THE DEPOSITION OF GOLD AND OTHER METALS: PRESSURE-INDUCED FLUID IMMISCIBILITY AND ASSOCIATED STABLE ISOTOPE SIGNATURES

Abstract

It is often proposed that the onset of fluid immiscibility may be a direct cause of ore deposition, with evidence cited from fluid inclusions associated with the ore minerals. A reaction-path model is used here to calculate fluid speciation, including pH, and mineral precipitation or dissolution as a function of volatile loss from a fluid undergoing phase separation. A modified Redlich-Kwong equation of state is used to model the H 2 O-CO 2 miscibility gap and calculate activity coefficients of H 2 O and CO 2 . This procedure is incorporated into the EQ3/6 chemical speciation and mass transfer computer codes to simulate the response of a mineral-fluid system to fluid immiscibility. Calculations for systems including Cu, Pb, Zn, Ag, and Au show that fluid immiscibility may induce metal deposition under a variety of conditions, but that simplistic assumptions regarding fluid response to volatile loss are not always correct because they may ignore the buffering capacity of a fluid-rock system.