Aresenopyrite-pyrite paragenesis in gold deposits (thermodynamic modeling)

The compositions of parageneses including arsenopyrite (Asp), pyrite (Py), and As-pyrite (As-Py) have been calculated by the solution of primal physicochemical modeling problems. The numerical models for the interaction of Py matrix with hydrothermal solution saturated with Asp are considered for three variants of the solution penetration into the Py matrix: percolation, spreading, and tightening at 100-300 °C and 300 bars. It is shown that Asp forms in the zone of an ore column where fluid (solution) is predominant, with the Py matrix being replaced independently of the type of the solution transfer. Prevailing As and Fe complexes are considered. The calculated models for the three types of interaction show that the redox potential in the solution varies from -0.055 V at the ends of the Asp-containing ore column to -0.55 V in its central zone. This difference makes an electrochemical geochemical barrier at the interface, where metallic gold is deposited with Asp-Py ores.