Abstract

In the San Juan volcanic area of southwestern Colorado, the isotopic composition of lead in ores and ore prospects of Cenozoic age ranges widely: 17.72 to 21.13 for 206 Pb/ 204 Pb; 15.50 to 15.81 for 207 Pb/ 204 Pb; and 37.21 to 38 for 208 Pb/ 204 Pb. Examination of the lead isotope data indicates that once deposition of lead minerals begins, further exchange of lead between fluid and wall rock is insignificant. This conclusion is supported by the relatively constant isotopic composition of the lead in these ores, which is not affected by the grade of ore mineralization or type of wall rock. The values of 206 Pb/ 204 Pb in some vein-type deposits exceed the maximum value known for all Mesozoic and Cenozoic igneous rocks of the Rocky Mountain region. These isotopic relations show that if ore-forming solutions are composed of meteoric water, as indicated in studies of light stable isotopes, they must have penetrated deep enough to acquire lead from Precambrian rocks or sediments derived from them. The fact that some of these vein ores are now in Cenozoic igneous rocks indicates the ore fluid had an upward vertical component to its movement. The lead isotope data therefore support a circulating cell hypothesis for these kinds of ores, as suggested by many recent studies of light stable isotopes as related to mineralization.Some other deposits (Summitville, Jasper, Red Mountain district) have values of 206 Pb/ 204 Pb similar to those of the large volumes of altered rock that enclose them ( 206 Pb/ 204 Pb approximately 18.5), suggesting that in places the lead may have been locally derived by leaching of the adjacent rocks or from magmatogenic fluids.When the lead isotope data are treated in detail, the rocks and galenas of the Platoro caldera complex, of the central San Juan caldera complex, and of the Baughman Creek center appear to contain significant components derived from 1,400- to 1,500-m.y.-old and 1,700- to 1,800 m.y.-old source materials. These also are the ages of the two main groups of rocks that comprise the Precambrian basement under the San Juan volcanic area. Although the data from the western San Juan caldera complex scatter considerably, the only obvious source for the lead seems to be the 1,700- to 1,800-m.y.-old rocks or detritus of such age in Phanerozoic sediments. Where the involvement of 1,400- to 1,500-m.y.-old sources is greatest, the Th/U ratio in the source material is small (calculated to be about 0.7), whereas the Th/U ratio of the 1,700- to 1,800-m.y.-old source material appears to be 2.3 to 3.3. The largest Th/U values are from the western San Juan caldera complex.

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