Lead isotope trends and metal sources in the Mississippi Valley-type districts from the mid-continent United States

Abstract

This study identifies metal-contributing source regions for Mississippi Valley-type (MVT) ore deposits in the Northern Arkansas district, the Tri-State district, and the Burkesville deposit in the Central Kentucky district. The Pb isotope ratios of sphalerite ores from the Tri-State district (208Pb/204Pb between 40.7443 and 41.2626; 207Pb/204Pb between 15.8633 and 15.9571; 206Pb/204Pb between 21.8373 and 22.1956) are congruent with the Pb isotope ratios defined by samples from the Central Missouri district, suggesting similar Pb sources. The ores from the Tri-State district plot in a narrow field and have higher 208Pb/204Pb values than samples from the Southeast Missouri district (Viburnum Trend and Old Lead Belt). The sphalerite samples from the Northern Arkansas district yield lower Pb isotope values (208Pb/204Pb between 39.4633 and 40.8863; 207Pb/204Pb between 15.8216 and 15.9176; 206Pb/204Pb between 20.2396 and 21.6438) than the Tri-State district ores and overlap the Southeast Missouri ores. The Northern Arkansas samples show a wide range of isotopic ratios with a linear trend, suggesting mixing of two end-member components. Age-corrected (250 Ma) Pb isotope ratios of Devonian-Mississippian Chattanooga shales from the Arkoma basin analyzed in this study plot close to the non-radiogenic end of the Northern Arkansas ores, implying that the shales may represent the non-radiogenic end-member that contributed Pb to the ores. The slope of the linear Pb isotope trend for the Northern Arkansas ores corresponds to an age of about 1.23 Ga, suggesting that the radiogenic Precambrian igneous basement may have supplied some of the ore Pb. Therefore, current data may indicate that the linear trend defined by Northern Arkansas ores may be the result of mixing of two end-members: the radiogenic basement rocks and the non-radiogenic shales.Pb isotope values of sphalerite from the Burkesville deposit are lower (208Pb/204Pb between 39.294 and 39.583; 207Pb/204Pb between 15.709 and 15.806; 206Pb/204Pb between 19.639 and 19.803) than those from the Northern Arkansas and Tri-State districts. The Pb isotope ratios of the Burkesville ores are similar to the Pb isotope values of the Central Tennessee (Elmwood deposit) and East Tennessee (Young deposit) ores and show evidence, the first of its kind for the region, of fluid mixing between the Illinois and Appalachian basins along the Cincinnati Arch that divides them.On the 208Pb/204Pb vs. 206Pb/204Pb diagram, the Pb isotope values of MVT ore samples from districts within the mid-continent US show two collinear, parallel trends, suggesting a different Th/U trend: (1) Tri-State and Central Missouri ore samples are collinear with Southeast Missouri and Northern Arkansas district samples; (2) Upper Mississippi Valley and Illinois-Kentucky ores are collinear with the Central Appalachian, Southern Appalachian (including East Tennessee), Central Tennessee, and Burkesville (Central Kentucky) ores.