LEAD ISOTOPE COMPOSITIONS OF CHEMICALLY SEPARATED SHALE COMPONENTS

Due to their metal-rich and/or organic-rich nature, certain black shale units in the mid-continent US have been often linked to hydrocarbon occurrences and ore fields in the region. Shale is composed of inorganics, extractable organics (bitumen), and ‘non-extractable’ organics (kerogen). This work is an attempt to separate these three fractions from shales sampled from the mid-continent region and analyze the Pb isotope composition of each fraction. All the fractions of Pennsylvanian-age shales from the Forest City and Cherokee basins show relatively low radiogenic signatures. However, kerogen and/or inorganic components of certain shale units from the Ouachita mountains and the Ozarks are highly radiogenic (e.g. Chattanooga shale-inorganic: ²⁰⁶Pb/²⁰⁴Pb 34.00, Fayetteville shale-kerogen: ²⁰⁶Pb/²⁰⁴Pb 20.69, Womble shale-inorganic: ²⁰⁶Pb/²⁰⁴Pb 21.27), suggesting a potential genetic relationship with the Mississippi Valley-type (MVT) Pb-Zn ores. Overall, the bitumen component tends to be the least radiogenic of all three fractions (Ozark: ²⁰⁶Pb/²⁰⁴Pb: 18.34-18.77, Ouachita: ²⁰⁶Pb/²⁰⁴Pb 18.56-19.79, Pennsylvanian: ²⁰⁶Pb/²⁰⁴Pb 18.62-19.42). The Pb isotope compositions of the various fractions of the same rock are different, and the bulk-rock isotope composition is often not representative of all the fractions. Therefore, characterizing the various fractions is essential in evaluating potential rock units that contributed metals to the MVT ores.