Diagenesis by burial fluids, Middle Ordovician platform to platform-marginlimestones, East Tennessee: Relationship to Mississippi valley-type deposits

We compare relative timing and geochemistry of different paragenetic phases in platform-margin and platform-interior limestones within a nearly 400-m- thick succession of Middle Ordovician strata, Emphasis is on late-stage cem ents that include ferroan, coarse sparry calcite and ferroan baroque dolomi te, Platform-interior rocks contain two late cements (clear, sparry calcite and nonferroan dolomite) not observed in platform-margin racks. Depleted O -18 and high iron content of these four calcite and dolomite phases indicat e a fluid that had interacted with siliciclastic material. Late-stage diage netic cements in the platform-margin limestones generally have more lower v alues of delta(18)O, higher iron and manganese contents, and more radiogeni c Sr-87 than their counterparts from the platform. Multiple interpretations for the different diagenetic phases and geochemist ry are possible, We favor a water-rock interaction hypothesis because it re quires a single source and is therefore the simplest explanation. Ferroan, coarse sparry calcite and ferroan baroque dolomite could have been generate d from brines expelled from the Sevier Basin before the end of the Middle O rdovician (ca, 450 Ma). These fluids could have moved up dip into platform carbonates under pressures induced by burial alone, This is based on an ana lysis of burial curves, possible temperatures, and an assumed range of like ly brine compositions. Bitumen associated with some ferroan baroque dolomit e may be synchronous with Mississippi Valley-type, main ore-stage sphalerit e, The timing for fluid migration that we suggest here contrasts with conve ntional thinking, which places the major fluid migration during the Alleghe nian orogeny (332-250 Ma) at the end of the late Paleozoic.