Graduate Thesis Or Dissertation
 

The geology and mineralization of the Peck Mountain area Hornet Quadrangle, Idaho

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  • The Peck Mountain area is located in west-central Idaho just 15 miles east of the Oregon-Idaho border. It is tectonically located within the "Columbia Arc" of the Nevadan oroaenic belt. The Seven Devils Volcanics, Permian to Traissic in age, are the oldest rocks in the area. They are composed of a eugeosynclinal sequence of metasedimentary and metavolcanic rocks. The North Hornet and Olive Creek intrusives are metamorphosed meta-quartz diorite plutons. They were guided in their emplacement by pre-existing northeast trending fractures in the Seven Devils Volcanics. Both intrusives are Triassic or older in age. The Seven Devils Volcanics and the meta-quartz diorite plutons have been cut by fine-grained mafic dikes some time before regional metamorphism. All of the pre-Tertiary rocks in the area have been subjected to low-grade regional metamorphism. This probably took place before Late Jurassic time. The highly altered, fine-grained, partially porphyritic rock in the central part of the Peck Mountain area is not a separate intrusive, as it has previously been described. It is, instead, a zone of hydrothermal alteration that parallels the contact between the meta-quartz diorite and the Seven Devils Volcanics. Mineralogical and textural evidence shows, however, that the hydrothermal fluids must have emanated from a younger (post Jurassic) source at depth and not from the meta-quartz diorite presently exposed in the area. The hydrothermal fluids were also guided along northeast trending fractures in the pre- Tertiary rocks. The Columbia River Basalt, of Miocene age, once covered the entire Peck Mountain area. Post-Miocene erosion has exposed the pre-Tertiary rocks as a window in the surrounding basalt. There is evidence in the Peck Mountain area that both the Yakima and the Picture Gorge formations of the Columbia River Basalt may be present. If this can be verified by further studies. the contact could prove invaluable in deciphering post-Miocene tectonism.
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