Abstract

The Johnson River prospect on the Alaskan peninsula is an unusually well preserved Jurassic example of gold-rich sea-floor mineralization accompanied by extensive anhydrite. The prospect is a gold-zinc-copper-lead deposit located within the dominantly dacitic middle portion of the lower Jurassic Talkeetna Formation, a complex succession of unmetamorphosed volcanic and volcaniclastic rocks. The mineralization at the Johnson prospect forms a discordant quartz-sulfide stockwork body in a series of subaqueous tuffs and debris flows.The mineralization and alteration can be divided into an early, pervasive nodular anhydrite-Mg chlorite-clay-sericite stage essentially barren of base metals and gold and a later quartz-sulfide gold stage associated with barite, Fe chlorite, sericite, and vein-type anhydrite. The abundance of sulfate far exceeds that of total sulfide at the Johnson River prospect. The anhydrite-bearing assemblages form a stratigraphically restricted zone in which anhydrite may constitute up to 80 percent of the rock. Textures indicating both replacement of fragments by anhydrite, and more commonly, open-space growth of anhydrite can probably be explained by anhydrite precipitation in highly porous unconsolidated tuffs. Early sulfide and oxide mineralization consists of jasper veins, sinuous pale sphalerite veins and chambers, and quartz-barite-sphalerite nodules accompanied by large volumes of chalcedonic quartz. Late sulfide mineralization includes chalcopyrite-rich veins and hydrothermal breccias accompanied by dark sphalerite, galena, pyrite, and native gold. High-grade gold is concentrated at the highest stratigraphic occurrences of significant copper mineralization.Whole-rock chemical analyses of anhydrite-chlorite rock shows that it is depleted in SiO 2 and enriched in MgO relative to stratigraphically equivalent unaltered tuff. The retrograde solubility of anhydrite, the prograde solubility of quartz, and the large Mg content of the chlorites suggest that this assemblage formed by heating of large volumes of cold seawater in a vigorous convection cell within the unconsolidated tuffs of the Talkeetna Formation. It is suggested that the convection cell was generated by the incursion of heated, metal-bearing fluids from below which eventually came to dominate the hydrothermal system with time as mineral precipitation gradually insulated the stockwork body from cold seawater.

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