Gold remobilization associated with Mississippi Valley-type fluids: A Pb isotope perspective

Abstract

Gold deposits are often observed to have complex histories with multiple mineralization and remobilization events. Lead (Pb) isotopes have been useful in identifying the source and timing of metals during the initial or "primary" mineralization in these deposits; however, the source and timing of metals associated with Pb that has more radiogenic isotope ratios as compared to the primary Pb are less definitive. Here, we used Pb isotope analyses, obtained by whole-rock, microdrilled, and in situ sampling techniques, coupled with detailed petrography to identify distinct fluid histories associated with gold mineralization in the Reef Deposit, which occurs in Penokean-age (1.8 Ga) rocks in north-central Wisconsin. Early Pb-rich minerals, encased in pyrite (as is some gold), have nonradiogenic Pb isotope ratios that match the Pb isotope composition of galena from volcanogenic massive sulfide deposits that occur locally north of the Reef Deposit. In contrast, late Pb-rich minerals, intergrown with gold, have highly radiogenic Pb isotope ratios that closely match those of galena from Paleozoic carbonate rocks in the region. A low-temperature brine fluid was found in fluid inclusion assemblages in late crosscutting carbonate veinlets. Based on these data, we infer that initial gold mineralization in the Reef Deposit was produced during the Penokean orogeny (ca. 1.8 Ga) and was subsequently overprinted by fluids similar to Mississippi Valley-type mineralization during the Permian (ca. 270 Ma). This temporal disparity between primary mineralization and remobilization, as identified by Pb isotopes, has important implications for our understanding of gold and other ore deposits. This study should prove useful to guide future Pb isotope investigations in various disciplines.