INVESTIGATING THE FLUID HISTORY OF QUARTZ VEINS USING PETROGRAPHY, SEM-CL, AND FLUID INCLUSION MICROTHERMOMETRY: EXAMPLES FROM THE PENNSYLVANIA VALLEY AND RIDGE PROVINCE

This study focuses on the morphology and fluid history of quartz veins from the central Pennsylvania Valley and Ridge province. We integrate vein petrography, SEM-CL data, structural orientation, and fluid inclusion microthermometry data to develop an understanding of the vein formation conditions and how these relate to the deformation history of the area.

Nearly all veins examined were cross-fold veins that have elongate-blocky crystals or blocky to euhedral crystals. Some veins have multiple morphologies due to multiple mineralization events. The SEM-CL response is integral in providing insight into the vein origin. Several types of responses were found. Some samples display little to no color variation in SEM-CL and are monotonously medium gray (typically elongate blocky veins). Other samples display early light gray to bright white areas concentrated near the host rock and significantly darker gray secondary crystals in the center of the vein, where bands of zoning can commonly be seen within them. A third response is found in samples that have an early dark gray area surrounded by a bright white rim.

Those samples with a uniform light gray response displayed little variation in microthermometric measurements. For example, along one crystal fiber in one sample, two-phase T_h values have a range of 123.6 to 132.4 °C. However, ice melting temperatures (T_mIce) varied along the length of the quartz fiber, indicating a change in salinity. The older material, closest to the host, had T_mIce values suggesting the fluids had a higher salinity than the younger material, closest to the tip of the fiber. In veins with both light and dark areas, the lighter quartz has single-phase homogenization with low temperatures (T_h) in the range of -95.7 to -107.4 °C, indicating burial depths of >~6.0 km. The darker quartz has single-phase T_h values in the range of -82.3 to -97.1 °C, indicating burial depths of ~3.0 – 4.0 km.

The data suggest that variations in SEM-CL response are correlated to variations in PT conditions and fluid composition during formation. In samples with noticeable variations in SEM-CL, results suggest that quartz precipitation occurred during protracted periods of either uplift and/or burial. Further research will include examining samples with significant zoning seen in SEM-CL with respect to trace element analysis.