Iodine-129, ⁸⁷Sr/⁸⁶Sr, and trace elemental geochemistry of northern Appalachian Basin brines: Evidence for basinal-scale fluid migration and clay mineral diagenesis

Abstract

Evidence for basin scale brine migration and clay mineral diagenesis in the northern Appalachian Basin was investigated using elemental and isotope (¹²⁹I/I, ⁸⁷Sr/⁸⁶Sr) geochemistry of formation waters collected from the Middle to Upper Devonian section of the northern basin margin in western New York, northwest Pennsylvania, and eastern Kentucky. One sample from each of the Mississippian Berea sandstone and the Silurian Medina sandstone were analyzed for comparison. Measured iodine ratios range between 28 to 1,890 × 10⁻¹⁵ and are anomalously high compared to cosmogenic iodine sourced from Devonian age organic matter. Iodine-129 in the waters was largely derived from fissiogenic sources, the spontaneous fission of ²³⁸U to produce ¹²⁹I, with estimated ¹²⁹I/I values up to 270 × 10⁻¹⁵, which occur locally in the organic-rich shales. There are three water samples that have values of 490 × 10⁻¹⁵, 860 × 10⁻¹⁵, and 1,890 × 10⁻¹⁵, which are above the range for local fissiogenic ¹²⁹I and may be accounted for by topographically driven, basin scale fluid flow through a regionally high fissiogenic source. Relatively large uranium occurrences lie along the structural front of the Appalachian Basin in the Blue Ridge Province and are situated within hypothesized flow paths parallel to the main compressional direction of the Alleghanian orogeny. Estimated ¹²⁹I/I values for these uranium occurrences are in excess of 55,000 × 10⁻¹⁵. The strontium isotope composition and Sr concentration of brines display a mixing trend between a highly radiogenic end-member (0.7210) with low Sr (51 mg/L) and a non-radiogenic (0.7100), high Sr (4789 mg/L) end-member. Potassium and boron concentrations are notably depleted relative to evaporated Paleozoic seawater, the hypothesized source of Appalachian Basin brines. The K/Rb values of formation waters are depleted relative to seawater values, but in some cases are well above values indicative of water-rock reactions. The Sr isotopic composition, K and B depletion, and intermediate K/Rb ratios are consistent with smectite diagenesis and paleo-temperatures that are likely greater than approximately 150 °C. These temperatures may be high given the burial history of the study area and support the flow of formation waters from deeper within the basin. The combined isotopic and elemental results of formation waters provide compelling evidence for basin scale fluid migration in the northern Appalachian Basin and are consistent with previously published evidence documented from the rock record, including clay mineral diagenesis and ore deposition.