Gold solubility in SiO₂-HCl-H₂O system at 200 °C: a preliminary assessment of the implications of silicification with regard to gold mineralization

Abstract

The complexation between gold and silica was experimentally, confirmed and calibrated at 200 °C:

$$\begin{gathered} Au^ + + H_3 SiO_4^ - \rightleftharpoons AuH_3 SiO_4^0 \hfill \\ \log K_{(200^\circ C)} = 19.26 \pm 0.4 \hfill \\ \end{gathered} $$

Thermodynamic calculations show that AuH₃SiO ⁰₄ would be far more abundant than AuCl ⁻₂ under physicochemical conditions of geological interest, suggesting that silica is much more important than chloride as ligands for gold transport. In systems containing both sulfur and silica, AuH₃SiO ⁰₄ would be increasingly more important than Au (HS) ⁻₂ as the proportion of SiO₂ in the system increases.

The dissolution of gold in aqueous SiO₂ solutions can be described by the reaction:

$$\begin{gathered} Au + 1/4O_2 + H_4 SiO_4^0 \rightleftharpoons AuH_3 SiO_4^0 + 1/2H_2 O \hfill \\ log K_{(200^\circ C)} = 6.23 \hfill \\ \end{gathered} $$

which indicates that SiO₂ precipitation is an effective mechanism governing gold deposition, and thus explains the close association of silicification and gold mineralization.