Lead-isotopic compositions of diverse igneous rocks and ore deposits from southwestern New Mexico and their implications for early Proterozoic crustal evolution in the western United States.

U/Pb zircon measurements from some of the basement rocks in SW New Mexico have ages that range from 1650-1450Ma. The largest ore deposits occur near Silver City in the SW part of the region studied. They are associated with Laramide alkali to calc-alkaline plutons, and their lead-isotopic compositions are the least radiogenic in the region. This lead exhibits lower-crustal-upper-mantle characteristics, and for the major producing porphyry copper deposits, the 206Pb/204Pb ratios are 208Pb/204Pb ratios that reflect a greater crustal involvement in their origin. On the 207Pb/204Pb-206Pb/204Pb plot, data from all of the rocks and ores that we have analyzed form an array that lies below the average orogene curve of Doe and Zartman (1979). A common source is implied for the lead, the isotopic composition of which was similar to that found in 1750Ma old stratiform deposits as far apart as Pecos, New Mexico, and Jerome, Arizona. Such a composition indicates that over a large region of the SW US, continental crust developed between 1750-1450Ma ago, possibly in an island-arc environment. -from Authors