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Steven, Thomas A

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...
Determining the precise age of ore deposition in many mining districts has been an elusive geochronologic problem. Paleontologic and isotopic determination of the ages of pre- and postore units commonly provide only broad limits, and in many deposits suitable materials for dating are unavailable. Especially promising is the possibility of direct radiometric dating of gangue and alteration minerals such as alunite, adularia, and coarse sericite, but in most previous attempts independent age control on the host rock and postore rocks has been lacking. However, a study on alunite (Silberman and Ashley, 1970) from the Goldfield district, Nevada, has demonstrated that K-Ar ages from hypogenea lunite can give reliable...
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