The solubility of Ag sub 2 S in near-neutral aqueous sulfide solutions at 25 to 300 degree C

The solubility of Ag{sub 2}S (acanthite/argentite) was measured in vapor-saturated aqueous sulfide solutions at 25-300{degree}C, total sulfide = 0.2-1.4 m, and pH{sub 25{degree}} = 5.8-7.3. Although Seward (1976) has shown that considerable silver can be dissolved as chloride complexes, the authors data indicate that Ag(HS){sup {minus}}{sub 2} is the dominant silver species in hydrothermal fluids with near-neutral to alkaline pH, relatively low oxidation state, high total sulfide, and T < 300{degree}C (e.g., Broadlands, N.Z.; Brown, 1986). Silver transported as Ag(HS){sup {minus}}{sub 2} is precipitated in response to a decrease in aqueous sulfide concentration, a change in pH away from the pK{sub 1} for H{sub 2}S or, in the native silver field, a decrease in oxidation state. Cooling is a less important depositional mechanism, particularly for solutions whose pH is buffered by feldspar alteration reactions.