SOLUBILITY LIMIT OF GOLD IN ARSENIAN PYRITE FROM CARLIN-TYPE AND EPITHERMAL DEPOSITS: EMPA, SIMS, HRTEM AND QUANTUM-MECHANICAL CONSTRAINTS

In an As-Au plot of new and previously published EMPA and SIMS analyses of arsenian pyrites from Carlin-type and epithermal deposits, almost all data points fall within a triangular cluster that is limited above by the line Au(wt%)=0.0516*As(wt%)-0.0126 (R2=0.9924), and to the right by a sharp vertical boundary at ~10wt% As. SIMS analyses define the boundaries of the triangular cluster, and show a positive correlation between Au and As at deposit and sample scales. EMPA analyses plot on the upper corner of the cluster, showing no evident correlation between Au and As.

A few high-Au grade samples from Screamer, Meikle (Nevada) and Emperor (Fiji) plot above the upper boundary line. Two of these samples show the highest Au contents ever documented for arsenian pyrite, reaching 0.8wt% (Screamer) and 1.106wt% (Emperor). EMPA data points of this anomalous arsenian pyrite from Screamer form a cluster with Au/As>0.0516. HRTEM observations show that elemental Au nanoparticles (~5 nm) are very abundant (~ 4% volume) in these samples. In addition, arsenic is heterogeneously distributed throughout a polycrystalline matrix, and many As-rich regions are located adjacent to Au nanoparticles. Quantum mechanical ab-initio calculations with different configurations of Au and As atoms in the pyrite structure show that the defect energy for replacing an Fe atom by a Au atom is lowered by ~ 1eV if an As atom is in the near vicinity. Approximately 50 As atoms are necessary to incorporate one Au atom into arsenian pyrite at the upper Au/As limit.

This new evidence suggests that the upper boundary line represents a solubility limit for Au in arsenian pyrite. Moreover, when extrapolated to higher As (arsenopyrite composition, ~46 wt%), this line also predicts the maximum solid solution Au content (~3wt%) of hydrothermally synthesized arsenopyrite. A strong dependence of Au upon As from the atomic level to the deposit scale is revealed. In consequence, stability (or metastablity) of arsenian pyrite structure may be affected if the maximum Au/As contents are exceeded. Preliminary observations suggest that arsenian pyrite with Au/As>0.0516 may not be a single-phase mineral, but rather a multiphase aggregate. Thus, when this limit is exceeded, the presence of Au nanoparticles may be expected as a result of direct deposition from Au-saturated fluids.