GAC 2025-2026 Howard Street Robinson Lecture Tour
Dr. Anthony (Willy) Williams-Jones
Earth and Planetary Sciences, McGill University
GAC Howard Street Robinson Lecture Tour
Title: Metals, Vapours and Volcanoes
Link to recorded lecture (YouTube)
Abstract: Until recently, conventional wisdom has held that the main agent of metal transport in hydrothermal systems is an aqueous liquid. However, there is increasing evidence from volcanic vapours, geothermal systems (continental and submarine), vapour-rich fluid inclusions, and experimental studies that the vapour may be an important and even dominant ore fluid in some hydrothermal systems, notably those that form porphyry and epithermal deposits. This is not a new idea. Indeed, as early as 1556, Georgius Agricola postulated that metal-bearing fumes were drawn up from the depths of the Earth to form orebearing veins. Support for the idea was provided by Sir Humphrey Davy, a British chemist and inventor who collected sublimates from Vesuvius during its 1820 eruption and showed them to contain cobalt chloride and copper sulphate. Further support came from the 1840 experiments of Auguste Daubrée, a French geologist and geochemist, who synthesised cassiterite, the main tin ore mineral, by reacting gaseous tin chloride (SnCl4) with steam. The tide changed when the father of modern economic geology and Director of the US Geological Survey, Waldemar Lindgren, declared “In ore deposits, there are vast quantities of non-volatile material, particularly silica, which can hardly be transported by vapour”. The idea that metals could be transported by aqueous vapour was dead! Here we present evidence for the transport of metals by vapor (an aqueous fluid of any composition with a density lower than its critical density), clarify some of the thermodynamic controls that may make such transport possible, and propose a model for the formation of porphyry and epithermal deposits that involves precipitation of the ores from vapour. Analyses of vapour (generally >90% water) released from volcanic fumaroles at temperatures from 500° to over 900°C and near-atmospheric pressure typically yield concentrations of ore metals in the parts per billion to parts per million range. These vapors also commonly deposit appreciable quantities of ore minerals as sublimates. Much higher metal concentrations (ppm to wt.%) are observed in vapour inclusions trapped at pressures of 200 to 1,000 bars in deeper veins at lower temperature (400°–650°C). Experiments designed to determine the concentration of Cu, Mo, Ag, and Au in HCl-bearing water vapor at variable although relatively low pressures (up to 180 bars) help explain this difference. These experiments show that metal solubility is orders of magnitude higher than predicted by volatility data for water-free systems. They also show that, in the case of Cu and Au, the solubility increases sharply with increasing water fugacity and fugacity of HCl. Thermodynamic analysis demonstrates that this high metal solubility is due to the reaction of the metal with HCl (Cu, Ag and Au) and hydration of the resulting chloride complex, leading to the formation of species such as MeClm.nH2O, or in the case of Mo, species such as MoO3.nH2O. Indeed, the solubility of Cu, Mo and Au, in vapour at 400 to 600°C and pressures corresponding to depths of 1km, can reach concentrations of several thousands of ppm, tens of ppm and a few ppm, respectively. In shallowly emplaced porphyry-epithermal systems, fluid inclusions provide evidence of the exsolution from the magma of a supercritical fluid of vapour-like density. On cooling and decompression, this fluid condenses a small fraction of brine by intersecting the two-phase surface on the vapour side of the critical curve, without significantly changing the composition of the expanding vapor. Vapour and brine reach Cu-Fe sulfide or MoS2 saturation as both phases cool below 425°C. Vapour, which is the dominant fluid in terms of the total mass of H2O, is interpreted to be the main agent of metal transport. Evolution of the system to lower temperature and pressure leads to a vapour that is enriched in H2S, SO2, Au, and variably enriched in Cu and As, which produces high sulphidation epithermal gold mineralisation or condenses in ground water to produce low sulphidation epithermal deposits.
Time
Location
Milligan Room, LSC-B8007, 8th floor of the Biology/EES wing of the Life Sciences Centre