Modeling of gold scavenging by bismuth melts coexisting with hydrothermal fluids

The effect of gold scavenging by bismuth melts is investigated using equilibrium thermodynamic modeling of an aqueous solution-mineral-melt system. The calculations for the Au-Bi-Na-CI-S-H-O system, performed at-temperatures between 300 and 450 degrees C, demonstrate that Au concentrations in the melt are several orders of magnitude higher than in the coexisting fluid, indicating the possible formation of economic gold deposits from undersaturated aqueous fluids, in which mineralization would not be expected in the absence of a bismuth melt. The model applies to any deposit where a Bi melt is stable and coexists with a hydrothermal fluid; examples of such deposits are known from skarn, intrusion-related, orogenic, and volcanogenic massive sulfide (VMS) gold systems. In sulfur-poor systems the partitioning curves presented here can be used directly to correlate the gold concentration in the fluid and the Au grade in the ore (e.g., Escanaba Trough VMS deposit). These results also illustrate important principles generally applicable to understanding magmatic-hydrothermal and metamorphic deposits that may have contained significant volumes of more complex polymetaltic melts.