MINEROLOGY AND ORIGIN OF ZN-RICH HORIZONS WITHIN THE ARCTIC VOLCANOGENIC MASSIVE SULFIDE DEPOSIT, AMBLER DISTRICT, ALASKA

The Arctic deposit in the Ambler district of the western Brooks Range, AK is a replacement-type Cu-Zn-Pb volcanic massive sulfide (VMS) deposit with an inferred resource of 36.3 million metric tonnes of 3.96% Cu, 5.95% Zn, 0.8% Pb, 54.9 grams/tonne Ag, and 0.7 grams/tonne Au (2007 Annual Exploration Report, NovaGold Resources Inc). Deposited in a back-arc basin environment during rifting of the Angayucham Basin in the Devonian, this region later underwent complex and extensive metamorphism due to obduction of the island arc onto the continental crust. The host rocks for this deposit consist of submarine bi-modal volcanic, pelagic and detrital rocks and various sands and silts metamorphosed at blue-schist, amphibolite, and greenschist-facies conditions as well as some assemblages that have undergone retrograde alteration.

Limited work has been done with the structural and depositional reconstruction of this deposit. Evidence from drilling and field mapping show sulfides existing in distinct layers and pods of varying thickness (cm to m) and in sulfide dominance of pyrite, chalcopyrite, or sphalerite. Due to metamorphic recrystallization, most sulfide layers show little to no evidence of foliation or linear texture despite their occurrence within strongly foliated units.

The Zn zones are particularly abundant spatially, both horizontally and vertically. Petrographic and SEM EDS analyses results show sphalerite to be particularly low in Fe and with few chalcopyrite inclusions, an especially rare occurrence in the presence of abundant chalcopyrite. Carbonate with varied Mg/Ca ratios, barite, micas often Ba-rich, quartz, and feldspars are the common associated gangue minerals. The lack of sulfide dissemination, lack of foliation, presence of metamorphic textures within the silicate minerals, and structural history of the region all suggest that sulfides experienced multiple stages of recrystalization and mobilization. The compositional variability of the sphalerite, the variations in mineralogy and composition of the accompanying gangue minerals, and their stratigraphic location give a clearer understanding of the zoning and geometry of the Arctic deposit.