Ion-microprobe analysis of pyrite, chalcopyrite and pyrrhotite from the Mobrun VMS deposit in northwestern Quebec; evidence for metamorphic remobilization of gold

Abstract

TheMobrunZn-Cu-Ag-Au deposit,located in theNorandaDistict, in the Quebec segmentof theAbitibi Greenstone Belt, is hosted mainly by felsic volcanic rocls of the Archean Blake Rivo Group. Primary facies of mineralization in the Mobrun oreMies resulted from deposition and reworking of sulfides by synvolcanic hydrothermal fluids. Secondary facies of mineralization resulted from greenschist-grade and related deformation, and locally overprinted the primary facies. The 'invisible contents ofpyrite, chalcopyrite and pyrrhotite in various facies were determined using a Cameca MS-4f ion microprobe. Samples were spuffered with a Cs+ pdmar/ bea:q and negative secondary ions were measured. External standards of sulfides wsre imflant€d with lqAu. Mass interferences were eliminaled by operating in high-mass-resolution mode (lvI/AIvI in the range 22100 to 3900), giving rise to minimum limits of detections of 50 ppbw. Primary pyrite contains up to 10 ppmw gold present as submicrometic inclusions of metallic gold, as well as very fine colloid-size or strucnrrally bound gold, Concentrations of in associared secondary (recrystallized) pyrite range from 1 to 67Vo of the concentrations in primary pyrite. The results indicate rhat syngenetic is present in the Mobrun orebodix, and that metamorphic recrystallization resulted in its release from pyrite. The remobilized was deposited in tectonic veins as easily recoverable electum and as invisible in secondary chalcopyrite. The partitioning of among various phases in secondary veins may have been influenced by the interaction between tie remobilizing fluid and the assemblages ofprimary ore minerals. Keyword.s:'invisible gold, refractory gol4 ion microprobe, SMS, pyrite, chalcopyrite, pyrrhotite, electrum, remobilization, metamorphic recrystallization, volcanogenic massive sulfide, Mobrun, Noranda, Quebec.