A Comparison of the Bisbee and Morenci, Arizona, Copper Ore Deposits
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?
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).
Now that I have generically explained how porphyritic intrusions occur, I will describe the two different strategies used to mine them, which I obtained from my research. If the mineralized area is horizontal to the surface, open pit mining is used. The porphyritic stock is generally mined in this method. If the ore is more vertical to the surface, you must dig down to the ore; this is referred to as underground mining. Shafts and levels are created in order to get to the heavily mineralized area. Mineralized veins in the softer rock formations surrounding the porphyritic stock are generally mined by this method. The Bisbee and Morenci areas have both open pit and underground mines. My first step in my research of these mines was to go out and obtain a geologic map of the state of Arizona. This map was able to give me a very general idea of the rock formations in these areas. After looking at this map, I felt I needed something a little more detailed. I decided to visit the Office of the Arizona Geological Survey to see if they had more detailed maps of the areas. They were able to come up with geologic maps of the Bisbee and Morenci mines. From these maps, and their explanations of the geologic time scale, I was able to determine the different rock formations in the two areas.
Bisbee: The first rock, Pinal Schist, I categorized as my "Base Rock" because it was the oldest rock in the Bisbee area, from the early Proterozoic Era (2.5 billion years ago). The next category was the "Older Sediments," which spanned from the Cambrian Period to the Permian Period (544 million to 290 million years ago). This group consisted of Bolsa Quartzite, Martin Limestone, Escabrosa Limestone, Horquilla Limestone, and the Colina Limestone Earp Formations. In the Jurassic Period (208 million years ago), the Juniper Flat Granite Porphyry intruded, which makes up the "Porphyritic Intrusion" of Bisbee. In the Cretaceous Period (146 million years ago), the Glance Conglomerate, Morita, and Mural Limestone Formations were shaped, making up the "Younger Sediments." Very recently (1 million years ago to the present), Pediment Gravels were introduced, which make up the "Recent Sediments" group at Bisbee. I categorized all these formations relative to the geologic time scale and placed them in a chart (see above).By observing the maps of this area, I found out that there was a lot of faulting and folding. The open pit area surrounded most of the Porphyritic Intrusion. Underground mine shafts were scattered around the Porphyritic Intrusion, in the formations of Older Sediments.
Morenci: The first rock was granite; I categorized that as my Base Rock because it was the oldest rock in the Morenci area, from the early to mid-Proterozoic Era (2.5 billion to 1.6 billion years ago). The Older Sediments spanned from the Cambrian Period all the way to the Cretaceous Period (544 million to 146 million years ago). This group consisted of Coronado Quartzite and Longfellow, Modoc, and Pinkard formations. In the Paleocene and Eocene epochs (65 million to 58 million years ago), the Diorite Monzonite and the Diabase Granite Porphyrys intruded, and make up the Porphyritic Intrusion
category. In the Neogene Period (1.6 million years ago), the Conglomerates, Tuffs, and Basalts formed, making up the Younger Sediments. Very recently (1 million years ago to the present), alluvium and conglomerates were introduced, making up the Recent Sediments group at Morenci. I categorized these formations and added them to the chart.
From various resources I collected a list of minerals that have been found in the Morenci area. Some of these minerals are turquoise, iron pyrite, chalcopyrite, native copper, malachite, and azurite. Bisbee: I made an observation trip to the town of Bisbee. Upon arriving in Bisbee, I participated in an underground mine tour of the Queen Mine. During the trip I observed how veining occurred inside these particular rock formations. The tour guide also explained about levels, shafts, and stoping. Shafts are the vertical tunnels created to gain access to the
mineralized areas, levels are the horizontal tunnels in the mine, and stopes are the dugout portions of heavily mineralized areas. After studying my geologic maps, I noted that this particular mine was located among the formations of Older Sediments.
I also had a chance to observe the Bisbee area from above ground at different locations. While studying my geologic map, I observed from the open pit lookout and its closely surrounding areas. I also had the opportunity to observe a wider view of the complete Bisbee area, from the eastern portion looking west and from
the western portion looking east. I used a digital camera to record all my observations and make real-life comparisons to my geologic map. I was able to see what the Base Rock, Older Sediments, Porphyritic Intrusion, Younger Sediments, and Recent Sediments actually looked like. I also observed a bright-colored copper solution at the bottom of the open pit.
At the open pit lookout, I found literature explaining the local geology. I obtained mineral samples that were collected from this area. Some of these mineral samples were chalcopyrite, native copper, bornite, chrysocolla, galena,
turquoise, pyrite, malachite, and azurite.
Morenci: I made the three-hour trip to Morenci in order to observe the mine. Previously, I had examined my geologic map to familiarize myself with the different rock formations, ages, faulting, pit area, and local mine shafts of that area. While there, I visually observed the Morenci area from many different locations. My first stop was at the open pit lookout, in which I observed the pit and surrounding areas. On my second stop I observed from the eastern portion of the area, looking west, and on my third stop I examined from the western portion of the area, looking
east. Again, I recorded and made comparisons to the geologic map in hand.I made real-life observations of the five general rock formations I had previously categorized. I observed the landscape in which the Younger Sediments dramatically changed into Base Rock. With the help of my geologic map, I saw the major fault that separated the two rock types. I also observed where erosion was taking place in the Younger Sediments. One of my observations that I found very interesting was of a herd of bighorn sheep bedded down inside the mine properties, atop
of a mine dump. The question that immediately came to mind was, "Have these sheep adjusted from their original habitat in order to coexist with this new mining environment?"
I found that the geology in both the Morenci and Bisbee areas were very similar in certain respects.
Although both areas contained various rock formations within various time zones, I was able to simplify these formations and group them into five major classes:
- Base Rock
- Older Sediments
- Porphyritic Intrusion
- Younger Sediments
- Recent Sediments
I found these five major classes in both areas.
One of the biggest differences I found was that the Porphyritic Intrusion in the Bisbee area, which dated to about 208 million years old, was much older than in the Morenci-area Porphyritic Intrusion, which dated to about 60 million years old. Although the Bisbee Porphyritic Intrusion was much older, I noticed it was much smaller. I would venture to guess that the Morenci ore deposit is about five times larger than the Bisbee ore deposit.
From the geologic maps I acquired, I derived my own maps (A and B) illustrating the five major categories, major faulting in the areas, and mine locations. It was very interesting to see from these maps the tremendous amount of faulting present in both areas. It was also very interesting to see where the underground mine shafts were located. Most of the underground mine shafts were located outside of the perimeter of the Porphyritic Intrusions, mostly in the Older Sediment formations. True to the research I had previously made on mining strategies, the perimeter of both open pits tended to surround the Porphyritic Intrusions.After examining how the ore deposits were formed in both these areas, it was not surprising to me that many of the popular minerals found in one location were also found in the other.I realize this is a very general description of what is a very complicated and complex science. Natural processes that take place over millions of years, processes which I had previously taken for granted or had not even thought about, are now coming into focus and comprehension. Now, when I watch the landscape while sitting in a vehicle traveling from here to there, many more questions and ideas come to my mind.
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More About This Resource...
This winning entry in the Museum's Young Naturalist Awards 2004 examines the geology of two mines in Arizona. Amanda's narrative essay (with photographs, illustrations, maps, and references) discusses:
- how her interest in Earth science stems from her family's work at Arizona mines and geologic samples given to her by her grandfather
- what she has learned so far about Earth science, which serves as a nice overview of the basic geologic concepts
- how porphyritic intrusions occur and the two different strategies used to mine them
- how the geologic maps of the Bisbee and Morenci mines that she got from the Office of the Arizona Geological Survey helped her determine the different rock formations in the two areas
Less than 1 period
Supplement a study of Earth science with an activity drawn from this winning student essay.
- Send students to this online article, or print copies of the essay for them to read.
- As a class, work together to research the geology of your region and create a sediment or geologic chart like the one in the essay.
OriginYoung Naturalist Awards
SubtopicMinerals and Resources