FAHLORE AS A PETROGENETIC INDICATOR: KENO HILL AG–PB–ZN DISTRICT, YUKON, CANADA

Fahlores [~(Cu,Ag)10(Zn,Fe)2Sb4S13] from the Keno Hill mining district, central Yukon, Canada record virtually the entire petrogenetic history of a Cretaceous hydrothermal system extending over 40 km outward from the Mayo Lake granitic pluton. These fahlores are an essential constituent of polymetallic sulfide veins developed in a graphitic Mississippian quartzite, where they occur in association with sphalerite, pyrargyrite, galena and siderite. Fahlores exhibit pronounced east–west zoning in average Ag/(Ag+Cu) and Zn/(Zn+Fe) values, with these simultaneously increasing and decreasing from east to west over 20 km of hydrothermal activity. These zonations are coupled with average Ag/(Ag+Cu) and Zn/(Zn+Fe) values in fahlore roughly paralleling the 300C isotherm for fahlores in equilibrium with pyrargyrite, miargyrite, and sphalerite in the simple system Ag2S–Cu2S–ZnS–FeS–Sb2S3. Early high–Ag, high–Zn fahlores from the eastern and western mines have Ag/(Ag+Cu) and Zn/(Zn+Fe) values requiring temperatures >=400C, in agreement with temperatures established from the As–content of arsenopyrite coexisting with pyrite, pyrrhotite, and sphalerite. Ag/(Ag+Cu) and Zn/(Zn + Fe) values in later, main–stage fahlores are consistent with the 250–310C range of temperatures established for boiling of Keno Hill fluids. Finally, Ag– and Fe–rich fahlores were produced by retrograde Fe–Zn exchange with sphalerite or crystallized from late–stage epithermal fluids which produced polybasite, stephanite, acanthite, and wire silver. One such fahlore exhibits unmixing into high–Ag and low–Ag varieties. This is the first reported miscibility gap for freibergite fahlores and confirms the earlier prediction of such gaps.