It is believed that four epochs of ore deposition are represented in Nevada; the earliest ore deposits are associated with granitic rocks, which in western Nevada were intruded near the close of the Jurassic and in eastern Nevada at the beginning of the Tertiary. Other ore deposits are associated with late Tertiary flows of two ages.In western Nevada the ore deposits associated with granitic masses satellitic to the Sierra Nevada batholith are predominantly argentiferous quartz veins, similar in appearance to the auriferous veins of California. These, however, have been valuable only for their oxidized and enriched silver ores. In eastern Nevada base metal replacement deposits prevail. These have also produced silver from the oxidized zone but in several districts base metal ores have proved workable in depth. It is thought that the difference between the two classes of deposits is a function of the different magmatic sources; that the western deposits are of late Jurassic or early Cretaceous age, dependent upon the Sierra Nevada batholith, while those in the eastern part of the State are of early Tertiary age. This implies that the western limit of the Tertiary intrusions is near the middle of the State.Several of the important precious metal deposits associated with the late Tertiary lavas are clearly older than upper Miocene sediments, but others are younger than upper Miocene. In the ore of the older group of deposits silver exceeds gold by weight, while in those of Pliocene age gold is commonly in excess. The Miocene deposits have on the whole proved far more productive than those of the younger group.From the available geologic data and a review of the history of production it is concluded that the western argentiferous quartz veins associated with granitic intrusives offer little hope of important future production but on the other hand the eastern deepseated deposits may continue to be important producers of base metals; and that, of the near-surface deposits associated with Tertiary lavas the silver and silver-gold veins of Miocene age are likely to prove of more importance than the Pliocene gold-bearing veins.

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