Abstract

We have measured the isotopic composition of lead in 34 Mesozoic and Cenozoic mineral deposits from northern Mexico. These deposits can be divided into massive sulfide, sedimentary, limestone replacement, and vein types and appear to have formed in four phases ranging from Upper Jurassic-Lower Cretaceous to Oligocene. The isotopic composition of lead from these deposits falls within a restricted range ( 206 Pb/ 204 Pb = 18.4-19.0), but individual deposits and districts show even smaller compositional ranges, which are approximately equivalent to the analytical uncertainty in this study. Groups of deposits from the same area and/or deposits of similar type exhibit isotopic compositional similarities. The Mexican lead isotope data conform most closely to the growth curve of Stacey and Kramers (1975) in terms of their 207 Pb/ 204 Pb ratios. The 30 Cenozoic deposits define a linear array with a slope of 0.092 + or - 0.017, which differs distinctly from mixing lines recently delineated for the Cascades and Aleutian volcanic rocks. The line coincides in part with the field delineated by Doe and Zartman (1979) for mature arcs but extends to more radiogenic compositions characterized by pelagic sediments and/or noncratonized continents and derived sediments. The Cenozoic linear array intersects the Stacey and Kramers growth curve at about 33 m.y., which is equivalent to the average time of mineralization, and at 1,438 m.y., which is the same as the average age of Precambrian basement rocks in northern Mexico. We interpret these observations to suggest that Precambrian basement rock extends beneath most of northern Mexico and contributed significant amounts of lead to magmas that formed the lead deposits.

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