PROVENANCE OF ORE METALS IN BASE AND PRECIOUS METAL DEPOSITS OF CENTRAL IDAHO AS INFERRED FROM LEAD ISOTOPES

Abstract

Sulfide ores containing principally argentiferous galena, sphalerite, and tetrahedrite occur both in Paleozoic sedimentary rocks and in Late Cretaceous granites along the eastern margin of the Idaho batholith in central Idaho. Two genetic models have been proposed for the sediment-hosted deposits, one involving vein and replacement styles of mineralization during the Cretaceous and Tertiary and the other representing initial Paleozoic sedimentary exhalative (sedex) style of mineralization and subsequent remobilization during the Cretaceous and Tertiary. There is also uncertainty about the sources of ore components; studies based on fluid inclusions and stable isotopes suggest a shallow crustal source, whereas the Pb isotope studies favor sources deeper than the exposed crustal rocks. We present new lead isotope data on 16 sulfide ore samples and 24 whole-rock and mineral separate samples, and compare them with data from the literature to assess the possible sources of base and precious metals in the deposits of central Idaho. Two potential ore metal sources are recognized: a sedimentary source and an igneous source. Deposits that derived their metals from a sedimentary source are characterized by radiogenic Pb and are subdivided into three types: Triumph ( 206 Pb/ 204 Pb = 19.4–20.1, 207 Pb/ 204 Pb = 15.7–15.9, and 208 Pb/ 204 Pb = 39.0–39.9), Minnimoore ( 206 Pb/ 204 Pb = 19.9–20.6, 207 Pb/ 204 Pb = 15.8–16.0, and 208 Pb/ 204 Pb = 40.1–41.3), and Pacific ( 206 Pb/ 204 Pb = 20.4–21.4, 207 Pb/ 204 Pb = 15.7–15.8, and 208 Pb/ 204 Pb = 40.8–41.1). The Pb isotope compositions of sediment-hosted deposits can be explained by the mixing of Pb derived from Paleozoic sedimentary rocks and Early Proterozoic crystalline rocks or by the mixing of Pb derived from the Cretaceous granite and Early Proterozoic crystalline rocks. Alternatively, the Pb in sediment-hosted deposits may represent a mixture of Pb derived from all three sources in varying proportions. None of these scenarios requires Pb from deep sources. The results of this study and prior fluid inclusion and stable isotope studies support a genetic model involving shallow crustal sources for metals and sulfur, mobilized by meteoric water-dominated hydrothermal systems. Deposits in which the metals were derived from an igneous source are subdivided into Carrietown and non-Carrietown types. Carrietown-type ores have relatively low uranogenic Pb isotope ratios ( 206 Pb/ 204 Pb = 17.3–19.1 and 207 Pb/ 204 Pb = 15.4–15.7) and variable thorogenic Pb isotope ratios ( 208 Pb/ 204 Pb = 38.9–41.0). The non-Carrietown–type ores are isotopically similar to their host granites and have higher uranogenic Pb isotope ratios ( 206 Pb/ 204 Pb = 19.3–20.6, 207 Pb/ 204 Pb = 15.7–15.8, and 208 Pb/ 204 Pb = 39.3–40.1) compared to the Carrietown-type ores. We propose that the two groups are dominated by Pb sources from different depths; the Carrietown-type ores from middle crustal sources (uranium-depleted) and the non-Carrietown–type ores from upper crustal sources (enriched in both U and Th). A magmatic hydrothermal origin for both types of ores is supported by their granitic host rocks, hydrothermal alteration of intrusive granite and surrounding rocks, high temperatures of ore formation, and similar radiometric ages for the granite and mineralized veins.