Thermodynamic modeling of metal transport in H2O-CO2 vapours: CO2-Fluxing Crashes Metal Mobility

English Abstract:

Magmatic systems host many types of ore deposits, including world-class deposits of copper and gold. It is now generally accepted that magmas are the source of metals and ore-forming fluids, although in some cases their input may be restricted to introducing thermal disturbances and associated hydrothermal circulation. In these magmatic-hydrothermal sys­tems, low-density solutions, or vapours, are an important carrier of metals. Such vapours are water-dominated at low pressure, but CO₂ becomes a progressevely important conpo­nent in vapours exsolved from magma at depth, especially for mafic magmas. Fluxing of these CO₂-rich vapours through the more shallow parts of the magmatic-hydrothermal plum­bing system is now recognized as ubiquitous during open-system magma degassing.

In this contribution, we show that such CO₂-fluxing leads to a dramatic drop in element solubility in the previously water-dominated vapour, up to a factor of 10,000 for Cu, as calculated from new thermodynamic stability constants for metal species in vapour, and a new model for understanding metal solvation. The predicted drop in metal solubility far exceeds that which would be calculated for the temperature and pressure gradients expec­ted in mag­matic-hydrothermal systems. CO₂-fluxing thus potentially repre­sents a highly efficient, but as of yet unrecognised mechanism for metal deposition in magmas and host rocks.