A Petrographic Study of Vein Mineralization within Paleozoic Rocks Units in the Central Pennsylvania Region

Monica McGrath (mcgramm09@juniata.edu), Larry Mutti, Department of Earth and Environmental Sciences, Juniata College, Huntingdon PA

High angle, cross cutting fractures found in Paleozoic rock outcrops in Central Pennsylvania are mineralized with quartz and/or calcite. Samples were collected from mineralized veins in several formations along the Tyrone-Mount Union Lineament, where these fractures are especially prevalent. Cathodoluminescence imaging of polished samples on the scanning electron microscope allowed for the recognition of multiple mineralization events with distinctive growth signatures. Calcite-only veins exhibited no recognizable growth features, suggesting a single, uniform calcite-precipitating event. Other material collected was dominated by quartz filled veins. Quartz-CL images, in contrast to calcite, show complex patterns. Most of the vein quartz grains showed quasi-concentric growth with alternating CL-bright and CL-dark bands foreshadowing final crystal morphology. Conditions during mineralization must have fluctuated as shown by the oscillatory CL patterns. There is also evidence of crack-seal textures, and refracturing and secondary mineralization. This complicates the story of simple progressive growth. Overgrowths of previously formed grains are seen along with asymmetric infilling of open fractures with quartz and other minerals. These patterns are distinct from those revealed in Curry's investigation of similar material from the Brallier Formation just east of Huntingdon (2011, 2012). Ongoing studies hope to define more clearly the mineralizing events and fluids that affected the Paleozoic rocks, as well as to geographically constrain the region of petroleum migration documented by Curry (2012) for the Braillier section

ABSTRACT

Fig. 3: (above) Predominantly quartz vein sample MU5. CL-bright and CL-dark bands alternate around grain centers. Overgrowth of grain boundaries and asymmetric infilling of fractures are evident.

Fig. 2: (above) Quartz vein sample MU-5. Crack-seal textures represent refracturing and infilling. Grains in the vein are distinct from those in the groundmass

Fig. 1: (on left) Quartz vein sample MP-4. This vein shows multiple fractures and fluid flow events. Parallel fracture traces can be seen.

•Samples were collected from outcrops located along Route 22 east and west of Huntingdon, PA. •Location, lithology, and strike and dip orientation of formations and veins were recorded. •Double polished thin sections were prepared and carbon coated for viewing on the JEOL 6460 SEM •SEI, CL, and BEC images were collected in transects across vein material. •CL images were gathered at 140x magnification in order to permit construction of composite images using the fewest number of raw pictures •SEI and BEC images were sometimes gathered at lower magnification to get a greater field of view image •Pictures were compiled as mosaics in Photoshop Adobe and compared to images from previous studies.

METHODS AND MATERIALS

INTRODUCTION

OBSERVATIONS

CONCLUSIONS

REFERENCES

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•Images of vein material vary depending on the mode of the SEM •Calcite CL images showed no growth patterns •Quartz CL images are complex showing quasi-concentric growth with alternating CL-bright and CL-dark bands •Quartz patterns show oscillatory patterns, refracture and infilling and crack-seal textures. •Detrital grains show different growth morphologies. Detrital grains are small, rounded, and show patterns relating to parent material, while vein quartz crystals are larger and intergrown. •Patterns differ from those observed by Curry (2011) in that they lack the prominent dissolution channels seen in that study.

Fig 4: (above) BEC picture of calcite vein. All calcite images were very uniform and showed no growth textures.

Fig. 6: (left) Images of calcite vein sample. Two veins are present in this sample.

Fig. 5: (far left) Image of quartz vein in red sandstone

Fig 8: Three images representing the three different modes of imaging. A) Shows a quartz vein and groundmass under CL; observe the differences in detritial and vein quartz. B) Shows the same field of view as A under SEI imaging. The distinction between vein and host rock is less clear. C) Shows the whole vein in BEC (back-scattered electron) view. This image is not the same field of view as A&B. The vein is clear, but growth patterns are not.

A B C Paleozoic rock outcrops in the Valley and Ridge Province of Central Pennsylvania were deformed during multiple orogenic events. This deformation caused the rock to be fractured numerous times that allowed the infilling and crystallization of fluid as mineral veins. Samples were collected along Route 22, both east and west of Huntington, PA, along a structure known as the Tyrone-Mount Union Lineament. The main Formations focused on are the Bald Eagle/Juniata and the Tuscarora. This study sought to extend geographically and stratigraphically the study undertaken by Curry (2011 ,2012) in order to see if the same growth patterns extended into other vein material. Also, it hopes to put physical constraints on the petroleum fluid flow across the Braillier Formation. Gathering CL images from vein material is the first step in extending Curry’s project. Using the growth patterns gathered from the CL and comparing them to the images collected from Curry (2011) will allow for the distinction of separate or the same growth events.

The quartz material showed multiple veining events that included refracturing and infilling by multiple fluids. Calcite crystals collected from rocks lower in the stratigraphic section recorded only one fluid flow event. Vein quartz displayed growth patterns that differed greatly from those observed in the Brallier (Curry 2011). This suggests that there were multiple and distinct veining events that affected the Tyrone-Mt. Union Lineament. Future study will extend this research geographically and stratigraphically to characterize these different veins events. A fluid inclusion study is also planned to define the type of fluids that flowed through these vein systems. It is important to complete this study to help constrain the extent of the petroleum fluid movement as well as to clarify the conditions affecting the Paleozoic rocks subsequent to their deposition.

Fig. 7: (left) CL images of quartz from Curry (2011). A) Shows repetitive light-dark banding and major growth that paralleled the crystallographic faces. B) Shows a dissolution channel, a commonly observed texture in Curry’s 2011 study which is absent in the quartz in this investigation

A

B B