Strontium isotopic geochemistry of Mississippi Valley-type deposits, East Tennesse: Implications for age and source of mineralizing brines

Strontium isotopic ratios of wall-rock, ore, and gangue minerals from the Mascot-Jefferson City, Copper Ridge, and Sweetwater Mississippi Valley-type (MVT) districts and from the Lost Creek barite deposit in East Tennessee were measured in an effort to determine the age of this mineralization and its relation to the tectonic evolution of the Appalachian orogen. The Lower Ordovician Knox Group, which hosts the MVT mineralization, contains limestones and dolomite karst breccias with ⁸⁷Sr/⁸⁶Sr ratios of 0.70900 to 0.70916 that are similar to those of Early Ordovician sea water. Primary (diagenetic) Knox dolomite, as well as "recrystal-line" dolomite, which formed by later alteration of limestone, exhibits ⁸⁷Sr/⁸⁶Sr ratios of 0.70930 to 0.71024, reflecting varying degrees of contamination by solutions with more radiogenic strontium. ⁸⁷Sr/⁸⁶Sr isotopic ratios of mineralization in the districts increase in the order Lost Creek, Sweetwater, Copper Ridge, Mascot-Jefferson City and extend to values significantly higher than those of the enclosing rocks.

Of the three possible source basins for the MVT brines, the Late Proterozoic Ocoee, Cambrian Luttrell, and Ordovician Sevier basins, the Sevier basin appears to be the only one that could have supplied brines of the appropriate composition at geologically reasonable times. Estimates of the change with time in the isotopic composition of Sevier basin brines indicate that the maximum ages of the Lost Creek, Sweetwater, Copper Ridge, and Mascot-Jefferson City mineralization are 520, 460, 405, and 395 m.y., respectively. Comparison of fluid-inclusion temperatures for the Sweetwater and Mascot-Jefferson City minerlization with the thermal evolution curve for the Sevier basin supports these estimates. These ages suggest that the Lost Creek barite minerlization formed as an exhaltive deposit in the Sevier basin and that the Sweetwater fluorite-barite mineralization formed during the later part of Taconic orogeny. The Copper Ridge and Mascot-Jefferson City minerlization could have formed either prior to or during the Alleghanian orogeny (but prior to deformation of the enclosing rocks), depending on the rate of istopic evolution of the Sevier basin brines.