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Major ore deposits are associated with late-stage intrusive phases of a magma system that may have been preceded by ash-flow eruption and caldera formation. The ash flows that erupt from the top of a magma chamber at shallow depth tend to be silicic and enriched in lithophile elements. If a mechanism exists for concentration of these elements, ash flows, especially thick accumulations of them such as exist in collapse calderas, have ore potential. Uranium exploration should focus on ash flows and calderas. Base and precious metals are not associated with silicic differentiates; they tend to be dispersed by eruption and are not common in ash-flow tuffs. Shallow-fracture systems such as calderas may be mineralized by later igneous activity, but most metal concentrations are in rocks emplaced at greater depths and at a significantly later time than caldera-forming activity.

In Nevada, Cenozoic ash-flow tuffs and the calderas formed by collapse during or after their eruption are in most places barren of economically important base and precious metals. Of 31 recognized calderas in Nevada, 2 have ore deposits that have produced $1 million or more in gold, silver, copper, lead, zinc, mercury, antimony, and iron. The ash-flow tuffs prove barren of the same metals. Of 98 districts with more than $1 million production, only 5 are in silicic tuffs, and these deposits probably are mineral accumulations unrelated to the ash-flow sheets. The largest production of gold and silver in Nevada is from Tertiary andesitic hypabyssal and extrusive rocks that show the effects of pervasive hydrothermal alteration. Where this type of system forms near or within existing caldera structures, mineralization may favor the old caldera faults; Goldfield is an example. Late-stage igneous activity from the relatively deep residual parts of a magma chamber or, more likely, from a new deep-level igneous system prove to be related most closely to hydrothermal metal deposits in many parts of the western United States.

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