LEAD AND BARIUM SOURCES IN CAMBRIAN SILICICLASTICS AND SEDIMENT PROVENANCE OF A SECTOR OF THE TACONIC OROGEN, QUEBEC - A MIXING SCENARIO BASED ON PB-ISOTOPIC EVIDENCE

To test the hypothesis that siliciclastic rocks constituted the major source of Pb and Ba in barite-galena deposits of the Taconic Orogen, w e determined Pb-isotope ratios in galena, barren rocks and contained m inerals, as well as concentrations of Pb, U, Th and Ba in the latter ( detrital feldspars, sandstones, mudstones, rock clasts and carbonate c ements and clasts). Ranges in Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/ Pb-204 of 28 galena samples are 17.96-18.05, 15.56-15.59 and 37.75-37. 93, respectively; ranges for 41 barren rocks and minerals are 16.17-23 .31, 15.26-15.86 and 35.98-42.51, respectively. The lowest ratios are in feldspar, and the highest in carbonate and mudstone. Values of the mudstones samples overlap those of galena when corrected for in situ d ecay of U and Th since galena precipitation (approximately 450 Ma). We thus propose that mudstones constituted a source of lead. Corrected r atios for anomalously Pb-rich mudstones are virtually identical to gal ena-Pb ratios and may be due to contamination by lead-bearing brines. Assuming that burial diagenesis did not disturb the Pb-isotope values of sandstones, these rocks contributed only a minor fraction of lead t o the galena, estimated at approximately 20% for one deposit. The sour ce of barite-Ba was probably perthite. Low Ba and Pb concentrations of sandstone adjacent to this deposit, compared to high concentrations r emote from it, support leaching of barium (and minor lead) from feldsp ar penecontemporaneous with feldspar dissolution. Geological data indi cate that the provenance of the siliciclastic rocks was mainly from Gr enville terrane. A comparison of our Pb-isotopic data for Taconic pert hite with those of Grenville K-feldspar, as well as ratios of trace el ements, support this provenance for both sandstones and mudstones. The presence of carbonate platforms peripheral to the orogen, and the Mid dle Ordovician-Middle Devonian depositional range of the studied and M ississippi Valley type deposits north (Newfoundland) and south (U.S.A. Appalachians) of our study area suggest targets for exploration in th ese platforms.