100th Anniversary Special Paper: Vapor Transport of Metals and the Formation of Magmatic-Hydrothermal Ore Deposits

Abstract

In most published hydrothermal ore deposit models, the main agent of metal transport is an aqueous liquid. However, there is increasing evidence from volcanic vapors, geothermal systems (continental and submarine), vapor-rich fluid inclusions, and experimental studies that the vapor phase may be an important and even dominant ore fluid in some hydrothermal systems. This paper reviews the evidence for the transport of metals by vapor (which we define as an aqueous fluid of any composition with a density lower than its critical density), clarifies some of the thermodynamic controls that may make such transport possible, and suggests a model for the formation of porphyry and epithermal deposits that involves precipitation of the ores from vapor or a vaporderived fluid.Analyses of vapor (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 (from ppm to wt %) are observed in vapor inclusions trapped at pressures of 200 to 1,000 bars in deeper veins at lower temperatures (400°-650°C). Moreover, concentrations of some metals, notably Cu and Au, are commonly higher in vapor inclusions than they are in inclusions of coexisting hypersaline liquid (brine). Experiments designed to determine the concentration of Cu, Sn, Ag, and Au in HCl-bearing water vapor at variable although relatively low pressures (up to 180 bars) partly explain this difference. These experiments show that metal solubility is orders of magnitude higher than predicted by volatility data for water-free systems, and furthermore that it increases sharply with increasing water fugacity and correlates positively with the fugacity of HCl. Thermodynamic analysis shows that metal solubility is greatly enhanced by reaction of the metal with HCl and by hydration, which results in the formation of species such as MeClm.nH2O. Nonetheless, the concentrations measured by these experiments are considerably lower than those measured in experiments involving aqueous liquids or determined for vapor fluid inclusions. A possible explanation for this and for the apparent preference of metals such as Cu and Au for the vapor over the coexisting brine in some natural settings is suggested by limited experimental studies of metal partitioning between vapor and brine. These studies show that, whereas Cu, Fe, and Zn all partition strongly into the liquid in chloride-bearing sulfur-free systems, Cu partitions preferentially into the vapor in the presence of significant concentrations of sulfur. We therefore infer that high concentrations of Cu and Au in vapor inclusions reflect the strong preference of sulfur for the vapor phase and the formation of sulfur-bearing metallic gas species.Phase stability relationships in the system NaCl-H2O indicate how vapor transport of metals may occur in nature, b...

Contrasting regimes of Cu, Zn and Pb transport in ore-forming hydrothermal fluids

“…The importance of bisulfide complexes for the transport of base metals (Cu, Pb and Zn) has been proposed in magmatic vapors and low density supercritical fluids, because reduced sulfur will partition preferentially into the low density phase during immiscibility (Heinrich et al, 1992;Mavrogenes et al, 2002;Williams-Jones and Heinrich, 2005;Liu et al, 2008;Seo et al, 2009). In contrast, the current consensus assumes that Cu, Pb, and Zn are transported as chloride complexes in high density magmatic fluids (e.g., Yardley, 2005;Lukanin et al, 2013).…”

Trace elements in the gas emissions from the Erta Ale volcano, Afar, Ethiopia

“…Elemental and mineral compositions and zonation of such sublimates and incrustations resemble endogenic ores (Williams-Jones and Heinrich, 2005). Deposition of minerals from fumarolic gases can be observed and is considered an accessible model of endogenic ore formation, although the conditions of mineral formation are different, especially in terms of pressure and fO 2 , and high-temperature fumaroles of quiescently degassing volcanoes never form ore deposits (Hedenquist and Lowenstern, 1994;Churakov et al, 2000).…”

Physical and chemical characteristics of the host rocks in controlling the mineralization of the Chinkuashih high-sulfidation gold–copper deposits, northeastern Taiwan

“…have involved either a metal-bearing saline magmatic fluid ascended to the level of a silicic alteration zone from depth (Hedenquist et al, 1994), or formation by low-salinity aqueous liquids derived by contractive cooling of metal-enriched magmatic vapors (Heinrich, 2005;Williams-Jones and Heinrich, 2005;Seo et al, 2009). During the main mineralization stage, the ore-metal bearing fluids subsequently precipitated gold, enargite, and pyrite by boiling, fluid mixing, or fluid de-sulfidation (Hedenquist et al, 1994;Williams-Jones and Heinrich, 2005). High-sulfidation mineral assemblages formed by crystallization directly from a magmatic volatile phase were suggested by Larocque et al (2008).…”