ION-MICROPROBE ANALYSIS OF PYRITE, CHALCOPYRITE AND PYRRHOTITE FROM THE MOBRUN VMS DEPOSIT IN NORTHWESTERN QUEBEC - EVIDENCE FOR METAMORPHIC REMOBILIZATION OF GOLD

The Mobrun Zn-Cu-Ag-Au deposit, located in the Noranda District, in th e Quebec segment of the Abitibi Greenstone Belt, is hosted mainly by f elsic volcanic rocks of the Archean Blake River Group. Primary facies of mineralization in the Mobrun orebodies resulted from deposition and reworking of sulfides by synvolcanic hydrothermal fluids. Secondary f acies of mineralization resulted from greenschist-grade metamorphism a nd related deformation, and locally overprinted the primary facies. Th e ''invisible'' (refractory) gold contents of pyrite, chalcopyrite and pyrrhotite in various facies were determined using a Cameca IMS-4f io n microprobe. Samples were sputtered with a Csf primary beam, and nega tive secondary ions were measured. External standards of sulfides were implanted with Au-197. Mass interferences were eliminated by operatin g in high-mass-resolution mode (M/Delta M in the range 2400 to 3900), giving rise to minimum limits of detections of 50 ppbw. Primary pyrite contains up to 10 ppmw gold, present as submicrometric inclusions of metallic gold, as well as very fine colloid-size or structurally bound gold. Concentrations of gold in associated secondary (recrystallized) pyrite range from 1 to 67% of the concentrations in primary pyrite. T he results indicate that syngenetic gold is present in the Mobrun oreb odies, and that metamorphic recrystallization resulted in its release from pyrite. The remobilized gold was deposited in tectonic veins as e asily recoverable electrum and as ''invisible'' gold in secondary chal copyrite. The partitioning of gold among various phases in secondary v eins may have been influenced by the interaction between the remobiliz ing fluid and the assemblages of primary ore minerals.