U-Pb zircon measurements from some of the basement rocks in southwestern New Mexico have ages that range from 1,650 to 1,450 m.y.
The largest ore deposits occur near Silver City in the southwest 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 less than 18.0.
Lead from more silicic mid-Tertiary volcanic rocks and associated ore deposits has somewhat higher 208Pb/204Pb ratios that reflect a greater crustal involvement in their origin. In a regional trend to the northeast, ore lead becomes more radiogenic, and at Hansonburg, 200 km from Silver City, lead in the Mississippi-type deposits is clearly derived from upper-crustal sources.
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 1,750-m.y.-old stratiform deposits as far apart as Pecos, New Mexico, and Jerome, Arizona. Such a composition indicates that over a large region of the southwestern United States, continental crust developed between 1,750 and 1,450 m.y. ago, possibly in an island-arc environment.
Basement rocks that are ∼ 1,750 m.y. old extend northward through Colorado to Utah. Galena data obtained in previous studies show that the fraction of older sialic lead in those rocks increases toward the Archean craton in Wyoming. The crust apparently developed southward from Wyoming in stages at 2,400 m.y. ago or before, 2,100 m.y. ago, and 1,750 m.y. ago, with incorporation of older sialic material in each stage as far south as Milford, Utah.