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Gray, T.E., 1988

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Gray, T.E., 1988, Origin and evolution of sub-seafloor Au-Zn-Cu-Pb mineralization and related alteration at the Johnson River prospect, south-central Alaska: University of Oregon, Eugene, M.S. thesis, 219 p., illust.


Concentrically-banded barite and sphalerite at the Johnson River Prospect may represent a complex of seafloor vent chimneys, but most mineralization formed below the seafloor in Jurassic submarine tuffs. Below a mudstone, mineralization occurs in brittle-fracture veins, but above the mudstone, disseminated textures and sinuous veins indicate that tuffs there were unconsolidated during mineralization. Massive anhydrite + Mg-chlorite alteration formed before mineralization, when heated seawater began circulating through the tuffs. At any given location, mineralization occurred in three successive stages: (1) sphalerite; (2) pyrite, chalcopyrite, Fe-chlorite, and hematite; (3) chalcopyrite, galena, and gold. Intense stage 1 precipitation apparently occurred when upwelling fluid entered the unconsolidated tuffs and mixed with cool seawater. Deficiency of sulfide in solution probably drove gold precipitation. A model in which stage 3 formed low in the system while stage 2 formed above, explains chemical trends in these stages and in related chloritic and sericitic alteration.

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