AN EXPERIMENTAL STUDY OF GOLD CONCENTRATIONS IN PYRITE AND ARSENOPYRITE FROM 200-500 DEGREES CELSIUS

Concentrations of submicrometer inclusions of gold (Au⁰) and structurally bound gold (Au⁺¹) are often higher in the rims of pyrite crystals relative to the core and are, in some cases, associated spatially with arsenic. We hypothesize that: 1) the addition of arsenic to pyrite increases the gold concentration contained within the arsenic-bearing pyrite crystal structure and 2) arsenic facilitates the adsorption of gold onto preexisting pyrite crystals.

Experiments to test these hypotheses were conducted in glass and silica tubes at 200-500 °C. The experimental load consisted of a pyrite seed crystal, crushed pyrite or arsenopyrite, native Au, a thin layer of crushed quartz between the seed crystal and native Au, and a 5 wt.% NaCl (equivalent) aqueous fluid. Experiments were run from 60 days at 500 °C to 378 days at 200 °C.

Following the experiments, the seed crystals were mounted in epoxy and polished to reveal the interiors of the crystals. Analyses were performed on the rims and cores of the crystals by using an electron microprobe (major elements) and laser-ablation inductively coupled plasma mass spectrometer (Au, trace elements). Generally, Au concentrations in the pyrite seed crystals increased with both temperature and the presence of arsenic, and were highest in the crystal rim.

For experiments devoid of arsenic, gold concentrations in the rims and cores increased from 0.04 ± 0.02 μg/g and 0.02 ± 0.01 at 200 °C to 0.3 ± 0.1 μg/g and 0.14 ± 0.05 μg/g at 500 °C. The addition of arsenic increased the concentration of Au in both the rims and cores of the pyrite crystals. Au concentrations in the rims and cores of 0.2 ± 0.1 μg/g and 0.08 ± 0.03 μg/g were found at 200 °C and increased to 0.9 μg/g and 0.2 μg/g at 500 °C in arsenic-bearing systems. These data suggest that more Au can be sequestered in pyrite as temperature and As increase. The experiments were conducted at Au saturation and yet the measured Au concentrations were less than that measured in some natural samples which suggests that mineral surficial controls and/or fluid composition may be more important than crystallography in controlling the concentration of Au in or on the surface of pyrite crystals in natural systems.