Clumped isotope geothermometry of Devonian brachiopods and cements from Iowa: Implications for emplacement of Upper Mississippi Valley Zn-Pb district

Understanding the thermal history of Mississippi Valley-type (MVT) ore deposits, such as the Upper Mississippi Valley (UMV) Zinc-Lead (Zn-Pb) district, is essential to understanding the complex interplay of tectonic, geochemical, and hydrothermal processes during mineralization. Prior investigations into the UMV district thermal history have yielded divergent findings. Fluid inclusions and biomarker thermal data propose a short heating event (~200 kyr), whereas paleomagnetic studies indicate prolonged heating over several million years. This difference is crucial to understanding the mineralization processes within the UMV district. Our study applies clumped isotope (Δ47) thermometry of articulated brachiopod shells and internal macropore calcite cements from north-central and southeastern Iowa to determine the UMV district thermal history. North-central Iowa brachiopods show warm but not unusual Δ47 temperatures (T(Δ47); 31-37°C), while the macropore cement shows a T(Δ47) of 70°C. In contrast, southeastern Iowa brachiopods yield unusually high T(Δ47) values (40-49°C), while macropore cements range from 46 to 77°C. The low T(Δ47) of northern Iowa brachiopods indicates minimal thermal alteration, consistent with their burial history, whereas southeastern Iowa brachiopod shells, located closer to the UMV district, show evidence of partial Δ47 reordering, indicating exposure to higher temperatures. Reordering kinetics sensitivity tests yielded a best-fit heating duration of 3.7 Myr at approximately 137°C, aligning with the prolonged basin-wide heating indicated by paleomagnetic data but contrasting with shorter timescales inferred from fluid inclusions and biomarkers. This discrepancy implies that different methods record the region’s thermal history at different scales: prolonged heating of the host carbonates by hot basinal fluids, versus short-term heating associated with discrete hydrothermal pulses that mineralized specific ore deposits.