TO LOG THIS EARTHCACHE INCLUDE
1. The name of this EarthCache on the first line.
2. The number of people in your group.
3. How wide is the excavated vein system? A) 20 feet; B) 40 feet; C) 50 feet; D) >100 feet
4. How long is the excavated vein system? A) 20 feet; B) 40 feet; C) 50 feet; D) >100 feet
5. What is the approximate orientation of the vein system? A) North-South; B) East-West; C) Northwest-Southeast
5. Which Big Ideas (1-9) are connected (list)?
6. Which GeoPrinciples are relevant (list)?
7. Include a photo or 2 if you're so inclined (optional).
Note: In order to manage email volume, you may assume your responses are accurate if you do not get an email after logging this EarthCache. If a response is grossly inaccurate, you will not receive credit for the cache.
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GEOLOGY OF GARNET HILL
Garnet Hill is located near the confluence of Moore Creek and the North Fork of the Mokelumne River, 2 miles west of the Salt Spring Reservoir dam. Tungsten, gold, copper, and molybdenum were all mined at this location. It has also been a source of fine andradite garnet and epidote crystals. In 1953 and 1954, the North American Tungsten Mining Company trucked ore from Garnet Hill to the Church mine, 5 miles south of Placerville in El Dorado County, where the ore was treated by flotation, which concentrated the tungsten metal, that was then shipped to Bishop, CA for sale. Production ceased in 1954 and the mine has not reopened since. Tungsten is used for its physical properties including hardness, density, and high melting point. It is an ingredient in cemented carbides, also called hardmetals, which are used for the shaping of metals, alloys, wood, composites, plastics and ceramics, as well as for the mining and construction industries. It is also used as lighting filaments, electrodes, electrical and electronic contacts, wires, sheets, rods, etc.
Roof Pendant
The ore deposit here is at the crest of Garnet Hill, a steep, dome-shaped peak that forms what geologists call a roof pendant. Magma intruded, or came up into, the older, overlying sedimentary rocks (layers of shale, siltstone, limestone, sandstone, etc., on the “roof” of the magma chamber). This heated and “baked” the rocks in a process called contact metamorphism, changing them into slates and marbles. The outline of the ore body formed a pendant-like shape on a geologic map (see Figures 1 & 2). Those metamorphic rocks were later uplifted, exposed to weathering and erosion, and all that was left behind here was a piece of roof pendant –metamorphic rock – surrounded by granitic rock. Some of the granitic rock closest to Garnet Hill was also metamorphosed into coarse-grained gneiss (looks like “stretched” granite…).
Figure 1: Geologic map of Garnet Hill and Walker Ridge showing the distribution of granodiorite, metasediments, skarn, and marble (from Brock, 1970).
Figure 2: Cross section through Garnet Hill and Walker Ridge showing the Garnet Hill Roof Pendant (from Brock, 1970). A cross section, which views the earth as if it were cut open and seen from the side, is an inference, since the relationships generally cannot be observed directly. They are helpful diagrams of slices through the earth used to clarify or interpret geological relationships (volumes of ore deposits, oil, natural gas, groundwater, etc.).
Tactites/Skarns
Tactite bodies, or skarn deposits, are formed when granitic intrusive rock contacts carbonate sedimentary rocks (such as limestone). There are multiple tactites scattered around Garnet Hill (see Figure 3), which mainly consists of medium-grained gneiss with small amounts of quartz-mica schist. Additionally, andradite garnet, epidote, and quartz are all present (see Figure 4). Tactites/skarns are the result of a process of geochemical transformation called metasomatism (from meta = change + sōma = ‘body’ - changing the body of the rock). During metasomatism, rock is altered and changed chemically by hydrothermal fluids (hot water) into different rock. This is similar to how clear water passed through the altered rocks are most often formed at the contact zone between intrusions of granitic magma bodies and carbonate sedimentary rocks such as limestone, dissolving the limestone and converting it into metamorphic rock; this is called contact metamorphism (see Figure 5). Skarns are sometimes associated with mineable accumulations of metallic ores of iron, copper, tungsten, zinc, lead, gold, and several others.
Figure 3: Geologic map of Garnet Hill showing tactite bodies, granodiorite, gneiss, and some schist (modified from Reynolds, 1953, in Ralph, 2017).
Figure 4: Clockwise from upper left: A) Rock = gneiss; B) Mineral = garnet (root beer brown minerals); C) Mineral = quartz; and D) Mineral = epidote (green mineral).
Figure 5: Skarns are associated with Contact Metamorphism (Briggs, 2016). The Garnet Hill roof pendant formed by contact metamorphism, from an intrusive magma body (shown in red above) creating the tactite/skarn deposit at this EarthCache site.
Lode gold also formed this way in the Sierra as sedimentary and igneous rock with very low concentrations of gold was buried by subduction and heated deep in the earth over a large region. Water in the rock was also heated and helped dissolve the low levels of gold along with silica. The hot, gold and silica-bearing fluids rose and migrated upward toward the earth’s surface along zones of weakness, and was forced into “lodes” or cracks. The fluids cooled and solidified into gold-rich quartz veins. Some of these veins were weathered and eroded and the gold accumulated in streams forming placers, or deposits of gold nuggets.
Regional vs. Contact Metamorphism
Two general types of metamorphism are contact metamorphism and regional metamorphism. Contact metamorphism is also known as thermal metamorphism and can occur when existing rocks are significantly altered due to the intense heat emanating from an intruding body of hot magma or lava. Unlike regional metamorphism, which affects vast areas due to mountain-building processes, contact metamorphism is localized to the area immediately surrounding the igneous intrusion (see Figure 5 above). Regional metamorphism occurs when the mineralogy and texture of rocks are changed over a wide area (i.e. the foothills of all of the Mother Lode) by deep burial and heating associated with the large-scale forces of plate tectonics (see Figure 6). In regional metamorphism, rocks that form closer to the margin of the tectonic plates, where the heat and pressure are greatest, often differ in their minerals and texture from those that form farther away. The contact metamorphism that occurred at Garnet Hill, is localized, and was caused by the heat from an igneous intrusion.
Figure 6: Regional Metamorphism and Tectonics (from Dickerson, 2017). Much of the metamorphism in the Mother Lode foothills formed due to regional heat, pressure, and fluid interactions with the rock along a subduction zone.
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EARTH SCIENCE BIG IDEAS
The Earth Science education community put together 9 “Big Ideas” for the Earth Science Literacy Initiative (ESLI), shown below. Their purpose was to highlight the main concepts and ideas a person should understand to be literate in the earth sciences:
An Earth-science-literate person:
• understands the fundamental concepts of Earth’s many systems
• knows how to find and assess scientifically credible information about Earth
• communicates about Earth science in a meaningful way
• is able to make informed and responsible decisions regarding Earth and its resources
Which of these Big Ideas below do you think are most relevant to this EarthCache?
Earth Science Literacy Project:
1. Big Idea 1: Earth scientists use repeatable observations and testable ideas to understand and explain our planet.
2. Big Idea 2: Earth is 4.6 billion years old.
3. Big Idea 3: Earth is a complex system of interacting rock, water, air, and life.
4. Big Idea 4: Earth is continuously changing.
5. Big Idea 5: Earth is the water planet.
6. Big Idea 6: Life evolves on a dynamic Earth and continuously modifies Earth.
7. Big Idea 7: Humans depend on Earth for resources.
8. Big Idea 8: Natural Hazards pose risks to humans.
9. Big Idea 9: Humans significantly alter the Earth.
For more details see: Earth Science Literacy Initiative
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GEOPRINCIPLES
There are several fundamental principles, developed over time, that guide geological reasoning and critical thinking, listed below. Read each short description, then use your best judgement to determine which principle, or principles, best relate to this EarthCache.
7 Principles in Geology:
1. Superposition – the oldest strata are at the bottom of the sequence
2. Original Horizontality - layers of sediment are originally deposited horizontally
3. Lateral Continuity - layers of sediment initially extend laterally in all directions
4. Faunal Succession - fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances
5. Law of Intrusive Relationships - the geologic feature which cuts another is the younger of the two features
6. Uniformitarianism - the assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe
7. Catastrophism - the theory that the Earth has been affected in the past by sudden, short-lived, violent events, possibly worldwide in scope
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LOGISTICS AND SAFETY
This site can be reached via Salt Springs Reservoir Road. Take USFS Forest Route 7N08 to the GPS parking location. Park your vehicle and continue on to the EarthCache location. Please note that the US Forest Service road is unpaved and will be closed during portions of the year in winter months due to snow and mud. You will likely need a high clearance vehicle on dirt Forest Service roads. There is a locked gate at the bottom of Garnet Hill and you will have to hike ~1.7 miles up to the EarthCache location. The elevation gain is approximately 1,100 feet (from ~3,300 to ~4,400 feet). This can be a strenuous challenge - be sure to be prepared for the challenges of this EarthCache!
Visitors to this site should plan ahead and prepare by:
• Knowing the regulations and special concerns for the area you are planning to visit (obeying laws that prohibit collection or destruction of artifacts);
• Carrying a map and a GPS unit and/or compass;
• Staying on existing roads and trails;
• Staying away from any/all mine shafts and adits;
• Planning for extreme weather, hazards, and emergencies;
• Being aware that cell phones DO NOT usually work in the rural areas away from the major highways;
• Leaving your travel plans with a responsible party, including the date and time of your return;
• Being aware of any natural hazards associated with the region (e.g. poison oak, rattlesnakes, mosquitoes, cliffs/steep slopes, etc., etc);
• Carrying a full-size spare tire, extra food, water, and warm clothing;
• Following the “Tread Lightly" and "Leave No Trace” philosophy.
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TO LOG THIS EARTHCACHE INCLUDE
1. The name of this EarthCache on the first line.
2. The number of people in your group.
3. How wide is the excavated vein system? A) 20 feet; B) 40 feet; C) 50 feet; D) >100 feet
4. How long is the excavated vein system? A) 20 feet; B) 40 feet; C) 50 feet; D) >100 feet
5. What is the approximate orientation of the vein system? A) North-South; B) East-West; C) Northwest-Southeast
5. Which Big Ideas (1-9) are connected (list)?
6. Which GeoPrinciples are relevant (list)?
7. Include a photo or 2 if you're so inclined (optional).
Note: In order to manage email volume, you may assume your responses are accurate if you do not get an email after logging this EarthCache. If a response is grossly inaccurate, you will not receive credit for the cache.
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Note: For a brief summary of the geologic history of the Central Sierra, see this EarthCache:
Dragoon Gulch EarthCache
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REFERENCES
1. Brock, Kenneth J., 1970, “Mineralogy of the Garnet Hill Skarns”, Ph.D. Dissertation submitted to the Department of Geology, Stanford University.
2. Briggs, P., 2016, June 15, “Unit 5 – Earth Resources Real-Life Applications: Skarns are Associated with Contact Metamorphism”, retrieved July 02, 2017, from http://slideplayer.com/slide/5848006/.
3. Busby, Cathy J., Andrews, G.D.M., Koerner, A.K., Brown, S.R., Melosh, B.L., and Hagan, J.C., 2016, “Progressive derangement of ancient (Mesozoic) east-west Nevadaplano paleochannels into modern (Miocene–Holocene) north-northwest trends in the Walker Lane Belt, central Sierra Nevada”, Geosphere 12, p. 135-175, 2016, http://www.geosphere.gsapubs.org.
4. Busby, Cathy J., Koerner, Alice, Hagan, Jeanette, and Andrews, Graham, 2012, “Sierra Crest graben: a Miocene Walker Lane Pull-apart in the Ancestral Cascades Arc at Sonora Pass”, in, N. Hughes and Garry Hayes (eds), “Geological Excursions, Sonora Pass Region of the Sierra Nevada”, Far Western Section, National Association of Geoscience Teachers field guide, p. 8-36.
5. Dickerson, B., June 27, 2017, “Metamorphic Facies and Mineral Assemblages”, retrieved July 02, 2017, from http://slideplayer.com/slide/6873498/.
6. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.
7. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.
8. Putnam, Roger, (pers. comm.), June 2017, Professor of Earth Science, 11600 Columbia College Drive, Sonora, CA, 95370.
9. Ralph, J., June 25, 2017, “Garnet Hill Mine (Rheona claim), Garnet Hill, Calaveras Co., California, USA - Geologic Map of Garnet Hill”, retrieved June 29, 2017, from https://www.mindat.org/photo-402994.html.
10. Rohlen, Ginger, (pers. comm.), June 2017, Teacher, Sierra Waldorf School,19234 Rawhide Rd., Jamestown, CA., 95327.
11. Schweickert, Richard, 2006, “Accretionary Tectonics of the Southern Part of the Western Sierra Nevada Metamorphic Belt” (modified from a 1999 guidebook article by Schweickert, Girty, and Hanson), in J. Tolhurst (ed), “Geology of the Central Sierra”, National Association of Geoscience Teachers Far Western Section Fall Conference field guide, p. 55-95.
12. Tolhurst, Courtney, (pers. comm.), July, 2017, Student, Sonora High School, 430 N Washington St,., Sonora, CA, 95370.
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