PRECIOUS VAPOR: METAL TRANSPORT IN MAGMATIC-HYDROTHERMAL SYSTEMS

Low- and intermediate-density fluids are the dominant volatile phases in magmatic-hydrothermal systems, and are the most likely agents of metal transport to sites of ore deposition [1]. These fluids are highly compressible and metal solubility depends on fluid density. Moreover, these fluids are characterized by molecular compounds and there are no free electrons. This suggests that metal precipitation can be related to decompression and is most likely decoupled from wall-rock alteration.

New formulations of metal solubility in low- and intermediate-density fluids based on experimental data [2-5], allow for the thermodynamic modeling of metal solubility and mineral precipitation during cooling and decompression of a magmatic high temperature volatile phase [6]. Geochemical simulations were performed using GEM-Selektor [7] and metal mobility was modeled in the context of Cu-Au-Mo porphyry and Au-Ag epithermal ore formation. Metal solubility as a function of T, P, redox condition, sulfur and HCl contents was evaluated, showing the interplay of these variables in dictating metal budgets and zoning in ore deposits. During cooling and decompression of an intermediate-density fluid, Au concentration reaches a maximum at 460 °C and Cu at 540 °C, whereas Mo and Ag concentrations decrease with decreasing T and P. The solubility maximum of gold occurs at 340 °C at lower fluid density indicating a potential for remobilization/redistribution of gold to shallower, and more distal environments. The redox condition at high temperatures is buffered by the vein mineral assemblage of magnetite-rutile-anhydrite or magnetite-rutile-anhydrite-pyrite. The presence of pyrite indicates that sulfur contents of the magmatic fluids are as high as 4-6 wt.%, whereas anhydrite stability occurs at sulfur contents of 0.2 wt.%. The sulfur content and redox condition control the metal ratios of the magmatic fluids and therefore the overall metal budget of the ore deposit.

[1] Weis et al. (2012), Science, 338, 1613-1616. [2] Hurtig and Williams-Jones (2014), GCA 127, 305-325. [3] Hurtig and Williams-Jones (2014), GCA 136, 169-193. [4] Migdisov and Williams-Jones (2013), GCA 104, 123-135. [5] Migdisov et al. (2014), GCA 129, 33-53. [6] Hurtig and Williams-Jones (2015), Geology 43, 587-590. [7] Kulik et al. (2013), Comput Geosci, 17, 1-24.