STRUCTURAL AND GEOCHEMICAL ANALYSIS OF VEINS FROM LIMESTONES IN THE WEST-CENTRAL VALLEY AND RIDGE PROVINCE OF PENNSYLVANIA

In order to gain insight on the interconnectivity of the fracture network and fluid system during deformation, oxygen and carbon stable isotope analyses are performed on Cambro-Ordovician and Siluro-Devonian age carbonate samples from the Valley and Ridge province of south-central and central Pennsylvania. Vein and host rock samples were collected from sites located within the major carbonate formations and veins are comprised of mainly blocky calcite with occasional blocky dolomite present. When comparing the δ¹⁸O and δ¹³C of the host rock to the isotope record of the Phanerozoic marine limestones the values match for the Ordovician and Devonian age limestones, however the Silurian age δ¹⁸O are much lower than indicated on the record.

The vein sets striking NW-SE (300° - 310°) and NNW-SSE (320° - 340°) in the Ordovician carbonates have δ¹⁸O values that are from 1.5 to over 3.0 per mil greater than the host rock, and δ¹³C values that are 1.0 to 3.0 per mil less than the host rock. Interestingly, in the Siluro-Devonian rocks, the trends are significantly different, with NW-SE veins and NE-SW veins having δ¹⁸O values that are from 1.5 to over 3.0 per mil greater than the host rock, and δ¹³C values that are either 1.0 to 3.0 per mil lesser or greater than the host rock. These data suggest open fluid network and fracture-fluid connectivity. Vein sets striking NE-SW (020° - 045°) and ENE-WSW (045° - 085°) have δ¹⁸O values and δ¹³C values that are similar to the host rocks, indicating relatively closed fluid systems.

Fractionation of oxygen isotopes between water and calcite occurs when carbonate minerals are precipitated and the calcite becomes enriched in ¹⁸O. However the extent of this fractionation is temperature dependent. Using fluid inclusion microthermometry the homogenization temperatures of the brine solution can be used to calculate the isotopic equilibrium values. Fluid inclusion homogenization values range from 104 to 158 °C. These give δ¹⁸O values that range from +7.0 to +11.5 per mil (V-SMOW). This indicates basinal brines are the source of the fluids that precipitated the vein calcite.