Mineralogy 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, Alaska 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. The sulfides show textural evidence for wide spread recrystallization and replacement. Yet the chemical zonation recorded across the deposit indicates preservation of original depositional zonation. Despite the retention of some original chemical gradation, the Arctic deposit is greatly folded, from micro- to deposit scale, with a large tight recumbent fold dominating the subsurface structure. The zonation at the Arctic deposit is identified from compositional analysis and variations of gangue mineral abundance. The changes in chemistry across the deposit occur in Fe/Mg percentages within gangue mineral compositions, Fe percent in sphalerite, and sheet silicate abundance. These variations are the evidence for locating the hydrothermal vents in the southeast zone of the volcanic massive sulfide deposit