LEAD-ISOTOPE GEOCHEMISTRY OF MISSISSIPPI VALLEY-TYPE DEPOSITS OF THE SOUTHERN APPALACHIANS

Lead isotope compositions of sphalerite, fluorite, barite and sparry dolomite in Mississippi Valley-type deposits of the southern Appalachians fall into four different isotopic groups. Most sphalerite forms two small clusters, whereas barite, fluorite, and sparry dolomite compositions are more scattered. These lead isotope compositions reflect the distribution of the deposits, which are hosted by two main paleoaquifers. Lower Cambrian clastic and carbonate rocks host the Austinville (Zn-Pb), Cartersville (Ba), Embreeville (Zn), and Doughtery (Zn) deposits, all of which fall in a cluster of lead isotope compositions near Pb-207/Pb-204 = 15.73 and Pb-206/Pb-204 = 19.45. Lower Ordovician carbonate rocks of the Knox Group host the Copper Ridge (Zn), Eve Mills (Zn), Greeneville-Fall Branch (Ba-F), Mascot-Jefferson City (Zn), Powell River (Zn-Pb), Russell-Tazewell (Ba-F), and Sweetwater (Ba-Zn-F) deposits, which have more complex compositions. Sphalerite from Mascot-Jefferson City, Eve Mills, Sweetwater, and Russell-Tazewell plots in a cluster around Pb-207/Pb-204 = 15.67 and Pb-206/Pb-204 = 19.0, whereas that from Copper Ridge plots in both clusters. Fluorite from Sweetwater falls on a long array in Pb-207/Pb-204-Pb-206/Pb-204 space that has a two-stage age of 1.55 to 1.9 Ga for t2 = 360 Ma, the previously published Sr-87/Sr-86 isochron age obtained for the Mascot-Jefferson City district. Barite and sparry dolomite compositions are relatively similar and have lower Pb-207/Pb-204 and Pb-208/Pb-204 ratios than most sphalerite and fluorite. The spatial distribution of lead isotope compositions indicates that there were at least two mineralizing solutions, represented by the two lead clusters; the upper cluster solution dominated the Lower Ordovician paleoaquifer whereas the lower cluster dominated the Lower Cambrian paleoaquifer; and that the two solutions mixed in most areas except the Mascot-Jefferson City district, where zinc mineralization is strongest. Although the specific sources of these leads cannot be determined from available data on possible source rocks, the two lead clusters probably came from relatively young, well-mixed sedimentary basins and the lead in the fluorite must have come from a source with an unusally high U/Th ratio such as phosphorite. This study confirms that the isotopic composition of lead in southern Appalachian Mississippi Valley-type deposits differs significantly from that found in mid-continent deposits.