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The state of Arizona is the biggest supplier of copper in the United States. My grandpa was employed at both the Bisbee and Morenci mines in Arizona, where he operated an ore dump truck for over 30 years. My father was also employed at the Morenci mine for about five years. When I was little, I was always surrounded by geologic samples from Arizona. When I became an adolescent, my grandpa gave me exquisite copper, turquoise, malachite, azurite, and pyrite mineral samples from the mines. I was always intrigued by how they were formed and how old they must have been. In order to set the scene for this report, I will go over what I have learned about Earth science. The Earth consists of a solid core surrounded by a liquid outer core, which is then surrounded by a solid mantle. The crust is the Earth's outermost shell. In this report I will be dealing mainly with the Earth's crust. Rocks are classified into three different types: igneous, metamorphic, and sedimentary. Igneous rocks are molten formations that harden at or below the Earth's surface. Metamorphic rocks form when other rocks either change in mineralogy, or surface as a result of changing chemical or physical surroundings. Sedimentary rocks are formed when rocks erode and are washed downstream; when layers of eroded gravel are dumped on top of each other, the layers are pressed together until the bottom layers turn into sedimentary rocks. This process can take millions of years. The geologic time scale is used when one wants to know which rocks came from what period in time. The geologic time scale is separated into eons, eras, periods, and epochs, which classify the ages of rocks. |
I decided I would observe the Morenci and Bisbee copper ore deposit areas. I needed to become a naturalist to investigate and answer the questions I had. What is the geology of these two areas? How were the copper deposits formed? How are these deposits mined? What are the similarities and differences between the rock formations in these two areas? Were similar minerals formed in both these areas?  How minerals form. (Click to enlarge) After researching copper deposits, I discovered that the Morenci and Bisbee mines are considered "porphyry copper deposits." A porphyry copper deposit is formed in many stages. First, the base rock is covered by sediments; we'll call them "older sediments." Folding and faulting of the Earth occurs. This folding either upthrusts or downthrusts the crust. After a long period of time, strong winds and water currents force the rocks to erode. A porphyritic igneous intrusion, which is a molten magmatic stock, intrudes the crust and almost gets to the surface. This stock contains all the minerals, and eventually mineralizes the surrounding rock formations. Younger sediments are deposited on top. More faulting occurs, and then erosion brings the porphyritic intrusion closer to the surface. Mineralization continues to occur, followed by more recent sediments covering the land. Mining operations are then used to get the minerals created from the intrusion. I created a series of drawings illustrating this sequence of events (see above). |
        
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