The significance of Rb-Sr glauconite ages, Bonneterre Formation, Missouri; late Devonian-early Mississippian brine migration in the Midcontinent

Rb-Sr mean model ages of 370 ?? 10 Ma for glauconites from the Cambrian lower Bonneterre Formation in southern and central Missouri (in contrast to mean model ages of 389 ?? 7 Ma from the middle and upper Bonneterre) are in excellent agreement with a Rb-Sr isochron age of 359 ?? 22 Ma for glauconites from the Magmont mine (Viburnum Trend) in the southeast Missouri lead district. The lower Bonneterre and Magmont mine ages probably reflect isotopic resetting, most likely associated with dolomitization and/or Mississippi Valley-type ore formation in the southeast Missouri lead district. The temporal relation between widespread dolomitization and ore formation in Missouri is unclear, but mounting evidence for two Devonian disturbances permits the following interpretation: 1) a Devonian timing (380-400 Ma) for widespread dolomitization, and 2) a Late Devonian-Early Mississippian timing (360-370 Ma) for ore formation in southeast Missouri. -from Authors