EVIDENCE FOR BASINAL-BRINE MIGRATION AND WATER ROCK INTERACTIONS FROM 129I/I, 87SR/86SR AND TRACE METAL GEOCHEMISTRY: NORTHERN APPALACHIAN BASIN

Tectonically- and topographically-driven basin scale brine migration has been demonstrated in the Appalachian Basin (AB) through numerous studies of ore-deposits, fluid inclusions, and clay mineral assemblages. This study provides further evidence of basin-scale fluid and solute transport from brine geochemistry across the northern basin margin. Iodine (¹²⁹I/I) and Sr (⁸⁷Sr/⁸⁶Sr) isotope ratios are combined with elemental (major, minor, trace) geochemistry to distinguish fluid flow paths and water-rock reactions. Fifty one brine samples were collected from active oil and gas wells producing from Mississippian, Devonian, and Silurian age formations at the shallow AB margin and analyzed for the elemental content. A subset of ten samples was analyzed for ¹²⁹I/I and ⁸⁷Sr/⁸⁶Sr.

Measured iodine ratios (28-1,890x10^-15) of AB brine samples are anomalously high compared to cosmogenic iodine sourced from Devonian age organic matter. A fraction of the elevated ¹²⁹I/I can be explained by fissiogenic ¹²⁹I/I (80-270x10^-15) locally sourced from shales. A regional source of fissiogenic ¹²⁹I/I (<17,000x10^-15) lies along a flow path parallel to the main compressional direction of the Alleghanian orogeny and may account for the high measured ¹²⁹I/I. Strontium concentrations and isotope ratios of shallow brines display a mixing trend between a highly radiogenic end-member with low Sr (0.7210, 51 mg/L) and a low ratio, high Sr (0.7100, 4789 mg/L) end-member. The highly radiogenic Sr may result from clay and/or feldspar diagenesis by fluids expelled from depth. The maximum Ba concentration in brine from the Silurian section (carbonates) is 2.73 mg/L, while the maximum Ba concentration in brine from the Devonian section (organic-rich shales) is 1,204 mg/L. The divergence in dissolved Ba may be controlled by lithology and is consistent with fluid flow within each section. Maximum concentrations for Fe, Mn, Cu, Pb, and Zn are 1005, 75.5, 1.03, 44.59, and 5.1 mg/L, respectively, above the 1 mg/L boundary defined for ore-forming fluids. Additionally, metal concentrations increase with increasing chloride concentration. The combined results indicate saline waters may have migrated from depth within each section and that diagenetic reactions along flow paths may have increased the dissolved trace metal concentration of AB brines.