenic concentrations correlating with increasing gold concentrations. The relationship between arsenic and gold was evaluated by conducting experiments at 200 °C, 300 °C, 400 °C, 500 °C and 600 °C. Pyrite or arsenopyrite seed crystals were 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. Run products were analyzed by electron microprobe and laser ablation inductively coupled plasma mass spectrometry. Gold concentrations in arsenopyrite and pyrite rims ranged from 0.058 to 0.685 and 0.011 to 0.073 microg/g at 200 °C, respectively. Gold concentrations were 0.194 to 2.641 and 0.076 to 0.324 microg/g at 600 °C in the rims of arsenopyrite and pyrite, respectively. Gold concentrations were higher in the rims of run product crystals and lower in the crystal cores and were found to increase with increasing temperature. This study showed that the presence of arsenic enhanced the concentration of gold in pyrite and arsenopyrite at the conditions of the experiments, but, the concentrations observed were lower than concentrations observed in some natural samples, which can reach up to 10,000 microg/g. This suggests that surficial processes, most likely through chemisorption and/or electrosorption during crystal growth, attract gold ions, atoms, and/or molecules to the surface of the mineral to aid in enhancing gold concentrations in arsenic-bearing minerals.
Mann, Matthew Francis, "An examination of the pyrite-gold and arsenopyrite-gold systems : implications for hydrothermal gold deposits" (2017). Graduate Research Theses & Dissertations. 1268.
viii, 117 pages
Northern Illinois University
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