Distribution and Controls on Silver Mineralization in the Hackett River Main Zone, Nunavut, Canada: An Ag- and Pb-enriched Archean Volcanogenic Massive Sulfide Deposit

摘要

The 2.67 Ga Hackett River volcanogenic massive sulfide (VMS) deposits located in the northeastern Slave province, Nunavut, Canada, are among the largest undeveloped massive sulfide resources in Canada and are silver rich compared to other such deposits of similar age, with Ag grades up to 3,000 g/t. The deposits are hosted by the Ignerit Formation of the felsic to intermediate calc-alkaline Hackett River Group metavolcanic rocks that are part of the province-wide supracrustal Yellowknife Supergroup. One of the most economically significant of the Hackett River deposits is the Hackett River Main zone (Main zone), which consists of two parts: a stratigraphically lower chalcopyrite-rich stringer zone and an upper massive to semi massive polymetallic sulfide lens. The mineralization is subdivided into five types based on mineralogy, textures, and approximate stratigraphic position: (1.) disseminated footwall sulfides, (2) copper-rich stringer sulfides, (3) pyrite-poor sphalerite-pyrrhotite-chalcoprite mineralization at the top of the stringer zone, (4) mineralization in calc-silicate altered calcareous tuff units, and (5) sphalerite-pyrite massive sulfide. In type 1 mineralization, disseminated pyrite, pyrrhotite, and sphalerite 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, Ag-rich tetrahedrite (freibergite) and galena are the main Ag hosts. In type 4, Ag is hosted in disseminated electrum and freibergite, and within type 5 mineralization, freibergite hosts 99% of the Ag. Overall within the Main zone, Ag-rich freibergite contains 79.4% of the Ag, whereas chalcopyrite hosts 6.3% and galena contains 1.8%. Trace minerals such as electrum host the remainder of the Ag, and these have a limited spatial distribution.