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Outline of the Geology of the Comstock Lode District, Nevada

1945; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr4529

2332-4899

Frank C. Calkins,

Geological and Geochemical Analysis

The term Comstock Lode district is here applied to the mapped area, which includes parts of the Virginia and Flowery Ranges of Nevada.The oldest rocks of the area are much altered sedimentary strata, probably Jurassic or older.These are overlain by metavolcanic rocks chiefly of basaltic composition, and both the sedimentary and the volcanic rocks have been invaded by quartz monzonite porphyry and granodiorite.These intrusive rocks may be contemporaneous with the great composite batholith of the Sierra Nevada and are certainly pre-Tertiary.Upon the pre-Tertiary rocks, after a long interval of erosion and probably in Eocene time, several hundred feet of rhyolitic lava was poured out.The rhyolite is overlain in turn, with apparent conformity, by the Alta andesite, nearly 3,000 feet thick.The Alta consists mainly of pyroxene andesite and hornblende-pyroxene andesite, in the form of lava and pyroclastics, but it includes a sedimentary member, the Sutro, containing Miocene fossil leaves.Marked hydro thermal alteration has affected a large part of the formation.Intrusive bodies of several kinds cut the Alta andesite.What may be the oldest of these are dome-like masses of the American Ravine andesite porphyry.The largest mass consists of the Eavidson diorite, from which Mount Davidson was carved.There are also many dikes, andesitic or dioritic in composition, some of which are later than the Davidson diorite, some contemporaneous with it, and some perhaps earlier.Another great body of andesite, the Kate Peak andesitic series, occurs mainly in the Flowery Range.^ The andesites of the Kate Peak are varied in composition, but are mostly characterized by the presence of biotite and horn-\ blende.They are of upper Miocene or Pliocene age, and they probably were erupted after the Aita andesite had been tilted and faulted.Still later, but probably Pliocene, are the Knickerbocker and Lousetown (?) andesites.These are chiefly dark pyroxene andesites locally passing into basalts.Latest among the eruptive rocks is an olivine basalt.Large areas of alluvium have been mapped, together with some landslides and some areas of made ground, which includes dumps and tailings.The pre-Tertiary sedimentary and volcanic rocks have been crumpled and sheared; the pre-Tertiary intrusives are unaffected by shearing.The Tertiary rhyolite and the Alta andesite have been tilted, mostly to the northwest, and dislocated by many faults, the most important of which are the Cornstock and Silver City faults, from both of which large ore bodies have been mined* The Silver City fault may be a branch of the Cornstock* A fault similar to these but less extensively mineralized is the Occidental.There are many other faults having various directions, and some of the latest cut the Kate Peak andesitic series* This series and the latter Pliocene volcanic rocks may have been involved in some of the later movement on the Cornstock fault and slightly tilted.