Presentation on theme: "Methods Field Sampling Samples were collected from the Bald Eagle portion of the outcrop about 10 feet to the NE of the major fault (380 feet from the."— Presentation transcript:
Methods Field Sampling Samples were collected from the Bald Eagle portion of the outcrop about 10 feet to the NE of the major fault (380 feet from the bridge shown in Figure 3) (Fault seen in Figure 4). They were preferentially picked for the presence of vein quartz with pyrite. (See Figure 5) Rock Sample Preparation I prepared 12 doubly polished thin sections. For fluid inclusion work, the thin slices of rock were dissolved off the glass slide. Analyses The polished rock chips were placed in a Fluid Inc.- adapted USGS style gas-flow heating and cooling stage and observed with an Olympus BX 51 microscope. Using a Fluid-Inc, Incorporated Trendicator, fluid inclusions were analyzed for homogenization and melting temperatures using a cycling technique. Some of the slides and rock samples were carbon coated and analyzed on a JEOL 6460 Scanning Electron Microscope to look at crystal morphology. One of the slides was analyzed on a Gatan PanaCL Cathodoluminescence Imager mounted on the SEM to determine growth histories. A number of samples were analyzed on a Scintag PAD-V powder X-ray diffractometer. Quartz Sulfide Mineralization of the Bald Eagle Formation of Skytop Mountain, Near State College, Pennsylvania Theresa Detrie, firstname.lastname@example.org, Larry Mutti, Ryan Mathurdetrita1@juniata.edu Juniata College Geology Dept, Huntingdon, PA 16652 Abstract The construction of I-99 in west central Pennsylvania recently opened a gash in Skytop Mountain that PennDOT wishes would have remained closed. Excavation there intersected a two hundred meter zone in which the Ordovician Bald Eagle Formation is laced with fracture-filled veins heavily mineralized with hydrothermal sulfide. Veins range in width from millimeter scale to several centimeters and are dominated by highly reactive pyrite, accompanied by quartz and minor sphalerite and galena. Surface runoff readily penetrates the fractures, producing substantial acid run-off and concern over acid and heavy metal contamination of surface and ground water. SEM images reveal pyrite in cubic and dodecahedral habit as well as distinctive thin tablets and needles. XRD analyses confirm that all of these forms have the pyrite crystal structure. Fluid inclusion homogenization and freezing studies were performed on the hydrothermal quartz. Most inclusions are too small (<2 m) for investigation. Sparse, larger inclusions, 3- 10 m, often occur as singlets, and are hard to treat as members of fluid inclusion assemblages. Most are 2-phase aqueous inclusions, and none bear daughter crystals. Microthermometry records a complex mineralization history, and indicates maximum temperatures higher than anticipated. Homogenization temperatures, T h, which record minimum entrapment temperature, range from 140 o C to greater than 375 o C, with most between 180-350 o C. No petrographic evidence exists to suggest heterogeneous entrapment. Pressure corrections are forthcoming, as are CL studies to clarify entrapment chronologies. Fluid salinities are also highly variable. The temperature of final ice melting T m ranges from -6.0 o C to -26 o C. Visible onset of melting in the latter samples occurred near -40 o C. These data indicate total salinities ranging from as little as 9.2% to as least as high as 25%. Salinities cluster at a number of values across this range. The data demonstrate that the solutions are not all simple NaCl brines but include a substantial divalent component, probably Ca. These studies point to the need for more thorough investigation of the thermal and hydrothermal history of rocks at the western margin of the Valley and Ridge in central PA. Introduction With its alternating karst limestone and shale valleys and sandstone topped ridges, the Valley and Ridge province overruns Central and Eastern Pennsylvania. The Bald Eagle Mountain stands prominently as the last ridge before reaching the Allegheny Front to the west. The Pennsylvania Department of Transportation (affectionately called PennDOT) is in the process of building a connection from I-70/I-76 in the south to I-80, the two the main highways that transect Pennsylvania east to west. Currently this connection, I-99, crosses many of the valleys and ridges and is under construction to meet at its final destination. During the construction of I-99, PennDOT contractors were cutting though the Bald Eagle Formation on Skytop Mountain (on the Bald Eagle Ridge) when they suddenly encountered a lot more pyrite than originally anticipated. Pyrite, FeS2, in high amounts can cause serious problems for nearby waterways due to the high iron and sulfur content. The acid rock drainage slipped into the nearby high quality stream, Buffalo Run, causing red water, a drop in pH, and an uproar. The area of exposure sits along a bend in current Route 322 between State College and the town of Port Matilda. A map of this area can be seen in Figure 1. The presence of pyrite and other sulfides is not a new discovery in the area. There have been a number of theses from Penn State University over the past 30 years studying the sulfide deposits at various locations in the surrounding area. Recently, a number of faculty and staff from Penn State University have studied the circumstances behind the abnormally large pyrite deposit as well as basic mineralogy of the entire outcrop. These include David P. Gold, Andrew Sicree, and H. L. Barnes. In December 2004, there was a conference at Penn State, “A Conference on Principles of Acid Pollution Control Along Highways”, specifically designed to share information about the acid rock drainage at Skytop Mountain. This challenge has gathered people from all over Central Pennsylvania and beyond to help come up with a solution. General Geology Skytop Mountain can be found within the Valley and Ridge province of Pennsylvania. The formations making up the ridges date to the late Ordovician and Silurian. The stratigraphic column to the left, Figure 2, details the basic geologic units of Bald Eagle Ridge and surrounding ridges in Central Pennsylvania as described in The Geology of Pennsylvania. (Shultz, 1999) The Reedsville Formation consists of cross-bedded sandstones, limestone, and shales. The sandstone content increases upward. The Bald Eagle Formation includes non-marine, gray, cross-bedded sandstone and some conglomerate. The boundary between the Bald Eagle Formation and Juniata Formation is defined by the color change from gray to red. The Juniata Formation is a red cross-bedded sandstone with gray and red mudstones. Overall, a general fining upward can be seen. As the Juniata transitions into the Tuscarora from the deep red color to a white sandstone, so does the Ordovician into the Silurian. The Tuscarora Formation is a medium-grained white sandstone that is the predominant ridge former in central Pennsylvania. The rocks visible on Skytop Mountain differ slightly from this general description. A picture of this outcrop can be seen in Figure 3. The local area is heavily fractured and faulted. The beds are nearly vertical and overturned. The Bald Eagle does not contain any appreciable amounts of conglomerate. Within the Bald Eagle Formation, hydrothermal fluids have infilled the fractures and precipitated a number of minerals including: Sulfides: primarily pyrite, sphalerite, galena, and a few trace minerals (see Figure 7) Quartz Figure 2: Stratigraphic Column of Late Ordovician to Silurian rocks in Central Pennsylvania. From The Geology of Pennsylvania Figure 3: A picture of the Skytop Mountain outcrop. The bridge is about 30 feet from Route 322. The picture looks southwest down the outcrop. Vein Paragenesis The veins ranged in size up to a few millimeters thick There were two basic vein types within the samples. (These can be seen in Figure 7.) Sulfide vein filling There were veins mineralized solely by pyrite. Sphalerite also precipitated along some of these veins. Quartz and sulfide vein filling Pyrite incorporating quartz and quartz incorporating pyrite growths are common. The pyrite and quartz precipitated simultaneously within the veins. (See Figures 7, 8) Most commonly, the veins were lined with quartz before pyrite infilling. Cathodoluminescence studies show multiple generations of quartz growth. Cross-cutting relationships among the veins are common, implying multiple episodes. (See Figure 8) Some of the veins indicated deformation and crack-seal textures. (See Figure 8) Figure 1: Maps showing the location of Skytop Mountain. A) Road map of 322- Skytop shown at red star (from Mapquest) B) Topographic map showing Skytop in the red circle (from Maptech) A) B) Figure 4: This is a picture of the portion of the outcrop where my samples originated. The people in the picture are at the major fault zone. Figure 5: An example of one of the samples I cut for fluid inclusion work. Figure 6: An example of a vein surface covered in pyrite. Figure 7: Point (a) shows a quartz and sulfide vein. Point (b) shows a pyrite vein. Figure 10: A) Striated pyrite cube B)Pyrite Morphologies (Modified from Murowchick and Barnes, 1979) C) Needled/Tabular Pyrite D) Cubic pyrite A) B) C) D) Sulfide and Quartz Growth Textures Powder XRD patterns were obtained for a number of minerals in the vein material Pyrite- needles, pyritohedrons, and a massive slickensided surface of pyrite Sphalerite The quartz is euhedral to subhedral crystals in open veins. In many cases, the quartz showed growth zoning, evident in CL images (Figure 15) The pyrite also exhibited a number of distinctive euhedral crystal forms. SEM images are shown in figures 10- A, C, and D. Pyritohedron Needles / Blades Cubic These morphologies have been investigated by Murowchick and Barnes. They showed that morphology correlates with temperature and degree of supersaturation of mineralizing solutions. (Figure 10-B) Fluid Inclusion Observations and Results Fluid inclusions within the quartz were analyzed. All had a visible vapor bubble at room temperature. No daughter crystals were visible. Most inclusions ranged from very small to a few microns in diameter (all were less than 10 microns). (See Figure 17) All of the inclusions are thought to be primary inclusions- the inclusions did not occur in recognizable strings or patterns, nor did they correspond to recognizable fracture fill in CL images. Homogenization temperatures (Th) (See Figure 18) Temperatures ranged from 110-350 o C. Temperatures not adjusted for pressure. Most temperatures fall between 160 and 220 o C. Also significant and worth noting are a number of inclusions with much higher temperatures than reported by others, ranging from 240 to 350 o C. These high temperatures are consistent with the morphological data provided by Murowchick and Barnes. (refer to Figure 10b) Melting temperatures (Tm) Freezing of the inclusions generally occurred only with substantial undercooling (–50 o C) 2 basic brine chemistries Fairly pure water, meteoric (?) (-6 to –1 o C) Highly saline brines Dominantly –25 o C +/- 1 o C Complete range from -22 to –33 o C The absence of daughter crystals implies were not saturated at trapping and do not exceed 23 wt% NaCl. Freezing point depression below the H20-NaCl eutectic requires that there is significant divalent solute present (Ca, Mg, SO4). (Salinity Ternary, Figure 19) The source(s) of these solutes is unknown. Although we can document a wide range of temperatures and involvement of varying fluids, we are not able to place these variations on a timeline. Bodnar, R.J., 2003, Introduction to aqueous-electrolyte fluid inclusions, In I. Samson, A. Anderson, D. Marshall, eds. Fluid inclusions, analysis and interpretation: Mineralogical Association of Canada, Short Course Ser. 32, p. 81-100. Lacazette, A. J., 1991, Natural hydraulic fracturing in the Bald Eagle Sandstone in central Pennsylvania and Ithaca Siltstone at Watkins Glen, New York [PhD Thesis]: State College, Penn State University. “Matternville, PA.” Mapquest 2005. Accessed Feb-05. “Matternville, PA.” Maptech 2004. Accessed Dec-04. Murowchick, J., Barnes, H., 1987, Effects of temperature and degree of supersaturation on pyrite morphology: American Mineralogist, v. 72, p. 1241-1250. Shultz, C., 1999, The Geology of Pennsylvania: Commonwealth of Pennsylvania. Tregaskis, S., 1979, Geological and geochemical studies of the Woodbury zinc and lead occurrences, Bedford County, PA [Masters Thesis]: State College, Penn State University. References Cited Figure 18: Homogenization Temperatures Figure 17: An image of fluid inclusion within a quartz sulfide vein. Figure 19: A ternary diagram representing salinity for highly saline brines. (from Bodnar, 2003) Figure 8: One of the thin sections showing deformation in the vein. Notice the offset of the pyritic vein. Figure 15: A CL image of quartz showing growth zoning.