Lead isotopic composition from data of high-precession MC-ICP-MS and sources of matter in the large-scale Sukhoi Log noble metal deposit, Russia

The lead isotopic composition of 33 sulfide samples from orebodies of the Sukhoi Log deposit was studied by high-precession MC-ICP-MS with a precision of +/- 0.02% (+/- 2SD from 120 analyses of the SRM 981 standard sample). The deposit is located in the Bodaibo gold mining district in the northern Baikal-Patom Highland. Gold mineralization is hosted in Neoproterosoic black slates. Variations of lead isotope ratios of the Sukhoi Log sulfides are generally typical of Phanerozoic deposits and ore fields. They are significant for Pb-206/Pb-204 (17.903-18.674), moderate for Pb-208/Pb-204 (37.822-38.457), and relatively narrow for Pb-207/Pb-204 (15.555-15.679). In the Pb-Pb isotope diagrams, the data points of pyrite and galena constitute a linear trend. The points corresponding to pyrite from metasomatic ore occupy the left lower part of the trend. Galena from late gold-quartz veins shows more radiogenic Pb, and corresponding data points are located in the upper part of the trend. According to the Stacey-Kramers model, the end points of the trend, which is regarded as a mixing line, have mu(2) = 9.6 and mu(2) = 13.2 and model Pb-Pb ages 455 and 130 Ma, respectively. The isotope characteristics of ore lead, their relationships in pyrite and galena, and the mixing trend of Pb isotopic compositions are clearly tied to two Paleozoic stages in the formation of the Sukhoi Log deposit (447 +/- 6 and 321 +/- 14Ma) and testify to the leading role of crustal sources, which are suggested as being the Neoproterozoic black-shale terrigenous-carbonate rocks.