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 cements that include ferroan, coarse sparry calcite and ferroan baroque dolomite. Platform-interior rocks contain two late cements (clear, sparry calcite and nonferroan dolomite) not observed in platform-margin rocks. Depleted 18 O and high iron content of these four calcite and dolomite phases indicate a fluid that had interacted with siliciclastic material. Late-stage diagenetic cements in the platform-margin limestones generally have more lower values of delta 18 O, higher iron and manganese contents, and more radiogenic 87 Sr than their counterparts from the platform. Multiple interpretations for the different diagenetic phases and geochemistry are possible. We favor a water-rock interaction hypothesis because it requires a single source and is therefore the simplest explanation. Ferroan, coarse sparry calcite and ferroan baroque dolomite could have been generated from brines expelled from the Sevier Basin before the end of the Middle Ordovician (ca. 450 Ma). These fluids could have moved up dip into platform carbonates under pressures induced by burial alone. This is based on an analysis of burial curves, possible temperatures, and an assumed range of likely brine compositions. Bitumen associated with some ferroan baroque dolomite may be synchronous with Mississippi Valley-type, main ore-stage sphalerite. The timing for fluid migration that we suggest here contrasts with conventional thinking, which places the major fluid migration during the Alleghenian orogeny (332-250 Ma) at the end of the late Paleozoic.