The precipitation mechanisms and stabilities of iron sulfide minerals are not completely understood. The conditions and rates of precipitation of marcasite and pyrite from hydrothermal solutions were studied experimentally by Schoonen and Barnes [1991] and Graham and Ohmoto [1994], who found that these minerals form via precursors of crystalline FeS or liquid S.
Relying on a computational approach, Bowers [1991] developed a
model for the deposition of Au and other metals during pressure-induced
fluid immiscibility. Her model used the EQ3/6 speciation and mass
transfer software package with extensions accounting for O, H and S
isotopic fractionation, plus the Redlich-Kwong equation of state for the
P-V-T propoerties of H
O-CO
mixtures. She found that fluid
immiscibility (volatile loss) could induce metal deposition under a variety
of conditions. However, she also showed that the influence of volatile
loss on metal deposition must be evaluated in the context of realistic
constraints: the effects of volatile loss on fluid pH and redox state must
be evaluated in light of the buffering capacity of the entire fluid-rock
system.