Abstract

The group of stratiform chalcopyrite-pyrite deposits at Denali, in the Talkeetna Range, Alaska, are not metamorphosed. The deposits are found along a zone of sediment-volcanic intercalation at a major east-west-trending contact. Finely disseminated pyrite and chalcopyrite locally appear almost massive, but on the average comprise approximately one-third of the unusually fine banded argillaceous, carbonaceous, and limy sediments in the mineralized beds. Some of the mineralized beds straddle the fronts of andesitic flows. The beds which lie under or in front (west) of the flow are squeezed and crumpled, presumably by the loading of the flows. Some of the mineralized beds are overlain by tuff or by unmineralized sediments, and these mineralized beds, together with the "Marker Lime" horizon, are much less disrupted.The deposits are postulated to have formed in a reducing environment in shallow water along a shoreline. The copper-rich solutions were probably derived from the leaching of the abundant copper mineralization in the volcanic flow tops or from solfataric vents. These solutions formed under subaerial oxidizing conditions and would deposit copper as soon as they entered the reducing conditions in the carbonaceous and limy marine environment. The postulation accounts for the rapid diminution of copper content toward the west (seaward) in the mineralized horizons.The deposits provide an excellent example of how a complicated mineralized structure can be developed through the processes of sedimentation and volcanism without "hard-rock" deformation.

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