AN EXPERIMENTAL INVESTIGATION OF GOLD INCORPORATION IN BORNITE, INTERMEDIATE SOLID SOLUTION (ISS), AND PYRRHOTITE: IMPLICATIONS FOR CONSTRAINING CONCENTRATIONS IN PORPHYRY ENVIRONMENTS

Magmatic hydrothermal ore deposits contain metals transported by a hydrothermal fluid to shallow areas where they can be deposited as native elements or sulfide minerals. A subclass of such are porphyry ore deposits which contain plentiful amounts of societally significant metals, including gold. This style of mineralization is found in many prominent deposits including Grasberg, Far Southeast and Bajo de la Alumbrera Cu-Au porphyry deposits. Common Au-bearing sulfide minerals found in the porphyry environment include bornite, pyrite, chalcopyrite, and pyrrhotite. Experiments have been conducted at 500, 600 and 700°C, and 50 and 100MPa, to represent a natural porphyry environment. Experimental systems were contained within a 99.95% pure gold capsule and filled with a 5wt% NaCl_eq fluid, characteristic of an exsolved magmatic volatile phase. Once experimental systems reached equilibrium, capsules were removed from the vessels, and sulfide crystals were extracted from the capsules. SEM-EDS techniques were used to create maps of individual grains, obtain relative abundances of major elements, and identify the presence and distribution of Au throughout. XRF was used to gather bulk elemental concentrations at the grain scale and to identify sulfide mineral phases present in the experimental run products. EPMA was conducted to examine areas of high and low Au occurrences in each grain and the associated sulfide mineral composition proximal to gold blebs. In experiments ran with a bornite + ISS + pyrrhotite assemblage, bornite and ISS post-experiment grains showed two phases distinguishable in SEM-EDS. In these grains native gold stringers and blebs were accompanied by bornite grains with ISS composition exsolution and ISS grains with bornite composition exsolution. In both bornite and ISS grains the phase richer in S and Cu hosted the most Au. Gold was also shown to increase with temperature and ƒO₂ in bornite and ISS showing that high gold grades in porphyry ore deposits form in the central portions of shallow magmatic bodies. No gold blebs or stringers were seen in pyrrhotite starting or run products. Maximum Au concentrations were as follows: (1) bornite composition grains up to 6250±1950 µg/g, (2) ISS composition grains up to 2130±200 µg/g and (3) pyrrhotite composition grains up to 90±140 µg/g.