UNDERSTANDING THE RELATIONSHIP BETWEEN GOLD, CHALCOPYRITE, BORNITE, AND PYRRHOTITE IN MAGMATIC-HYDROTHERMAL SYSTEMS

The low concentrations of gold in magmatic-hydrothermal systems make exploration a challenge, so the mineral assemblages documented as containing gold in other systems become a proxy for potential gold-rich zones in new systems. In porphyry systems, gold is often found in its native metallic form with bornite and chalcopyrite and appears to decrease with decreasing temperature and changes in the host mineral assemblage (e.g., Bingham; Redmond and Einaudi, 2010, Econ. Geol., v. 105, 43-68). A fundamental question remains, do gold concentrations in the porphyry environment follow select mineral assemblages or is the gold distribution controlled solely by the characteristics of the hydrothermal fluid from which it precipitates? To address this question, bornite (Bn), high-temperature chalcopyrite, (referred to as intermediate solid solution (ISS)), and pyrrhotite were experimentally equilibrated with gold at 500-700 °C, 50-100 MPa, and over a wide range of oxygen and sulfur fugacities. Our working hypothesis is that gold can be incorporated into the crystal structures of select sulfide minerals at high-temperatures and exsolve during cooling, thus, the location of gold will be controlled partially by select mineral assemblages. Gold concentrations in bornite were found to be 1800 to 5500, 500 to 3800, and 100 to 1400 µg/g at 700, 600, and 500 °C, respectively. Gold in ISS was determined to be 1300 to 4900 and 340 to 3600 µg/g at 700 and 600 °C, respectively. Gold concentrations were found to generally increase in both ISS and Bn with increasing temperature and oxygen fugacity. Gold concentrations in pyrrhotite were always below 100 µg/g. These data indicate that gold can be incorporated into the structures of bornite and high-temperature chalcopyrite (ISS) at conditions equivalent to those under which porphyry copper and gold deposits are thought to form. Thus, the distribution of gold within the system will be influenced, at least partially, by the sulfide mineral assemblage.