Evaluation of the chemical reactivity of the fluid phase through hard–soft acid–base concepts in magmatic intrusions with applications to ore generation

Abstract

Ore genesis, when associated with felsic magmatism, develops from metals scavenging from the melt into the fluid, gaseous, phase. Its composition includes water, CO 2, sulphur under several possible species, and halogens, mainly F and Cl. The respective influence of those elements is examined by computing the theoretical electronegativity and chemical hardness of the fluid phase. Those parameters are commonly larger for the fluid phase than for the silicate melt. Indeed a common hardness value for the melt is around 3.5 eV, whereas the computed values for the fluid phase are around 7.5 eV for pure water, with departure ranging from 6.9 eV in case of S-rich fluid, to 8.8 eV in case of a F-rich fluid. Since metals show tendency to present electronegativity above 15 eV and high hardness, the fluid phase is very attractive for metals. The influence of S, under its various non-detailed species, is to decrease both electronegativity and hardness. It therefore favours segregation of soft metals, as Cu, Ag and Au. Since F − is the hardest base, it increases both electronegativity and hardness, making the fluid phase attractive to Sn and W. Cl − presents contrasted effects, since it decreases the hardness, but increases the electronegativity. It could be of influence in the segregation of Fe in iron-oxide–copper–gold (IOCG) porphyry deposits, though mixing between magmatic and evaporitic fluids makes the situation quite complex. The chemical character of the fluid phase also explains the discrepancy existing for metal solubility, as well as for the redox conditions, between the melt and the fluid phase. The change in oxidation state induced by a hard fluid, i.e. F-rich, promotes oxidation, for instance from Sn(II) to Sn(IV) or reduction in case of a soft, i.e. S-rich, fluid phase, from Mo(VI) to Mo(IV). The bulk electronegativity and hardness of the fluid phase modify the redox state of the metals during transportation, before condensation. The semi-quantitative model provides a new insight on the chemical conditions of metals segregation and transportation through the magma before ore formation.