Abstract

The Helen iron formation consists of an upper zone of banded chert-carbonate and a lower siderite zone rich in pyrite near the top. The footwall rocks are volcanics, rhyolitic to the west and dacitic to the east, and are intensely altered to a distance of about 150 feet stratigraphically below the iron formation. The areal patterns of chemical changes in the footwall rocks are clearly similar to the patterns of compositional variation in the iron formation, indicating a common origin. The same volcanic solutions are believed to have altered the footwall rocks and deposited the iron formation.

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