IMPACT OF SULFUR ON GOLD AND COPPER TRANSPORT AND FRACTIONATION IN VAPOR-BRINE SYSTEMS: INSIGHTS FROM IN SITU EXPERIMENTS AND PHYSICAL-CHEMICAL MODELING

The growing geological evidence that fluid boiling and immiscibility phenomena largely impact the formation of magmatic-hydrothermal Au-Cu deposits has motivated the need for better understanding physical-chemical and molecular mechanisms that control the metal transport by the fluid and vapor phases.

Recent laboratory experiments and thermodynamic models of Au and Cu speciation and solubility show that reduced sulfur is likely to play a key role in the distribution of these metals in vapor-brine systems, by enhancing Au, Cu (and Pt) partitioning into the low-density acidic vapor phase in comparison to the coexisting brine, while the fractionation of other metals (e.g., Zn, Pb, Fe, Ag, As) is almost unaffected by the presence of sulfur. The formation of volatile complexes of chalcophile metals with reduced sulfur ligands, presumably H₂S, is likely responsible for their selective partitioning into the vapor phase. However, the values of vapor-brine distribution coefficients of Cu and Au from these experiments are typically 10-100 times lower than those inferred by analyses of natural coexisting brine and vapor inclusions from many magmatic-hydrothermal deposits. The major limitation for quantifying Au and Cu behavior in such systems is the paucity of high-temperature data on the amount and speciation of sulfur itself.

Our in situ Raman and XAFS spectroscopy measurements show that aqueous sulfur forms like sulfite, polysulfides or polythionates are far more abundant at elevated temperature and pressure than predicted from available low-temperature thermodynamic data; they may compete efficiently with the sulfide ligand (H₂S/HS^-) for Au and Cu in the fluid and vapor phases. Our solubility measurements and theoretical predictions indicate that carbonic fluids like CO₂-CH₄ may further enhance the vapor-phase solubility of chalcophile metals in the form of sulfur-bearing complexes, in line with few natural observations. The development of in situ spectroscopic methods and molecular modeling approaches, together with the continuing progress of analytical techniques to accurately determine metal and sulfur contents and speciation in natural fluid and melt inclusions, will provide a major advance in our understanding of the impact of sulfur on the fate of chalcophile metals in geological fluids.