The distribution and controls on silver mineralization in the Main Zone of the 2.68 Ga Archean Hackett River Zn-Pb-Cu-Ag volcanogenic massive sulfide (VMS) deposit, Nunavut, Canada

Abstract

The 2.68 Ga Zn-Pb-Cu-Ag Hackett River Main Zone (HRMZ) volcanogenic massive sulfide (VMS) deposit, within the Hackett River Greenstone Belt of the Archean Slave Craton is highly enriched in Ag (and Pb) compared to other VMS deposits of a similar age and type. The mineralization has been sub-divided into five categories based on mineralogy, textures and stratigraphic location: 1) disseminated footwall sulfides, 2) copper-rich stringer sulfides, 3) pyrite-poor sphalerite-pyrrhotite-chalcopyrite mineralization located at the top of the stringer zone, 4) mineralization in calc-silicate altered units and 5) sphalerite-pyrite massive sulfide mineralization. Using a mass-balance for Ag calculated from electron microprobe analyses, pyrrhotite and chalcopyrite in type 1 mineralization contain negligible Ag and in type 2, Bi-Ag-(Pb) sulfides, Ag-Bi-Se enriched galena and chalcopyrite are the dominant Ag hosts. Within type 3, freibergite and galena are the main silver hosts. In type 4, Ag is hosted in disseminated electrum and freibergite while freibergite in type 5 hosts 99% of the Ag. Overall, Ag-rich freibergite contains 79.4% of the total Ag, chalcopyrite hosts 6.3% and galena contains 1.8% of the Ag. Trace minerals such as electrum, stephanite, acanthite and Bi-bearing sulfides host the remainder of the Ag (12.5%) and have a restricted spatial distribution. Mineral assemblages have undergone pervasive recrystallization and annealing during amphibolite grade metamorphism with localized redistribution of base and precious metals from metamorphism at a grain scale only. Within freibergite and chalcopyrite, Ag directly substitutes for Cu within the mineral lattice and replaces Pb in galena by coupled substitution with Bi and to a lesser extent, Sb. The principal controls on Ag residence in the HRMZ are temperature and redox conditions (which varies with distance to the hydrothermal vent) and the ratio of Bi and Sb available for coupled substitution with silver within galena. Subsequent deposit-scale zone refining is the principal factor influencing the distribution of Ag. Lower temperatures and more oxidizing conditions favour partitioning of Ag into freibergite and less oxidizing conditions favour galena. At higher temperatures, the most reducing conditions favour incorporation of Ag in Ag-Bi rich galena (plus Se) and Bi-bearing sulfides or Ag-rich chalcopyrite under lesser reducing conditions.