MODELLING THE As-Au ASSOCIATION IN HYDROTHERMAL GOLD MINERALIZATION: EXAMPLE OF ZARSHURAN DEPOSIT, NW IRAN

摘要

The paragenetic sequence at the Zarshuran arsenic deposit, NW Iran demonstrates that during the late stage of mineralization, gold precipitated with orpiment, realgar and arsenian pyrite. Thermodynamic calculations have been used to model sulphur and arsenic speciation and solubility at the P-T conditions of the late stage mineralization, reconstructed based on fluid inclusion studies. Arsenic was probably transported as the As~(+3) complex; H_3AsO_3~0, while the dominant Au complex was Au(HS)_2~-. The calculations indicate that both pH and fO_2 have opposing effects on gold and arsenic solubilities. Hence co-precipitation was not simply a response to changes in pH and fO_2 Experimental data and geochemical modelling results indicate that pH, fO_2, P, T and S availability are the main equilibrium factors controlling gold and arsenic transport and precipitation. The proposed geochemical model suggests that partial reduction of an oxidizing As-rich fluid of partial magmatic origin (As~S), leached gold from the Precambrian metamorphic rocks as it was reduced, but further reduction by the black shale resulted in gold and arsenic co-precipitation. An alternative model for As and Au co-precipitation is a kinetic one, with Au being adsorbed onto growing orpiment surfaces despite being undersaturated in the fluid. The result shows the condition in which an arsenic saturated fluid with mAs=mS may persist to a point where gold bisulphide has maximum solubility, and so may leach gold. Further reduction from that point may causes gold and orpiment co-precipitation. The high level As to S concentrations is one of the main features here, and is not known for the Zarshuran fluid, although data from different magmatic sources suggest that the requirements are within the range of magmatic fluids.