EXAMINATION OF CRYSTAL GROWTH AND FLUID HISTORY IN HYDROTHERMAL QUARTZ VEINS USING VEIN PETROGRAPHY, FLUID-INCLUSION MICROTHERMOMETRY, AND SEM-CL: EXAMPLES FROM THE PENNSYLVANIA VALLEY AND RIDGE PROVINCE

The growth of quartz crystals in hydrothermal veins can be a complicated process, with multiple growth phases, zones, and textures, as well as dissolution events that reflect multiple fluid flow events and changes in pressure, temperature, and chemical conditions that occurred during mineral growth. Multiple methods of analysis must be combined to best understand the history of the vein’s growth.

In this study, bedding and joint orientation data, and vein samples were collected from the Pine Grove syncline at the boundary between the Valley and Ridge and Great Valley provinces, eastern Pennsylvania. This structure is a thrusted frontal syncline that has been uplifted approximately 10 km, with an average bedding trend of 060° indicating shortening directed toward 330°. Joint and vein orientations are best grouped upon rotation to bedding horizontal, and show four groups based on strike. A nearly orthogonal 045°±10° set and a 305°±12° set, and a second nearly orthogonal 074°±10° set and a 338°±10° set may be related to an early pre-folding shortening events.

The 074°±10° joint set contains the most well developed veins, including some over 1 cm thick. Petrography of the thick sections made from the vein samples showed that all of the veins are composed of blocky quartz and elongate blocky quartz. The thick sections were then examined using SEM-Cathodoluminescence (SEM-CL) to differentiate the growth zones in the quartz by looking at variation in zones of heavy and light elements, which display as different shades of gray in SEM-CL. Most of the veins were brecciated along the margin of the host rock and then rehealed. In addition, some veins show several cycles of growth then brecciation and/or dissolution near the host rock before an area of new growth formed in the middle of the vein. The veins commonly contained crystal growth zones with anhedral light gray luminescence within a medium gray luminescent subhedral to euhedral zone. These zones are occasionally surrounded by a dark gray luminescent euhedral growth zone. Many fluid inclusions in these veins are decrepitated with a ragged morphology, suggesting rapid decompression during uplift. These observations indicate that numerous separate periods of quartz growth occurred before and during the uplift of these rocks.