Abstract

Sulfide minerals in Mesozoic replacement, skarn, porphyry, and vein deposits in lower Paleozoic rocks in central and eastern Nevada have sulfur isotope compositions (10‰ ≤ δ34S < 20‰) and radiogenic lead ratios (206Pb/204Pb ∼ 19) that are elevated relative to the range of S and Pb isotope compositions in eastern Great Basin metal deposits. The S and Pb isotope compositions of central and eastern Nevada Mesozoic metal deposits (e.g., Eureka) are similar to the S and Pb isotope compositions of pyrite disseminated in the thick (is less than or equal to 8 km) terrigenous detrital succession (TDS) of siliciclastic rocks of Late Proterozoic–Early Cambrian age subjacent to the deposits. TDS rocks are, therefore, a possible source for most if not all S and Pb in these deposits. To the south and east in southern Nevada, southeastern California, and western Utah, progressively thinner TDS rocks correlate with lower δ34S values (<10‰) and lower 206Pb/204Pb ratios (<18.6) in overlying Mesozoic metal deposits. These relationships suggest that TDS rocks supplied S and Pb to overlying deposits in amounts proportional to TDS thickness and that some S and Pb in the southern and eastern Great Basin deposits in lower Paleozoic rocks came from more isotopically homogeneous and presumably deeper sources, most likely Early and Middle Proterozoic crystalline rocks. Possible S and Pb sources for eastern Great Basin metal deposits in middle and upper Paleozoic rocks include, in addition to TDS pyrite and Early and Middle Proterozoic crystalline rocks, Paleozoic sedimentary pyrite that has S and Pb isotope compositional ranges similar to, as well as lower than, TDS pyrite isotope ranges.

S and Pb isotope compositions of sulfide minerals in metal deposits that are temporally related to middle Tertiary granitic intrusions also vary geographically and are generally lower than isotope compositions of Mesozoic metal deposits, regardless of Paleozoic host-rock age. Compared to the Mesozoic deposits, middle Tertiary deposits in central and eastern Nevada apparently derived significant, but mostly smaller, amounts of S and Pb from TDS rocks and/or Paleozoic rocks. Tertiary metal deposits in western Utah may have obtained nearly all their S and Pb from older Precambrian crystalline rocks or from magmas and virtually none from TDS and Paleozoic rocks.

Semiquantification of source-rock contributions of S and Pb to metal deposits is based on average S and Pb isotope compositions of possible source rocks and simple mixing calculations. Possible source rocks are somewhat isotopically inhomogeneous, but their S and Pb isotope compositional ranges largely bracket the S and Pb isotope compositions of metal deposits in the eastern Great Basin, thus facilitating determination of end-member contributions. Geologic factors that cause isotope inhomogeneity in both source rocks and metal deposits include different source-rock provenances, particularly for Pb isotopes, isotope mixing and fractionation by unrecognized hydrothermal processes, metamorphism, and tectonism that has juxtaposed potential source rocks of differing ages and isotope compositions.

TDS pyrite formed from processes that produced S with high δ34S values—including diagenesis involving seawater sulfate and, at higher temperatures and greater depths, thermochemical sulfate reduction. Radiogenic Pb in TDS pyrite was derived from leaching of quartzofeldspathic sedimentary rocks. Granitic melts acquired S and Pb, and possibly other ore-forming components, by bulk assimilation of TDS and/or Paleozoic sedimentary rocks, Proterozoic crystalline rocks, and possibly older Precambrian rocks; by volatilization of disseminated pyrite in source rocks during ascent; and by hydrothermal circulation near the sites of 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 number of granitic intrusions, but also of intrusion age and the thickness and type of Precambrian crust. S and Pb isotope compositions in eastern Great Basin metal deposits support a proposed origin for Jurassic, Cretaceous, and Tertiary intrusions that involves generation of magmas at different crustal levels and variable amounts of magmatic contamination by Precambrian rocks.

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