Frank, Mark R.
M.S. (Master of Science)
Department of Geographic and Atmospheric Sciences
Magmatic-hydrothermal ore deposits contain metals sourced from a melt and transported to shallower levels by a magmatic volatile phase (MVP) where they can be deposited as native elements or structurally bound in sulfide or oxide minerals. Porphyry ore deposits are a major subclass of such and contain plentiful concentrations of societally significant metals including copper and gold. These deposits include the Grasberg porphyry Cu-Au-(Mo) deposit in Indonesia, the Bingham Canyon porphyry Cu-Mo-Au deposit in Utah, USA, and the Far Southeast porphyry Cu-Au deposit in the Philippines. In these systems gold is often found associated with bornite, intermediate solid solution (ISS, high-temperature chalcopyrite), and pyrrhotite. The relationship between gold concentrations and changes in temperature, pressure, mineral assemblage, and oxygen and sulfur fugacity of porphyry systems was evaluated in this study by conducting experiments simulating the porphyry ore environment. Experiments were performed at 700°C, 600°C, and 500°C, and 100 MPa or 50 MPa, with oxygen fugacity buffered at either MnO-Mn3O4 or Ni-NiO. The sulfur fugacity of the system was buffered by the 2 starting sulfide mineral assemblages, either chalcopyrite + pyrrhotite or chalcopyrite + pyrrhotite + bornite. Once experiments reached equilibrium, they were quenched, capsules were removed from the vessels, and sulfide crystals extracted from the capsules. Run-products were analyzed using a CAMECA SXFive Field Emission (SN 944) Electron Probe Microanalyzer, JEOL JSM-5610LV Scanning Electron Microscope, Shimadzu XRF-1800 Wavelength Dispersive X-ray Fluorescence Spectrometer, and Photon Machines Thermo Scientific ELEMENT 2 and 193 nm Excimer Laser Ablation Inductively Coupled Plasma Mass Spectrometer. Experiments conducted with starting chalcopyrite + pyrrhotite assemblage produced ISS with average gold concentration of 2.1(±0.7)x103 µg/g , 5.8(±0.7)x102 µg/g, and 1.5(±0.5)x102 µg/g at 700°C, 600°C, and 500°C, respectively and average gold concentrations in pyrrhotite of 0.4(±1)x102 µg/g at 700°C and below detection limit in experiments at 600°C and 500°C. Similarly, experiments with starting chalcopyrite + pyrrhotite + bornite mineral assemblage yielded average gold concentrations in ISS of 2.0(±0.6)x103 µg/g , 5(±1)x102 µg/g, and 3(±3)x101 µg/g at 700°C, 600°C, and 500°C, respectively. Average gold concentrations in bornite were measured to be 5(±1)x103 µg/g, 2.5(±0.7)x103 µg/g, and 0.5(±0.7)x103 µg/g at 700°C, 600°C, and 500°C, respectively. Exsolution lamellae and gold blebs and stringers were observed in bornite and ISS from experiments run with bornite included in the starting assemblage. Gold blebs and stringers were hosted central to lamellae or along lamellae and primary sulfide mineral boundaries. The presence of lamellae and metallic gold increased with increasing temperature in both bornite and ISS. Exsolution lamellae was not observed in experiments run with chalcopyrite + pyrrhotite starting assemblage. Very fine gold blebs were rarely observed using SEM-EDS and EPMA in ISS run-products from chalcopyrite + pyrrhotite starting assemblage experiments but not in pyrrhotite. Gold concentrations in bornite and ISS increased with increasing temperature and possibly with f_(S_2)^sys. Gold concentrations in pyrrhotite did not seem to be affected by temperature, f_(S_2)^sysor Cu concentration. Partition coefficients between bornite and ISS were calculated to be 2.45±1.01 at 700°C and 100 MPa, 3.25±1.31 at 600°C and 100 MPa, 5.00±2.19 at 600°C and 50 MPa, and 6.12±1.90 at 500°C and 100 MPa. The data in this study suggest that high-grade gold in porphyry ore deposits is most likely to be found associated with deep, high-temperature portions of the system that are central to shallow, magmatic, porphyritic intrusions and with elevated bornite to chalcopyrite ratios.
Schmidt, Carli Lynn, "An Experimental investigation of Gold incorporated into Bornite, intermediate Solid Solution (iss), and Pyrrhotite: Implications for Porphyry Ore Deposits" (2023). Graduate Research Theses & Dissertations. 7185.
Northern Illinois University
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