131 Andesites of Mount Mazama and Scott – Alterations of the Mazama Andesites

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

Microscopic Petrography


Andesites of Mount Mazama and Scott

     Alterations of the Mazama Andesites

Where fumaroles, solfataras, and hot springs were active, the Mazama andesites have been variously decomposed. In such places, the rocks assume rich orange, yellow, brown, red, and greenish tints. Alteration is especially marked near the vents of the Phantom and Hillman cones, but is also conspicuous at many horizons on the Eagle Crags. Signs of alteration beyond the rim of the caldera are rare.

A common effect of decomposition was to convert the ferromagnesian minerals to serpentine, and to a lesser degree to chlorite and talc. A second effect was to deposit quartz and calcite in open cavities. Where solfataric action was intense, the lavas were converted to masses of milky opal and kaolin, like those so extensively developed around the hot springs of the Lassen region. Here and there chalcedony occurs. Specular hematite and limonite are also characteristic of the solfatarized rocks. Pyrite is surprisingly rare, and pyrrhotite has been detected at only one locality, on the walls of Chaski Bay. The presence of alunite has not been confirmed. Cristobalite and tridymite, as sublimates of fumarolic vapors, are widespread.


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