Calibration of the ion microprobe for the determination of silver in pyrite and chalcopyrite from the Mobrun VMS deposit, Rouyn-Noranda, Quebec

The Ag contents of pyrite and chalcopyrite from the Mobrun volcanogenic massive-sulphide deposit near Noranda were determined by ion microprobe; external standards of sulphides implanted with (super 107) Ag were used for calibration. The yields of secondary ions obtained with Cs (super +) and O (super + ) (sub 2 ) primary beam sources were compared; the highest yields and best peak/background ratio were obtained with O (super + ) (sub 2 ) and an energy offset of 90 V to eliminate mass interference by (super 75) As (super 16) O (sub 2) and (super 75) As (super 32) S. Minimum detection limits of 60 ppb were obtained routinely. Ag contents range 0.1-1426 ppm in pyrite and 1-200 ppm in chalcopyrite; Au/Ag values range 0.002-1.273 in pyrite and 0.001-1.000 in chalcopyrite. The Ag contents of primary pyrite deposited by synvolcanic hydrothermal fluids are higher than those of secondary recrystallized pyrite formed during metamorphism and deformation, indicating that metamorphic recrystallization led to the release of Ag from pyrite; the remobilized Ag was deposited subsequently in tectonic veins in secondary chalcopyrite and with gold as electrum.