Subjacent crustal sources of sulfur and lead in eastern Great Basin metal deposits

Sulfide minerals in Mesozoic replacement, skarn, porphyry, and vein deposit s in lower Paleozoic rocks in central and eastern Nevada have sulfur isotop e compositions (10 parts per thousand less than or equal to delta(34)S < 20 parts per thousand) and radiogenic lead ratios (Pb-206/Pb-204 > 19) that a re elevated relative to the range of S and Pb isotope compositions in easte rn Great Basin metal deposits. The S and Ph isotope compositions of central and eastern Nevada Mesozoic metal deposits (e.g., Eureka) are similar to t he S and Pb isotope compositions of pyrite disseminated in the thick (18 km ) terrigenous detrital succession (TDS) of siliciclastic rocks of Late Prot erozoic-Early Cambrian age subjacent to the deposits. TDS rocks are, theref ore, a possible source for most if not all S and Pb in these deposits. To t he south and east in southern Nevada, southeastern California, and western Utah, progressively thinner TDS rocks correlate with lower delta(34)S value s (<10 parts per thousand) and lower Pb-206/Pb-204 ratios (<18.6) in overly ing Mesozoic metal deposits. These relationships suggest that TDS rocks sup plied S and Fb to overlying deposits in amounts proportional to TDS thickne ss and that some S and Pb in the southern and eastern Great Basin deposits in lower Paleozoic rocks came from more isotopically homogeneous and presum ably deeper sources, most likely Early and Middle Proterozoic crystalline r ocks. Possible S and Pb sources for eastern Great Basin metal deposits in m iddle and upper Paleozoic rocks include, in addition to TDS pyrite and Earl y and Middle Proterozoic crystalline rocks, Paleozoic sedimentary pyrite th at has S and Pb isotope compositional ranges similar tot as well as lower t han, TDS pyrite isotope ranges. S and Pb isotope compositions of sulfide minerals in metal deposits that ar e temporally related to middle Tertiary granitic intrusions also vary geogr aphically and are generally lower than isotope compositions of Mesozoic met al deposits, regardless of Paleozoic host-rock age. Compared to the Mesozoi c deposits, middle Tertiary deposits in central and eastern Nevada apparent ly derived significant, but mostly smaller, amounts of S and Pb from TDS ro cks and/or Paleozoic rocks. Tertiary metal deposits in western Utah may hav e obtained nearly all their S and Pb from older Precambrian crystalline roc ks or from magmas and virtually none from TDS and Paleozoic rocks. Semiquantification of source-rock contributions of S and Pb to metal deposi ts is based on average S and Pb isotope compositions of possible source roc ks and simple mixing calculations. Possible source rocks are somewhat isoto pically inhomogeneous, but their S and Pb isotope compositional ranges larg ely bracket the S and Ph isotope compositions of metal deposits in the east ern Great Basin, thus facilitating determination of end-member contribution s. Geologic factors that cause isotope inhomogeneity in both source rocks a nd metal deposits include different source-rock provenances, particularly f or Pb isotopes, isotope mixing and fractionation by unrecognized hydrotherm al processes, metamorphism, and tectonism that has juxtaposed potential sou rce rocks of differing ages and isotope compositions. TDS pyrite formed from processes that produced S with high delta(34)S value s-including diagenesis involving seawater sulfate and, at higher temperatur es and greater depths, thermochemical sulfate reduction. Radiogenic Ph in T DS pyrite was derived from leaching of quart-zofeldspathic sedimentary rock s. Granitic melts acquired S and Pb, and possibly other ore-forming compone nts, by bulk assimilation of TDS and/or Paleozoic sedimentary rocks, Proter ozoic crystalline rocks, and possibly older Precambrian rocks; by volatiliz ation of disseminated pyrite in source rocks during ascent; and by hydrothe rmal circulation near the sites of ore ore deposition. The high density of eastern Great Basin metal deposits and the sources of S and Pb for these deposits appear to be a function not only of the large nu mber of granitic intrusions, but also of intrusion age and the thickness an d type of Precambrian crust. S and Pb isotope compositions in eastern Great Basin metal deposits support a proposed origin for Jurassic, Cretaceous, a nd Tertiary intrusions that involves generation of magmas at different crus tal levels and variable amounts of magmatic contamination by Precambrian ro cks.