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

The paragenetic sequence at the Zarshuran arsenic deposit, NW Iran demonstrates thatduring the late stage of mineralization, gold precipitated with orpiment, realgar andarsenian pyrite. Thermodynamic calculations have been used to model sulphur andarsenic speciation and solubility at the P-T conditions of the late stage mineralization,reconstructed based on fluid inclusion studies. Arsenic was probably transported as theAs+3 complex; H3AsO30, while the dominant Au complex was Au(HS)2−. The calculationsindicate that both pH and fO2 have opposing effects on gold and arsenic solubilities.Hence co-precipitation was not simply a response to changes in pH and fO2. Experimentaldata and geochemical modelling results indicate that pH, fO2, P, T and S availability arethe main equilibrium factors controlling gold and arsenic transport and precipitation. Theproposed geochemical model suggests that partial reduction of an oxidizing As-rich fluidof partial magmatic origin (As~S), leached gold from the Precambrian metamorphic rocksas it was reduced, but further reduction by the black shale resulted in gold and arsenic coprecipitation.An alternative model for As and Au co-precipitation is a kinetic one, withAu being adsorbed onto growing orpiment surfaces despite being undersaturated in thefluid. The result shows the condition in which an arsenic saturated fluid with mAs=mSmay persist to a point where gold bisulphide has maximum solubility, and so may leachgold. 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 knownfor the Zarshuran fluid, although data from different magmatic sources suggest that therequirements are within the range of magmatic fluids.