STABLE ISOTOPE GEOCHEMISTRY OF CALCITE AND DOLOMITE VEINS FROM SILURIAN AND DEVONIAN LIMESTONES IN THE WESTERN VALLEY AND RIDGE PROVINCE OF PENNSYLVANIA

Stable isotope and fluid inclusion microthermometry provide insight on the paleo-fluid systems and associated fluid fracture interconnectivity during deformation. 15 host rock samples and 54 vein samples were collected from Silurian and Devonian limestones from the Wills Mountain anticline and southern Nittany anticline of the western Valley and Ridge province of Pennsylvania.

The veins are 1 - 10 mm thick and consist of one or more stages of blocky calcite and blocky dolomite. Vein orientations include three sets of cross-fold veins and three sets of nearly strike-parallel veins. Thirteen host rock samples from the Devonian Helderberg Formation have δ¹⁸O values of -11.2 to -5.8 ‰ and δ¹³C values of -1.6 to 3.8 ‰; while two samples from the Tonoloway Formation have δ¹⁸O values of -9.4 to -5.1 ‰ and δ¹³C values of -1.8 to -0.9 ‰. The δ¹³C values are in the range of Phanerozoic marine limestones, while the δ¹⁸O values are lower.

Several end member relationships exist between the vein data and host rock data. Where vein isotopic values are close to the host rock, it suggests a closed fluid system with low fracture connectivity; while vein isotopic values that are far from the host rock suggest an open fluid system with high fracture connectivity. Most sites exhibit some open system behavior. For sites around the Nittany anticline, veins tend to have lower δ¹⁸O and δ¹³C values compared to the host rock, which may be due to fold-related fracturing. δ¹⁸O values are 1 to 4 ‰ less than the host rock while δ¹³C values are 0 to 2 ‰ less. One sample indicated a closed fluid system. Sites along the Wills Mountain Anticline have similar values, except for two sites that have markedly lower δ¹³C values that are up to 8 ‰ less than the host rock. Two sites are nearly completely closed systems. Fluid mixing may have occurred with fluids from adjacent formations.

The homogenization temperatures from fluid inclusion microthermometry indicate fluid temperatures of 104 to 158 °C, with salinities of 19.8 to 24.0 wt. % NaCl equiv. The extent of ¹⁸O fractionation between water and calcite is temperature dependent. Calculating the fluid oxygen isotopic values in equilibrium with the calcite give δ¹⁸O values that range from +5.0 to +11.5 ‰ (V-SMOW). This indicates basinal brines are the source of the fluids that precipitated the vein calcite.