ALLEGHENIAN AGE OF THE UPPER MISSISSIPPI VALLEY ZINC LEAD DEPOSIT DETERMINED BY RB-SR DATING OF SPHALERITE

MISSISSIPPI Valley type (MVT) ore deposits in North America are generally thought to have formed from warm brines which migrated from sedimentary basins to sites of deposition in sedimentary strata of the neighbouring craton long after lithification of the sediments 1-3. When and how these migrations occurred is not well understood, but recent geological evidence suggests that continental-scale brine migration can be triggered by tectonic collisions 4-6. Indirect evidence 7-9 has suggested an association of North American MVT mineralization with the Late Palaeozoic Alleghenian/Ouachita orogeny; recently, however, Nakai et al. 10 obtained a Rb-Sr age of 377 +/- 29 Myr for sphalerites from the East Tennessee MVT district, which they linked to the Middle Palaeozoic Acadian orogeny. Here we report Rb-Sr dates for sphalerites from the MVT 'type section'- the Upper Mississippi Valley (UMV) zinc-lead district. Rb-Sr data from two distinct sphalerite bands from the West Hayden ore body yield two independent but indistinguishable isochrons of Permian age (269 +/- 6 Myr and 270 +/- 4 Myr), consistent with a brine migration episode initiated by the Alleghenian/Ouachita orogeny. In conjunction with the previous data indicating a link to the Acadian orogeny 10,11, our results indicate that North American MVT deposits formed in at least two distinct tectonic episodes.