GOLD SEQUESTRATION IN CHALCOPYRITE, BORNITE, AND PYRRHOTITE AT MAGMATIC CONDITIONS

The low concentrations of Au in nature make exploration of any given prospect a challenge. Au can be found in orthomagmatic, magmatic-hydrothermal, or hydrothermal systems. In both the orthomagmatic and magmatic-hydrothermal systems, Au is commonly associated with chalcopyrite, bornite, and/or pyrrhotite, but why is debated. To address this question, bornite (Bn), high-temperature chalcopyrite (intermediate solid solution, ISS), and pyrrhotite (Po) were experimentally equilibrated with gold at 500-700°C, 50-100 MPa, and over a range of oxygen and sulfur fugacities. Stringers and blebs of native Au were observed within ISS and Bn grains at the conclusion of the experiments. Our analyses indicate that Au can be incorporated into the crystal structures of select sulfide minerals at high-temperatures and exsolve during cooling. Au concentrations in Bn were found to be 1800 to 5500, 500 to 3800, and 100 to 1400 µg/g at 700, 600, and 500°C, respectively, and were always higher than those of coexisting ISS. The highest concentrations were found in experiments conducted at the highest oxygen fugacity. Au in ISS was determined to be 1300 to 4900 and 340 to 3600 µg/g at 700 and 600°C, respectively. Au concentrations increased in both ISS and Bn with increasing temperature. Au concentrations in Po were always < 100 µg/g. The precipitation of ISS and Bn from a low-viscosity mafic magma will effectively strip Au from the melt, thus, periods of sulfide saturation would produce gold-rich zones within the magma through crystal settling. Crystallization of Bn and ISS in a high-viscosity granite would sequester Au from the melt and volatile phase and limit its ability to reach the porphyry or epithermal zones; resulting in a relatively Au-poor hydrothermal system. Conversely, the absence of sulfides in the melt, but presence in the porphyry or epithermal environment can result in elevated Au grades through incorporation into the crystals during formation from a hydrothermal fluid.