When you hike throughout the eastern piedmont, and especially around the Uwharrie Mountains and the area of Gold Hill, you'll see quartz everywhere. Folks are fond of lining driveways with it, you'll see it crushed to make driveways, and you'll definitely see big hunks of it in many front lawns. But what is quartz?
Under some conditions, extreme heat and high pressure changes the structure of the existing rocks. In doing so, "volatiles" such as water, carbon dioxide, and other gases and fluids are driven out of those rocks. Quartz, known chemically at silicon dioxide, or SiO2, is a very abundant mineral throughout the Earth's crust, and those volatiles carry a lot of quartz with them. Quartz veins are deposited as rising hot fluid solutions seek cooler, lower pressure environments. While the predominant mineral in such quartz veins is silicon dioxide, these deposits often lay down metal-heavy ore deposits. We'll return to such deposits later.
OK. We know that quartz is a mineral, but is it a rock? Those chunks lying in someone's front lawn certainly look like a rock. The Dictionary of Geological Terms says a rock is an aggregate of one or more minerals, so... can you have a rock made out of just one mineral? Sure, and that might be what we're looking at. But here's where it starts to get puzzling: Quartz is first a mineral. Mineral grains, crystals or zones of quartz can be formed under igneous, metamorphic and sedimentary processes. Right now, we'll just say this "rock" we are looking at is quartz, and that the gold- and copper-bearing ores under today's Gold Hill are contained in quartz veins. We will discuss the specific forces that created them in another EarthCache.
To be classified as a mineral, a compound must:
Be naturally occurring
Be a solid at or near the surface of the Earth
Be inorganic (coal is a major exception)
Have a fixed chemical formula
Have an orderly crystalline structure
Quartz meets all of these criteria. With respect to having "an orderly crystalline structure", quartz has a structure that is very strong. In fact, the reason you see all those pieces of quartz lying around the piedmont is because the crystalline structure of quartz is much harder than the rocks in the area, and remains after those have weathered away.
Igneous and metamorphic rocks are formed from pressure and heat -- so much pressure and heat that the chemical composition of the original rock can change. We mentioned that as the super-hot liquids and gases escape from the original rock, great amounts of quartz are carried with them. When the pressure is reduced, the quartz crystallizes and solidifies within fissures, joints, fault zones and other weak areas. Thus, a zone of quartz minerals is formed. The globs of quartz seen throughout this area are probably best thought of as massive mineral formations, rather than rocks, but "quartz-rock" is what they are usually called. It might be helpful to note that quartz-rocks formed under the conditions we've described are considered neither igneous nor metamorphic rocks by geologists.
Quartz is not the only thing swept out of the parent rock material during intense pressure and heat. Iron, magnesium, titanium and many other metals are swept along by the volatiles. Two very important metals which are often carried along are copper and gold. Both are in the quartz veins in the Gold Hill area. Copper joins with iron and sulfur to form a mineral called chalcopyrite. If you see country rocks or chunks of quartz in the area that are stained green, they contained chalcopyrite, and the copper has weathered out to form the green stain. Gold, however, is a "noble" element, and that's another way of saying it's a snob. It rarely joins with other elements to form minerals, and is present in the veins as mostly minute bits and pieces. Gold mining has always come down to the economic cost of separating those minute bits and pieces from the mass of quartz or other rock within which it has been found, versus the value of the separated bits and pieces of gold.
Yes, trying to come to grips with whether those blobs of quartz are rightly called rocks, minerals, or quartz-rocks is a bit of a puzzler, but they are also the providers of some of the most valuable metals on earth.
Note: For other EarthCaches in the Gold Hill mining district, go here.
To receive credit for visiting this EarthCache, please send me an e-mail not part of your log entry, answering the following questions -- remember, I'm not looking for scholarly answers, I just want to see if you've really thought about what you are seeing here at Gold Hill:
1. Make the first line of your e-mail: "Gold Hill: Quartz -- Puzzler and Provider" GC218JC
2. How many people are in your party?
3. Look closely, both up close and at a distance of a few feet, at the large quartz-rock at the coordinates given. Describe the following for me:
b. Colors observed
c. "Pureness" of the quartz-rock, i.e., is is pure quartz mineral, or are other compounds or minerals apparently in the rock?
4. On a few surfaces of this quartz-rock and most others in the Gold Hill area, you'll see at least some reddish staining. The same red staining colors the clay soils throughout the entire piedmont, from Alabama to beyond the Mason and Dixon Line. What is the origin of the red stains on the quartz and clay soils?
5. Some quartz crystals, grown under "ideal" conditions, produce very beautiful and valuable gemstones, such as amethyst, citrine, and onyx. Gold Hill quartz-rocks are interesting, but no one would consider them "gemstone" quality. Why do you suppose such beautiful and valuable gemstones are not present in Gold Hill?
6. Please post a photo (with your log entry) of you and your party at the quartz-rock at ground zero.
The author thanks Phil Bradley, Senior Geologist, North Carolina Geological Survey, for his valuable corrections, additions and suggestions relative to this write-up. Any mistakes herein, however, are solely the responsibility of the author.
The author thanks Vivian Hopkins, Vice President, The Historic Gold Hill and Mines Foundation, Inc., and Chair of the Foundation's History Committee. She has been a tour guide, source of knowledge, and careful fact checker for the author.
Wikipedia -- fact checking concerning several specific topics
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