Abstract

To provide an improved understanding of the genesis of high-temperature Ag-Pb-Zn ore deposits we have obtained revised estimates for the thermodynamic properties for (Cu, Ag)10(Fe, Zn)2Sb4S13 fahlore endmember components. Calculations of aqueous solution-mineral equilibria (250°–300°C) for fahlore-bearing assemblages from Ag-Pb-Zn ore deposits from the Coeur d’Alene (Idaho, United States) and Keno Hill (Yukon Territory, Canada) mining districts demonstrate consistency between predicted and observed mineral assemblages and hydrothermal fluids based on these new estimates. These calculations confirm that much of the Ag mined from the Coeur d’Alene district was originally present in galena. They also confirm that sphalerite was typically undersaturated in fluids crystallizing Fe-rich fahlores in the Coeur d’Alene Ag-Pb-Zn ores and constrain the parameters of the fluids equilibrated with these ores to within narrow limits. For the Keno Hill deposits these calculations demonstrate that the early, high-temperature (300°–310°C) fluids (0.07 < XCO2Fluid <0.42) were highly undersaturated with respect to graphite in the quartzite host and that only the most CO2 poor of these CO2-rich fluids represent the CO2 supercritical fluid phase at hydrostatic pressure. The calculations also concur with the results of fluid inclusion and petrologic studies, which suggest that later, lower temperature (240°–260°C) fluids were CO2 poor (XCO2Fluid <0.01) and saturated with respect to graphite. Finally, they suggest that very Ag rich galenas (XAgSbS2Gn >0.7 in Pb2S2-AgSbS2 galena) or their compositionally equivalent sulfosalt assemblages, diaphorite (Pb2Ag3Sb3S8) + miargyrite ([(AgSb),Pb2]S2) or diaphorite (Pb2Ag3Sb3S8) + frieslebenite (PbAgSbS3), were deposited initially in Ag-rich fahlore-bearing ores. These minerals were replaced by Ag-poor galena and epithermal Ag minerals that include acanthite, stephanite, and polybasite.

You do not currently have access to this article.