RETROGRADE AG-ENRICHMENT IN FAHLORES FROM THE COEUR D'ALENE MINING DISTRICT

Microprobe analyses, corona textures, mass–balance constraints and mineral assemblages in sulfide ores from the Coeur d’Alene mining district (Idaho) demonstrate that tetrahedrite fahlores [~ (Cu,Ag)10(Zn,Fe)2Sb4S13] in these ores have been enriched in Ag by the Ag–Cu exchange reaction 0.1 Cu10(Zn,Fe)2Sb4S13 + AgSbS2 + PbS=CuPbSbS3 + 0.1 Ag10(Zn,Fe)2Sb4S13 which occurred during cooling following galena mineralization. This solid–state reaction resulted in quantitative removal of Ag from galena and can account for all of the bournonite mineralization. In galena–rich samples it resulted in the highest Ag tetrahedrite fahlores found in the district. Fahlore molar Ag/(Ag+Cu)=0.303±0.011 and 0.336±0.011 were found in samples which achieved saturation with respect to other Ag–sulfosalts, namely pyrargyrite [~ Ag3SbS3] and polybasite [~ (Ag,Cu)16Sb2S11] in the West Chance and diaphorite [~ Pb2Ag3Sb3S8] in the Gold Hunter vein systems, respectively. This solid–state reaction may explain why previous district studies have been unable to convincingly demonstrate primary fahlore zoning. Based on thermodynamic analysis of Ag net–transport reactions involving Ag10Zn2Sb4S13 and other Ag–sulfosalts and the Fe–Zn exchange reaction between fahlore and “sphalerite”, it is estimated that these reactions ceased below about 175°C. From a comparison of the results of this thermodynamic analysis with composition data for other Ag tetrahedrite fahlores found in the Coeur d’Alene district and elsewhere, we conclude that the thermodynamic mixing properties of (Cu,Ag)10(Zn,Fe)2Sb4S13 fahlores are adequately characterized, but that further study of those of Fe–rich “sphalerites” are warranted.