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Geology, alteration, lithogeochemistry and hydrothermal fluid characterization of the Neoproterozoic Niblack polymetallic volcanic-hosted massive sulfide camp, southeast Alaska, USA McNulty, Brian


The Neoproterozoic Alexander terrane Niblack Cu-Au-Zn-Ag volcanic-hosted massive sulfide camp is on Prince of Wales Island, southeast Alaska. Multiple massive sulfide deposits occur at different stratigraphic levels in 565 ±1.25 Ma felsic volcanic strata. Host rocks are vent-proximal felsic pyroclastic volcanic breccia, lapilli tuff, coherent flows (Lookout deposit, Trio zone) and vent-distal felsic ash tuffs (Niblack Mine, Mammoth and Dama zones). New geochemical data and volcanic lithofacies indicate the host stratigraphy was deposited in a juvenile oceanic back-arc basin. Sub-seafloor mineralization (Lookout deposit, Trio zone) consists of 15-75% sulfide with disseminated to net-textured to semi-massive sulfide textures. These ores precipitated in unconsolidated water-laden vent-proximal felsic volcanic stratigraphy. Seafloor exhalative mineralization (Niblack Mine, Mammoth and Dama zones) is comprised of massive (>90%) poorly-banded sulfide. This type of sulfide precipitated on the paleo-seafloor in, or above, less-permeable vent-distal felsic ash tuff stratigraphy. Sub-seafloor sulfide, related hydrothermal alteration assemblages, and variations in metal ratios represent hydrothermal fluid flow and cooling through the host stratigraphy. Temperature estimates from chlorite microprobe data indicate sub-seafloor 1:1 Cu:Zn and 1:10 Au:Ag chlorite-rich alteration (Lookout deposit) formed at 321 ±19°C and seafloor exhalative 3:1 Cu:Zn and 1:10 Au:Ag magnesium-rich chlorite alteration (Niblack Mine) formed at 307 ±16°C. Sulfur, oxygen, deuterium and carbon stable isotope results are used to identify the origin of hydrothermal fluids. Sulfide mineral separates have δ³⁴S values of +6.6 to +10.5‰. Calculated fluid compositions of chlorite and sericite have values of +2.3 to +5.9‰ δ¹⁸O(water) and -24.9 to -77.1‰ δD(water). Calculated δ¹⁸O(water) values of magnetite are +8.1 to +15.5‰ and values of carbonate are -13.5 to +9.0‰ δ¹⁸O(water) and -4.4 to -0.5‰ δ¹³C(calcite). The sulfur isotope results indicate sulfur was derived from leached igneous sulfide and minor seawater sulfate reduction. The oxygen, deuterium and carbon isotopic signatures indicate evolved seawater and magmatic hydrothermal fluid sources. The Lookout deposit is the largest in the camp and has the largest magmatic fluid component. Seafloor exhalative sulfide mineralization (Niblack Mine) formed from an evolved seawater-dominant hydrothermal fluid. This suggests magmatic fluids contributed extra metals and sulfur to the hydrothermal system that enhanced the size of the Lookout deposit.

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