Lead isotope studies of Precambrian basement in the midcontinent United States: Implications for metal sources in Mississippi Valley-type ores

Fifty-three basement samples from the Central Plains Orogen (1.65-1.8 Ga) and Southern Granite-Rhyolite Province (1.3-1.4 Ga) have been analyzed for Pb isotope ratios and for U, Th, and Pb concentrations to re-evaluate the possibility of a basement Pb source for Mississippi Valley-type (MVT) ores throughout the greater midcontinent region and to develop a schematic model pertaining to the ore-forming mechanisms. Current results for the Granite-Rhyolite Province basement rocks are all collinear with the array defined by published data and show higher Pb isotope ratios. This suggests mixing between a more depleted source, potentially a reservoir in the upper mantle, and a more enriched source that requires a crustal/sediment contribution. The Pb isotope ratios of the Central Plains Orogen rocks plot between those of the Granite-Rhyolite Province, indicating source(s) of intermediate enrichment. An important finding of this study is that several basement samples that yield highly radiogenic Pb isotope ratios ( 208 Pb/ 204 Pb between 40.534 and 45.810) originate from shallower depths (between around 650 m and 1050 m), and those that yield low Pb isotope ratios ( 208 Pb/ 204 Pb between 37.171 and 37.860) originate from deeper depths (between around 2200 m and 3800 m). While the current results agree with the description of the midcontinent basement being heterogeneous, there are likely specific, localized features (e.g., younger intrusion or a basement high) that impart certain rocks their unique, highly radiogenic Pb signatures. The samples that display the high Pb isotope ratios plot near the radiogenic end-member of the MVT ore array. The Pb isotope patterns noticed in basement rocks originating from various levels, together with available Pb isotopes and fluid inclusions data in the MVT ores, support the interpretation that metal-rich fluids emanating from local, shallower, more radiogenic basement rocks migrated along deep-seated faults directly to deposition zones, without mixing with basin-derived fluids (e.g., Northern Arkansas District). Other basement-derived fluids emanating from deeper, less radiogenic basement rocks might have migrated upward along deep-seated faults, then laterally, potentially mixing with basin-derived fluids enriched in radiogenic Pb and contributing to isotopic diversity (e.g., Tri-State and Southeast Missouri districts). The possible presence of unique features within the basement mentioned above may explain why the 1.05 Ga and 1.01 Ga model Pb source ages for midcontinent ores are younger than the ages of the Granite Rhyolite Province and the Central Plains Orogen.