Trigonal and pentagonal shaped plates and prism, and octahedra of Au crystals 3-40 mu m in dimension were found on the inner wall of a silica tube inserted into a 800 degrees C fumarolic vent Of Colima volcano, Mexico. Gold precipitates from the high-temperature and highly oxidized volcanic vapor (a mixture of magmatic gas with more than 90% of air) cover a narrow temperature range of 550-600 degrees C, and occur in association with V-rich Na-K- sulfates. The Au concentration in the volcanic gas condensate is between 0.1 and 0.5 mu g/kg. Using thermodynamic data for Au(c), Au(g), AuH(g) and AuS(g), open- and closed-system cooling of a simplified volcanic gas has been modeled with the following characteristics of volcanic gas + air mixture: P = 1 bar, f(SO2) = 0.01 bar; f(SO2) fixed by Fe2O3-Fe3O4 or Cu2O-CuO pairs for open-system cooling, and f(SO2) = 0.1 bar for closed-system cooling. Volcanic vapor released from the shallow magma body transports Au as AuH(g) and AuS(g). According to calculations, after mixing with air, AuS(g) and AuH(g) oxidize to Au(g), and the temperature of the Au deposition depends only on the initial total concentration of Au species in the vapor. The temperature range of 550-600 degrees C for Au precipitation at a high f(SO2) corresponds to a very low initial Au concentration, about ng/kg in the volcanic gas condensate. This is at least two orders of magnitude lower than the observed Au content in the Colima gas, indicating the presence of other volatile gold species, e.g. AuClx, Au(OH)(x) etc., or Au precipitation under non-equilibrium conditions from a volcanic gas + air mixture with coexisting free H-2 and O-2 (C) 1999 Elsevier Science Ltd. All rights reserved.
