CALIBRATION OF THE ION MICROPROBE FOR THE DETERMINATION OF SILVER IN PYRITE AND CHALCOPYRITE FROM THE MOBRUN VMS DEPOSIT, ROUYN-NORANDA, QUEBEC

The silver contents of pyrite and chalcopyrite from the Mobrun volcano genic massive-sulfide deposit near Noranda, Quebec, were determined us ing a Cameca IMS-4f ion microprobe, External standards of sulfides imp lanted with Ag-107 were used for calibration. Secondary-ion yields obt ained with Cs+ and O-2(+) primary-beam sources were compared; the high est yields and best peak/background ratio were obtained with O-2(+) an d an energy offset of 90 V to eliminate mess interference by (AsP2)-As -75-P-16 and (AsS)-As-75-S-32. Minimum detection-limits (MDL) of 60 pp bw were routinely obtained, and are much lower than the MDL cited by p revious investigators. Silver contents range from 0.1 to 1426 ppmw in pyrite and from 1 to 200 ppmw in chalcopyrite. Au/Ag values range from 0.002 to 1.273 in pyrite and from 0.001 to 1.000 in chalcopyrite. Sil ver contents of primary pyrite deposited by synvolcanic hydrothermal f luids are higher than those of secondary recrystallized pyrite formed during metamorphism and deformation, indicating that metamorphic recry stallization led to the release of silver from pyrite. The remobilized silver was deposited subsequently in tectonic veins in secondary chal copyrite and with gold as electrum.