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

Lead isotope compositions of sphalerite, fluorite, barite and sparry d olomite in Mississippi Valley-type deposits of the southern Appalachia ns fall into four different isotopic groups. Most sphalerite forms two small clusters, whereas barite, fluorite, and sparry dolomite composi tions are more scattered. These lead isotope compositions reflect the distribution of the deposits, which are hosted by two main paleoaquife rs. Lower Cambrian clastic and carbonate rocks host the Austinville (Z n-Pb), Cartersville (Ba), Embreeville (Zn), and Doughtery (Zn) deposit s, 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 carbon ate rocks of the Knox Group host the Copper Ridge (Zn), Eve Mills (Zn) , Greeneville-Fall Branch (Ba-F), Mascot-Jefferson City (Zn), Powell R iver (Zn-Pb), Russell-Tazewell (Ba-F), and Sweetwater (Ba-Zn-F) deposi ts, which have more complex compositions. Sphalerite from Mascot-Jeffe rson City, Eve Mills, Sweetwater, and Russell-Tazewell plots in a clus ter around Pb-207/Pb-204 = 15.67 and Pb-206/Pb-204 = 19.0, whereas tha t from Copper Ridge plots in both clusters. Fluorite from Sweetwater f alls on a long array in Pb-207/Pb-204-Pb-206/Pb-204 space that has a t wo-stage age of 1.55 to 1.9 Ga for t2 = 360 Ma, the previously publish ed Sr-87/Sr-86 isochron age obtained for the Mascot-Jefferson City dis trict. Barite and sparry dolomite compositions are relatively similar and have lower Pb-207/Pb-204 and Pb-208/Pb-204 ratios than most sphale rite and fluorite. The spatial distribution of lead isotope compositio ns indicates that there were at least two mineralizing solutions, repr esented 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 miner alization is strongest. Although the specific sources of these leads c annot be determined from available data on possible source rocks, the two lead clusters probably came from relatively young, well-mixed sedi mentary basins and the lead in the fluorite must have come from a sour ce with an unusally high U/Th ratio such as phosphorite. This study co nfirms that the isotopic composition of lead in southern Appalachian M ississippi Valley-type deposits differs significantly from that found in mid-continent deposits.