Source Constraints of Ore Metals In Mississippi Valley-Type Deposits in Central and Eastern Tennessee Using Pb Isotopes.

摘要

The Mississippi Valley-Type (MVT) Central and East Tennessee (TN) Districts contain economically significant lead-zinc deposits that occur in the Early Ordovician carbonates of the Knox Group. Although both districts share similar host rock, have similar temperatures of formation, and typically fill open spaces of collapse breccias or replace their host carbonates, previous studies may suggest that these ores did not form from the same mineralizing fluids and may have different Pb sources. Nu Plasma MC-ICP-MS Pb isotopic analyses on sulfides from Central and East TN were conducted and the metal sources evaluated. Results were plotted on covariation diagrams with respect to the average crustal growth curve from Stacy and Kramers (1975) and the orogene curve from Zartman and Doe (1981). Lead isotope data from both Central and East TN plot beyond the present day age (0 m.y. ago) indicative of a crustal source of Pb-Zn. Central TN 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios range from 19.334 -- 20.128, 15.548 -- 16.034, and 38.837 -- 40.034 respectively. Eastern TN 206Pb/ 204Pb, 207Pb/204Pb, and 208Pb/ 204Pb ratios range from 19.341 -- 19.455, 15.638 -- 15.789, and 39.045 -- 39.591 respectively. Pb isotope ratios from the Young Mine in the East TN District plot within ranges defined by other East TN District samples from previous studies. Three Pb isotope ratios from the Elmwood Mine in the Central TN District plot near these East TN samples, while the remaining Elmwood samples remain suspect to analytical error as they plot away from any other recorded Central TN deposits. It appears that the thick, black, metal-rich shales of the Appalachian Basin have been the source of the Pb-Zn and ore fluids of MVT deposits that formed the East TN deposits. When East TN Pb data from this study are compared to Central TN Pb data from previous studies, Central TN Pb is commonly more radiogenic, suggestive of a mineralizing fluid with different compositions. Further analysis of Central TN Pb deposits is needed to further constrain this source.