A LEAD-ISOTOPIC STUDY OF GALENA FROM EASTERN WISCONSIN: EVIDENCE FOR LEAD SOURCES IN PRECAMBRIAN BASEMENT ROCKS

Galena and other Mississippi Valley-type (MVT) minerals are found in Paleozoic sedimentary rocks throughout Wisconsin, northern Illinois, and northeastern Iowa, including areas well away from the Upper Mississippi Valley ore district. The MVT mineralization, hydrothermal dolomitization, and K-silicate mineralization in the region were the result of at least two hydrothermal events during the middle and late Paleozoic Era. In eastern Wisconsin, petrographic and fluid-inclusion data reveal a history of water-rock interaction with aqueous Na-Ca-Mg-Cl-H2O brines (13 - 28 wt. %, NaCl equivalent) between 65 and 120°C.

Here we report the first lead (Pb)-isotopic data for galena from eastern Wisconsin and have placed these data into a regional context of MVT mineralization along with data from the entire Upper Mississippi Valley region. An unusually well developed, regionally mappable Pb-isotopic pattern is evident, with the highest lead ratios occurring in east-central Wisconsin. The junction between the Penokean and Eastern Granite-Rhyolite provinces occurs in basement rocks throughout the central part of the study area. Pb-isotopic compositions of galena in Paleozoic rocks increase from 38.24 to 52.36 for ²⁰⁸Pb/²⁰⁴Pb and from 18.29 to 34.70 for ²⁰⁶Pb/²⁰⁴Pb across the region, with a larger isotopic range present over the Penokean province. These results indicate the Pb-isotopic composition of ore-stage fluids in the region was influenced by interaction with Precambrian basement rocks, instead of solely by leaching of metals from Paleozoic aquifer sediments during fluid expulsion from adjacent sedimentary basins. This interpretation is consistent with previous interpretations that lead was leached from Precambrian basement rocks between 400 and 250 Ma. The requirement for a substantial exchange of fluids with Precambrian basement rocks has important implications for diagenetic models and heat flow constraints placed on hydrogeologic models used to explain MVT ore deposition in the region.