Ore deposits in the western San Juan Mountains formed intermittently in middle to late Tertiary time, from about 30 to 10 m.y. ago, during essentially the same span as that of associated igneous activity, as indicated by 31 new K-Ar and fission-track ages. Mineralization occurred recurrently during the waning stages of evolution of several precaldera central volcanoes and also after formation of the Uncompahgre, San Juan, Silverton, and Lake City calderas. The richest ore deposits are associated with structures of the Silverton caldera, but they were emplaced 5 to 15 m.y. after the caldera formed. This mineralization appears genetically unrelated to evolution of the caldera and its associated magmatic system but seems closely related in space and time to volumetrically minor intrusions of quartz-bearing silicic porphyry. This association is common in many mining districts in the Rocky Mountains region.

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