EXPLORING FOR COPPER DEPOSITS
(teacher's guide)

M. R. Farr
Department of Geology
University of Kansas
Lawrence, KS 66045


Level: Senior high

Anticipated Learning Outcomes

Background

Background for the activity is given in the student handout. Students should know the three major rock classes and their origin and understand the concept of a geologic map.

Material

Procedures

  1. Students should read the background information in the handout.
  2. Ask the students to look up the current price per lb. for copper in the commodity section of a newspaper such as the Wall Street Journal or New York Times. (Alternatively, you can provide them the current price. The price at the time this exercise was written was $1.18 per lb.)
  3. In order to help visualize ore deposits, show students specimens of ore minerals (especially copper minerals such as chalcopyrite, bornite, or azurite) if these are available. Such specimens may be obtained from mineral supply houses or state geologic surveys.
  4. Students solve the problems in the handout. You may ask the students to show their work for question 9 on a separate sheet.


Results and Discussion

  1. Ore should occur along the edges of the stocks within the altered zone in Figure 4.
  2. Students should color in the symbol keys on both the sediment and soil maps.
  3. Students should use Table 1 to color in the squares and circles on both maps to correspond to the symbol keys.
  4. One deposit is located within the altered zone on the northwestern margin of the large igneous stock (upstream from stream sediments 3 and 7 and roughly between soil samples B3 and A5). The other is located on the southeastern margin of the same stock (upstream from stream sample 19 between soil samples D6 and C7).
  5. Cupriferous Creek is flowing east (or east-southeast) at sediment sample 21. The "v's" point to the east.
  6. The anomaly (stream sample 18) is probably due to pollution from industrial waste, sewage, or a landfill.
  7. The anomaly southeast of the railroad (sample 32) is probably due to construction contamination from the bridge.
  8. The anomalies in the southwestern portion of the map (e. g. stream sample 23) are probably associated with vein deposits in the host sandstone. As stated in the handout, these vein deposits are not commonly economical.
  9.  Total production cost = $25/ton x 20 million tons = $500 million
     1 % Cu = 1 tons Cu / 100 tons of ore
     tons of Cu = 1 tons Cu/100 tons of ore x 20 million tons of ore = 0.20 million tons
     lbs. of Cu = 0.20 million tons x 2000 lbs./ ton = 400 million lbs.


    The following assumes a $1.18 per lb. price for copper. The answers will vary depending on the price used by the students.

Gross income = $1.18/lb x 400 million lbs. = $472 million

The gross income ($472 million) is less than the production costs ($500 million). Therefore the deposit will not be profitable.


References

Evans, Anthony M., 1980, An introduction to ore geology: Elsevier, New York, 231 p.


EXPLORING FOR COPPER

 

Background

Ores are rocks and minerals that can be mined and sold for a profit. The primary products of ores are metals such as iron, gold, zinc, and copper. These metals are recovered from the ores through special kinds of industrial processes. Such metals occur at low concentrations in all rocks. However, they can be mined only when they occur at high concentrations.

The concentration of a metal in an ore is called its grade. Grade is usually expressed as a weight percentage of the total rock. For example, 1000 kilograms (kg) of iron (Fe) ore that contains 300 kg of iron metal has a grade of 30%:

Grade = (kg metal / kg rock ) x 100

Fe grade = (300 kg Fe/1000 kg ore) x 100 = 30%

Metals occur at much lower grades in most rocks, sediments, and soils. A common way to express these lower concentrations is in terms of parts per million (ppm). If a rock has 1 ppm zinc (Zn), then 1 million grams of the rock (1000 kg) contains 1 gram of Zn.

To determine whether a potential ore deposit will be profitable, mining companies must consider a number of factors including: the size of the deposit, its average grade, mining and refining costs, and environmental-related costs. The estimated gross income must be greater than the total costs in order for the deposit to be considered a true ore deposit.

There are many types of ore deposits. They occur in all sorts of sedimentary, igneous, and metamorphic rocks and form in many different ways. However, some key requirements for the formation of most ores include a source for the metals, a mechanism for transporting the metals, and a mechanism for precipitating the ore minerals.

In this exercise, you will learn about a particular kind of ore deposit known as a porphyry copper deposit. Most of the world's copper comes from such deposits located primarily in South America, New Guinea, Indonesia, the United States, and Canada. Copper (Cu) occurs primarily in the mineral chalcopyrite (CuFeS2) within these deposits. The porphyry deposits occur underground on the edges of intrusive igneous bodies known as stocks.

Figure 1 is a cross section of the subsurface that shows how these ore bodies form. Think of this diagram as a picture of a vertical slice of the earth. As a hot igneous stock intrudes into the rock already present, it encounters underground water derived from rainfall. The stock heats this water, and the water begins to move in large circular paths. As the water moves downward, it becomes hotter and leaches copper and other metals from the different rocks it encountered. As the metal-rich water moves back upward, it cools and changes its chemistry, so that chalcopyrite and other ore minerals are precipitated at the edge of the stock. In this model, the immediate sources of the metals are the rocks surrounding the igneous stock, the circulating groundwater is the transporting mechanism, and the cooling and changing composition of the groundwater is the precipitation mechanism.


Figure 1. Vertical cross section showing a porphyry copper deposit as it occurs deep within the earth. (Modified from Evans, 1980)

In addition to forming ore deposits, this circulating water can form large bodies of altered rocks surrounding the stocks known as alteration zones. Minor copper mineralization can be formed away from the stocks within thin planar bodies known as veins. However, this mineralization does not usually contain enough copper to be considered ore.

As water and wind erode the surface of the earth, they remove the tops of the igneous stock, alteration zone, and porphyry copper deposit. Figure 2 is a geologic map that shows what these eroded rocks look like from the air. A central stock is surrounded by copper ore bodies, then an alteration zone, and finally unaltered sedimentary and/or volcanic rocks with some minor copper mineralization.


Exploration Techniques

Exploration geologists use a variety of techniques to find such ore deposits. One important technique is geologic mapping. A geologic map such as Figure 2 shows the distribution of the various rocks at the surface of the earth. In the case of porphyry copper deposits, geologists know that such deposits usually form on the outer edges of the igneous stocks and within alteration zones. Once a map such as Figure 2 is constructed, the geologists can focus their exploration activity in these areas.


Figure 2. Geologic map showing the aerial view of a porphyry copper deposit

Another common exploration technique is called geochemical exploration. A type of geochemical exploration is gold panning [see another on-line activity called Panning for Gold and Magnetite]. Prospectors and geologists have long used this technique to find gold deposits. A large pan is filled with sediment and water from a stream and swirled around, so that the moving water sorts the grains by their density. Because the gold grains are very dense and easy to recognize, the geologist can quickly isolate these grains. They often count or weigh the grains to determine the approximate gold content of the stream sediments at that particular site. After doing this at many different locations in an area, the geologist makes a map of the gold content of the stream sediments to help find gold deposits.


Figure 3. Map showing a stream and sediment survey

Such a map may look like Figure 3. The circles along the streams are locations at which gold (Au) has been panned. The numbers refer to the number of gold grains that were found at these locations. The arrows point in the direction that the stream is flowing (the "v's" formed by the joining streams always point downstream). After studying the map, geologists would predict that gold deposits may occur upstream from the two highest gold values (35 and 21). Unusually high concentrations such as these are termed geochemical anomalies by exploration geologists. The number of grains decreases downstream from these anomalous values. Upstream from the predicted gold deposit, there are few gold grains in the sediments. This is because the stream can only carry the gold grains downstream from the deposit, not upstream.

Geologists also sample stream sediments to explore for porphyry copper deposits. Instead of panning for copper, the geologists take samples of the sediments to a laboratory to determine their copper concentrations. They then make a map similar to Figure 3, but the numbers at each sample location would refer to copper concentrations.

Another commonly used geochemical exploration technique is soil geochemistry. Geologists establish a sampling grid over an area of interest. Figure 3 shows such a grid. It is defined by the letters A through D on the north-south axis, and the numbers 1 through 5 are on the east-west axis. Geologists analyze soil samples at each node of the grid (where the lines cross). They then construct a map showing the concentration of gold at each location. On this map, the highest value of gold (4.3 ppm) occurs at node B3. Node B4 has a lower gold value than B3 (0.53 ppm), but higher than all of the other soil samples in the area. Geologists could use these anomalous values, together with the anomalous stream sediment values to predict that an ore body was present below the soil somewhere in the blackened area.

One difficulty in using sediment and soil geochemistry to explore for ore deposits is the occurrence of anomalies related to human activities. Construction of bridges often produces high concentrations of metals in sediments. Pollution from industry or landfills can impart high metal content to soils, streams, or the atmosphere. Such geochemical anomalies produced by human activities can be confused with anomalies that might indicate the presence of ore deposits.


ORE DEPOSITS PROBLEM

The Copper Kettle Mineral Exploration Company has decided to explore for porphyry copper deposits in the country of Oro in an area likely to contain ore deposit. The geologists first made a geologic map of the area (Figure 4). The company then collected and analyzed 36 stream sediment samples (open circles in Figure 5) and 88 soil samples (Figure 6). The soil samples are from a sample grid defined by the letters A-H and 1-11 in Figure 5. Table 1 gives the copper concentrations for these sediment and soil samples.

  1. Based on your knowledge of how porphyry copper deposits form, what portion of the geologic map (Figure 4) would you expect to find ore deposits?


  2. Color in the squares and circles in the symbol key indicating the concentration of copper in the sediment and soil samples in figures 5 and 6. Use red for samples with greater than 500 ppm and blue for samples containing 200 to 500 ppm copper. Don't color the symbols for samples with less than 200 ppm copper.
  3. Color in the squares and circles on the map to correspond to their value in the table. For example, sample 5 in Figure 4 contains 231 ppm copper. Therefore, you should color it blue because it falls within the 200 and 500 ppm category on the symbol key. Circle the anomalies indicated from the sediment and soil maps.
  4. There are two major copper deposits in the area. Based on the overlap in anomalies and the geologic map, outline the areas where you would expect to find the deposits. You may find it helpful to use the maps as overlays and hold them up to a light.
  5. What direction is Cupriferous Creek flowing at sediment sample 21? How do you know?


  6. What is the significance of the stream sediment anomaly west of the city of Pollutia?


  7. What is the likely cause for the stream sediment anomaly southeast of the railroad bridge?


  8. What is the likely origin for other anomalies on the map?


  9. One of the deposits contains 20 million tons (U. S.) of potential ore. The cost of producing the ore (mining, refining, environmental reclamation, etc.) will be $25 per ton. What will be the total production cost?

    You will be given a price per lb. for copper by your teacher, or you will be asked to look up the current price of copper in a journal or newspaper. What is this price?

    Given an average copper grade of 1%, how many tons of copper will be produced? How many lbs. of copper is this?

    Will the deposit be profitable?

 



Figure 4. Geologic map of the area

 

Figure 5. Map showing location of stream sediment samples

 


Figure 6. Map showing soil sample grid

Table1. Copper concentrations in sediment and soil samples.

 Stream Sediment Samples

Soil Samples

 Sample

Copper Conc. (ppm)

Sample

Copper Conc. (ppm)

Sample Copper Conc. (ppm)
1 120 A1 122 E1 125
2 115 A2 102 E2 188
3 550 A3 305 E3 31
4 422 A4 589 E4 52
5 231 A5 505 E5 88
6 255 A6 300 E6 105
7 625 A7 125 E7 136
8 198 A8 175 E8 155
9 105 A9 165 E9 105
10 106 A10 152 E10 111
11 135 A11 141 E11 105
12 115 B1 122 F1 160
13 108 B2 108 F2 250
14 95 B3 355 F3 189
15 98 B4 305 F4 120
16 55 B5 201 F5 108
17 68 B6 106 F6 111
18 735 B7 101 F7 105
19 750 B8 99 F8 58
20 86 B9 91 F9 85
21 78 B10 81 F10 52
22 86 B11 83 F11 25
23 513 C1 177 G1 113
24 105 C2 165 G2 125
25 123 C3 155 G3 355
26 165 C4 143 G4 105
27 105 C5 122 G5 117
28 533 C6 222 G6 125
29 350 C7 955 G7 165
30 105 C8 555 G8 105
31 102 C9 125 G9 102
32 355 C10 105 G10 172
33 51 C11 111 G11 58
34 85 D1 108 H1 43
35 105 D2 95 H2 28
36 76 D3 82 H3 101
    D4 73 H4 108
    D5 53 H5 98
    D6 771 H6 92
    D7 443 H7 82
    D8 44 H8 77
    D9 38 H9 75
  D10 21 H10 73
    D11 22 H11 71

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