LEAD AND SULFUR ISOTOPE INVESTIGATION OF PALEOZOIC SEDIMENTARY-ROCKS FROM THE SOUTHERN MIDCONTINENT OF THE UNITED-STATES - IMPLICATIONS FORPALEOHYDROLOGY AND ORE GENESIS OF THE SOUTHEAST MISSOURI LEAD BELTS

This study utilizes Pb isotope data to identify flow paths and Pb sour ces for Mississippi Valley-type ores in the midcontinent of the United States with emphasis on the Old and Nerv Lead Belt subdistricts bf so utheast Missouri. Nearly 150 new analyses are reported which character ize the,isotope systematics of both ore samples and Pb hosted by sedim entary rocks away from ore districts. A subset of the samples has been analyzed for sulfur isotopes. The nonore samples contain trace amount s of Pb largely hosted by trace metal-rich FeS2 (termed ''sulfide trac e Pb''). These metal-rich sulfide phases are constrained by previous p etrographic studies to have precipitated from the mineralizing fluids responsible for Mississippi Valley-type ore formation. Sampling was de signed to evaluate the role of potential Cambrian and Ordovician aquif ers as transport paths for Pb and S in an area broadly surrounding the locus of lead belt mineralization in southeast Missouri. Analyses are also presented for various leach fractions of rocks (termed ''rock Pb '') associated with these same potential aquifer units. Analyses of or e samples establish important benchmarks with which to compare the non ore samples. Ore samples were analyzed from southeast Missouri and inc lude new data for both galena and nongalena phases (chalcopyrite, spha lerite, dolomite) from the New Lead belt subdistrict (Viburnum Trend s ubdistrict), and galena from the Old Lead Belt subdistrict and the out lying subdistricts of Annapolis and Indian Creek. Also studied were ga lena samples from the Central Missouri and Tri-State districts. The ne w data plus published values from the Northern Arkansas and Upper Miss issippi Valley districts are nearly collinear on a uranogenic Pb isoto pe plot (Pb-206/Pb-204 vs. Pb-207/Pb-204). The slope of the linear reg ression line through this data is consistent with all the ores having been derived from the approximately 1450 Ma basement in the midcontine nt area. However, the ores are very clearly separated on a plot of tho rogenic isotope data (Pb-206/Pb-204 vs. Pb-208/Pb-204). Ores from eith er end of the Illinois basin, the Upper Mississippi Valley and Illinoi s-Kentucky fluorspar districts, have markedly higher Pb-208/Pb-204 for a given Pb-206/Pb-204 compared to ores from the ore districts in the Ozark region. Conversely, data from main-stage galena samples from the Viburnum Trend (which are commonly cube octahedral in morphology but which may also be disseminated in the host rock) and limited data from the Old Lead Belt are notable for having a lower Pb-208/Pb-204 for a given Pb-206/Pb-204 compared not only to Illinois basin-associated ore s but also compared to data from other districts in the Ozark region a nd even to paragenetically later, and volumetrically minor, cubic-stag e mineralization data from the Viburnum Trend itself. Identifying the transport path and source for the Pb-208-depleted main-stage ores of t he lead belts was a major goal of this study. Uranogenic Pb isotope da ta for sulfide trace Pb samples are generally collinear with ore Pb da ta. This similarity indicates that a basement source is also possible for sulfide trace Pb; however,the thorogenic isotopic plot shows much more complexity. Pb isotope data from samples of dominantly dolomitic Cambrian and Ordovician units younger than the Upper Cambrian Bonneter re Formation form an array which is collinear with data from the North ern Arkansas, Tri State, and Central Missouri districts and with the m ost radiogenic values from the late-stage cubic mineralization. This a rray is Significantly enriched in Pb-208 compared to main-stage Viburn um Trend mineralization. Data from the Bonneterre Formation, which hos ts ore in the lead belts, show more scatter but tend to show even more Pb-208 enrichment compared to main-stage lead belt data. This trend t oward increased scatter and increased Pb-208 enrichment continues to b e supported by isotopic data from the underlying, coarse elastic Lamot te Sandstone, which many have postulated as a major aquifer for ore ge nesis in southeast Missouri. Thus, an extensive data set from a large area surrounding the lead belts failed to identify the isotopic tracks of Pb-208-depleted main-stage lend belt mineralization in aquifer uni ts hosting sulfide-associated Pb, but did identify potential aquifers associated with dolomitic, post-Bonne-terre Cambrian rocks for Pb tran sport to less Pb-rich Mississippi Valley-type districts such as Tri-St ate, Northern Arkansas, and Central Missouri. The sulfide trace Pb sam ples present data which show that they are systematically enriched in S-32 (i.e., delta(34)S < 10%) compared to samples of main-stage ores o f the Viburnum Trend (delta(34)S > 10 parts per thousand), although is otopically heavy trace sulfides are localized in the upper Bonneterre Formation. Combined Pb and S isotope data for sulfide trace Pb samples do not reflect the signature expected of a single, unique Mississippi Valley-type ore-forming fluid for the lead belts but are indicative o f a fluid which could have represented the isotopically light S and sl ightly Pb-208-enriched end member of a mixture. Pb isotope data from l eaches of rock samples from the Bonneterre Formation (a predominantly carbonate unit) and the elastic Lamotte Sandstone are easily distingui shable. On a thorogenic isotope plot, the Bonneterre Formation results , from both limestones and dolomites, form a linear array that is in g ood agreement with the array formed by sulfide trace Pb samples hosted by carbonate rock units (i.e., exclusive of the Lamotte Sandstone). T he combined Pb isotope array, which we term the ''midcontinent carbona te Pb line'' is interpreted to reflect both sources of Pb (primary lim estone and premineralization dolomite) and sinks (trace Pb,in sulfides and ore-stage hydrothermal dolomite). The slope of the uranogenic plo t for the midcontinent carbonate Pb line, which gives an age similar t o that of both ores and basement rocks, is interpreted to indicate an ultimate basement source for the uranium in these units, but some pref erential transport and incorporation of U over Th did occur during for mation of these rocks. A major finding of this study is that thorogeni c Pb isotope data on sock Pb from the Lamotte Sandstone (supported by published data on Pb trapped in presumed ore-stage authigenic K feldsp ar) yield the Pb-208-depleted isotope signature expected for rocks of a transport path of main-stage Pb. The marked contrast betwe