Some mining districts of eastern Oregon

This report presents the results of a reconnaissance of most of the mining districts of Oregon east of the Cascade Range, with the exception of the districts in the Sumpter quadrangle. The districts described are distributed through an area roughly coincident with the Blue Mountains, which extend over much of the northeast quarter of the State.

The geology of the Blue Mountains, except for certain small areas, is known only in its most general outlines. The most widespread rocks are the Tertiary volcanic rocks, which extend over a very large part of the State. They are separated by a profound angular and erosional unconformity from the pre-Tertiary rocks.

The pre-Tertiary rocks include representatives of all the geologic periods from Carboniferous to Cretaceous, and earlier periods may also be represented.

The known Carboniferous rocks include argillite, chert, greenstone, and subordinate limestone of uncertain age and a thick series of Permian greenstone, tuff, and limestone. The Triassic rocks include shale, slate, and limestone; the Jurassic rocks are shale and slate; and the Cretaceous are conglomerate and sandstone. Schist and phyllite with some limestone and greenstone of uncertain age occur along the Burnt River west of Durkee and in the Mormon Basin.

Intrusive rocks of at least two magmatic cycles are widespread in the Blue Mountains, and the stratigraphy suggests that several cycles may be represented. The older intrusive rocks include gabbro, pyroxenite, norite, dunite, hornblende-quartz diorite, and albite granite and are characterized by a considerable degree of shearing and both cataclastic and metasomatic metamorphism. The age of some of these rocks in the Canyon Range has been fixed as Lower or Middle Triassic, and others are known to be post-Triassic. Younger biotite-quartz diorite and granodiorite of post-Triassic (possibly post-Jurassic) age are widespread, arid the ore deposits of pre-Tertiary age are believed to be genetically related to these rocks.

Representatives of the Tertiary period include Eocene volcanic rocks and sediments, Oligocene fluviatile deposits, Miocene basic lavas and fluviatile and lacustrine sediments, and Pliocene tuff and gravel. The Quaternary deposits include glacial moraines in the higher mountains, stream gravel, and volcanic ash.

The structure of the pre-Tertiary rocks is complex. Folds are characteristically close or isoclinal and strike eastward. The post-Permian rocks appear to be less metamorphosed and less strongly deformed than the earlier rocks, and the Jurassic less than the Triassic. More than one epoch of diastrophism is therefore involved.