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

Oxygen and carbon stable isotope analyses along with fluid inclusion microthermometry are used to investigate the paleofluid system and deformation conditions operating during Late Paleozoic Alleghenian deformation. Vein mineral samples were collected from major carbonate units across the Valley and Ridge province, including the Devonian Helderberg Fm., Silurian Tonoloway Fm., and Ordovician Nittany Fm.

In all formations, dominant vein sets strike NNW (340° – 350°) and ENE (080° – 085°), NW (315° – 335°) and NE (020° – 030°), and WNW (270° – 290°). Most veins are mineralized with blocky calcite along with uncommon blocky dolomite. Preliminary fluid inclusion data from the Tonoloway Fm. indicate fluid trapping at between 99° to 113°C. Fluid salinity is 19.5 to 23.7 wt. % equiv. Carbonate host rock δ¹⁸O values are between 21.6 and 24.5 ‰ V-SMOW and δ¹³C values are between -0.8 and 4.4 ‰ V-PDB. In general, vein isotopic values are slightly to significantly different form host rock values, indicating open fluid systems. Several sites in particular have wide differences. On the western margin of the Valley and Ridge, two Tonoloway Fm. sites have multiple vein sets that are 2-5 ‰ lower carbon and oxygen values than the host rock. Another site on the western flank of the Friend's Cove anticline has ENE vein carbon values 7 ‰ lower than the host rock. On the other hand, a sample from the Helderberg Fm. in the southern part of the study area has NE, NNW, and fault-related veins with values that are 1.5 ‰ higher in oxygen and 1.5 ‰ lower in carbon. Other Tonoloway Fm. and Nittany Fm. Sites have vein isotopic values that are similar to the host rock indicating closed fluid systems. Whether a fluid system is open or closed depends upon the interconnectivity of the fracture network during deformation.