The geology of the Khayyam and Stumble-On deposits, Prince of Wales Island, Alaska



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The Khayyam and Stumble-On deposits are stratiform copper-zinc-gold-silver massive sulfide deposits within the late Proterozoic(?) to pre-Middle Ordovician Wales Group, a metamorphosed volcanic arc complex within the Alexander tectonostratigraphic terrain. These massive sulfide deposits are interbedded with schist layers of siliceous, felsic, intermediate and mafic composition from 1 m to 30 m thick. Within the study area, these strata strike N 70° - 80° W and dip from 70° S to 70° N. Whole rock geochemistry of five samples indicate that the host rocks are metamorphosed, high-sodium low-potassium tholeiitic to calc-alkalic volcanic rocks. Electron microprobe energy dispersive analysis and petrographic observations show that fine-grained mafic schist contains plagioclase with compositions ranging from An₁₉ to An₃₁, and amphiboles that plot in the compositional fields of calcic ferro-and magnesio-hornblendes. These mineral compositions indicate metamorphism to the lower amphibolite facies. The Khayyam deposit was mined from 1901 to 1907, when approximately 250,000 metric tons of ore, including 3.2 million kg copper, 36,600 g gold and 48,600 g silver were recovered. It is comprised of multiple stacked sulfide lenses up to 80 m long and 7 m thick. Base and precious metal zonation is apparent in the Khayyam mine area, with copper, gold and silver concentrated along the main ore horizon; zinc is in relative abundance along the main ore horizon's western edge. The Stumble-On deposit, 2 km to the east, is a single sulfide lens greater than 170 m long and up to 2 m thick, with a local parting of schist of intermediate composition. A ground VLF-EM survey in the Stumble-On mine area indicates that the orebody may continue along strike in the subsurface to the east. Metal ratio plots show that the relative abundances of copper and gold within this sulfide lens appear to increase in the same direction. Correlation coefficients calculated for massive sulfide samples taken from both mines show strong positive correlation between copper and silver and between gold and silver. Both orebodies have the same mineral assemblage, with pyrite, chalcopyrite, sphalerite, pyrrhotite and magnetite comprising the bulk of the ore. Pyrite usually occurs as euhedral porphyroblasts, but locally it exhibits textures indicative of rolling and abrasion or of brittle deformation. Chalcopyrite, sphalerite and pyrrhotite have deformed in a ductile manner and are found infilling around pyrite porphyroblasts and along fractures within pyrite. Magnetite occurs as non-crystalline masses or as euhedral porphyroblasts that have replaced sulfides. Minor amounts of gahnite, hematite, quartz, plagioclase, chlorite, hornblende, garnet and stilpnomelane are present, and arsenopyrite is found in trace amounts. Several features are consistent with models for volcanic-associated syngenetic sulfide precipitation from submarine hydrothermal vents. Coarse fragmental textures in the host metavolcanic rocks indicate the proximity of an active volcanic center contemporaneous with ore formation. Strong chloritic alteration in the footwall rocks adjacent to the orebodies is evidence for alteration of sea floor volcanic rocks by hydrothermal fluids. The lens-like shape of the orebodies is typical of sulfide deposits that formed by emanation onto the sea floor from hydrothermal vents. The presence of siliceous schist adjacent to and gradational with the orebodies indicates that silica-rich fluids emanated from the same sulfide-precipitating hydrothermal vents. Sulfide occurrences present in exposures of the Wales Group along strike to the west may have been deposited concurrently with the Khayyam and Stumble-On deposits