Abstract

Tertiary ore deposits and prospects in Trans-Pecos Texas and adjacent Mexico have Pb isotope ratios that indicate probable sources of metals. Sulfides from these deposits have a wide range of Pb isotope ratios: 206 Pb/ 204 Pb, 17.74 to 20.04; 207 Pb/ 204 Pb, 15.46 to 15.69; 208 Pb/ 204 Pb, 37.52 to 40.37. In part, this variability reflects derivation of Pb from contrasting basement and Tertiary igneous rocks across a major lithospheric boundary between the North American craton on the northwest and terranes accreted during the late Paleozoic Ouachita orogeny to the southeast. Precambrian basement and Tertiary igneous rocks northwest of a northeast-trending zone through Trans-Pecos Texas and Chihuahua, Mexico, are less radiogenic than Phanerozoic and Precambrian basement and Tertiary igneous rocks to the southeast.Although isotopic ratios from ore deposits and prospects in the region generally reflect basement type, steep arrays on 206 Pb/ 204 Pb versus 207 Pb/ 204 Pb diagrams from some deposits in the northwest basement province indicate mixing with additional, more radiogenic, sources of Pb. We infer that these sources are local sedimentary rocks or basinal fluids. Vein- and porphyry-type prospects hosted by igneous rocks show the least influence from these radiogenic sources. They have Pb isotope ratios that fall within the range of Pb isotope ratios of local Tertiary igneous rocks. In contrast, red-bed-hosted Ag-Cu-Pb deposits in the northwest, with no apparent igneous source of fluids, generally have relatively high 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios, which are apparently inherited from their host sediments. Veins hosted by sedimentary rocks and high-temperature carbonate-hosted Ag-Pb-Zn deposits (manto, chimney, and skarn) have intermediate Pb isotope ratios that range between the lower 207 Pb/ 204 Pb igneous-hosted deposits and the higher 207 Pb/ 204 Pb sedimentary-hosted deposits. They appear to contain variable mixtures of Pb from mid-Tertiary igneous sources and sedimentary rocks. In the northwest basement province Ag-Pb-Zn deposits at Shafter, Texas, have low Pb isotope ratios derived from associated igneous rocks and indicative of underlying Precambrian crust with low 206 Pb/ 204 Pb and 207 Pb/ 204 Pb. Similar, but larger, deposits at Santa Eulalia, Chihuahua, contain a mixture of Pb from at least two sources. It appears that a low 207 Pb/ 204 Pb component associated with Tertiary igneous rocks and ultimately derived from Precambrian basement has mixed with a higher 207 Pb/ 204 Pb component derived from the sedimentary wall rock. Published fluid inclusion and S isotope studies also suggest that mixing of magmatic and sedimentary components is common in the genesis of high-temperature carbonate-hosted Ag-Pb-Zn deposits.

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