HARD-SOFT ACID-BASE CONCEPTS APPLIED TO METAL SEGREGATION INTO THE FLUID (VAPOUR) PHASE DURING THE MAGMATIC STAGE

Ore genesis associated with felsic magmatism develops from metals scavenging from the melt into the gaseous, phase. Its composition includes water, CO₂, sulphur under several possible species, and halogens F and Cl. The respective influence of those elements is examined by computing their equivalent electronegativity and chemical hardness. For a silica content ranging from 45 to 78 w%, the electronegativity values linearly increase from 6.0 and 6.6 eV whereas hardness values increase 4.5-5.0 eV, given the vol% in each component of ranging from 10^-4 to 10^-2, to be compared with usual analyses. Isovalue surfaces are drawn for a fixed content in water and CO₂, but varying concentration of halogens and S, whatever its species. The bulk “neutral” values of water and CO₂ are 7.37 eV for electronegativity and 5.94 eV for hardness. With F, those values increase up to a little more than 8.0 eV and 6.4 eV respectively, when H₂O is fixed at 2.5 %. This results from the content in F, the hardest halogen. Conversely, when S is added to the fluid phase electronegativity and hardness reduce, owing to the soft character of HS^-. Since high-valency metals present electronegativity above 15 eV and various hardness values, the fluid phase is often more attractive for them than the melt. S, under its various non-detailed species, decreases both electronegativity and hardness. It 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^- present contrasted effects, since it decreases the hardness, but increases the electronegativity. It could segregate Fe in iron-oxide-copper-gold (IOCG) porphyry deposits, though mixing between magmatic and evaporitic fluids make 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 semi-quantitative model provides a new insight on the chemical conditions of metals segregation and transportation through the magma before ore formation.