Abstract

Strata-bound sulfide deposits at Ducktown, Tennessee, have many characteristics of volcanogenie exhalative deposits but occur in an overwhelmingly sedimentary environment. Lead isotope ratios of galenas and massive pyritic and pyrrhotitic ores fall into three distinct groups that are not related to wall-rock type, type of ore, or to orebody. A positive correlation between the lead and sulfur isotope ratios of galena indicates a relation between sulfur and lead that does not seem to be an effect of metamorphism but is rather an original feature of the ore. The groupings of lead isotope ratios and a possible bimodality of sulfur isotope compositions, as well as the above-mentioned positive correlation, suggest that Ducktown lead and sulfur represent the mixing of two sources. The dominant source of lead was probably U/Pb-enriched clastic sediments. Seawater sulfate may have dominated sulfur contributions from this source. The second source could have been a deep-seated, mafic igneous source tapped by a deep fault associated with late Precambrian rift basin development. The deep source could have provided heat to drive the hydrothermal cell in the sediments, a relatively nonradiogenic component of lead, sulfur with delta 34 S (sub Sigma ) [asymp] 0 per mil, a considerable amount of Cu and Zn, as well as the small amphibolite pods at Ducktown and along strike.Lead isotope studies of contemporaneous rift volcanics, the Mount Rogers Formation, indicate that lead in Ducktown ores could not have been derived from a similar source. Mount Rogers felsic volcanics contain lead derived from a relatively U/Pb-poor, Th/Pb-enriched source that appears to have been (granulitized?) Grenville-age basement.Lead-isotope ratios of galenas from the Gossan Lead are similar to those at Ducktown. Other Blue Ridge massive sulfide deposits have less radiogenic 206 Pb/ 204 Pb values, but none contain any discernable basement component similar to that of Mount Rogers. Reconnaissance sulfur isotope ratios from other Blue Ridge massive sulfides suggest a tendency toward heavier values than those at Ducktown.

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