SULFIDATION STATE OF FLUIDS IN ACTIVE AND EXTINCT HYDROTHERMAL SYSTEMS

Sulfidation and oxidation states of sulfide assemblages in porphyry Cu, base-metal, and epithermal deposits are contrasted with measured fluid compositions from active hydrothermal systems on log fS₂ – 1/T, Rh – 1/T, and Rs – 1/T diagrams, where Rh = log (X H₂ / X H₂O), Rs = log (X H₂ / X H₂S), and X is mole fraction.

Rh and Rs values of volcanic fumaroles are similar to those estimated from opaque mineral assemblages in early high-temperature stages of porphyry-Cu deposits. The values lie on the isomolar SO₂/H₂S curve from 1000 to 400^oC and are interpreted to result from exsolution and cooling of volatiles from magmas.

Sulfidation state of magmatic-hydrothermal vapors at 400 to 100^oC varies widely from low to very high, reflecting variable degrees of quenching and interaction with wall rock. These vapors are considered proxies for high sulfidation (HS) copper and HS epithermal  gold deposits that display the most complete range of sulfidation states of any deposit-type. Gold deposition in HS deposits occurs at intermediate sulfidation states similar to those in intermediate sulfidation (IS) deposits, and there may be a closer affiliation between these two types than commonly thought.

Geothermal samples from near-neutral pH systems have lower sulfidation and oxidation states than magmatic-hydrothermal fluids, perhaps the result of a greater degree of fluid-rock interaction, a smaller magmatic component, or a distinct magmatic component. Simple cooling, even accentuated by boiling, is unlikely to result in any appreciable increase in sulfidation state in geothermal systems, which are interpreted to be analogs to either IS Au-Ag-base metal deposits and in back arc settings, to low-sulfidation (LS) epithermal deposits, the latter with monotonous sulfide assemblages and very low sulfide contents.

The pattern of active hydrothermal fluid compositions if viewed as individual time slices through systems at various stages in their life cycle, mimics the looping path in log fS₂-1/T space recognized in ore deposits: an early trend toward high sulfidation state on cooling from 500 to 350ºC, followed by an abrupt decline as equilibrium with rock buffer is achieved. At any given time, many active hydrothermal systems span the complete range of chemical and physical states that are commonly relegated to changing time in ore deposits.